Experimentally determined K-NH4 partitioning between feldspars, muscovites and aqueous chloride solutions
vorgelegt von Diplom-Mineralogin Birgit Pöter aus Waltrop
Vom Fachbereich VI -Bauingenieurwesen und Angewandte Geowissenschaften- der Technischen Universität Berlin zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften -Dr. rer. nat.-
genehmigte Dissertation
Promotionsausschuß: Vorsitzender: Prof. Dr. H. Wolff Berichter: Prof. Dr. W. Heinrich Berichter: Prof. Dr. G. Franz Berichter: Prof. Dr. E. Althaus
Tag der wissenschaftlichen Aussprache: 24. April 2003
Berlin 2003
D 83 Vorwort
Ohne die Hilfestellung und Ermutigung vieler würde diese Arbeit wohl mit der Einleitung beginnen und auch gleichzeitig schließen. Ich möchte daher allen, die mich während dieser Zeit unterstützt haben, herzlich danken. Im einzelnen gilt mein Dank Herrn Prof. Dr. Wilhelm Heinrich, der mir die Möglichkeit gab, am GFZ Potsdam die (K-NH4) Verteilung zwischen Feldspäten, Glimmern und chloridischen Lösungen zu untersuchen. Ich danke ihm für seine Offenheit und Diskussionsbereitschaft, die mir Denkanstöße gab und zu neuen Anregungen verhalf. Der DFG sei für die Finanzierung des Projektes über drei Jahre gedankt. Weiterhin möchte ich mich bei Herrn Dr. Matthias Gottschalk für die zahlreichen Begebenheiten bedanken, in denen er sich die Zeit nahm, sich mit meinen Fragestellungen auseinander zu setzen. Diese Hilfe war vor allem im Bereich der thermodynamischen Auswertungen für mich unverzichtbar. Darüber hinaus verdanke ich ihm das Erlernen der Rietveldstrukturverfeinerung und die Unterstützung bei Problemen, die sich bei diesem Prozess zwangsläufig auftaten. Danken möchte ich Herrn Dr. Daniel E. Harlov für die gewinnbringende Zusammenarbeit an zwei Publikationen und seine vielfältigen Anregungen. Von seiner unermüdlichen Bereitschaft mein Englisch zu korrigieren, habe ich sehr profitiert. Herzlichen Dank auch an Dr. Stefan Melzer und Dr. Jens Najorka, die aufgrund ihrer gutmütigen Art stets die ersten waren, die von mir mit Fragen bombardiert wurden. Ich danke ihnen für ihre Geduld und ihre hilfreichen Antworten. Herrn Dr. Michael Andrut möchte ich danken für die gute Zusammenarbeit an zwei Publikationen, die zahlreichen per Email geführten Diskussionen und für die Einführung in die IR-Spektroskopie. Mein Dank für das Erlernen der Auswertung meiner IR-Daten gilt Herrn Dr. Christian Schmidt. Außerdem sei ihm für viele anregende Gespräche gedankt. Ohne Hilfe von Herrn Reiner Schulz bei der Durchführung meiner Experimente wäre es wohl zu größeren Katastrophen gekommen. Ihm habe ich die Erkenntnis „Technik statt Kraft“ im Umgang mit experimentellem Gerät zu verdanken. Bei der Durchführung der Hochdruckexperimente war mir Dr. Bernd Wunder behilflich. Ihm sei vor allem auch für seine Freundschaft gedankt und für das Korrekturlesen meiner Arbeit unter Zeitdruck kurz vor der Abgabe. Dafür danke ich auch Herrn Dr. Axel Liebscher, der sich die Zeit nahm, sich kurzfristig mit meiner Arbeit auseinander zu setzen. Für die Verbesserungsvorschläge und die Hilfe bei den Formalitäten herzlichen Dank. Die analytischen Erkenntnisse dieser Arbeit basieren auf den Fähigkeiten der Mitarbeiter des GFZ Potsdams. Großen Dank an Frau Inka Bauer, die mich bei den Laborarbeiten unterstützte und die Röntgenbeugungsaufnahmen inklusive der Probenpräparation für mich durchführte. Weiterhin bedanken möchte ich mich bei Herrn Dr. Dieter Rhede und Frau Oona Appelt, die mich in die Mikrosondenanalytik einführten und mir bei den teils langwierigen und schwierigen Messungen stets beratend zur Seite standen. Frau Erika Schemmert sei für die gute Präparation meiner doch recht winzigen Proben ein Dank ausgesprochen. Für die Einarbeitung in die Ionenchromatographie und seine anregenden Diskussionen bedanke ich mich herzlich bei Herrn Dr. Georg Schettler. Mein Dank für viele gemeinsam am Rasterelektronenmikroskop verbrachte Stunden gilt Frau Ursula Glenz und Frau Dr. Helga Chemnitz. Bedanken möchte ich mich auch bei Dr. Richard Wirth und Frau Karin Paech, die mir bei dem Versuch halfen, meinen Muskoviten mittels Transmissionselektronenmikroskopie zu Leibe zu rücken. Für die angenehme, freundschaftliche und offene Arbeitsatmosphäre, die ich am GFZ Potsdam erlebt habe, danke ich allen Mitarbeitern des Projektbereichs 4.1 recht herzlich. Weiterhin ein großer Dank an Prof. Dr. Sumit Chakraborty, der mir die Möglichkeit gab, nach meiner Zeit in Potsdam an der Ruhr-Uni-Bochum zu arbeiten. Seiner Begeisterungsfähigkeit, Motivationskraft und Diskussionsfreudigkeit habe ich viel zu verdanken.
Zusammenfassung
Experimente wurden nach dem Prinzip der „Austauschsynthese“ bei 400-600 °C / 400 MPa und 500-600 °C / 1500 MPa durchgeführt, um Mischkristalle im System Buddingtonit – K-Feldspat bzw. Tobelit – Muskovit herzustellen. Die Syntheseprodukte wurden mittels Elektronenstrahlmikrosonde (EMS), IR Spektroskopie und Rietveldstrukturverfeinerung von Röntgenpulverdiffraktogrammen analysiert. Als Grundlage dieser Untersuchung dienten die reinen NH4-Endglieder Buddingtonit NH4[AlSi3O8] und Tobelit NH4Al2[AlSi3O10](OH)2. Buddingtonit als auch die synthetisierten (K-NH4)-Feldspäte weisen eine monokline Kristallstruktur vergleichbar dem Sanidin (RG C2/m) auf. Dabei variiert die Dimension der + Elementarzelle mit dem NH4 -Gehalt der Feldspatmischkristalle. Zunehmende Ammoniumkonzentration führt aufgrund des größeren Ionenradius zur Expansion der Elementarzelle, was sich insbesondere auf die Gitterkonstante a auswirkt. Durch EMS Analysen an (K-NH4)-Feldspäten wurde weiterhin gezeigt, dass + eine lineare Korrelation zwischen der Gitterkonstante a und dem NH4 -Gehalt besteht. Diese Abhängigkeit wurde genutzt, um die Werte fsp = [K/(K+NH )]fsp der synthetisierten Feldspäte zu ermitteln. In den X K 4 untersuchten Tobelit und (K-NH4)-Muskovit Proben tritt bevorzugt der monokline Glimmerpolytyp 1M (RG C2/m) auf. Auch hier nimmt die Elementarzelle mit steigender Ammoniumkonzentration in den Mischkristallen zu, was sich vor allem in der Gitterkonstante c zeigt. Eine lineare Abhängigkeit vorausgesetzt konnten so die musc Werte bestimmt werden. Über das Verhältnis der N-H Bande bei 1430 X K cm-1 und der O-H Bande bei 3635 cm-1 in den aufgenommenen IR-Spektren ließ sich ebenfalls musc X K berechnen. Ein Vergleich beider Methoden zeigt eine gute Übereinstimmung der erzielten Daten im Rahmen der Standardabweichung. + + Insgesamt weisen die Ergebnisse auf eine vollständige Substitution von K durch NH4 in der Feldspat- und + Muskovitstruktur hin. Lediglich bei 600 °C / 400 MPa und hohem NH4 -Gehalt der Ausgangszusammen- setzung konnten keine Feldspatmischkristalle mit hohen Ammoniumkonzentrationen synthetisiert werden. Dies ist auf eine bevorzugte Bildung von (K-NH4)-Muskoviten und Quarz zurückzuführen und zeigt an, dass NH4-reiche Feldspäte weniger termperaturstabil sind als NH4-reiche Muskovite. Aus den ermittelten (K-NH4)-Zusammensetzungen der Feldspat- und Muskovit- Mischkristalle ergibt sich in Verbindung mit den (K-NH4)-Gehalten der koexistierenden Fluidphasen das Verteilungsverhalten. Im untersuchten P-T Bereich wird Ammonium bevorzugt in die chloridische Fluidphase fraktioniert; synthetisierte Feldspäte und Muskovite weisen höhere fsp bzw. musc Werte auf als in der X K X K Ausgangszusammensetzung vorgegeben. Eine Temperaturabhängigkeit der (K-NH4)-Verteilung im Intervall 400-600 °C ist nicht zu beobachten, während steigender Druck von 400 auf 1500 MPa zu einer leichten Abnahme der (K-NH4)-Fraktionierung zwischen Feldspäten bzw. Muskoviten und Fluidphase + führt. Für geringe NH4 -Konzentrationen -relevant bei gesteinsbildenden Prozessen- konnten konstante − Verteilungskoeffizienten D solid fluid = solid / fluid abgeleitet werden. Diese betragen 0.22 ± 0.08 bzw. NH4 X NH4 X NH4 0.25 ± 0.07 für den Feldspat-Fluid Austausch bei 400-600 °C / 400 MPa bzw. 500-600 °C / 1500 MPa und 0.19 ± 0.10 bzw. 0.26 ± 0.08 für den Muskovit-Fluid Austausch bei 400-600 °C / 400 MPa bzw. 500-600 °C / 1500 MPa. Die erhaltenen Ergebnisse korrelieren mit Beobachtungen an natürlichen Metasedimenten des Erzgebirges (Granat-Phyllite und Granat-Phengit-Schiefer), die mit zunehmendem Metamorphosegrad abnehmende + Ammoniumgehalte zeigen (Mingram und Bräuer, 2001). Stickstofffreisetzung in Form von NH4 ist somit möglich und lässt sich mit Hilfe der gewonnen experimentellen Daten erklären. Weiterhin konnte die (K-NH4)-Verteilung zwischen Feldspäten und Muskoviten aus den Experimenten abgeleitet werden. Es zeigt sich, dass die synthetisierten (K-NH4)-Feldspäte mehr Ammonium aufnehmen als die Muskovitmischkristalle. Dies gilt insbesondere im Bereich von 0.1 < fsp < 0.9 und könnte eine X K Erklärung dafür liefern, warum bislang in der Natur vorgefundene Buddingtonite NH4-reicher sind als natürliche Tobelite. Abstract
Experiments using the „exchange synthesis“ technique were performed at 400-600 °C / 400 MPa and at 500-600 °C / 1500 MPa, respectively and yielded a variety of solid solutions (ss) along both, the (K-NH4)- feldspar and the (K-NH4)-muscovite joins. Different analytical methods including Rietveld refinement of XRD spectra, EMP analysis and IR spectroscopy were used on the synthesised solid solutions for comparison. First, emphasise was laid on a thorough characterisation of the NH4-endmembers buddingtonite NH4[AlSi3O8] and tobelite NH4Al2[AlSi3O10](OH)2. Buddingtonite as well as the synthesised (K-NH4)- feldspar ss are isostructural to sanidine (space group C2/m). The size of the corresponding unit-cells + + depends on the NH4 concentration in feldspars. With increasing NH4 content the unit-cells expand especially with regard to lattice parameter a. EMP analyses of (K-NH4)-feldspars demonstrate that a linear correlation between a and NH + content exists. This dependence was used to derive fsp = [K/(K+NH )]fsp 4 X K 4 + values for solid solutions along the buddingtonite – K-feldspar join. The detection limit for NH4 by EMP + analyses using a standard equipped Cameca SX50 was estimated to 0.8 wt% NH4 . In tobelite and synthesised (K-NH4)-muscovites mica polytype 1M (space group C2/m) is dominating. Lattice constant c of + the corresponding unit-cell increases linearly with increasing NH4 content and was used to determine musc values for the muscovite ss. Additionally, ratios of the N-H stretching band at 1430 cm-1 and the X K OH-band at 3635 cm-1 obtained from IR spectra allowed for calculation of musc . Both methods show an X K excellent agreement within the standard deviation 2σ( musc ) = 0.06. X K + + The analytical data demonstrates that a continuous substitution of NH4 for K exists for the (K-NH4)- + feldspar and the (K-NH4)-muscovite series, respectively. However, at 600 °C / 400 MPa and high NH4 content in the starting mixture, no feldspar ss with low fsp were obtained due to preferred formation of X K muscovite ss and quartz from feldspar stoichiometry. This indicates that NH4-rich muscovite is more stable under elevated temperatures than NH4-rich feldspar. The (K-NH4) concentration in feldspar and muscovite solid solutions along with the (K-NH4) content of the coexisting fluids determined by ion chromatography yielded the (K-NH4) distribution behaviour. In all experimental runs ammonium preferentially fractionates into the fluid phase. No temperature dependence exists, whereas with increasing pressure from 400 to 1500 MPa the (K-NH4) partitioning between solids + and fluids slightly decreases. For low NH4 concentrations, relevant for rock-forming processes, constant − partition coefficients D solid fluid = solid / fluid are derived. They are 0.22 ± 0.08 and 0.25 ± 0.07, NH4 X NH4 X NH4 respectively for the feldspar-fluid series at 400-600 °C / 400 MPa and 500-600 °C / 1500 MPa. For the − muscovite-fluid series D solid fluid values are 0.19 ± 0.10 and 0.26 ± 0.08, respectively at 400-600 °C / 400 NH4 MPa and 500-600 °C / 1500 MPa. These experimental results fit very well with natural observations of ammonium release from garnet phyllites and garnet-phengite schists during metamorphism (Mingram and Bräuer, 2001). Loss of nitrogen + as NH4 from metasediment sequences during prograde metamorphism is possible and may be related to the fluid-solid fractionation behaviour observed within this study. + The obtained data also indicates that synthesised (K-NH4)-feldspars incorporate more NH4 in comparison to (K-NH )-muscovites. This is especially seen in the range of fsp between 0.1 and 0.9 and could explain, 4 X K + why natural buddingtonites found so far are richer in NH4 than natural tobelites.
Content
1. INTRODUCTION...... 11
1.1 BUDDINGTONITE AND TOBELITE IN NATURE AND EXPERIMENT ...... 11
1.2 AMMONIA IN CRUSTAL ROCKS...... 13
1.3 AIM OF THIS STUDY ...... 15
2. EXPERIMENTAL AND ANALYTICAL PROCEDURE ...... 18
2.1 SYNTHESIS OF PURE ENDMEMBERS BUDDINGTONITE AND TOBELITE ...... 18
2.2 SYNTHESIS OF BUDDINGTONITE-K-FELDSPAR SS AND TOBELITE-MUSCOVITE SS ...... 18
2.3 EXPERIMENTAL METHODS ...... 20
2.4 ANALYTICAL METHODS...... 21
2.4.1 FLUID COMPOSITIONS ...... 21
2.4.2 SOLID COMPOSITIONS ...... 21
3. STRUCTURAL PARAMETERS OF SYNTHETIC BUDDINGTONITE NH4[ALSI3O8] AND TOBELITE
NH4AL2[ALSI3O10](OH)2 ...... 25
3.1 BUDDINGTONITE ...... 25
3.2 TOBELITE ...... 33
4. RESULTS OF “EXCHANGE SYNTHESIS EXPERIMENTS” ...... 38
4.1 THE SYSTEM BUDDINGTONITE – K-FELDSPAR – NH4CL – KCL ...... 38
4.2 THE SYSTEM TOBELITE – MUSCOVITE – NH4CL – KCL...... 44
5. STRUCTURAL AND COMPOSITIONAL VARIATION OF BUDDINGTONITE-K-FELDSPAR AND TOBELITE-MUSCOVITE SOLID SOLUTIONS ...... 47
5.1 BUDDINGTONITE – K-FELDSPAR SOLID SOLUTIONS ...... 47
5.1.1 RESULTS OBTAINED BY EMP...... 47
5.1.2 RESULTS OBTAINED BY XRD ...... 49
5.1.3 COMPARISON BETWEEN RESULTS OBTAINED BY EMP AND XRD ...... 55
5.2 TOBELITE – MUSCOVITE SOLID SOLUTIONS...... 57
5.2.1 RESULTS OBTAINED BY EMP...... 57
5.2.2 RESULTS OBTAINED BY XRD ...... 58
5.2.3 RESULTS OBTAINED BY FTIR ...... 63
5.2.4 COMPARISON BETWEEN RESULTS OBTAINED BY EMP AND FTIR AS WELL AS XRD AND FTIR...... 65
+ + 6. THE FRACTIONATION BEHAVIOUR OF NH4 AND K IN THE SYSTEMS BUDDINGTONITE – K-
FELDSPAR – NH4CL – KCL AND TOBELITE – MUSCOVITE – NH4CL – KCL ...... 67
6.1 THE NH4 – K PARTITIONING BETWEEN FELDSPARS AND (K-NH4)CL FLUIDS...... 67
6.2 THE NH4 – K PARTITIONING BETWEEN MUSCOVITES AND (K-NH4)CL FLUIDS ...... 74
6.3 THE NH4 – K PARTITIONING BETWEEN FELDSPARS AND MUSCOVITES...... 80
7. DISCUSSION ...... 84
7.1 EQUILIBRIUM CONDITIONS IN "EXCHANGE SYNTHESIS EXPERIMENTS" ...... 84
7.1.1 THE SYSTEM BUDDINGTONITE - K-FEDLSPAR - NH4CL - KCL ...... 84
7.1.2 THE SYSTEM TOBELITE - MUSCOVITE - NH4CL - KCL...... 88 7.2 THERMODYNAMIC EVALUATION OF THE BUDDINGTONITE - K-FELDSPAR AND TOBELITE - MUSCOVITE
SOLID SOLUTIONS SERIES ...... 90
7.3 COMPARISON TO OTHER K-NH4 EXCHANGE DATA ...... 95 7.4 GEOLOGICAL APPLICATIONS...... 98
8. REFERENCES...... 100
1. Introduction
+ Most of the nitrogen in the earth’s crust is stored in minerals as ammonium NH4 . Due to the + + + similar cation radius of NH4 (1.69 Å) in comparison to K (1.52 Å) (Shannon, 1976), NH4 substitutes for K+ in alkali-bearing silicates such as clay minerals (e.g. illite), feldspars, muscovites, biotites and amphiboles. In general, ammonium bounded in crustal rocks derives from breakdown of organic matter deposited in sediments. Under anoxic conditions of methane + formation and temperatures around 150°C, amino-acids decompose and NH4 is released as a fluid component (Williams et al., 1992). These ammonium-bearing fluids might precipitate authigenic
(K-NH4)-bearing silicates like clay minerals and feldspars or react with sedimentary K-bearing + minerals via cation exchange. As a consequence, sedimentary rocks exhibit a wide range of NH4 contents, depending not only on the redox conditions, but also on the availability of ammonium during formation of clay minerals and on the fixing capacity of these clay minerals (Schroeder and Ingall, 1994). During metamorphism of sediments, ammonium is either incorporated into newly formed alkali-bearing silicates and / or released as N2, which may eventually be trapped in fluid inclusions. Depending on the bulk composition of the rocks and fluids involved as well as current
P-T and redox conditions, a broad variety of (K-NH4) solid solutions can be present in crustal rocks.
1.1 Buddingtonite and tobelite in nature and experiment
Buddingtonite (NH4)[AlSi3O8], the natural NH4-analogue of K-feldspar, occurs in highly reducing, low-grade environments associated with organic-rich source rocks such as black shales, coal or oil deposits. In these environments, buddingtonite either forms diagenetically or by hydrothermal alteration of feldspars. Generally, the exchange of alkaline ions and ammonium is favoured by near-surface conditions (low P and T ≤ 200 °C) coupled with a reducing atmosphere. The first + natural occurrence of buddingtonite, containing nearly 0.80 NH4 pfu, was described by Erd et al. (1964) and Barker (1964) from hydrothermally altered andesitic rocks from a hot-spring located near a mercury ore deposit at Sulphur Bank, Lake County, California. Since then, buddingtonite has been found in several other localities. In gold and mercury-bearing hydrothermal systems in the western U.S. the maximum concentration of (NH4)2O in feldspars reaches 5.6 wt% which is + equivalent to 0.57 NH4 pfu (Krohn and Altaner, 1987; Krohn et al., 1993). Buddingtonite is also the primary constituent of the largest known ammonium deposit which extends over 10 km at
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Cedar Mt., Nevada and was discovered using satellite imagery (Krohn and Altaner, 1987; Krohn et al., 1993). In arkosic sandstones of the San Joaquin and Los Angeles Basins, buddingtonite with + up to 0.80 NH4 pfu is the product of early diagenesis under anoxic conditions and occurs as authigenic overgrowth on detrital K-feldspar and as microfracture filling (Ramseyer et al., 1993). The source of the ammonium in these basins is either internal, i.e. decay of organic matter present in the sandstones or external, i.e. oil-bearing fluids that migrated through the investigated basins. In oil-shale deposits, with high bulk rock ammonium contents, from Queensland, Australia buddingtonite averages 10 % of the strata (Loughnan et al., 1983). Buddingtonite is also found widely distributed in the rocks of the Meade Peak Member of the Phosphoria Formation in SE Idaho where it can make up to 50 % of the bulk rock composition (Gulbrandsen, 1974). The + buddingtonite - K-feldspar solid solutions in this area range from 13 to 82 mol% NH4 component suggesting a continuous solid solutions series.
Tobelite (NH4)Al2[AlSi3O10](OH)2, named after the type locality Tobe, Japan, is the NH4-rich analogue of muscovite and was first described by Higashi (1982) from a hydrothermally altered biotite andesite dike. Like buddingtonite, tobelite is either formed diagenetically or by hydrothermal exchange of alkaline ions and ammonium. In natural samples, tobelite generally + + contains less NH4 than buddingtontite. For example, tobelite from Tobe, Japan, with 0.53 NH4 pfu equivalent to 3.6 wt% (NH4)2O has the highest ammonium content reported for natural tobelite so far (Higashi, 1982). Besides Tobe, Japan, some other tobelite localities are known. Veins of tobelite in black shales which outcrop in the Oquirrh Mountains, Utah contain ~ 0.36 pfu ammonium and are explained as a product of hydrothermal exchange (Wilson et al., 1992). In contrast, tobelite associated with illite-smectite layers in North Sea oil source rocks is believed to have formed during diagenesis and subsequent oil generation (Drits et al., 1997). Additionally, tobelite is found associated with coal seams of varying degrees of coalification in a variety of widely separated locations including northern China (Liang et al., 1997) and northeast Pennsylvania (Juster et al., 1987). In these two cases, tobelite most likely is the product of low grade metamorphism of illite to tobelite with the source of ammonium coming from breakdown of amino-acids in coal layers during the coalification process. So far, pure buddingtonite and tobelite are only known from synthesis experiments with either gels or oxide mixes in an NH3-rich environment buffered to highly reducing conditions (Hallam and Eugster, 1976; Voncken et al. 1987, 1988). They can also be produced by cation exchange between natural muscovite or K-feldspar, respectively and an NH3-rich solution (Barker, 1964;
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Eugster and Munoz, 1966). Synthesis conditions range from 250 to 600 °C and 50 to 200 MPa. Hallam and Eugster (1976) showed that at 200 MPa and below 600 °C tobelite is stable even at -4 extremely low levels of fNH3 whereas buddingtonite requires fNH3 ≥ 10 bars. Moreover, tobelite appears to be stable at higher temperatures than buddingtonite. Between 300 to 350 °C buddingtonite breaks down to form tobelite according to the reaction
(1) 6 NH4 AlSi3O8 + 3 O2 = 2 NH4Al2AlSi3O10(OH)2 + 12 SiO2 + 2 N2 + 6 H2O.
1.2 Ammonia in crustal rocks
Generally, the ammonium contents of metasedimentary and other crustal rocks do not exceed some hundreds to thousands of ppm. This is shown e.g. by Honma and Itihara (1981) for coexisting micas and feldspars in metamorphic and granitic rocks from the Ryoke belt in Japan. + Biotite exhibits the highest NH4 content, followed by muscovite, K-feldspar and plagioclase. In gneisses from the Dôme de l’Agout, France, ammonium concentrations of micas range up to 1140 ppm (Duit et al., 1986). Here, a profile from the outer biotite zone into the inner gneisses shows a + steep decrease in the NH4 content of the micas and an increase in the N2 concentration in associated fluid inclusions with increasing metamorphic grade. Whether ammonium and molecular nitrogen in metamorphic rocks are inherited from the protolith or are the product of infiltration from an external source, is often discussed. For the Dôme de l’Agout e.g. Kreulen and Schuiling
(1982) argue for an external origin to explain the increasing N2 contents in fluid inclusions, whereas Duit et al. (1986) suggest a loss of nitrogen from mica phases. In case of the Moine + Succession, Scotland, high NH4 concentrations found in a series of amphibolite facies Precambrian metasediments are considered to be derived from organic matter originally present in the sedimentary rocks (Boyd and Philippot, 1998). An organic origin is also suggested for ammonium observed in the 3800 Ma Isua supracrustal rocks of central West Greenland (Honma, 1996). These rocks consist predominantly of a volcanogenic, basic component and a seawater- derived clay component, which could have been a major sink for nitrogen compounds present on the Earth´s surface 3800 Ma ago. Magmatic rocks, such as basalts and granites, normally have negligible ammonium contents. During hydrothermal alteration, these rocks can acquire additional + amounts of ammonium from interaction with NH4 -rich hydrothermal brines. This is shown by + Hall (1989) on spilitized basalts from S.W. England, which contain 54 ppm NH4 on average. Hall (1993) also describes the occurrence of hydrothermally altered granitic rocks from the Rosses complex of Donegal in which the K-feldspars are enriched in ammonium.
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In recent years the importance of nitrogen and ammonium as tracers in fluid-rock interaction + processes has been recognised. Concentrations of N2 in fluids and NH4 in minerals and rocks as well as their isotopic compositions give important informations about devolatilization processes during metamorphism (e.g. Haendel et al., 1986; Bebout and Fogel, 1992; Boyd and Philippot, 1998; Sadofsky and Bebout, 2000). A general observation is that the higher the metamorphic grade the lower the ammonium content in micas and feldspars (Bebout and Fogel, 1992; Boyd and Philippot, 1998; Mingram and Bräuer, 2001). During metamorphism nitrogen loss results from redox reactions (2), simple thermal decomposition (3) and cation exchange via coexisting fluids (4) (Eugster and Munoz, 1966). For tobelite, the most important reactions are:
(2) 2 NH4Al2AlSi3O10(OH)2 + 1.5 O2 ↔ 3 Al2SiO5 + 3 SiO2 + N2 + 6 H2O
(3) 2 NH4Al2AlSi3O10(OH)2 ↔ 3 Al2SiO5 + 3 SiO2 + 2 NH3 + 3 H2O + + (4) NH4Al2AlSi3O10(OH)2 + K ↔ KAl2AlSi3O10(OH)2 + NH4
Breakdown of ammonium-bearing minerals according to reaction (2) leads to release of N2 which may be trapped in fluid inclusions. N2-rich fluid inclusions are reported from low-grade black shales (Bottrell et al., 1988) but are also found in high-grade metamorphic rocks (e.g. Andersen et al., 1989; Herms and Schenk, 1992; Klemd et al., 1992). Though a mantle origin of N2 was + considered for high-grade metamorphic rocks, it is now generally accepted that NH4 -bearing silicates are the principle sources of nitrogen inclusions in crustal rocks (Bottrell et al., 1988; Andersen et al., 1993). Elevated water activities and low oxygen fugacities favour the formation + and stability of NH4 -containing micas up to high T and P (Elvevold and Andersen, 1993; Andersen et al., 1995). This allows for muscovite and biotite-phlogopite solid solutions to serve as a major storehouse for nitrogen in the deep crust. For example, biotites from upper amphibolite- facies rocks in the Bamble sector of South Norway, equilibrated at 700-800 °C and 6-8 kbar, still + contain up to 3000 ppm NH4 (Visser, 1992). Decreasing water activity during granulite-facies metamorphism combined with oxygen fugacities near the Ni-NiO equilibrium within most rocks is probably an efficient way to release nitrogen as N2. In granulites and eclogites from metapelitic terranes, nitrogen inclusions are often found in combination with CO2 and/or brines (e.g. Althaus and Istrate, 1989; Visser, 1992; Herms and Schenk, 1992; Klemd et al., 1992; Andersen et al., 1993). A recent study on metasedimentary rocks from the Erzgebirge and the Zone of Erbendorf- Vohenstrauss, Germany indicates that nitrogen loss with increasing metamorphism does not only
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occur because of redox reactions (Mingram and Bräuer, 2001). Isotope fractionation data suggest a + change from NH3 / NH4 release for the garnet phyllites (≈ 9 kbar / 470 °C) to N2 release for the + mica schists (≈ 12 kbar / 550 °C) again to NH3 / NH4 release for the garnet-phengite schists (> 12 kbar / 730 °C). Whether the observed nitrogen loss in the garnet phyllites as well as the garnet- phengite schists results from simple thermal decomposition according to reaction (3) or from cation exchange via coexisting fluids according to reaction (4) is not yet clarified. However, simple thermal decomposition leads to breakdown of mica phases and loss of nitrogen as NH3, + whereas via cation exchange mica phases remain stable and nitrogen is released as NH4 . The data obtained by Mingram and Bräuer (2001) suggest a combination of both, thermal decomposition and cation exchange. But so far, only few experimental data exist to describe the decomposition of + + NH4 -bearing minerals as well as the fractionation behaviour of NH4 between micas, feldspars and coexisting fluids at P and T.
1.3 Aim of this study
Cation exchange experiments are a convenient means of determining cation distribution coefficients between fluids and solids. For example, studies on exchange reactions of the general form Xmineral + Yfluid = Ymineral + Xfluid are available for alkali feldspars (Orville, 1963, 1972; Saxena and Ribbe, 1972; Bos et al., 1988; Schliestedt and Johannes, 1990; Lagache, 1993), muscovite-paragonite solid solutions (Pascal and Roux, 1985; Flux and Chatterjee, 1986; Chatterjee and Flux, 1986) and tremolite-richterite solid solutions (Zimmermann et al, 1997b). The
K-NH4 fractionation between feldspar and chloridic fluids was first reported by Lorch (1978). He investigated the exchange reaction
(5) buddingtonite + KCl = K-feldspar + NH4Cl in aqueous solutions between 400 - 600 °C and 200 MPa using hydrothermally grown K-feldspar and buddingtonite synthesised at 450 °C / 200 MPa as starting materials. Bos et al. (1988) analysed the exchange reaction
(6) NH4-phlogopite + KCl = phlogopite + NH4Cl + at 200 MPa and 550 to 650°C. Under these conditions NH4 is preferentially incorporated into
NH4/ K phlogopite as indicated by a distribution coefficient K D between phlogopite and fluid of 1.29 (0.30) at 550 °C and 1.44 (0.20) at 650 °C. The experimental strategy in these studies uses the “rotating tie-line“ technique described by Schliestedt and Johannes (1990). The basic assumption
15
is that in a reciprocal ternary system with one fluid and one solid solution at fixed P and T only one univariant equilibrium tie-line exists between the two phases. Any tie line between a starting fluid and solid reactant should then rotate around the bulk composition of the system into the equilibrium position. However, due to fractional crystallisation this technique often leads to disequilibrium assemblages (Zimmermann et al., 1997a) and the determined cation distribution coefficients are not precise enough. Therefore, in the present study the fractionation behaviour of potassium and ammonium between feldspars, micas and aqueous chloride solutions was investigated using the different experimental set-up described by Zimmermann et al. (1997b). The aim was to determine the equilibrium + + distribution coefficients for NH4 and K between buddingtonite - K-feldspar and tobelite - muscovite solid solutions, respectively, and 2 to 3 molal chloridic aqueous fluids in the system N- K-Al-Si-O-H-Cl. The following exchange reactions
(7) K-feldspar + NH4Cl = buddingtonite + KCl and
(8) muscovite + NH4Cl = tobelite + KCl were investigated using a combination of synthesis and exchange reaction in one experiment.
Synthesised (K-NH4)-feldspar and (K-NH4)-muscovite solid solutions grew in equilibrium with an excess exchange fluid. Details about the experimental procedure as well as the analytical methods used to determine the composition of solid and fluid products are given in Chapter 2. In Chapter 3, a thorough crystal- chemical characterisation of the NH4-endmembers buddingtontite NH4[AlSi3O8] and tobelite
NH4Al2[AlSi3O10/(OH)2] will be presented including analytical techniques such as electron microprobe analysis, Rietveld refinement of X-ray diffraction spectra and infra-red spectroscopy. The results obtained from the “synthesis exchange” experiments are given in Chapter 4. In Chapter 5, different analytical techniques to characterise the synthesised solid solutions along both the (K-
NH4)-feldspar as well as the (K-NH4)- muscovite join are compared. It will be demonstrated that + + for both series a continuous substitution of NH4 for K exists. In Chapter 6, the temperature and pressure dependence of the fractionation behaviour is analysed using Roseboom diagrams as well as the reciprocal ternaries buddingtonite-K-feldspar-NH4Cl-KCl and tobelite-muscovite-NH4Cl- KCl, respectively. It will be shown that equilibrium conditions were achieved in all runs and that + NH4 preferentially fractionated into the fluid phase in both systems. Chapter 7, compares the derived muscovite-fluid, feldspar-fluid and muscovite-feldspar exchange coefficients with those found in natural NH4-bearing minerals and demonstrates that cation exchange is an efficient way
16
+ to release nitrogen from rocks during prograde metamorphism as NH4 via cation exchange. Additionally, mixing models for the muscovite-tobelite and K-feldspar-buddingtonite solid solution series are presented, which can be used to estimate the stability of (K-NH4)-bearing feldspars and muscovites.
17
2. Experimental and analytical procedure
2.1 Synthesis of buddingtonite, tobelite and K-feldspar endmembers
The buddingtonite and tobelite endmembers characterised in this study were kindly provided by D. E. Harlov. They were synthesised at GeoForschungsZentrum Potsdam from 150 mg of a stoichiometric mixture of Al2O3 and SiO2 plus a 25% NH3 solution in excess of 50 mol%. The experiments were carried out in welded Au capsules placed in cold seal hydrothermal autoclaves with a Ni-NiO filler rod and an external thermocouple. Run conditions were 400 °C / 500 MPa for buddingtonite and 600 °C / 500 MPa for tobelite syntheses. The duration of each experiment varied between 1 and 3 weeks. Further details of the syntheses conditions of the buddingtonite and tobelite endmembers are given in Harlov et al. (2001a, 2001b).
Pure K-feldspar (Kfsp) was synthesised at 500 °C / 200 MPa from a stoichiometric mixture of
SiO2, Al2O3 and K(OH) using the same experimental equipment and technique as described for the buddingtonite and tobelite syntheses. Run durations were 10 days.
2.2 Synthesis of buddingtonite-K-feldspar and tobelite-muscovite solid solutions
In order to investigate the exchange reactions (7) and (8) the “synthesis exchange“ technique described by Zimmermann et al. (1997b) was used. The advantage of this technique is that the solid solutions (ss) are directly synthesised from oxide mixtures and an aqueous Cl-bearing fluid which contains the exchangeable cations in excess. Therefore, disequilibrium problems typical for the “rotating tie-line“ method are avoided. So far, the “synthesis exchange“ technique has been successfully applied to Na-K exchange between richterites and chloride solutions (Zimmermann et al., 1997b), to Rb-K and Rb-Na exchange between richterites and chloride solutions (Melzer et al., 1998) as well as to Ca-Sr exchange between amphiboles, clinopyroxenes and chloride solutions
(Najorka et al., 1999). Syntheses of (K-NH4)-feldspar and muscovite solid solutions were performed using the two reactions:
aq aq (9) 2 (NH4,K)OH + 6 SiO2 + Al2O3 + n (NH4,K)Cl = aq 2 (NH4,K) AlSi3O8 + n (NH4,K)Cl + H2O
aq aq (10) 2 (NH4,K)OH + 6 SiO2 + 3 Al2O3 + n (NH4,K)Cl + H2O = aq 2 (NH4,K) Al2AlSi3O10(OH)2 + n (NH4,K)Cl
18
Starting materials consisted of a stoichiometric mixture of solid Al2O3 and SiO2 and a 5 molar (K-
NH4)OH solution plus a 3 molal (K-NH4)Cl solution. The experiments were set up in such a way that during the runs micas or feldspars grew in equilibrium with a chloride (K-NH4)-solution of
NH4 almost constant concentration. Variations of the ratio of the bulk composition were achieved K by adjusting the starting fluid compositions. In Figure 1 the experimental approach is illustrated for (K-NH4)-feldspars. Since no feldspar is present at the beginning of the synthesis, only the bulk
bulk composition, X K = K/(K+NH4), can be shown.
fsp XK = K / ( K + NH4 ) Buddingtonite K-Feldspar 1 J J Figure 1 Experimental 0,9 1: bulk composition 1 approach of „exchange 0,8 2: bulk composition 2
l a t o syntheses“ illustrated by the t 0,7 )
4 reciprocal ternary 0,6 buddingtonite - K-feldspar – 0,5 / (K+NH J
sp 0,4 1 f 2 J
NH4Cl – KCl ) 4 0,3
0,2 (K+NH
0,1
0 J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 NH Cl KCl 4 X fluid = K / ( K + NH ) K 4
+ + At the start of each run all K and NH4 ions are dissolved in the fluid and the feldspars grow from
fluid bulk fsp fluid a fluid composition of X K = X K . During the reaction the mole fractions X K and X K + + + change depending on the fractionation behaviour of K and NH4 . As shown in Chapter 6, NH4
fsp preferentially partitions into the fluid and consequently the synthesised feldspars have X K >
bulk X K . At the end of each experiment, product feldspar and fluid should ideally be connected by a tie-line through the bulk composition (cf. bulk compositon 1 in Fig.1). However, such an ideal tie- line will only exist if no fractional crystallisation occurs and/or no other (K-NH4)-bearing phases form. For example, if (K-NH4)-muscovite volunteers during a feldspar exchange experiment, the tie-line connecting the feldspar and fluid composition misses the bulk composition and consequently expands into the shaded triangle (cf. bulk composition 2 in Fig.1). However, the
19
determined distribution behaviour is still valid since the formation of other (K-NH4)-minerals, in addition to feldspar, simply leads to a displacement of the bulk composition in the reciprocal ternary buddingtonite-K-feldspar-NH4Cl-KCl.
2.3 Experimental methods
Experiments were performed at 400 MPa and 400 to 600 °C using standard cold seal hydrothermal vessels. The temperature was recorded with an uncertainty of less than ±5 °C using internal Ni- CrNi thermocouples which closely adjoined the sample position. Pressure was measured using a calibrated strain gauge within the range of ±10 MPa. After 10 to 24 days, the samples were quenched by cooling the autoclaves with compressed air. Temperatures of less than 100 °C were reached in about 3 minutes. High pressure experiments were done at 1500 MPa and 500 to 600 °C in a non-end-loaded piston-cylinder apparatus using NaCl assemblies. The temperature was recorded using Ni-CrNi thermocouples with an accuracy of ±10 °C. Pressure was measured with a total uncertainty of approximately ±50 MPa. Runs lasted 5 to 10 days and were quenched to less than 200 °C within 15 seconds.
For hydrothermal experiments at 400 MPa, gold capsules were used whereas the piston-cylinder runs at 1500 MPa were performed in platinum capsules. Depending on the size of the capsule, approximately 10 to 80 mg of the stoichiometric oxide mixture plus 1 to 5 mol% SiO2 in excess were used for the feldspar experiments (reaction 9) and additionally 5 mol% Al2O3 in excess for the muscovite experiments (reaction 10). The solid/fluid ratio was ~ 0.35-0.50 for syntheses at 400 MPa and >0.50-0.63 for syntheses at 1500 MPa. All capsules were sealed using a plasma welder while partially immersed in an ice/water mix. After the runs the capsules were cleaned with hot distilled water, weighed to check for leakage and then cut open in doubly distilled water to avoid fluid loss. The solids were separated from the fluid by filtration and washed out with doubly distilled water. Fluids obtained from hydrothermal experiments were diluted to a total of 100 ml aqueous solution whereas fluids from the piston cylinder runs were diluted to 50 ml.
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2.4 Analytical methods
2.4.1 Fluid compositions
Ion exchange chromatography
+ + K and NH4 concentrations in the product fluids were measured by isocratic ion exchange chromatography coupled with micro membran suppression and conductivity detection. The cations were separated via a chromatographic column from Dionex (CS12A, 4x250 mm) due to different + + retention times of K and NH4 to the stationary phase. The injection volume was 25 µl. As mobile phase, also called eluent, a 22 mN H2SO4 with a flow rate of 1 ml/min was chosen. Under these + + conditions the retention times were about 6 minutes for NH4 and 7.5 minutes for K . For micro membran suppression a 100mN tetrabutylammoniumhydroxid solution (TBAOH) was used as regenerent in order to neutralize the acid eluent and reduce the background conductivity to < 2 µS. One analysis required 0.6 ml solution; each sample was measured three times. The achieved + + relative mean standard deviation was below 2%. On basis of the amount of K and NH4 − determined by ion exchange chromatography the Cl content of the fluid was calculated.
2.4.2 Solid compositions
Solid phases were examined by scanning electron microscopy (SEM), electron microprobe (EMP), powder X-ray diffraction (XRD) with Rietveld analysis and Fourier-transform infrared spectroscopy (FTIR). The SEM used was a Zeiss DSM962 equipped with an energy dispersive (EDX-) system.
Electron microprobe EMP analyses were performed on polished grain mounts with a Cameca SX50 at GFZ Potsdam using wavelength dispersive spectrometry (WDS) and the PAP correction program (Pouchou and Pichoir, 1984). For pure buddingtonite as well as the buddingtonite-K-feldspar solid solutions the elements N, (K), Al and Si were measured whereas O and H were added stoichiometrically. Operating conditions used an accelerating potential of 10 kV, a beam current of 10 nA and a 5 µm beam diameter. Counting times for all elements on peak and background positions as well as the spectrometer crystals used are given in Table 1.
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Table 1 Counting times and spectrometers for EMP analyses of synthetic buddingtonite (excluded K) and buddingtonite-K-feldspar ss
Elements Counting time-peak (s) Counting time-background (s) Spectrometer N 120 60 LDE1 / LDEN K 120 60 PET Al 40 20 TAP Si 40 20 TAP
Quantitative EMP analysis of nitrogen was faced with problems such as low count rates, high absorption of generated characteristic X-rays and irregular and variable peak shapes (Armstrong, 1988; Raudsepp, 1995). In order to optimise the diffraction of low-energy radiation, layered synthetic microstructure analysers like the W/Si-multilayer LDE1 (2d=60Å) were used. However, count rates for pure buddingtonite averaged not more than 30 counts s-1. Improvement on nitrogen measurement was reached by using the JEOL JXA 8900 RL at the University of Göttingen equipped with a LDEN (2d=80Å) analyser which consists of Cr/Sc-multilayers with low mass absorption coefficients (MAC) on N-Kα radiation. The intensity yield obtained with the LDEN is around ten times higher in comparison to the LDE1 multilayer crystal (Kronz and Pöter, 1999). To reduce the absorption of nitrogen radiation by carbon which has a high MAC of 25500 cm2g-1 on N-Kα (Bastin and Heijligers, 1988), the carbon coating was kept thin to approximately 5-10 nm and samples and standards were coated simultaneously. Additionally, carbon contamination on the sample surface during analysis was minimised by using a liquid-nitrogen-trap. However, the highest loss of intensity occurred within the polypropylene detector window. Since irregular and variable peak shapes depend on the chemical state and sometimes on the crystallographic orientation, good standard materials similar to the analysed material were required. Synthetic buddingtonite NH4AlSi3O8 was used for calibration of N (Harlov et al., 2001a) and well-defined natural orthoclase for calibration of K, Al and Si. Since NH4-rich phases tend to lose nitrogen under the electron beam, standardising and measuring conditions were always kept the same. For pure buddingtonite, the intensity loss of N-Kα reached a plateau of 80 % of its initial value after 100 seconds. Therefore, the counting time for the nitrogen peak was set to 120 s (Table 1). Since synthesised tobelite and tobelite-muscovite ss did not exceed 5 µm and were more sensitive to the electron beam than buddingtonite, only Al and Si as well as K for the solid solutions were analysed. For measurement of Al and Si two separate TAP spectrometers were used whereas K
22
was measured on a PET spectrometer. The operating conditions were 15 kV, 20 nA and 2 µm beam diameter. In order to reduce the damage of mica crystals due to nitrogen loss under the electron beam, counting times were kept short (peak position: 20 s ; background: 5 s). Again, + orthoclase was used as standard for K, Al and Si. The amount of NH4 present on the interlayer site for the tobelite-muscovite solid solutions was estimated by charge balance assuming completely occupied octahedral and tetrahedral sites by a total of six atoms (Al+Si) pfu. For pure + tobelite crystals the amount of NH4 present on the interlayer site is 1 in case the tetrahedral and octahedral sites are fully occupied by Al and Si.
X-ray diffractometry The experimental products including pure buddingtonite and tobelite as well as buddingtonite-K- feldspar and tobelite-muscoite solid solutions were ground in an agate mortar for several minutes, diluted with Elmer´s White glue and evenly spread on a circular foil. To minimise preferential orientation the powder was stirred during drying. Finally, the foil was placed into the transmission sample holder and covered with a second empty foil. Powder XRD patterns were recorded in transmission using a fully automated STOE STADI P diffractometer (CuKα1 radiation), equipped with a primary monochromator and a 7°-wide position sensitive detector (PSD). The normal-focus Cu X-ray tube was operated at 40 kV and 40 mA, using a take-off angle of 6°. Intensities were recorded in the range of 5 to 125 ° (2θ) with a detector step size of 0.1° and a resolution of 0.02°. Counting times were selected to yield a maximum intensity of 2000 to 3000 counts for each sample, resulting in 5 to 20 s per detector step. Unit cell dimensions, additional structural parameters and phase quantities were determined using the GSAS software package for Rietveld refinements (Larson and Von Dreele, 1987). The peaks were defined as pseudo-Voigt with a variable Lorentzian character. The peak full-width at half maximum height (FWHM) was varied as a function of 2θ using the parameters ‘U’, ‘V’, ‘W’ of Caglioti et al. (1958). For the Lorentzian character the parameters ‘X’ and ‘Y’ were used. The recorded peaks were highly symmetric due to the geometry of the STADI P diffractometer, therefore no parameters describing the asymmetry of the peaks had to be used. Background was fitted with a real space correlation function which is capable of modelling the diffuse background from the amorphous foil and glue used for the preparation. The preferred orientation was corrected with the formulation of March (1932) and Dollase (1986). During the refinement of the XRD spectra, scale factor, background, zero-point correction, phase fractions, lattice parameters, profile parameters, preferred orientation and atomic
23
positions were all taken into account. The atom fractions and isotropic displacement factors were not refined. As starting values the unit-cell parameters, atomic coordinates and isotropic displacement factors given for orthoclase by Chao et al. (1940), for the 1M polytype of phlogopite by Rayner (1974), for the 2M1 polytype of muscovite by Richardson and Richardson (1982) and for the 2M2 polytype of lepidolithe by Sartori et al. (1973) were used.
Fourier-transformed infra-red spectroscopy FTIR analyses were performed on pure buddingtonite and tobelite as well as tobelite-muscovite solid solutions. Since the size of the synthesised crystals was too small for single-crystal measurements, powdered samples were investigated. All samples for FTIR investigation were prepared by grinding 2 mg of the product and dispersing it into 450 mg KBr. The homogenised mixture was subsequently pressed under vacuum to transparent pellets 13 mm in diameter and then dried for several days at 170 °C. Within the methodical resolution, the IR spectra of a powdered sample without KBr taken with an IR microscope and a sample dispersed in a KBr pellet were + + identical, suggesting that no measurable exchange between K in KBr and NH4 in the sample material took place. The absorption measurements were carried out in the spectral range 3800 to 1000 cm-1 with a resolution of 2 cm-1 using a Bruker IFS 66v FTIR spectrometer equipped with a globar as the light source, a KBr beam-splitter and DTGS-detector. Spectra were averaged over 256 scans. Phase correction mode of the interferogram was performed using the procedure after Mertz (1965) and Griffiths and de Haseth (1986). Norton-Beer weak mode was chosen as the apodization function. The sample chamber of the Bruker IFS 66v was evacuated down to 200 Pa.
Therefore, the influence of H2O vapour and CO2 was negligible. The spectra were displayed in the form of absorption spectra versus wavenumber. After background correction, the band centre, FWHM and integral intensity were determined with the program PeakFit by Jandel Scientific.
24
3. Structural parameters of synthetic buddingtonite NH4[AlSi3O8] and tobelite
NH4Al2[AlSi3O10](OH)2
3.1 Buddingtonite
Synthetic buddingtonites were investigated using EMP analyses, Rietveld refinement of XRD spectra and IR spectroscopy (Harlov et al., 2001a). The buddingtonites typically occur in clumps of intergrown idiomorphous crystals with monoclinc symmetry. Generally, the grains are up to 50 µm long and 20 µm wide (Figure 2 a, b). Chemical compositions of three synthesised budding- tonite samples determined by EMP are given in Table 2 and indicate no deviation from the theoretical composition NH4AlSi3O8.
50 µm 20 µm a) b)
Figure 2 SEM images of synthetic buddingtonite crystals (sample Budd 2) with monoclinc symmetry; from Harlov et al., 2001a
Rietveld analyses were performed for all synthesis runs yielding nearly pure buddingtonite. In addition, natural buddingtonite from Menlo Park, California, and two synthetic potassium feldspars were refined for comparison. Each buddingtonite and the K-feldspar samples are isostructural with sanidine (Z = 4, space group = C2/m ). All data concerning the Rietveld analysis of either phase, along with the obtained phase proportions, are given in Table 3. For all refinements, the obtained least-squares parameters are within the range which indicates a good fit (Table 3). In Table 4, cell dimensions for each buddingtonite sample, refined using the Rietveld technique, are listed. A corresponding XRD spectrum for buddingtonite is given in Figure 3 and exhibits an excellent agreement between the observed and calculated spectrum.
25
Table 2 Microprobe analyses of synthetic buddingtonites (2σ standard deviation of EMP measurement)
Sample Budd 1 Budd 2 Budd 4 No. of analyses 10 9 7
Oxides wt%
(NH4)2O 10.38 (13) 10.42 (13) 10.32 (21) SiO2 68.51 (55) 68.50 (42) 68.44 (45) Al2O3 19.49 (60) 19.57 (34) 19.75 (36) Total 98.38 98.49 98.51
Atoms pfu *
NH4 1.05 1.05 1.04 Si 2.99 2.98 2.98 Al 1.00 1.01 1.01
*calculated on basis of 8 O atoms
Table 3 Rietveld refinement parameters for buddingtonite and K-feldspar
χ2 Sample P T Duration RP Rwp DW Quantitative phase analysis [MPa] [°C] [d]
Budd 1 500 400 20 0.062 0.086 1.20 1.63 99 wt% Budd; 1 wt% Qtz Budd 2 500 500 22 0.077 0.106 1.24 1.63 99 wt% Budd; 1 wt% Qtz Budd 3 500 500 22 0.077 0.103 1.36 1.54 84 wt% Budd; 16 wt% Qtz Budd 4 500 500 21 0.051 0.067 1.23 1.55 100 wt% Budd Budd 5 500 500 20 0.071 0.095 1.33 1.48 99 wt% Budd; 1 wt% Qtz Budd 6 500 500 22 0.062 0.081 1.42 1.38 99 wt% Budd; 1 wt% Qtz 1 nat. Budd - - - 0.061 0.079 1.47 1.34 60 wt% Budd; 35 wt% Qtz; 2 4 wt% Illite K-feldspar 1 200 500 10 0.062 0.083 1.19 1.71 100 wt% Sanidine K-feldspar 2 200 500 10 0.067 0.091 1.18 1.68 86 wt% Sanidine; 14 wt% Qtz
1 natural sample from Menlo Park, California; 2 plus 1 wt% minor phases which could not be identified; obs calc obs obs calc obs 0.5 Budd = buddingtonite; Qtz = quartz; Rp = ∑ | yi - yi | / ∑ yi ; Rwp = [ ∑ wi( yi - yi ) ² / ∑ wi (yi ) ² ] ;
obs calc χ2 = wi( yi - yi ) ² / (N-P) ; DW = d statistics (Durbin-Watson) Note: (N-P) = observations (step intervals); y = intensity; w = 1/y (weighting factor)
26
Table 4 Unit-cell dimensions of buddingtonite and K-feldspar along with 2σ standard deviation
Sample a [Å] b [Å] c [Å] β [°] vol [Å3] Source
Budd 1 8.8268(7) 13.0641(9) 7.1935(5) 116.108(3) 744.87(11) this study Budd 2 8.8251(6) 13.0553(8) 7.1896(5) 116.142(3) 743.60(11) this study Budd 3 8.8262(7) 13.0574(9) 7.1882(6) 116.121(6) 743.81(12) this study Budd 4 8.8326(12) 13.0445(15) 7.1875(8) 116.220(6) 742.91(20) this study Budd 5 8.8347(8) 13.0574(10) 7.1926(6) 116.162(6) 744.72(13) this study Budd 6 8.8398(11) 13.0411(14) 7.1868(8) 116.285(6) 742.84(18) this study 1 nat. Budd 8.804(2) 13.040(3) 7.193(2) 116.075(24) 741.8(3) this study nat. Budd 8.804(3) 13.024(3) 7.183(1) 116.105(18) 739.6(3) Kimball & Megaw (1974) K-feldspar 1 8.6050(8) 13.0154(11) 7.1851(6) 116.017(3) 723.17(14) this study K-feldspar 2 8.6054(9) 13.0209(12) 7.1857(6) 116.032(6) 723.47(15) this study nat. K-feldspar 8.600 13.020 7.220 116.1 726.3 Chao et al. (1940)
1 natural sample from Menlo Park, California
Sample Budd 2
99 wt% budd 1 wt% qtz
qtz budd Iobs – Icalc
Figure 3 XRD spectrum of synthetic buddingtonite sample containing 99 wt% buddingtonite (budd) and 1 wt% quartz (qtz); phase proportions determined by Rietveld analysis
27
+ According to the Rietveld analyses performed, the NH4 ion in monoclinic buddingtonite replaces the K+ ion in the sanidine structure on the nine-fold coordinated A position with m site symmetry. + + Due to the larger radius of NH4 in comparison to K , i.e. 1.69 vs. 1.52 Ǻ (Shannon, 1976), the + polyhedron enclosing the NH4 molecule is expanded with increasing A-Oi distances (Table 5). The corresponding structure model for buddingtonite is shown in Figure 4.
Figure 4 Structure of monoclinc buddingtonite (similar to sanidine): A = NH4 (nine-fold coordinated site);
Oi = O; Ti = Si resp. Al (tetrahedral site); unit-cell indicated by dashed lines
28
Table 5 Interatomic distances and angles determined for synthesised buddingtonite and K-feldspar samples after Rietveld analyses
A Budd 1 Budd 2 Budd 4 K-feldspar 1 K-feldspar 2
2 A-O1 3.089 Ǻ 3.069 Ǻ 3.078 Ǻ 2.936 Ǻ 2.930 Ǻ
A-O2 2.953 Ǻ 2.970 Ǻ 3.002 Ǻ 2.742 Ǻ 2.739 Ǻ
2 A-O3 3.160 Ǻ 3.134 Ǻ 3.144 Ǻ 3.063 Ǻ 3.040 Ǻ
2 A-O4 3.140 Ǻ 3.133 Ǻ 3.144 Ǻ 3.113 Ǻ 3.097 Ǻ
2 A-O5 3.043 Ǻ 3.025 Ǻ 3.036 Ǻ 2.962 Ǻ 2.970 Ǻ
T1-O2 1.643 Ǻ 1.647 Ǻ 1.625 Ǻ 1.638 Ǻ 1.642 Ǻ
T1-O3 1.637 Ǻ 1.644 Ǻ 1.618 Ǻ 1.634 Ǻ 1.651 Ǻ
T1-O4 1.645 Ǻ 1.637 Ǻ 1.641 Ǻ 1.638 Ǻ 1.627 Ǻ
T1-O5 1.642 Ǻ 1.638 Ǻ 1.675 Ǻ 1.642 Ǻ 1.625 Ǻ
T2-O1 1.644 Ǻ 1.649 Ǻ 1.648 Ǻ 1.645 Ǻ 1.652 Ǻ
T2-O3 1.627 Ǻ 1.641 Ǻ 1.638 Ǻ 1.626 Ǻ 1.632 Ǻ
T2-O4 1.648 Ǻ 1.651 Ǻ 1.646 Ǻ 1.660 Ǻ 1.680 Ǻ
T2-O5 1.646 Ǻ 1.651 Ǻ 1.624 Ǻ 1.655 Ǻ 1.658 Ǻ
T2-O1-T2 146.6° 145.1° 146° 144.7° 144.1°
T1-O2-T1 143.6° 142.9° 146.3° 139.4° 137.2°
T1-O3-T2 151.3° 150.3° 150.7° 154.3° 151.9°
T1-O4-T2 130.8° 130.2° 131.2° 130.3° 130.1°
T1-O5-T2 141.4° 140.6° 141.4° 141.2° 141.9°
A = Interatomic distances for the 9-fold coordinated A position;
B = Interatomic distances for the tetrahedral site T1;
C = Interatomic distances for the tetrahedral site T2;
D = T1,2-Oi-T1,2 angles
29
The individual interatomic distances A-Oi in buddingtonite do not expand as a function of their absolute length. Rather, the shorter the original A-Oi distance in K-feldspar, the larger the relative expansion of the corresponding A-Oi distance in buddingtonite. Figure 5 shows e.g. that the + shortest interatomic distance A-O2 in K-feldspar (2.74 Ǻ) increases the most when K is replaced + by NH4 (Table 4). The difference between A-O2 in buddingtonite and A-O2 in K-feldspar is in average about 0.23 Ǻ, followed by the interatomic distance A-O1 which increases about 0.14 Ǻ from K-feldspar to buddingtonite. The one exception to this trend is the A-O5 distance, which exhibits with 0.07 Ǻ a smaller increase in length than observed for the A-O3 distance with 0.08 Ǻ increase from K-feldspar to buddingtonite.
0,3
Budd1 A-O 2 Budd2 Ǻ 0,25 Budd4
Kfsp ) i 0,2
A-O1 – (A-O 0,15 Budd ) i A-O3 0,1 (A-O
0,05 A-O5
A-O4 0 2,7 2,75 2,8 2,85 2,9 2,95 3 3,05 3,1 3,15
Kfsp Ǻ Interatomic distance (A-O ) i
Figure 5 Expansion of the A-Oi interatomic distance in buddingtonite relative to pure K-feldspar (Kfsp) with sanidine structure for three experimentally produced buddingtonites (average values obtained for synthetic K-feldspar 1 and 2 used as reference, cf. Table 5)
30
In general, the A-site polyhedron in buddingtonite is expanded in such a way that it becomes more regular in comparison to the A-site polyhedron in sanidine. To compensate for this expansion, the Al-Si framework responds via rotation of the rigid Al and Si tetrahedra. This is obvious from the changes in the T2-O1-T2 and T1-O2-T1 angles, which increase due to an increase in the interatomic distances (Table 5). Because of this expansion in the interatomic distances and angles, the unit-cell parameters for buddingtonite are larger when compared to natural and synthesised potassium feldspars (Table 4). This is especially seen with respect to the lattice parameters a and β which are + + the most influenced by the substitution of NH4 for K . In contrast, the magnitude of the c lattice parameter remains nearly unchanged within the analytical error of XRD analysis. + + The strong influence of the substitution of NH4 for K on the magnitude of a and β is also evident in natural buddingtonites. For example, buddingtonite from Sulphur, Bank, Lake County, California (Kimball and Megaw, 1974), which contains a 5 mol% K-feldspar component, has significantly smaller values for a and β compared to pure end-member synthetic buddingtonite (Table 4). This effect is also seen for natural buddingtonite from Menlo Park, California (Table 4). The obtained cell dimensions for this sample are in good agreement with those given by Kimball and Megaw (1974) for the buddingtonite from Sulphur Bank and suggest the presence of a minor K-feldspar component similar to that seen in the Sulphur Bank sample. Lattice parameters obtained for a series of synthetic buddingtonites (Budd 1 to Budd 6) are variable with respect to the lattice constants a and b and also to β, lying outside the analytical error of XRD analyses (Table 4). These variations are unlikely to be due to differences in the chemical composition since all buddingtonites were synthesised under almost identical conditions from the same stoichiometric SiO2-Al2O3 oxide mix and 25 % NH3 solution (Table 3). Moreover, within analytical error, chemical compositions of samples Budd 1, Budd 2 and Budd 4 are identical + (Table 2) and IR measurements of buddingtonite Budd 3 indicate no incorporation of H3O instead + of NH4 (Harlov et al. 2001a, Figure 6) as observed by Laricheva et al. (1993, 1996) in buddingtonites synthesised at 250-450 °C and 50-100 MPa.
31
1,8
1,6 /I)
o + no indication of H3O band 1,4
ν4
1,2 ν3
ν + ν 1 2 4 2ν 0,8 4 2ν2 (shoulder) 0,6 absorbance log (I absorbance
0,4 4500 4000 3500 3000 2500 2000 1500 1000 wavenumber [cm-1 ]
ν (A ) ν (E ) ν (T ) ν (T ) 11 22 3 2 4 2
Figure 6 IR spectra of synthetic buddingtonite Budd 3 at 298 K. IR-active: triply generated fundamental states ν3 and ν4; combination mode ν2 + ν4; overtone mode 2ν4 and overtone mode 2ν2 (weak); no -1 indication of νOH (~ 3600 cm ) (from Harlov et al., 2001a)
Consequently, it is quite possible that the differences in a, b and β observed between synthetic buddingtonites are caused by structural variations. Various degrees of Al-Si ordering/disordering in sanidine for example lead to variations in the lattice constants b and c (Kroll and Ribbe, 1987). Since buddingtonite exhibits a structure comparable to sanidine, the variations in b are likely to be caused by differences in Al-Si ordering/disordering. However, the lattice parameters a and β remain nearly unaffected by the Al-Si ordering/disordering phenomena and consequently, variations in a and β have to result from differences in the expansion of the A site polyhedron.
32
Table 5 and Figure 5 e.g. show that the difference in A-O2 between Budd 1 and Budd 4 is 0.049 Å which is correlated to variations in the lattice constant a (Table 4). Generally, structural variations such as odering/disordering phenomena and polyhedron expansion are likely to be caused by small variations in the experimental procedure (e.g. differences in run duration, heating and cooling rate).
3.2 Tobelite
Synthetic tobelites were investigated using EMP analyses, Rietveld refinement of XRD spectra and IR spectroscopy (Harlov et al., 2001b). They form masses of euhedral to semieuhedral crystals less than 2 µm in size (Figure 7 a, b). The chemical compositions of synthetic tobelites determined by EMP are given in Table 6 and indicate no deviation from the theoretical composition
NH4Al2[AlSi3O10](OH)2. Due to the small size of the individual tobelite crystals coupled with the sensitivity of the samples to the electron beam, only the elements Al and Si were measured. However, taking into account the cation proportions of Al and Si, within microprobe error, all of + the 12-fold coordinated interlayer sites A for tobelite are fully occupied by NH4 . This is also confirmed by IR spectroscopy in the OH-stretching region, since any deviation from fully occupancy of the interlayer site A would lead to an additional OH-band in the region at about 3600 cm-1 (Harlov et al., 2001b; Figure 8).
20 µm 10 µm a) b)
Figure 7 SEM images of synthetic tobelites (a) sample Tob 2; (b) sample Tob 3 (from Harlov et al., 2001b)
33
Table 6 Microprobe analyses of synthetic tobelite samples
Sample Tob 1 Tob 2 Tob 3 No. of analyses 5 7 5
Oxides wt%
SiO2 46.18 44.15 48.19 Al2O3 40.43 38.13 41.21 Total 86.61 82.28 89.40
Atoms pfu *
IVSi 2.95 2.97 2.99 IVAl 1.05 1.03 1.01 VIAl 2.00 2.00 2.00 Total 6.00 6.00 6.00
Oxygens 10.48 10.49 10.50 + NH4 ** 1.05 1.03 1.01
* Calculated on the assumption of a total of 6 atoms on the tetrahedral and octahedral sites ** Calculated on the assumption of 11 oxygens pfu
All tobelite samples analysed via XRD and Rietveld refinement consist predominantly of the monoclinic one-layer mica polytype 1M with space-group symmetry C2/m (Table 7). Additionally, the two-layer polytypes 2M1 and 2M2 each having the space-group symmetry C2/c are present in the tobelite samples. However, it could not be resolved whether the three polytypes form single crystals or are intergrown domains within one single crystal. The variation in polytype concentration observed between sample Tob 1 in comparison to Tob 2 and Tob 3 is not likely to result from synthesis conditions since all samples were produced at 600 °C and 500 MPa using the same starting mixture. Unit-cell dimensions for tobelite are only presented for the 1M polytype since it is the dominating polytype in all samples (Table 8). For synthetic 1M tobelites from different runs, the variability of the a, b and c lattice constants and the monoclinic angle β is extremely small and falls within the analytical error of XRD analysis. In comparison to synthetic muscovite, the lattice parameters for tobelite are lager, especially in the c direction. This increase in the c lattice constant is approximately one order of magnitude larger than that observed in the a and b directions and is due to an increase in the interlayer spacing between the T-O-T sheets perpendicular to the c axis (Figure 9). It results from replacing the smaller K+ ion with the larger + NH4 ion on the 12-fold coordinated interlayer site A. Accordingly, the structural parameters for
34
synthetic tobelite, particulary the lattice constants c and b, are larger than reported for type natural tobelite (Table 8; Higashi, 1982). This is not surprising, because natural tobelite is a muscovite- tobelite solid solution with XK = 0.20, XNH4 = 0.57 and XVacancies = 0.27 for the A site cation. In Figure 10 the XRD-spectrum for synthetic tobelite is shown. The position of reflex (00l) serves as + + an indicator for NH4 - K substitution because it is shifted the most to smaller 2θ values with + increasing NH4 content due to the expansion of the unit-cell.
Table 7 Rietveld refinement parameters for synthetic tobelite and muscovite
Sample P T Duration RP Rwp χ2 DW Quantitative phase analysis [MPa] [°C] [d]
Tob 1 500 600 20 0.062 0.083 1.46 1.34 47 wt% Tob-1M; 23 wt% Tob-2M1; 30 wt% Tob-2M2
Tob 2 500 600 22 0.084 0.111 1.34 1.56 81wt% Tob-1M; 12 wt% Tob-2M1; 7 wt% Tob-2M2
Tob 3 500 600 19 0.091 0.118 1.54 1.36 81wt% Tob-1M; 15 wt% Tob-2M1; 4 wt% Tob-2M2
Syn. 400 500 10 0.046 0.061 1.32 1.45 76wt% Musc-1M; 10wt% Musc- Musc 2M1; 14wt% Fsp Tob = Tobelite; Musc = Muscovite; Fsp = K-Feldspar;
obs calc obs obs calc obs 0.5 obs calc Rp = ∑ | yi - yi | / ∑ yi ; Rwp = [ ∑ wi( yi - yi ) ² / ∑ wi (yi ) ² ] ; χ2 = wi( yi - yi ) ² / (N-P) ; DW = d statistics (Durbin-Watson) Note: (N-P) = observations (step intervals); y = intensity; w = 1/y (weighting factor)
Table 8 Unit-cell dimensions of 1M tobelite and 1M muscovite along with 2σ deviation
Phase a [Å] b [Å] c [Å] β [°] vol [Å3] Source
Tob 1 5.2184(7) 9.009(1) 10.541(2) 101.41(1) 485.8(1) this study Tob 2 5.2183(5) 9.0089(9) 10.544(1) 101.40(1) 485.9(1) this study Tob 3 5.2175(7) 9.007(1) 10.540(2) 101.39(1) 485.6(1) this study syn.Tob 5.217(3) 9.001(3) 10.540(2) 101.37 485.33 Eugster & Munoz (1966) nat. Tob 5.219(4) 8.986(3) 10.447(2) 101.31(1) 480.44 Higashi (1982) syn. Musc 5.2064(3) ..8.9818(4) 10.278(1) 101.686(6) 470.66(4) this study (sample 23-99)
Tob = Tobelite; Musc = Muscovite
35
Figure 8 IR spectrum of synthetic tobelite (sample Tob 2) at 298 K. IR-active: triply generated fundamental states ν3 and ν4; combination mode ν2 + ν4; overtone mode 2ν4 and overtone mode 2ν2 (weak); -1 no indication of additional νOH bands at 3600 to 3700 cm (from Harlov et al., 2001b)
Figure 9 Structure image of monoclinc 1M tobelite: A = 12-fold coordinated (NH4); M = octahedral site
(Al); Ti = tetrahedral site (Si resp. Al); red balls = O; unit-cell indicated by dashed lines
36
(00l) Sample Tob 2
81wt% Tob-1M
12 wt% Tob-2M1
4 wt% Tob-2M2
2M2 2M1 1M
Figure 10 XRD spectrum of tobelite (sample Tob 2) synthesised at 600 °C / 500 MPa; mica polytypes 1M, 2M1 and 2M2 identified by Rietveld analysis
37
4. Results of “Exchange synthesis experiments”
4.1 The system buddingtonite – K-feldspar – NH4Cl – KCl
In Table 9 a-d the results of all experiments performed in the buddingtonite – K-feldspar – NH4Cl – KCl system along with the starting materials and run conditions are presented. The synthesised
(K-NH4)-feldspar solid solutions form, similar to pure buddingtonite, clumps of intergrown idiomorphous crystals with monoclinic symmetry. In high pressure runs the individual grains generally are about 20 to 40 µm long and ~ 10 µm wide (Figure 11a). Feldspar grains from hydrothermal experiments do not exceed 5 to 10 µm and additionally show a thin film of an amorphous phase on the crystal edges (Figure 11b). Despite the fact that all bulk compositions are within the feldspar-fluid reciprocal ternary, run products consist not only of (K-NH4)-feldspar but of additional (K-NH4)-muscovite and quartz in varying amounts. Figure 12 shows the obtained phase proportions for the exchange series at all run conditions. Increasing synthesis temperature + and NH4 concentration in the starting composition lead to an increase in the formation of muscovite and quartz. For example, in runs with high ammonium contents performed at 600 °C and 400 MPa no feldspar is observed at all. The synthesised tobelite-muscovite solid solutions within the feldspar-fluid exchange series occur in small clumps of semieuhedral crystals less than 2 µm in size, comparable to synthetic tobelite.
20 µm 6 µm a) b)
Figure 11 SEM images of synthetic (K-NH4) solid solutions with monoclinc symmetry (a) sample 45-99 synthesised at 500 °C / 1500 MPa; (b) sample 5-99 synthesised at 500 °C / 400 MPa
38
Fsp-Fluid (400/4) Fsp-Fluid (500/4) 400 °C / 400 MPa 500 °C / 400 MPa 1 1 0,8 0,8 fsp ss 0,6 0,6 musc ss 0,4 0,4 qtz 0,2 phase prop. prop. phase 0,2 phase prop. 0 0 0 1 0 1 XK (bulk)bulk XK (bulk)bulk XK XK
Fsp-Fluid (600/4) 600 °C / 400 MPa 1 0,8 fsp ss 0,6 musc ss 0,4 0,2 qtz phase prop. 0 0 1 XK (bulk)bulk XK
Fsp-Fluid (500/15) Fsp-Fluid (600/15) 500 °C / 1500 MPa 600 °C / 1500 MPa 1 1 0,8 fsp ss 0,8 0,6 0,6 musc ss 0,4 0,4 0,2 qtz 0,2 phase prop. prop. phase prop. phase 0 0 0 1 0 1 XK bulk(bulk) XK (bulk)bulk XK XK
Figure 12 Phase proportions obtained by Rietveld analysis of XRD spectra for the feldspar-fluid exchange + series at 400-600 °C / 400 MPa and 500-600 °C / 1500 MPa; increasing NH4 content of the bulk bulk composition (low X K ) as well as higher temperatures lead to formation of (K-NH4)-muscovite and quartz instead of (K-NH4)-feldspar; run durations are 10-24 days for hydrothermal and 5-10 days for piston- cylinder runs
39
Table 9a Experimental data for "exchange synthesis" experiments in the buddingtonite - K-feldspar - NH4Cl - KCl reciprocal ternary at 400°C and 400 MPa
Run No. 38-99 39-99 40-99 43-99 44-99 Duration [days] 12 10 21 21 12 Starting materials Solids mg SiO2 18.88 7.87 15.91 19.52 18.76 mg Al2O3 5.29 2.21 4.45 5.46 5.25 OH-fluids
µl NH4(OH) 19.08 7.40 11.19 9.69 5.84 µl K(OH) 6.19 2.76 7.18 13.39 17.03 Molarity 5.0 5.0 5.0 5.0 5.0
K / (K+NH4) 0.245 0.272 0.391 0.580 0.745 Cl-fluids mg NH4Cl 35.66 8.55 12.82 11.62 9.65 mg KCl 12.27 3.46 10.10 21.23 27.34 Molality 3.1 3.1 3.1 3.1 3.1
K / (K+NH4) 0.256 0.288 0.441 0.646 0.739
(K+NH4)solid / (K+NH4)total 0.459 0.577 0.564 0.531 0.499 Total mmol Si 0.314 0.131 0.265 0.325 0.312 mmol Al 0.104 0.043 0.087 0.107 0.103 mmol K 0.069 0.025 0.067 0.133 0.170 mmol NH4 0.206 0.064 0.096 0.084 0.059 mmol Cl 0.149 0.037 0.071 0.102 0.115 mmol H2O 3.426 1.053 1.965 2.663 2.852 expected Cl- molality 2.41 1.96 2.01 2.12 2.23
Products Feldspar ss (wt%) 80.2 78.3 80.6 88.0 88.5 Musc ss (wt%) 6.3 3.2 3.1 0 0 Quartz (wt%) 13.5 18.5 16.3 12.0 11.5 feldspar XK (XRD) 0.36 0.37 0.55 0.81 0.91 musc XK (XRD) 0.56 0.67 0.73 - - Fluid fluid XK 0.21 0.20 0.30 0.49 0.64 mmol K 0.036 0.009 0.025 0.058 0.088 mmol NH4 0.138 0.037 0.059 0.061 0.050 calculated Cl- molality 2.46 2.13 2.10 2.17 2.32 + + (from K and NH4 under the + + assumption that K and NH4 are exclusively balanced by Cl-)
40
Table 9b Experimental data for "exchange synthesis" experiments in the buddingtonite - K-feldspar - NH4Cl - KCl reciprocal ternary at 500°C and 400 MPa
Run No. 1-99 4-98 9-98 2-99 3-99 2a-98 10-98 4-99 5-99 5-98 11-98 6-99 Duration [days] 13 12 12 11 11 10 12 13 25 10 12 24 Starting materials Solids mg SiO2 20.05 19.14 21.09 19.79 19.36 19.11 20.86 19.33 19.15 19.44 19.86 18.90 mg Al2O3 5.61 5.36 5.11 5.54 5.42 5.15 4.92 5.41 5.36 5.44 4.68 5.29 OH-fluids
µl NH4(OH) 20.16 18.38 16.39 15.29 13.29 11.09 10.39 9.29 6.88 5.44 4.42 2.35 µl K(OH) 3.54 6.49 5.34 7.41 9.99 11.23 10.99 13.78 16.11 18.39 15.76 19.94 Molarity 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
K / (K+NH4) 0.149 0.261 0.246 0.327 0.429 0.503 0.514 0.597 0.701 0.772 0.781 0.895 Cl-fluids mg NH4Cl 38.74 33.88 30.77 29.80 25.79 20.63 19.46 16.90 12.78 9.03 8.07 4.95 mg KCl 5.55 10.26 8.47 13.48 17.87 20.29 19.85 25.01 29.27 32.29 29.12 36.54 Molality 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1
K / (K+NH4) 0.125 0.232 0.216 0.311 0.409 0.496 0.505 0.597 0.696 0.781 0.783 0.881
(K+NH4)solid / (K+NH4)total 0.463 0.476 0.472 0.458 0.462 0.468 0.467 0.470 0.469 0.482 0.467 0.464 Total mmol Si 0.334 0.319 0.351 0.329 0.322 0.318 0.347 0.322 0.319 0.323 0.331 0.315 mmol Al 0.110 0.105 0.100 0.109 0.106 0.101 0.096 0.106 0.105 0.107 0.092 0.104 mmol K 0.035 0.064 0.053 0.079 0.105 0.119 0.116 0.146 0.171 0.192 0.169 0.213 mmol NH4 0.221 0.197 0.177 0.169 0.146 0.119 0.112 0.099 0.074 0.055 0.047 0.027 mmol Cl 0.137 0.137 0.122 0.134 0.135 0.127 0.122 0.130 0.130 0.128 0.115 0.129 mmol H2O 3.326 3.408 3.006 3.275 3.351 3.186 3.060 3.299 3.329 3.358 2.955 3.312 expected Cl- molality 2.29 2.23 2.25 2.27 2.24 2.21 2.21 2.19 2.17 2.12 2.17 2.16
Products Feldspar ss (wt%) 13.0 28.0 35.0 39.5 64.0 63.0 55.0 80.0 90.0 95.0 88.0 95.0 Musc ss (wt%) 29.0 27.0 23.0 21.5 10.0 11.0 12.0 4.0 1.5 0.0 0.0 0.0 Quartz (wt%) 58.0 45.0 42.0 39.0 26.0 26.0 33.0 16.0 8.5 5.0 12.0 5.0 feldspar XK (XRD) 0.25 0.41 0.36 0.49 0.62 0.76 0.74 0.81 0.87 0.93 0.90 0.99 musc XK (XRD) 0.34 0.53 0.42 0.68 0.78 0.75 0.87 0.72 1.0 - - - Fluid fluid XK 0.131 0.213 0.197 0.257 0.336 0.440 0.404 0.495 0.601 0.679 0.674 0.818 mmol K 0.027 0.041 0.034 0.051 0.062 0.066 0.066 0.082 0.098 0.099 0.092 0.126 mmol NH4 0.178 0.151 0.136 0.148 0.123 0.084 0.097 0.084 0.065 0.047 0.044 0.028 calculated Cl- molality 3.01 2.74 2.76 2.96 2.69 2.28 2.58 2.43 2.36 2.10 2.23 2.23 + + (from K and NH4 under + the assumption that K and + NH4 are exclusively balanced by Cl-)
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Table 9c Experimental data for "exchange synthesis" experiments in the buddingtonite - K-feldspar - NH4Cl - KCl reciprocal ternary at 600°C and 400 MPa
Run No. 7-99 6-98 8-99 9-99 7-98 10-99 8-98 12-99 Duration [days] 12 11 13 12 12 12 11 11 Starting materials Solids mg SiO2 19.94 19.37 19.73 19.11 19.41 18.83 19.22 18.62 mg Al2O3 5.59 5.42 5.52 5.35 5.44 5.27 5.38 5.22 OH-fluids
µl NH4(OH) 20.66 18.45 15.12 11.78 11.06 9.24 5.58 2.16 µl K(OH) 3.53 6.38 7.24 9.85 11.55 13.67 18.06 20.10 Molarity 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
K / (K+NH4) 0.146 0.257 0.324 0.456 0.511 0.597 0.764 0.903 Cl-fluids mg NH4Cl 38.25 34.00 29.31 25.39 20.84 16.61 8.97 4.63 mg KCl 4.76 10.31 13.53 18.24 21.40 25.20 32.10 36.06 Molality 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1
K / (K+NH4) 0.111 0.233 0.316 0.418 0.507 0.603 0.782 0.886
(K+NH4)solid / (K+NH4)total 0.476 0.475 0.457 0.444 0.463 0.469 0.481 0.469 Total mmol Si 0.332 0.322 0.328 0.318 0.323 0.313 0.320 0.310 mmol Al 0.110 0.106 0.108 0.105 0.107 0.103 0.106 0.102 mmol K 0.032 0.064 0.078 0.106 0.124 0.146 0.190 0.212 mmol NH4 0.222 0.198 0.166 0.138 0.120 0.098 0.056 0.025 mmol Cl 0.133 0.137 0.133 0.135 0.131 0.130 0.127 0.126 mmol H2O 3.286 3.413 3.235 3.269 3.268 3.287 3.334 3.273 expected Cl- molality 2.25 2.23 2.28 2.30 2.22 2.19 2.12 2.14
Products Feldspar ss (wt%) 0.0 0.0 0.0 19.0 35.0 65.0 93.0 93.0 Musc ss (wt%) 39.0 44.0 43.0 28.0 23.0 9.0 0.0 0.0 Quartz (wt%) 61.0 56.0 57.0 53.0 42.0 26.0 7.0 7.0 feldspar XK (XRD) - - - 0.69 0.77 0.82 0.96 0.99 musc XK (XRD) 0.28 0.57 0.64 0.78 0.83 0.88 - - Fluid fluid XK 0.130 0.247 0.297 0.399 0.504 0.566 0.697 0.841 mmol K 0.025 0.047 0.062 0.080 0.082 0.091 0.111 0.124 mmol NH4 0.168 0.144 0.148 0.121 0.081 0.070 0.048 0.023 calculated Cl- molality 2.87 2.74 3.15 2.98 2.42 2.37 2.30 2.18 + + (from K and NH4 under + the assumption that K and + NH4 are exclusively balanced by Cl-)
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Table 9d Experimental data for "exchange synthesis" experiments in the buddingtonite - K-feldspar - NH4Cl - KCl reciprocal ternary at 1500 MPa
Run No. 17-00 19-00 41-99 42-99 28-99 45-99 46-99 13-00 14-00 15-00 16-00 T [°C] / Duration [days] 500 / 6 500 / 6 500 / 10 500 / 6 500 / 6 500 / 6 500 / 6 600 / 5 600 / 5 600 / 8 600 / 8 Starting materials Solids mg SiO2 8.38 8.03 7.73 6.18 8.52 7.07 7.04 7.93 8.06 7.97 8.43 mg Al2O3 2.26 2.16 2.08 1.73 2.39 1.98 1.97 2.13 2.17 2.15 2.27 OH-fluids
µl NH4(OH) 9.28 8.74 7.69 6.12 5.02 4.84 3.35 7.92 6.04 4.72 2.83 µl K(OH) 1.43 1.71 3.08 3.46 5.18 6.23 7.23 2.95 4.09 6.25 7.70 Molarity 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
K / (K+NH4) 0.134 0.164 0.286 0.361 0.508 0.563 0.683 0.271 0.404 0.570 0.731 Cl-fluids mg NH4Cl 11.86 11.39 8.35 6.47 4.57 4.76 3.33 8.42 6.72 4.64 2.64 mg KCl 2.29 2.40 3.95 5.74 5.22 7.91 9.67 3.72 6.34 7.87 9.70 Molality 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1
K / (K+NH4) 0.162 0.174 0.321 0.470 0.533 0.624 0.744 0.306 0.485 0.629 0.786
(K+NH4)solid / (K+NH4)total 0.550 0.550 0.586 0.558 0.627 0.585 0.568 0.591 0.556 0.586 0.579 Total mmol Si 0.140 0.134 0.129 0.103 0.142 0.118 0.117 0.132 0.134 0.133 0.140 mmol Al 0.044 0.042 0.041 0.034 0.047 0.039 0.039 0.042 0.043 0.042 0.045 mmol K 0.014 0.016 0.028 0.035 0.042 0.056 0.066 0.026 0.040 0.056 0.069 mmol NH4 0.083 0.079 0.064 0.051 0.039 0.039 0.027 0.066 0.051 0.038 0.022 mmol Cl 0.044 0.043 0.038 0.038 0.030 0.039 0.040 0.038 0.041 0.039 0.038 mmol H2O 1.170 1.143 1.097 1.032 0.967 1.141 1.136 1.094 1.102 1.130 1.107 expected Cl- molality 2.08 2.07 1.93 2.04 1.74 1.91 1.97 1.91 2.04 1.91 1.92
Products Feldspar ss (wt%) 15.0 5.5 62.0 74.0 67.0 86.0 91.0 21.0 48.0 74.0 86.0 Musc ss (wt%) 51.0 64.5 17.0 21.0 14.0 2.0 0.0 38.0 32.0 20.0 9.0 Quartz (wt%) 34.0 30.0 21.0 5.0 19.0 12.0 9.0 41.0 20.0 6.0 5.0 feldspar XK (XRD) 0.24 0.29 0.43 0.64 0.73 0.85 0.93 0.50 0.68 0.87 0.93 musc XK (XRD) 0.45 0.46 0.72 0.85 0.99 - - 0.77 0.90 1.0 - Fluid fluid XK 0.181 0.199 0.292 0.443 0.466 0.577 0.716 0.297 0.429 0.575 0.744 mmol K 0.010 0.011 0.015 0.021 0.018 0.030 0.036 0.014 0.024 0.030 0.034 mmol NH4 0.044 0.042 0.037 0.026 0.021 0.022 0.014 0.033 0.032 0.022 0.012 calculated Cl- molality 2.29 2.28 2.33 2.25 2.03 2.21 2.21 2.15 2.46 2.26 2.01 + + (from K and NH4 under + the assumption that K and + - NH4 are balanced by Cl )
43
4.2 The system tobelite – muscovite – NH4Cl – KCl
In Table 10 a-b the results of all experiments performed in the tobelite – muscovite – NH4Cl – KCl system along with the starting materials and run conditions are presented. All runs of the muscovite-fluid exchange series performed at 400 and 500 °C / 400 MPa yield between 85 and
100 wt% (K-NH4)-muscovite. Small amounts of quartz are detectable in most samples, some + experiments with only minor or no NH4 in the starting composition contain few amounts of feldspar. At 600 °C / 400 MPa, quartz and corundum up to 16-18 wt.% each are present + additional to muscovite. Only run number 4-00, with low NH4 concentration in the starting composition yields pure (K-NH4)-muscovite. Experimental products obtained at 500 °C and 1500 MPa show no other phases besides tobelite-muscovite solid solutions and probably a minor quartz component. Generally, the hydrothermally synthesised (K-NH4)-muscovites show the same appearance as synthetic tobelites and tobelite-muscovite solid solutions from feldspar-fluid exchange experiments. The (K-NH4)-muscovite crystals are extremely flat with grain sizes of 1-2 µm and stick together in masses (Figure 13). Only the high-pressure experiments performed at 500 °C / 1500 MPa yield slightly larger tobelite-muscovite solid solutions with grains of about 5 µm in diameter.
20 µm
Figure 13 SEM image of muscovite solid solutions synthesised at 500 °C / 400 MPa (sample 14-99)
44
Table 10a Experimental data for "exchange syntheses" in the tobelite - muscovite - NH4Cl - KCl reciprocal ternary at 400 MPa
Run No. 7-00 8-00 9-00 10-00 13-99 14-99 15-99 16-99 16a-99 17-99 18-99 19-99 20-99 21-99 22-99 23-99 1-00 2-00 3-00 4-00 T [°C] / Duration [days] 400/18 400/14 400/12 400/11 500/11 500/11 500/11 500/11 500/12 500/12 500/12 500/12 500/12 500/13 500/13 500/13 600/13 600/13 600/12 600/12 Starting materials Solids
mg SiO2 46.30 45.71 45.14 44.75 9.60 9.53 9.48 9.36 9.39 9.36 9.29 9.26 9.18 9.18 9.11 9.10 69.32 68.54 67.80 67.21
mg Al2O3 41.25 40.72 40.22 39.87 8.06 8.01 7.96 7.87 7.88 7.86 7.80 7.78 7.72 7.72 7.65 7.65 61.76 61.07 60.41 59.88 OH-fluids
µl NH4(OH) 42.01 31.26 20.86 10.77 12.05 10.17 9.16 8.16 8.39 7.47 6.51 5.55 4.50 3.22 2.35 0.00 63.24 45.51 30.76 15.86 µl K(OH) 10.90 21.29 31.22 40.97 0.00 1.92 3.21 4.25 4.33 5.27 6.15 7.17 8.22 9.14 9.85 13.25 15.70 30.97 45.67 61.81 Molarity 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
K / (K+NH4) 0.206 0.405 0.599 0.792 0.000 0.159 0.260 0.342 0.341 0.414 0.486 0.564 0.646 0.740 0.808 1.000 0.199 0.405 0.598 0.796 Cl-fluids
mg NH4Cl 81.72 50.02 33.39 16.72 27.40 23.79 21.01 19.01 18.84 15.81 13.32 10.57 8.53 6.48 3.51 0.00 99.66 74.88 48.70 24.61 mg KCl 27.95 44.26 67.97 87.87 0.00 5.15 8.49 12.16 12.04 14.52 17.76 21.55 24.70 27.56 30.65 33.09 33.64 66.21 97.36 118.51 Molality 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1
K / (K+NH4) 0.255 0.469 0.671 0.840 0.000 0.178 0.288 0.390 0.390 0.479 0.571 0.671 0.743 0.810 0.897 1.000 0.252 0.469 0.667 0.828
(K+NH4)solid / (K+NH4)total 0.438 0.473 0.453 0.444 0.415 0.402 0.403 0.391 0.399 0.404 0.397 0.390 0.382 0.369 0.365 0.347 0.489 0.466 0.458 0.467 Total mmol Si 0.771 0.761 0.751 0.745 0.160 0.159 0.158 0.156 0.156 0.156 0.155 0.154 0.153 0.153 0.152 0.152 1.154 1.141 1.128 1.119 mmol Al 0.809 0.799 0.789 0.782 0.158 0.157 0.156 0.154 0.155 0.154 0.153 0.153 0.151 0.151 0.150 0.150 1.211 1.198 1.185 1.175 mmol K 0.141 0.244 0.367 0.477 0.000 0.026 0.042 0.059 0.059 0.071 0.086 0.103 0.118 0.131 0.144 0.157 0.183 0.360 0.530 0.676
mmol NH4 0.463 0.311 0.208 0.106 0.145 0.125 0.111 0.100 0.100 0.086 0.074 0.060 0.049 0.036 0.023 0.000 0.625 0.460 0.305 0.156 mmol Cl 0.340 0.292 0.314 0.324 0.085 0.090 0.091 0.097 0.096 0.094 0.096 0.100 0.103 0.106 0.106 0.103 0.413 0.437 0.453 0.444
mmol H2O 7.575 6.890 7.200 7.350 1.835 1.907 1.947 2.023 2.025 2.001 2.029 2.076 2.126 2.145 2.139 2.029 9.937 10.200 10.449 10.442 expected Cl- molality 2.58 2.44 2.51 2.53 2.65 2.69 2.68 2.72 2.69 2.68 2.70 2.73 2.76 2.80 2.81 2.88 2.40 2.47 2.49 2.45
Products Feldspar ss (wt%) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.0 8.0 15.0 16.0 (*) 10.0 (*) 4.0 (*) 0.0 Musc ss (wt%) 99.0 99.0 99.0 97.0 97.0 97.0 96.0 97.0 100.0 98.5 98.7 98.0 98.0 94.0 92.0 85.0 66.0 78.0 91.0 100.0 Quartz (wt%) 1.0 1.0 1.0 3.0 3.0 3.0 4.0 3.0 0.0 1.5 1.3 2.0 2.0 2.0 0.0 0.0 18.0 12.0 5.0 0.0 feldspar XK ------1.00 - - - - musc XK (XRD) 0.35 0.60 0.83 0.92 0.0 0.30 0.49 0.62 0.65 0.76 0.85 0.91 0.94 0.95 0.99 1.00 0.42 0.71 0.86 0.97 Fluid fluid XK 0.167 0.315 0.517 0.760 0.0 0.125 0.196 0.290 0.286 0.347 0.436 0.549 0.651 0.736 0.827 1.000 0.202 0.364 0.536 0.729 mmol K 0.055 0.092 0.168 0.264 0.0 0.013 0.022 0.034 0.034 0.040 0.052 0.067 0.081 0.092 0.102 0.110 0.095 0.179 0.266 0.340
mmol NH4 0.276 0.199 0.157 0.084 0.099 0.088 0.089 0.083 0.085 0.075 0.067 0.055 0.044 0.033 0.021 0 0.376 0.313 0.231 0.126 calculated Cl- molality 2.10 2.03 2.15 2.23 2.62 2.54 2.72 2.73 2.81 2.72 2.76 2.77 2.74 2.70 2.65 2.49 2.31 2.32 2.26 2.33 + + (from K and NH4 concentration)
(*) formation of corundum
45
Table 10b Experimental data for "exchange syntheses" in the tobelite - muscovite - NH4Cl - KCl reciprocal ternary at 500°C and 1500 MPa
Run No. 20-00 5-00 6-00 11-00 12-00 Duration [days] 6 8 10 10 10 Starting materials Solids
mg SiO2 5.12 5.54 5.57 4.96 4.95
mg Al2O3 4.52 4.89 4.91 4.38 4.36 OH-fluids
µl NH4(OH) 6.27 6.06 4.44 2.60 1.61 µl K(OH) 1.37 2.21 3.52 4.23 4.99 Molarity 5.0 5.0 5.0 5.0 5.0
K / (K+NH4) 0.179 0.267 0.442 0.619 0.756 Cl-fluids
mg NH4Cl 7.94 6.22 4.79 4.57 2.19 mg KCl 2.24 2.94 5.06 8.54 10.62 Molality 3.1 3.1 3.1 3.1 3.1
K / (K+NH4) 0.220 0.321 0.514 0.651 0.829
(K+NH4)solid / (K+NH4)total 0.548 0.593 0.566 0.456 0.454 Total mmol Si 0.085 0.092 0.093 0.083 0.082 mmol Al 0.089 0.096 0.096 0.086 0.086 mmol K 0.014 0.020 0.033 0.048 0.058
mmol NH4 0.056 0.050 0.037 0.027 0.015 mmol Cl 0.030 0.028 0.031 0.041 0.040
mmol H2O 0.839 0.828 0.849 0.938 0.913 expected Cl- molality 2.16 1.98 2.07 2.48 2.49
Products Musc ss (wt%) * > 98 > 98 > 98 > 98 > 98 musc XK (IR) 0.34 0.40 0.65 0.86 0.94 Fluid fluid XK 0.190 0.235 0.400 0.577 0.712 mmol K 0.007 0.007 0.014 0.026 0.033
mmol NH4 0.029 0.023 0.021 0.019 0.014 calculated Cl- molality 2.12 1.79 2.04 2.33 2.44 + + (from K and NH4 concentration)
* evaluation of XRD pattern without Rietveld analyses
46
5. Structural and compositional variation of buddingtonite-K-feldspar and tobelite-muscovite solid solutions
5.1 Buddingtonite – K-feldspar solid solutions
Solid products of the feldspar-fluid exchange series were investigated using EMP analyses and Rietveld refinement of XRD spectra. IR spectroscopy was not performed because nearly all run products of this series contain (K-NH4)-muscovite and quartz additional to or instead of buddingtonite - K-feldspar solid solution.
5.1.1 Results obtained by EMP
Tables 11 and 12 present EMP analyses of buddingtonite-K-feldspar solid solutions synthesised at varying bulk compositions and temperatures at 400 MPa and 1500 MPa, respectively. EMP measurements were obtained with a Cameca SX50 at the GFZ Potsdam and with a JEOL JXA 8900 RL at the University of Göttingen. At least 3 spots were measured on each sample. Feldspar analyses are accepted if the oxid sums exceed 93 wt%. The formulae are calculated on the basis of 8 oxygens pfu; hydrogen is added stoichiometrically with respect to
fsp the measured N content. Compositions range from X K 0.14 to 0.89 for the (K-NH4)-feldspar solid solutions of various runs. Despite the higher precision of the JEOL JXA 8900 RL with respect to N measurement, analyses are in good agreement to each other (Figure 16, Chapter + 5.1.3). The main difference between both electron microprobes is the detection limit for NH4 determination. For a standard equipped Cameca SX50 it is approximately 0.8 wt% whereas the + JEOL JXA 8900 RL, with a LDEN multilayer crystal, is able to measure NH4 down to 0.1 wt%.
47
Table 11 Microprobe analyses of hydrothermally synthesised feldspar solid solutions at 400 MPa Sample 38-99 39-99 40-99 43-99 3-99 10-98 5-99 5-98 9-99 8-98 T [°C] 400 400 400 400 500 500 500 500 600 600 no. of analyses 10 a 8 a 8 a 10 a 8 a 3 b 3 b 3 b 3 b 3 b wt% of oxides
(NH4)2O 6.32 5.26 3.58 1.21 2.99 2.28 1.46 0.94 2.92 0.75
K2O 5.08 4.27 7.46 9.13 8.62 11.73 12.23 14.14 8.88 10.57 SiO2 62.38 64.34 63.71 64.09 63.69 66.92 67.36 65.54 65.90 67.91 Al2O3 19.36 19.95 19.97 19.21 19.74 19.59 20.02 19.16 19.72 19.23 total 93.14 93.83 94.72 93.65 95.03 100.52 101.07 99.78 97.42 98.47 atoms pfu *
NH4 0.68 0.56 0.39 0.13 0.32 0.24 0.15 0.10 0.31 0.08 K 0.31 0.25 0.44 0.55 0.51 0.67 0.69 0.83 0.52 0.61 Si 2.95 2.98 2.97 3.03 2.98 3.00 3.00 3.00 3.00 3.06 Al 1.07 1.08 1.09 1.07 1.09 1.03 1.05 1.03 1.06 1.02
fsp (EMP) 0.31 0.31 0.54 0.81 0.61 0.74 0.82 0.89 0.63 0.89 X K *calculated on basis of 8 O atoms a measured with Cameca SX50; b measured with JEOL JXA 8900RL
Table 12 Microprobe analyses of feldspar solid solutions synthesised at 1500 MPa Sample 17-00 41-99 42-99 28-99 13-00 14-00 T [°C] 500 500 500 500 600 600 no. of analyses 14 a 11 a 16 a 15 a 11 a 8 a wt% of oxides
(NH4)2O 8.31 5.87 3.16 2.08 4.70 2.41
K2O 2.40 4.18 9.73 11.17 4.51 6.88 SiO2 66.87 66.89 65.48 65.80 67.22 65.62 Al2O3 19.96 19.48 19.75 19.04 19.30 20.22 total 97.54 96.55 98.12 98.10 95.72 95.12 atoms pfu *
NH4 0.85 0.61 0.33 0.22 0.49 0.26 K 0.14 0.24 0.57 0.65 0.26 0.41 Si 2.97 3.01 2.98 3.01 3.04 3.02 Al 1.04 1.03 1.06 1.03 1.03 1.09
fsp (EMP) 0.14 0.28 0.63 0.75 0.35 0.62 X K *calculated on basis of 8 O atoms; a measured with Cameca SX50
48
5.1.2 Results obtained by XRD
For all solid products of the feldspar-fluid exchange experiments XRD spectra were taken and Rietveld analyses performed. The statistical parameters obtained for each refinement are shown in Table 13 a-c and indicate good fits. Additionally, the quantitative phase relations of the products feldspar, quartz and different mica polytypes are given (Table 13 a-c). It is obvious that muscovite solid solutions synthesised from feldspar-fluid exchange experiments at hydrothermal conditions consist of the mica polytype 1M whereas at 1500 MPa both mica polytypes 1M and 2M1 are present. (K-NH4)- feldspars exhibit a monoclic structure similar to sanidine as observed for the buddingtonite samples. Besides quantitative determination of phase proportions, Rietveld analysis is also used to derive unit-cell dimensions for the synthesised buddingtonite-feldspar solid solutions (Table 14). As already shown in Chapter + + 3.1, the substitution of NH4 for K influences the size of the feldspar unit-cell. Especially, lattice constant a in buddingtonite – K-feldspar solid solutions increases significantly with + increasing NH4 content. The obtained values for a exhibit an increase from 8.6063 Å for nearly pure K-feldspar to 8.770 Å for NH4-rich feldspar. Under the assumption that cell
fsp fsp volumes vary linearly with the K-NH4 composition in feldspar, the mole fractions X K = K / fsp (K+NH4) are derived from the two endmembers buddingtonite and K-feldspar according to:
fsp fsp budd Kfsp budd (11) X K = (a ss – a ) / (a – a ).
Since pure buddingtonites yield a relatively wide range of values for the lattice constant a, the average from Budd 1 to Budd 6 is used in equation (11) (Table 4, Chapter 3.1). In Table 15 the fsp σ X K results obtained from cell parameter a are given along with the calculated 2 deviations.
fsp The values for X K range from 0.24 to 0.99 and indicate that an almost complete solid solution series exists between K-feldspar and buddingtonite. In order to prove the assumption that cell parameters vary linearly with the K-NH4 composition in feldspar, a, b, c, β and V are plotted
fsp versus the X K values determined according to reaction (11) (Figure 14). It is not surprising
fsp that a linear relationsship exists between a and X K since this was assumed in order to fsp β calculate X K . However, a linear correlation is also observed for lattice parameters b, c, and
fsp V versus X K suggesting that the excess volume within the buddingtonite – K-feldspar solid solution series is negligible.
49
Table 13a Rietveld refinement parameters for solid solutions of the feldspar-fluid exchange series at 400 MPa (phase proportions also given in Table 9a-c)
Sample RP Rwp χ2 DW Quantitative phase analysis Buddingtonite – K-feldspar at 400 °C and 400 MPa 38-99 0.065 0.087 1.31 1.55 80wt% Fsp; 6wt% Musc-1M; 14wt% Qtz 39-99 0.072 0.095 1.29 1.57 78wt% Fsp; 3wt% Musc-1M; 19wt% Qtz 40-99 0.065 0.088 1.19 1.65 81wt% Fsp; 3wt% Musc-1M; 16wt% Qtz 43-99 0.063 0.087 1.08 1.82 88wt% Fsp; 12wt% Qtz 44-99 0.060 0.082 1.16 1.75 89wt% Fsp; 11wt% Qtz Buddingtonite – K-feldspar at 500 °C and 400 MPa 1-99 0.119 0.162 1.48 1.42 13wt% Fsp; 29wt% Musc-1M; 58wt% Qtz 4-98 0.085 0.115 1.08 1.83 28wt% Fsp; 27wt% Musc-1M; 45wt% Qtz 9-98 0.077 0.103 1.17 1.73 35wt% Fsp; 23wt% Musc-1M; 42wt% Qtz 2-99 0.087 0.120 1.11 1.78 40wt% Fsp; 21wt% Musc-1M; 39wt% Qtz 3-99 0.094 0.129 1.14 1.78 64wt% Fsp; 10wt% Musc-1M; 26wt% Qtz 2a-98 0.082 0.114 1.09 1.80 63wt% Fsp; 11wt% Musc-1M; 26wt% Qtz 10-98 0.079 0.110 1.10 1.76 55wt% Fsp; 12wt% Musc-1M; 33wt% Qtz 4-99 0.081 0.111 1.12 1.80 80wt% Fsp; 4wt% Musc-1M; 16wt% Qtz 5-99 0.058 0.078 1.18 1.62 90wt% Fsp; 2wt% Musc-1M; 8wt% Qtz 5a-98 0.055 0.074 1.17 1.68 95wt% Fsp; 5wt% Qtz 11-98 0.058 0.078 1.16 1.74 88wt% Fsp; 12wt% Qtz 6-99 0.055 0.075 1.22 1.58 95wt% Fsp; 5wt% Qtz Buddingtonite – K-feldspar at 600 °C and 400 MPa 7-99 0.114 0.156 1.30 1.65 39wt% Musc-1M; 61wt% Qtz 6-98 0.100 0.136 1.28 1.52 44wt% Musc-1M; 56wt% Qtz 8-99 0.105 0.145 1.24 1.61 43wt% Musc-1M; 57wt% Qtz 9-99 0.117 0.161 1.26 1.67 19wt% Fsp; 28wt% Musc-1M; 53wt% Qtz 7-98 0.093 0.125 1.28 1.59 35wt% Fsp; 23wt% Musc-1M; 42wt% Qtz 10-99 0.084 0.115 1.19 1.74 65wt% Fsp; 9wt% Musc-1M; 26wt% Qtz 8-98 0.054 0.073 1.25 1.61 93wt% Fsp; 7wt% Qtz 12-99 0.060 0.082 1.23 1.59 93wt% Fsp; 7wt% Qtz
50
Table 13b Rietveld refinement parameters for solid solutions of the feldspar-fluid exchange series at 1500 MPa (phase proportions also given in Table 9d)
Sample RP Rwp χ2 DW Quantitative phase analysis Buddingtonite – K-feldspar at 500 °C and 1500 MPa 17-00 0.050 0.067 1.29 1.51 15wt% Fsp; 21wt% Musc-1M; 30wt% Musc-2M1; 34wt% Qtz 19-00 0.047 0.068 1.76 1.16 6wt% Fsp; 24wt% Musc-1M; 40wt% Musc-2M1; 30wt% Qtz 41-99 0.057 0.077 1.57 1.30 62wt% Fsp; 8wt% Musc-1M; 9wt% Musc-2M1; 21wt% Qtz 42-99 0.061 0.082 1.59 1.23 74wt% Fsp; 10wt% Musc-1M; 11wt% Musc-2M1; 5wt% Qtz 28-99 0.084 0.115 1.38 1.51 67wt% Fsp; 6wt% Musc-1M; 8wt% Musc-2M1; 19wt% Qtz 45-99 0.071 0.094 1.48 1.42 86wt% Fsp; 2wt% Musc-1M; 12wt% Qtz 46-99 0.056 0.074 1.32 1.47 90wt% Fsp; 1wt% Musc-1M; 9wt% Qtz Buddingtonite – K-feldspar at 600 °C and 1500 MPa 13-00 0.062 0.083 1.37 1.50 21wt% Fsp; 8wt% Musc-1M; 30wt% Musc-2M1; 41wt% Qtz 14-00 0.058 0.079 1.72 1.16 48wt% Fsp; 14wt% Musc-1M; 18wt% Musc-2M1; 20wt% Qtz 15-00 0.058 0.076 1.70 1.25 74wt% Fsp; 3wt% Musc-1M; 17wt% Musc-2M1; 6wt% Qtz 16-00 0.044 0.061 1.73 1.13 86wt% Fsp; 2wt% Musc-1M; 6wt% Musc-2M1; 6wt% Qtz
51 Table 14 Unit-cell dimensions of buddingtonite – Kfsp solid solutions along with 2σ deviations
Sample a [Å] b [Å] c [Å] β [°] V [Å3]
Buddingtonite - K-feldspar at 400 °C and 400 MPa 38-99 8.7456(3) 13.0457(5) 7.1866(3) 116.097(2) 736.35(5) 39-99 8.7438(5) 13.0515(6) 7.1921(4) 116.091(4) 737.12(7) 40-99 8.7047(6) 13.0453(7) 7.1890(4) 116.066(4) 733.32(7) 43-99 8.6464(4) 13.0466(5) 7.1832(3) 116.016(2) 728.20(6) 44-99 8.6246(3) 13.0452(4) 7.1821(2) 116.013(2) 726.19(5) Buddingtonite - K-feldspar at 500 °C and 400 MPa 1-99 8.770(2) 13.056(3) 7.192(1) 116.13(2) 739.3(2) 4-99 8.736(1) 13.046(1) 7.1881(7) 116.088(8) 735.81(9) 9-98 8.746(1) 13.060(1) 7.1879(8) 116.03(1) 737.7(1) 2-99 8.718(1) 13.045(1) 7.1844(7) 116.080(8) 733.84(9) 3-99 8.6901(7) 13.0438(7) 7.1842(4) 116.077(5) 731.44(7) 2a-98 8.6574(7) 13.0465(7) 7.1850(4) 116.008(5) 729.36(8) 10-98 8.6630(7) 13.0347(7) 7.1773(4) 116.041(5) 728.17(8) 4-99 8.6481(6) 13.0459(6) 7.1871(4) 115.994(4) 728.83(8) 5-99 8.6343(4) 13.0373(5) 7.1820(3) 116.034(2) 726.42(6) 5a-98 8.6214(3) 13.0349(4) 7.1775(2) 116.030(2) 724.78(6) 11-98 8.6264(5) 13.0325(6) 7.1803(3) 116.038(3) 725.30(7) 6-99 8.6063(3) 13.0278(4) 7.1804(2) 116.027(2) 723.43(5) Buddingtonite - K-feldspar at 600 °C and 400 MPa 7-99 - - - - - 6-98 - - - - - 8-99 - - - - - 9-99 8.673(2) 13.039(2) 7.179(1) 116.028(5) 729.5(2) 7-98 8.655(1) 13.045(1) 7.1857(6) 116.013(8) 729.1(1) 10-99 8.6452(6) 13.0443(7) 7.1852(3) 116.015(4) 728.19(7) 8-98 8.6148(3) 13.0305(4) 7.1783(2) 116.009(2) 724.19(5) 12-99 8.6070(3) 13.0408(4) 7.1872(2) 115.985(2) 725.15(5) Buddingtonite - K-feldspar at 500 °C and 1500 MPa 17-00 8.772(2) 13.050(2) 7.1945(9) 116.08(1) 739.7(1) 19-00 8.762(2) 13.049(3) 7.198(2) 116.13(2) 738.8(3) 41-99 8.7302(4) 13.0542(5) 7.1900(3) 116.056(3) 736.13(5) 42-99 8.6838(4) 13.0466(5) 7.1846(3) 116.057(2) 731.24(6) 28-99 8.6651(5) 13.0477(6) 7.1862(3) 116.049(3) 729.93(7) 45-99 8.6393(4) 13.0404(5) 7.1837(3) 116.040(2) 727.15(6) 46-99 8.6195(3) 13.0350(4) 7.1830(2) 116.044(2) 725.10(5) Buddingtonite - K-feldspar at 600 °C and 1500 MPa 13-00 8.7148(7) 13.049(1) 7.1847(6) 116.055(6) 734.00(7) 14-00 8.6748(5) 13.0445(5) 7.1850(3) 116.064(3) 730.36(6) 15-00 8.6330(3) 13.0320(5) 7.1832(3) 116.032(2) 726.15(6) 16-00 8.6464(4) 13.0466(5) 7.1832(3) 116.016(2) 728.20(6)
52 Table 15 fsp values determined from XRD data of feldspar solid solutions along with 2σ deviations X K
Sample a [Å] fsp [XRD] X K Buddingtonite-K-feldspar at 400 °C and 400 MPa 38-99 8.7456 (3) 0.36 (4) 39-99 8.7438 (5) 0.37 (4) 40-99 8.7047 (6) 0.55 (3) 43-99 8.6464 (4) 0.81 (3) 44-99 8.6246 (3) 0.91 (2) Buddingtonite-K-feldspar at 500 °C and 400 MPa 1-99 8.770 (2) 0.25 (4) 4-99 8.736 (1) 0.41 (4) 9-98 8.746 (1) 0.36 (4) 2-99 8.718 (1) 0.49 (3) 3-99 8.6901 (7) 0.62 (3) 2a-98 8.6574 (7) 0.76 (3) 10-98 8.6630 (7) 0.74 (3) 4-99 8.6481 (6) 0.81 (3) 5-99 8.6343 (4) 0.87 (3) 5a-98 8.6214 (3) 0.93 (2) 11-98 8.6264 (5) 0.90 (2) 6-99 8.6063 (3) 0.99 (3) Buddingtonite-K-feldspar at 600 °C and 400 MPa 7-99 - - 6-98 - - 8-99 - - 9-99 8.673 (2) 0.69 (3) 7-98 8.655 (1) 0.77 (3) 10-99 8.6452 (6) 0.82 (2) 8-98 8.6148 (3) 0.96 (2) 12-99 8.6070 (3) 0.99 (2) Buddingtonite-K-feldspar at 500 °C and 1500 MPa 17-00 8.772 (2) 0.24 (4) 19-00 8.762 (2) 0.29 (4) 41-99 8.7302 (4) 0.43 (3) 42-99 8.6838 (4) 0.64 (3) 28-99 8.6651 (5) 0.73 (3) 45-99 8.6393 (4) 0.85 (2) 46-99 8.6195 (3) 0.93 (2) Buddingtonite-K-feldspar at 600 °C and 1500 MPa 13-00 8.7148 (7) 0.50 (3) 14-00 8.6748 (5) 0.68 (3) 15-00 8.6330 (3) 0.87 (3) 16-00 8.6464 (4) 0.93 (3)
53
8,85 13,15
8,80 13,10
8,75 13,05
8,70 13,00 a [A] b [A]
8,65 12,95
8,60 12,90
8,55 12,85 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00
fsp fsp XK XK
7,35 116,25
7,30 116,20
7,25 116,15
7,20 116,10 ß [°] c [A]
7,15 116,05
7,10 116,00
7,05 115,95 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00
fsp fsp XK XK
745
740
735 V [A³] V 730
725
720 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00
fsp XK
Figure 14 Lattice parameters of the buddingtonite – K-feldspar solid solutions series at 400-600 °C /
400 MPa and 400-500 °C / 1500 MPa determined from Rietveld analysis versus fsp values obtained X K from lattice constant a; error limits of lattice constants a, b, c, β and V are within size of the symbols
54
5.1.3 Comparison between results obtained by EMP and XRD
EMP and XRD analyses are used to determine the K-NH4 concentrations of the synthesised buddingtonite – K-feldspar solid solutions. For each (K-NH4)-feldspar obtained, the mole
fsp fraction X K are calculated on basis of the lattice parameter a determined via Rietveld analysis of XRD spectra. This implies a thorough knowledge of both the lattice constants of the solid solutions as well as the K- and NH4- endmembers. Rietveld analyses yield unit-cell parameters within an error of 2σ = 0.002 Å. However, along the (K-NH4)-feldspar join, the main uncertainty occurs within the variations observed for the lattice constant a in different pure buddingtonite samples (Table 4, Chapter 3.1). Taking this into account, the analytical error for
fsp + the determined X K (XRD) values increases with increasing NH4 content of the feldspars and σ fsp reaches up to 2 (X K (XRD)) = 0.04. The main disadvantages of the XRD analysis is that it only yields the average K- and NH4- concentration of all feldspar solid solutions within one sample. EMP analyses obtained for various buddingtonite – K-feldspar solid solutions show that the samples are homogeneous with respect to K, N, Al and Si and that no compositional zoning as well as deviation from Al-Si stoichiometry occurs. However, not all samples could be measured via electron microprobe. (K-NH4)-feldspars analysed with the Cameca SX50 at + the GFZ Potsdam have to contain at least 0.8 wt% NH4 due to the detection limit. Morever, experimental runs of the feldspar-fluid exchange series with high ammonium concentration in the bulk composition yield large amounts of quartz and (K-NH4)-muscovites and only few or no feldspar at all. However, for 16 samples along the buddingtonite – K-feldspar join EMP
fsp data are obtained. Figure 15 shows a good agreement between the mole fractions X K (EMP)
fsp fsp fsp and X K (XRD) for X K > 0.5. Three samples at low X K yield slightly higher values determined by XRD than by EMP analyses. This is probably caused by synthetic buddingtonite used as standard material for the EMP measurement of nitrogen. Since buddingtonite is more sensitive to the electron beam than (K-NH4)-feldspars, the propotional loss of nitrogen is + higher for buddingtonite than for (K-NH4)-feldspars and thus NH4 contents derived by EMP for the feldspar ss could be to high. However, synthetic buddingtonite is the best standard material for EMP analyses of NH4-bearing phases found so far. In Figure 16, the lattice
fsp constant a is plotted versus X K (EMP) and proves that -with the exception of the three samples mentioned above- a linear correlation between a and the K-NH4 composition of feldspar ss indeed exists. These obervations confirm the reliablity of both EMP and XRD data
55
fsp and thus, mole fractions X K determined from XRD analyses are used to investigate the K-
NH4 fractionation behaviour presented in Chapter 6.1 (see also Table 9 a-d, Chapter 4.1).
1,0 Figure 15 Cameca SX 50 fsp values obtained from XRD X K 0,8 JEOL JXA 8900 RL and EMP data on synthesised
(K-NH4)- feldspar ss 0,6
XRD
K
X 0,4
0,2
0,0 0,0 0,2 0,4 0,6 0,8 1,0 EMP XK
8,85 Figure 16 Cameca SX 50 X fsp values obtained from EMP 8,80 K JEOL JXA 8900 RL data vs. lattice constant a of
8,75 (K-NH4)-feldspar ss
8,70
a [A] 8,65
8,60
8,55 0,0 0,2 0,4 0,6 0,8 1,0 EMP XK
56
5.2 Tobelite – muscovite solid solutions
Tobelite-muscovite solid solutions synthesised at 500 °C / 1500 MPa were investigated using EMP analyses and IR spectroscopy. Hydrothermally produced solids of the muscovite-fluids series were analysed via Rietveld refinement of XRD spectra and IR spectroscopy.
5.2.1 Results obtained by EMP
In contrast to hydrothermally synthesised tobelite-muscovite solid solutions, (K-NH4)- muscovites obtained from high-pressure experiments were large enough for EMP analysis
(~ 5µm). Due to distinct problems associated with the measurement of NH4, only the elements K, Al and Si were analysed using well-defined orthoclase as standard material. At least 5 spots were measured on each sample. The formulae are calculated on the basis of 11 oxygen pfu and on the assumption of a total of six atoms (Al + Si) pfu on the octahedral and tetrahedral sites. Results are given in Table 16 and show only a slight deviation from the Al-Si stoichiometry of 1:1.
Table 16 Microprobe analyses of muscovite solid solutions synthesised at 1500 MPa Sample 5-00 6-00 11-00 12-00 T [°C] 500 500 500 500 no. of analyses 9 a 9 a 5 a 15 a wt% of oxides
K2O 3.83 4.68 7.94 7.66 SiO2 38.77 28.36 37.95 31.86 Al2O3 32.01 23.20 31.84 26.83 total 74.61 56.24 77.73 66.35 atoms pfu *
NH4 0.57 0.30 0.18 0.07 K 0.39 0.64 0.81 0.92 Si 3.05 3.05 3.02 3.01 Al 2.95 2.95 2.98 2.99
musc 0.41 0.68 0.82 0.93 X K (EMP) * calculated on the assumption of a total of 6 Al and Si atoms on the tetrahedra and octahedral sites; calculated on basis of 11 O atoms p.f.u. a measured with Cameca SX50
57
5.2.2 Results obtained by XRD
For all solid products of the muscovite-fluid exchange series XRD spectra were taken. Rietveld analyses were only performed for the hydrothermally synthesised tobelite-muscovite solid solutions. Refinements of (K-NH4)-muscovites from high-pressure synthesis runs failed because the peaks in the corresponding XRD spectra were too broad to be fitted. In Table 17 the statistical parameters determined for each refinement of the hydrothermally obtained products along with the quantitative phase proportions are given. The least-squares parameters are in the range which indicates a good fit. Tobelite-muscovite solid solutions consist predominantely of polytype 1M followed by polytype 2M1. In no sample, the mica polytype
2M2 occurs. Derived unit-cell dimensions for the hydrothermally synthesised 1M tobelite- muscovite solid solutions are presented in Table 18. As already shown in Chapter 3.2, the + + substitution of NH4 for K influences the size of the unit cell. Especially, lattice constant c in + (K-NH4)- muscovite increases significantly with increasing NH4 content. The obtained values for c exhibit an increase from 10.278 Å for pure muscovite to 10.537 Å for pure tobelite. As for (K-NH4)-feldspar solid solutions, it is assumed that unit-cell parameters vary linearly with
musc musc the K-NH4 composition in muscovites. Consequently, the mole fractions X K = K / (K + musc NH4) are derived using the lattice constant c of the endmembers tobelite and muscovite:
musc musc tob musc tob (12) X K = (c ss – c ) / (c – c ).
musc In Table 19 the X K results calculated from cell parameter c are given. The values range from
musc 0.02 to 1.00 for X K suggesting a complete solid solution series along the muscovite-tobelite join. This corresponds to the data obtained by Shigorova et al (1981) on (K-NH4)-muscovite solid solutions synthesised from kaolinite and NH4-bearing solutions. Plots of the unit-cell parameters a, b, c, β and V are presented in Figure 17. A linear relationsship is observed
musc between c and X K due to the assumption made. However, within analytical error of XRD β musc analyses, lattice parameters a, b, and V also vary linearly with X K indicating that no excess volumes exists within the tobelite – muscovite solid solution series.
58
Table 17 Rietveld refinement parameters for solid solutions of the muscovite-fluid exchange series (phase proportions also given in Table 10 a)
Sample RP Rwp χ2 DW Quantitative phase analysis Tobelite – Muscovite at 400 °C and 400 MPa
7-00 0.075 0.097 2.00 1.00 55wt% Musc-1M; 42wt% Musc-2M1; 3wt% Qtz
8-00 0.064 0.084 1.57 1.27 67wt% Musc-1M; 32wt% Musc-2M1; 1wt% Qtz
9-00 0.053 0.069 1.57 1.24 67wt% Musc-1M; 32wt% Musc-2M1; 1wt% Qtz
10-00 0.055 0.074 1.72 1.19 64wt% Musc-1M; 35wt% Musc-2M1; 1wt% Qtz Tobelite – Muscovite at 500 °C and 400 MPa
13-99 0.073 0.097 1.56 1.28 59wt% Musc-1M; 38wt% Musc-2M1; 3wt% Qtz
14-99 0.065 0.086 1.86 1.06 55wt% Musc-1M; 42wt% Musc-2M1; 3wt% Qtz
15-99 0.067 0.088 1.75 1.15 55wt% Musc-1M; 41wt% Musc-2M1; 4wt% Qtz
16-99 0.057 0.075 1.58 1.22 62wt% Musc-1M; 36wt% Musc-2M1; 2wt% Qtz
16a-99 0.056 0.075 1.74 1.14 53wt% Musc-1M; 47wt% Musc-2M1
17-99 0.062 0.081 1.79 1.14 71wt% Musc-1M; 27wt% Musc-2M1; 2wt% Qtz
18-99 0.052 0.069 1.59 1.24 79wt% Musc-1M; 20wt% Musc-2M1; 1wt% Qtz
19-99 0.046 0.062 1.39 1.35 72wt% Musc-1M; 26wt% Musc-2M1; 2wt% Qtz
20-99 0.056 0.076 1.63 1.28 76wt% Musc-1M; 23wt% Musc-2M1; 1wt% Qtz
21-99 0.048 0.065 1.30 1.48 79wt% Musc-1M; 15wt% Musc-2M1; 2wt% Qtz; 4wt% Fsp
22-99 0.049 0.065 1.36 1.44 79wt% Musc-1M; 14wt% Musc-2M1; 7wt% Fsp
23-99 0.046 0.061 1.32 1.45 76wt% Musc-1M; 10wt% Musc-2M1; 14wt% Fsp Tobelite – Muscovite at 600 °C and 400 MPa
1-00 0.085 0.112 1.86 1.10 39wt% Musc-1M; 27wt% Musc-2M1; 18wt% Qtz; 16wt% Corundum
2-00 0.077 0.102 1.76 1.14 53wt% Musc-1M; 25wt% Musc-2M1; 12wt% Qtz; 10wt% Corundum
3-00 0.060 0.080 1.47 1.39 60wt% Musc-1M; 31wt% Musc-2M1; 5wt% Qtz; 4wt% Corundum
4-00 0.061 0.081 1.57 1.22 81wt% Musc-1M; 19wt% Musc-2M1
59
Table 18 Unit-cell dimensions of 1M (K-NH4)- muscovite solid solutions along with 2σ deviations
Sample a [Å] b [Å] c [Å] β [°] V [Å3]
Tobelite-Muscovite at 400 °C and 400 MPa
7-00 5.2134(4) 9.0046(9) 10.449(1) 101.305(8) 480.99(8) 8-00 5.2091(3) 8.9971(6) 10.381(1) 101.442(7) 476.85(7) 9-00 5.2064(3) 8.9873(5) 10.324(1) 101.519(5) 473.33(7) 10-00 5.2036(3) 8.9833(6) 10.298(1) 101.562(6) 471.62(8)
Tobelite-Muscovite at 500 °C and 400 MPa
13-99 5.2138(3) 9.0037(5) 10.537(1) 101.313(7) 485.03(7) 14-99 5.2111(2) 8.9997(5) 10.461(1) 101.373(6) 480.99(6) 15-99 5.2100(3) 9.0022(6) 10.413(1) 101.335(8) 478.86(7) 16-99 5.2070(3) 8.9929(6) 10.378(1) 101.370(7) 476.43(7) 16a-99 5.2067(3) 8.9906(5) 10.371(1) 101.407(7) 475.89(7) 17-99 5.2048(2) 8.9857(4) 10.340(1) 101.504(4) 473.87(5) 18-99 5.2037(2) 8.9833(3) 10.319(1) 101.573(3) 472.56(3) 19-99 5.2026(2) 8.9811(3) 10.301(1) 101.626(3) 471.45(4) 20-99 5.2026(3) 8.9816(4) 10.295(1) 101.658(4) 471.12(3) 21-99 5.2048(2) 8.9818(3) 10.290(1) 101.640(3) 471.17(4) 22-99 5.2036(3) 8.9793(5) 10.282(1) 101.671(5) 470.49(5) 23-99 5.2064(3) ..8.9818(4) 10.278(1) 101.686(6) 470.66(4)
Tobelite-Muscovite at 600 °C and 400 MPa
1-00 5.2106(5) 8.9988(6) 10.432(2) 101.37(1) 479.53(9) 2-00 5.2077(3) 8.9906(4) 10.354(1) 101.498(8) 475.06(7) 3-00 5.2041(2) 8.9837(3) 10.314(1) 101.651(3) 472.28(3) 4-00 5.2025(3) 8.9789(6) 10.287(1) 101.671(2) 470.61(4)
60
musc Table 19 X K values determined from XRD data of 1M (K-NH4)- muscovite solid solutions along with 2σ deviations
Sample c [Å] musc X K [XRD]
Tobelite-Muscovite at 400 °C and 400 MPa
7-00 10.449 (1) 0.35 (2) 8-00 10.381 (1) 0.60 (2) 9-00 10.324 (1) 0.83 (2) 10-00 10.298 (1) 0.92 (2)
Tobelite-Muscovite at 500 °C and 400 MPa
13-99 10.537 (1) 0.02 (3) 14-99 10.461 (1) 0.30 (2) 15-99 10.413 (1) 0.49 (2) 16-99 10.378 (1) 0.62 (2) 16a-99 10.371 (1) 0.65 (2) 17-99 10.340 (1) 0.76 (2) 18-99 10.319 (1) 0.85 (2) 19-99 10.301 (1) 0.91 (2) 20-99 10.295 (1) 0.94 (2) 21-99 10.290 (1) 0.95 (2) 22-99 10.282 (1) 0.99 (2) 23-99 10.278 (1) 1.00 (2)
Tobelite-Muscovite at 600 °C and 400 MPa
1-00 10.432 (2) 0.42 (2) 2-00 10.354 (1) 0.71 (2) 3-00 10.314 (1) 0.86 (2) 4-00 10.287 (1) 0.97 (2)
61
5,35 9,15
5,30 9,10
5,25 9,05
5,20 9,00 a [A] b [A]
5,15 8,95
5,10 8,90
5,05 8,85 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 musc musc XK XK
10,55 101,70
101,65 10,50 101,60 10,45 101,55
10,40 101,50 ß [°] c [A] 101,45 10,35 101,40 10,30 101,35
10,25 101,30 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 musc musc XK XK
490
485
480
475 V [A³]
470
465
460 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 musc XK
Figure 17 Lattice parameters of the tobelite – muscovite solid solutions series at 400-600 °C/400 MPa
musc determined from Rietveld analysis versus X K values obtained from lattice constant c; error limits of lattice constants a, b, c, β and V are within size of the symbols; for (K-NH4)-muscovite solid solutions synthesised at 500 °C / 1500 MPa Rietveld analyses could not be performed (see text) and unit-cell dimensions are not available
62
5.2.3 Results obtained by FTIR
(K-NH4)-muscovites of the muscovite-fluid exchange series at 400 to 500 °C / 400 MPa and 500 °C / 1500 MPa were measured via FTIR. Run prodcuts obtained at 600 °C and 400 MPa were not analysed by FTIR due to major amounts of corundum and quartz within the samples. The determined FTIR spectra are similar to those obtained for pure synthetic tobelite (Figure 18). They show one band in the spectral region around 1430 cm-1 caused by the N-H -1 deformation vibration (ν4); a system of overlappig bands from 2800 to 3300 cm caused by N-
H stretching (ν3), a combination mode (ν3 +ν4) and two first overtone vibrations (2ν4 and 2ν2). -1 -1 Additionally, one stretching band for O-H (νOH) at 3630 cm with a shoulder at 3665 cm is present. No further OH-bands occur indicating a full occupation of the interlayer site A within the tobelite-muscovite solid solutions. Some of the recorded spectra are shown in Figure 18 + with decreasing NH4 concentration in (K-NH4)-muscovites from bottom to top. It is directely obvious that the intensity of the N-H bands decreases with decreasing NH4 content of the tobelite-muscovite solid solutions. This dependence is used to calculate the mole fraction musc ν X K by determining the ratio of the integrated intensities fo the N-H deformation band ( 4) at -1 -1 1430 cm and the O-H band (νOH) at 3630 cm . For reference, the I(ν4) / I(νOH) ratios obtained for tobelite samples previously characterised by Harlov et al. (2001b) are used. In Table 20 the
musc derived X K values are listed.
musc Table 20 X K values determined from IR data musc Sample I (ν4) I (νOH) I (ν4) / I (νOH) XK [IR] Tobelite-Muscovite at 400 °C and 400 MPa 7-00 29 37 0.79 0.39 8-00 14 30 0.47 0.64 9-00 6 29 0.21 0.84 10-00 2 29 0.07 0.95 Tobelite-Muscovite at 500 °C and 400 MPa 13-99 44 34 1.29 0.00 14-99 31 33 0.94 0.28 15-99 21 30 0.70 0.46 16-99 18 32 0.56 0.57 17-99 13 37 0.35 0.73 18-99 9 36 0.25 0.81 21-99 1 25 0.06 0.96 22-99 1 24 0.02 0.98 23-99 0 25 0 1.00 Tobelite-Muscovite at 500 °C and 1500 MPa 20-00 29 34 0.85 0.34 5-00 29 37 0.78 0.40 6-00 17 37 0.46 0.65 11-00 6 36 0.18 0.86 12-00 3 40 0.08 0.94
63
18-99
16-99
15-99
14-99
13-99 νOH ν4 ν2+ν4 ν3
2ν4
2ν2
musc Figure 18 IR spectra of (K-NH4)-muscovite solid solutions synthesised at 500 °C / 400 MPa; X K values determined from FTIR data (Table 20); abbreviation: mus = muscovite
64
5.2.4 Comparison between results obtained by EMP - FTIR and XRD - FTIR
In case of the tobelite-muscovite solid solutions, only the samples synthesised at 500 °C and 1500 MPa were large enough for EMP analyses. Results indicate neither zoning nor deviation from the Al-Si stoichiometry. However, nitrogen could not be analysed reliably, most likely due to the fact, that buddingtonite was used as standard material for N measurement. This is because buddingtonite is sensitive to the electron beam, which results in the loss of ammonium during standardisation and analysis. As observed for buddingtonite and the (K-NH4)-feldspar solid solutions the loss of ammonium is not the same and consequently influences the nitrogen measurement. Tobelite and (K-NH4)-muscovite solid solutions might also react differently under the electron beam. This is one explanation for bad results obtained from N measurement within the tobelite-muscovite solid solutions. Since no other elements besides N, K, Al, Si, O + and H are present in the experimental runs, the amount of NH4 can be calculated on basis of
musc measured K, Al and Si compositions. In order to cross-check the mole fractions X K determined for the high pressure samples via EMPA, FTIR analyses were performed. Due to the small size of the crystals only powdered samples were investigated. The disadvantage of + this method is the same as for the XRD analysis - only the average NH4 concentration of all tobelite-muscovite solid solutions within one sample is determined. Moreover, a linear + correlation between the NH4 content and the ratio of the integrated intensities of the N-H -1 -1 deformation band (ν4) at 1430 cm and the O-H band (νOH) at 3630 cm has to be assumed to
musc determine absolute X K values from FTIR spectra. This assumption seems to be correct, because the results obtained by EMP analyses and FTIR method on the high pressure tobelite- muscovite solid solutions are in good agreement to each other (Figure 19) and indicate the reliability of the FTIR investigation. In contrast to the high-pressure samples, the hydrothermally produced tobelite-muscovite solid solutions were analysed via Rietveld refinement of XRD data and FTIR. Figure 20 demonstrates an excellent agreement between
musc X K values obtained from both methods. The possibility of determining the ammonium content of micas by IR spectroscopy was also shown by Shigorova (1982).
In conclusion, to investigate the K-NH4 fractionation behaviour between (K-NH4)-muscovites
musc and chloride fluids for 500 °C and 1500 MPa, the mole fractions X K derived from FTIR data
musc are used whereas for 400 / 500 / 600 °C and 400 MPa, X K values calculated on basis of lattice constant c are used.
65
1 J Figure 19 0,9 500 °C J musc 0,8 1500 MPa X K values obtained from EMP and FTIR data on 0,7 J
(K-NH )- muscovite ss 0,6 4
EMP synthesised at 500 °C and
K 0,5 1500 MPa X 0,4 J 0,3 0,2 0,1 0 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
IR XXK IR K
1 JJ J J Figure 20 0,9 400 / 500 °C JJ musc 400 MPa X K values obtained from 0,8 J 0,7 XRD and FTIR data on
J (K-NH )- muscovite ss 0,6 J 4
XRD synthesised at 400 / 500 °C and 0,5 J K 400 MPa X 0,4 J 0,3 J 0,2 0,1 0 J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
X IR IR XK K
66
+ + 6. The fractionation behaviour of NH4 and K in the systems buddingtonite –
K-feldspar – NH4Cl – KCl and tobelite – muscovite – NH4Cl – KCl
6.1 The NH4 – K partitioning between feldspars and (K-NH4)Cl fluids
Results of the feldspar-fluid exchange experiments are summarised in Table 9 a-d along with
fluid + + the corresponding mole fractions X K . In all runs the NH4 and K concentrations of product + + fluids range between 5 to 50 ppm. On basis of the amount of NH4 and K measured in the product fluid, the Cl molality is calculated. It varies between 2.10 – 2.46 for runs at 400 °C / 400 MPa, between 2.10 – 3.15 for runs at 500 and 600 °C / 400 MPa and between 2.01 – 2.46 for runs at 500 and 600 °C / 1500 MPa. Generally, the calculated Cl molality should agree with the expected Cl molality which is derived from the bulk composition assuming 100 % feldspar formation. In feldspar-fluid exchange experiments performed at 400 °C / 400 MPa as
bulk well as at 500 and 600 °C / 400 MPa with high X K > 0.7 in the starting mixture, the calculated Cl molalities are in good agreement with those expected from the bulk composition.
bulk In contrast, exchange experiments at 500 and 600 °C / 400 MPa with low X K , which contain large amounts of quartz and (K-NH4)-muscovite additional to (K-NH4)-feldspar, yield calculated Cl molalities up to 25 % higher than expected from the bulk composition. This is because the expected molalities are determined assuming 100 % feldspar formation. Thus any deviation from that stoichiometry (e.g. because of mica formation) results in considerable differences between the expected and calculated Cl molalities. The obtained phase relations for buddingtonite-feldspar solid solutions and their corresponding fluids are presented in the reciprocal ternary buddingtonite - K-feldspar - NH4Cl - KCl (Figure
21 a-e). At 400 °C and 400 MPa, the amount of (K-NH4)-muscovite obtained from feldspar stoichiometry does not exceed 6 wt% and the tie-lines between (K-NH4)-feldspars and (K-
NH4)Cl fluids are close to the corresponding bulk compositions or even coincide with them. Run 38-99 and 39-99 performed under similar conditions yield nearly identical results and demonstrate an excellent reproducibility. For each exchange experiment within this series,
+ fsp NH4 preferentially fractionates into the fluid phase; e.g. in run 44-99 a feldspar of X K = 0.91
fluid coexists with a fluid of X K = 0.64 (Table 9a). At 500 °C and 400 MPa, the amount of (K-
bulk NH4)-muscovite obtained from feldspar stoichiometry varies from 29 wt% for X K = 0.13 to
bulk 1.5 wt% for X K = 0.70. Thus, all experiments within this range (1-99 to 5-99) show an offset between tie-lines and corresponding bulk compositions. This offset increases with increasing
67
+ NH4 content of the starting mixture and is correlated to an increasing (K-NH4) muscovite formation from feldspar stoichiometry.
fsp X = K / (K+NH ) Buddingtonite K 4 K-Feldspar 1 JJJ JJ
400 °C / 0,9 400 MPa
0,8
l a t
o 0,7 t ) 4 0,6 39-99 Ö 40-99Ö 43-99Ö 44-99 0,5 Ö / (K+NH Ö
lid 38-99 so
) 0,4 4
0,3 (K+NH 0,2
0,1
0 JJJ J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 NH Cl fluid KCl 4 X = K / (K+NH ) K 4 Figure 21 a Feldsparss-, fluid- and bulk composition at 400 °C / 400 MPa (Table 9a) in the reciprocal ± fsp ± fluid ternary (NH4-K)-feldspar – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for X K ; (K-NH4)-muscovite formation in runs 38-99 to 40-99 up to 6 wt%
fsp Buddingtonite XK = K / (K+NH4) K-Feldspar 1 JJJJJJJJJ J JJ
500 °C / 0.9 400 MPa
0.8
l a t
o 0.7 t ) 4 0.6 5-98 4-98 10-984-99 0.5 3-99 5-99 / (K+NH Ö 2-99 Ö 1-99 Ö Ö Ö Ö ÖÖ Ö Ö Ö Ö lid 9-98 2a-98 so 4-99 11-98 6-99
) 0.4 4
0.3 (K+NH 0.2
0.1
0 JJJJ JJJ J JJJJ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
NH Cl fluid KCl 4 XK = K / (K+NH4) Figure 21 b Feldsparss-, fluid- and bulk composition at 500 °C / 400 MPa (Table 9b) in the reciprocal ± fsp ± fluid ternary (NH4-K)-feldspar – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for X K ; (K-NH4)-muscovite formation in runs 1-99 to 5-99 up to 29 wt%.
68
+ bulk However, for runs with high K concentration in the bulk composition (X K > 0.7), which yield only feldspar and no additional muscovite, the tie-lines deviate only slightly from the corresponding starting mixture indicating that an equilibrium distribution is attained. The NH4- K partioning between feldspars and fluids is similar to the partitioning observed at 400 °C and
fsp 400 MPa. Buddingtonite-feldspar solid solutions of X K > 0.9 coexist with fluids ranging from
fluid X K = 0.68 to 0.82 (Table 9b).
fsp Buddingtonite XK = K / (K+NH4) K-Feldspar 1 JJJ JJ
0,9 600 °C / 400 MPa 0,8
l a t o
t 0,7 ) 4 0,6 7-99 8-99 7-98 8-98 0,5 7-98 O O O O O / (K+NH O O O lid 6-98 so 0,4 9-99 10-99 12-99 ) 4 0,3
(K+NH 0,2
0,1 no fspss
0 JJJJJJJ J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
NH Cl fluid KCl 4 X = K / (K+NH ) K 4
Figure 21 c Feldsparss-, fluid- and bulk composition at 600 °C / 400 MPa (Table 9c) in the reciprocal ± fsp ± fluid ternary (NH4-K)-feldspar – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for X K ; runs 7-99 to 8-99: (K-NH4)-muscovite + quartz, no feldsparss; runs 9-99 to 10-99: feldsparss + muscovitess + quartz; runs 8-98 to 12-99: feldsparss + quartz
At 600 °C / 400 MPa, the amount of (K-NH4)-muscovite obtained from feldspar stoichiometry
bulk is even higher than at 500 °C / 400 MPa. Runs 7-99, 6-98 and 8-99 with X K ranging from 0.11 to 0.32 yield no buddingtonite-feldspar solid solutions at all. The tie-lines between (K-
NH4)-feldspar and (K-NH4) fluids for the exchange experiments 9-99, 7-98 and 10-99 show a large offset regarding the corresponding bulk compositions due to muscovite contents up to 28 wt% within the run products. However, similar to experiments at 500 °C / 400 MPa, high K+ concentrations in the starting mixtures of 8-98 and 12-99 lead to nearly pure feldspar formation. Thus, the resulting tie-lines between (K-NH4)-feldspars and (K-NH4) fluids deviate only slightly from the corresponding bulk compositions. The NH4-K partitioning between feldspars and fluids at 600 °C / 400 MPa is similar to the partitioning observed at 400 and 500 °C / 400 MPa.
69
High pressure experiments performed at 500 / 600 °C and 1500 MPa also show an increasing + formation of muscovite ss with increasing NH4 concentration of the starting mixture.
fsp X = K / (K+NH ) Buddingtonite K 4 K-Feldspar 1 JJ J J J J J
500 °C / 0,9 1500 MPa
0,8
l a t o
t 0,7
) 28-99 4 41-99 Ö 0,6 Ö 17-00 Ö Ö 46-99 ÖÖ 42-99Ö 45-99 0,5 19-00 / (K+NH lid so
) 0,4 4
0,3 (K+NH 0,2
0,1
0 JJ J J J J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 NH Cl fluid 4 X = K / (K+NH ) KCl K 4 Figure 21 d Feldsparss-, fluid- and bulk composition at 500 °C / 1500 MPa (Table 9d) in the reciprocal ± fsp ± fluid ternary (NH4-K)-feldspar – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for X K
fsp Buddingtonite X = K / (K+NH ) K-Feldspar K 4 1 JJ JJ
600 °C / 0,9 1500 MPa
0,8
l a t o
t 0,7 ) 4 13-00 14-00 0,6 Ö Ö Ö Ö 15-00 16-00 0,5 / (K+NH lid so
) 0,4 4
0,3 (K+NH 0,2
0,1
0 JJ J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
NH Cl fluid KCl 4 X = K / (K+NH ) K 4 Figure 21 e Feldsparss-, fluid- and bulk composition at 600 °C / 1500 MPa (Table 9d) in the reciprocal ± fsp ± fluid ternary (NH4-K)-feldspar – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for X K
70
The tie-lines at 500 and 600 °C / 1500 MPa between buddingtonite-feldspar solid solutions and
(K-NH4) fluids deviate strongly from the corresponding bulk compositions. In contrast to the
bulk low pressure runs, this deviation remains even at high X K when no muscovite is formed from the feldspar stoichiometry (cf. run 46-99). However, the NH4-K partitioning between feldspars and fluids does not change significantly with respect to the low pressure exchange
fsp series. At 500 and 600 °C / 1500 MPa a buddingtonite-feldspar solid solution of X K = 0.93
fluid coexists with a fluid of X K = 0.72 and 0.74, respectively (Table 9d).
Figure 22 a Diagram showing the experimentally
determined K-NH4 partitioning between feldspars and fluids at 400 to 600 °C /
400 MPa; error limits only given for 500
fluid °C / 400 MPa; K
X distribution curves calculated according to data obtained in chapter 7.2; equal K-
NH4 partitioning indicated by the dashed line
Figure 22 b Diagram showing the experimentally
determined K-NH4 partitioning between feldspars and fluids at 500-600 °C /
1500 MPa; error limits only given for
fluid 500 °C / 1500 MPa; K
X distribution curves for 400 °C to 600 °C calculated according to data obtained in
chapter 7.2; equal K-NH4 partitioning indicated by the dashed line
71
In Figure 22 a, b the resulting K-NH4 distribution between feldspars and fluids is shown using Rooseboom diagrams. For all experimental runs a similar fractionation behaviour is observed with about the same negative deviation from equal distribution. K+ is preferentially + incorporated into the feldspar solid solutions and thus NH4 fractionates into the (K-NH4) fluid phase. Within the error limits, temperature increase from 400 to 600 °C at 400 MPa does not change the K-NH4 partitioning. Obviously, the formation of (K-NH4)-muscovite from feldspar stoichiometry at 500 and 600 °C / 400 MPa has no influence on the K-NH4 fractionation between buddingtonite-feldspar solid solutions and (K-NH4) fluids. Increasing pressure from
400 to 1500 MPa slightly decreases the deviation from equal K-NH4 exchange. Distribution coefficients for each run are calculated using the following relationsship:
fsp XNH4 fsp ⋅ fluid fsp− fluid XXNH4 K XK (13) K D = fsp fluid = fluid ⋅ XNH4 XXK NH4 XK
fsp− fluid The determined K D values are listed in Table 21 and generally decrease with increasing
+ fsp− fluid K concentration in the bulk composition. For example, K D values for the feldspar-fluid
bulk series at 500 °C and 400 MPa range from 0.45 ± 0.14 for low X K to 0.05 ± 0.27 for high
bulk X K . This is not surprising since only for ideal mixing the distribution coefficient KD is a
fsp constant and does not vary with changing X K . However, a linear correlationship between fsp fluid + fluid ≥ X K and X K is observed for low NH4 concentrations within the range of X K 0.95 to
fsp− fluid 1.0. The corresponding constant partition coefficient D NH4 is defined as
fsp fsp− fluid X NH4 (14) D NH4 = fluid with X NH4
fsp fsp (15) X NH4 = 1-X K and
fluid fluid (16) X NH4 = 1-X K .
fsp− fluid D NH4 is derived for 400-600 °C / 400 MPa and 1500 MPa, respectively by linear regression to the fitted curves shown in Figure 22 and is given in Table 22 as the reciprocal slope. With respect to the error limits, a similar K-NH4 exchange behaviour between feldspars and fluids + for low NH4 concentrations is observed for 400 and 1500 MPa.
72
Table 21 KD values (along with calculated 2σ standard deviations) determined for all runs of the feldspar-fluid exchange series
Run No. T (°C) P (MPa) fsp fluid X K X K K D 38-99 400 400 0.36 0.21 0.46 (± 0.10) 39-99 400 400 0.37 0.20 0.42 (± 0.10) 40-99 400 400 0.55 0.30 0.35 (± 0.07) 43-99 400 400 0.81 0.49 0.22 (± 0.08) 44-99 400 400 0.91 0.64 0.17 (± 0.12) 1-99 500 400 0.25 0.13 0.45 (± 0.14) 4-98 500 400 0.41 0.21 0.40 (± 0.09) 9-98 500 400 0.36 0.20 0.43 (± 0.10) 2-99 500 400 0.49 0.26 0.36 (± 0.07) 3-99 500 400 0.62 0.34 0.32 (± 0.07) 2a-98 500 400 0.76 0.44 0.24 (± 0.07) 10-98 500 400 0.74 0.40 0.24 (± 0.06) 4-99 500 400 0.81 0.50 0.24 (± 0.08) 5-99 500 400 0.87 0.60 0.23 (± 0.11) 5-98 500 400 0.93 0.68 0.17 (± 0.14) 11-98 500 400 0.90 0.67 0.22 (± 0.14) 6-99 500 400 0.99 0.82 0.05 (± 0.27) 9-99 600 400 0.69 0.40 0.29 (± 0.07) 7-98 600 400 0.77 0.50 0.30 (± 0.09) 10-99 600 400 0.82 0.57 0.29 (± 0.10) 8-98 600 400 0.96 0.70 0.10 (± 0.15) 12-99 600 400 0.99 0.84 0.05 (± 0.32) 17-00 500 1500 0.24 0.18 0.70 (± 0.20) 19-00 500 1500 0.29 0.20 0.61 (± 0.15) 41-99 500 1500 0.43 0.29 0.54 (± 0.11) 42-99 500 1500 0.64 0.44 0.44 (± 0.09) 28-99 500 1500 0.73 0.47 0.33 (± 0.08) 45-99 500 1500 0.85 0.58 0.25 (± 0.10) 46-99 500 1500 0.93 0.72 0.18 (± 0.16) 13-00 600 1500 0.50 0.30 0.42 (± 0.08) 14-00 600 1500 0.68 0.43 0.35 (± 0.08) 15-00 600 1500 0.87 0.58 0.20 (± 0.10) 16-00 600 1500 0.93 0.74 0.23 (± 0.19)
73
fsp− fluid Table 22 Experimentally determined constant partition coefficients D NH4 for the feldspar-fluid fluid ≥ exchange series in the range of X K 0.95 to 1.0
Experimental conditions fsp− fluid D NH4
400-600 °C / 400 MPa 0.22 ± 0.08
400-600 °C / 1500 MPa 0.25 ± 0.07
6.2 The NH4 – K partitioning between muscovites and (K-NH4)Cl fluids
Results of the muscovite-fluid exchange experiments are summarised in Table 10 a-b along
fluid + + with the corresponding mole fractions X K . In all runs the NH4 and K concentrations of product fluids range between 5 to 50 ppm. On basis of the amount of these cations measured in the product fluid, the Cl molality is calculated. It varies between 2.03 – 2.23 for runs at 400 °C / 400 MPa, between 2.49 – 2.81 for runs at 500 °C / 400 MPa; between 2.26 – 2.33 for runs at 600 °C / 400 MPa and between 1.79 – 2.44 for runs at 500 °C / 1500 MPa. Generally, the calculated Cl- molality should agree with the expected Cl molality, which is derived from the bulk composition assuming 100% muscovite formation. In most of the muscovite-fluid exchange experiments the expected and calculated Cl molalities differ by not more than ± 10 % relative which is within the analytical error range. A significantly lower Cl molality up to 25 % relative is only observed for runs performed at 400 °C and 400 MPa. The obained phase relations for tobelite-muscovite solid solutions and their corresponding fluids are presented in the reciprocal ternary tobelite - muscovite - NH4Cl - KCl (Figure 23 a- d). At 400 and 500 °C / 400 MPa the tie-lines between (K-NH4)-muscovites and (K-NH4)Cl fluids are close to the corresponding bulk compositions or even coincide with them. Moreover, for each series, the tie-lines are nearly parallel to each other indicating that equilibrium distribution exists between muscovites and coexisting fluids. Run 16-99 and 16-99a performed at 500 °C / 400 MPa under similar starting conditions yield nearly identical results and demonstrate an excellent reproducibility. For each exchange experiment within both series, + NH4 preferentially fractionates into the fluid phase. At 400 °C / 400 MPa e.g. a tobelite-
musc fluid muscovite solid solution of X K = 0.92 coexists with a fluid of X K = 0.76;
74
musc at 500 °C / 400 MPa a similar K-NH4 distribution results in a muscovite of X K = 0.95
fluid coexisting with a fluid of X K = 0.74 (Table 10a).
At 600 °C and 400 MPa the tie-lines between (K-NH4)-muscovite and (K-NH4)-fluid correlate quite excellent to the corresponding bulk compositions of runs 3-00 and 4-00. In contrast, for runs 1-00 and 2-00 an offset between tie-lines and starting mixtures is observed lying outside the analytical error. This deviation is probably caused by corundum and quartz formation additional to (K-NH4)-muscovite. However, the NH4-K partioning between muscovites and fluids is similar to the partitioning observed at 400 and 500 °C / 400 MPa. A tobelite-
musc fluid muscovite solid solution of X K = 0.97 coexists with a fluid of X K = 0.73 (Table 10a). For the high pressure exchange experiments performed at 500 °C and 1500 MPa the tie-lines between (K-NH4)-muscovites and (K-NH4)Cl fluids are close to the corresponding bulk + compositions or even coincide. It is obvious that again NH4 preferentially fractionates into the
musc fluid phase. Here, e.g. a tobelite-muscovite solid solution of X K = 0.94 coexists with a fluid
fluid of X K = 0.71 (Table 10b).
X musc = K / (K+NH ) Tobelite K 4 Muscovite 1 JJJJ
0,9 400 °C / 400 MPa 0,8
l a t o t 0,7 ) 4 0,6
0,5 7-00 8-00 9-00 10-00
/ (K+NH O O O lid O
so 0,4 ) 4 0,3
(K+NH 0,2
0,1
0 JJ J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 NH Cl fluid 4 X = K / (K+NH ) KCl K 4
Figure 23 a Muscovitess-, fluid- and bulk composition at 400 °C / 400 MPa (Table 10a) in the ± musc ± reciprocal ternary (NH4-K)-muscovite – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for fluid X K
75
X musc = K / (K+NH ) Tobelite K 4 Muscovite 1 JJJJJJJJJJJJ
0.9 500 °C / 400 MPa 0.8
l a t 0.7 o t ) 4 0.6
0.5
/ (K+NH 16a-99 17-99
lid 13-99 15-99 21-99 O O O O 19-99 so O O 0.4 O O O 23-99 ) 14-99 O O 4 18-99 16-99 20-99 O 22-99 0.3
(K+NH 0.2
0.1
0 JJJJJJ J J J J J J 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 fluid KCl NH Cl X = K / (K+NH ) 4 K 4
Figure 23 b Muscovitess-, fluid- and bulk composition at 500 °C / 400 MPa (Table 10a) in the ± musc ± reciprocal ternary (NH4-K)-muscovite – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for fluid X K
musc Tobelite XK = K / (K+NH4) Muscovite 1 JJJJ
0,9 600 °C / 400 MPa
0,8
l a t 0,7 o t ) 4 0,6
0,5 1-00 O 2-00 3-00 4-00
/ (K+NH O O O lid
so 0,4 ) 4
0,3 (K+NH 0,2
0,1
0 JJ J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 KCl NH Cl X fluid = K / (K+NH ) 4 K 4
Figure 23 c Muscovitess-, fluid- and bulk composition at 600 °C / 400 MPa (Table 10a) in the ± musc ± reciprocal ternary (NH4-K)-muscovite – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for fluid X K ; formation of corundum and quartz in runs 1-00 and 2-00
76
musc X = K / (K+NH ) Tobelite K 4 Muscovite 1 JJ J JJ
0,9 500 °C / 1500 MPa 0,8
l a t o t 0,7 ) 4
0,6 O 5-00 20-00 O O 6-00
/ (K+NH 0,5 11-00
lid O O 12-00 so
) 0,4 4
0,3 (K+NH 0,2
0,1
0 JJ J J J 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 fluid KCl NH Cl X = K / (K+NH ) 4 K 4 Figure 23 d Muscovitess-, fluid- and bulk composition at 500 °C / 1500 MPa (Table 10b) in the ± musc ± reciprocal ternary (NH4-K)-muscovite – (NH4-K)Cl; error limits are 0.04 for X K and 0.01 for fluid X K
In Figure 24 a, b the resulting K-NH4 distribution between muscovites and fluids is shown using Rooseboom diagrams. Similar to the feldspar-fluid series, K+ is preferentially + incorporated into the muscovite solid solutions and thus NH4 fractionates into the (K-NH4) fluid phase. All experimental runs performed at 400 MPa exhibit about the same fractionation behaviour with negative deviation from equal distribution. Within the error limits, temperature increase from 400 to 600 °C does not change the K-NH4 partitioning between tobelite- muscovite solid solutions and (K-NH4) fluids. Increasing pressure from 400 to 1500 MPa decreases the deviation from equal K-NH4 exchange. Distribution coefficients for each run are calculated using the following relationsship:
musc XNH4 musc fluid − XX⋅ XK (17) K musc fluid = NH4 K = D musc fluid fluid ⋅ XNH4 XXK NH4 XK
musc− fluid The determined K D values are listed in Table 24. As for the feldspar-fluid series, the
musc− fluid + distribtution coefficients K D generally decrease with increasing K concentration in the fluid ≥ bulk composition. Within the range of X K 0.95 to 1.0 a linear correlationship between
77
musc fluid musc− fluid X K and X K is observed and a constant partition coefficient D NH4 is defined for 400 and 1500 MPa (Table 24):
musc musc− fluid X NH4 musc musc fluid fluid (18) D NH4 = fluid with X NH4 = 1-X K and X NH4 = 1-X K . X NH4
+ At 1500 MPa the K-NH4 exchange behaviour between muscovites and fluids for low NH4 concentrations deviates less from equal distribution than at 400 MPa.
Figure 24 a Diagram showing the experimentally
determined K-NH4 partitioning between muscovites and fluids at 400-
600 °C / 400 MPa; error limits only
fluid given for 500 °C / 400 MPa; K
X distribution curves for 400 °C- 600 °C calculated according to data obtained in chapter 7.2;
equal K-NH4 partitioning indicated by the dashed line
Figure 24 b Diagram showing the experimentally
determined K-NH4 partitioning between muscovites and fluids at 500 /
1500 MPa; fluid
K distribution curves for 400 °C- 600 °C X calculated according to data obtained in chapter 7.2;
equal K-NH4 partitioning indicated by the dashed line
78
Table 23 KD values (along with calculated 2σ standard deviations) determined for all runs of the muscovite-fluid exchange series
Run No. T (°C) P (MPa) X musc fluid K X K K D 7-00 400 400 0.35 0.17 0.37 (± 0.11) 8-00 400 400 0.60 0.32 0.31 (± 0.08) 9-00 400 400 0.83 0.52 0.23 (± 0.11) 10-00 400 400 0.92 0.76 0.26 (± 0.28) 14-99 500 400 0.30 0.13 0.33 (± 0.12) 15-99 500 400 0.49 0.20 0.26 (± 0.07) 16-99 500 400 0.62 0.29 0.25 (± 0.07) 16a-99 500 400 0.65 0.29 0.22 (± 0.06) 17-99 500 400 0.76 0.35 0.16 (± 0.06) 18-99 500 400 0.85 0.44 0.14 (± 0.08) 19-99 500 400 0.91 0.55 0.12 (± 0.11) 20-99 500 400 0.94 0.65 0.13 (± 0.16) 21-99 500 400 0.95 0.74 0.14 (± 0.24) 22-99 500 400 0.99 0.83 0.12 (± 0.39) 1-00 600 400 0.42 0.20 0.36 (± 0.10) 2-00 600 400 0.71 0.36 0.23 (± 0.08) 3-00 600 400 0.86 0.54 0.18 (± 0.11) 4-00 600 400 0.97 0.73 0.10 (± 0.22) 20-00 500 1500 0.34 0.19 0.46 (± 0.13) 5-00 500 1500 0.40 0.24 0.46 (± 0.12) 6-00 500 1500 0.65 0.40 0.36 (± 0.10) 11-00 500 1500 0.86 0.58 0.22 (± 0.13) 12-00 500 1500 0.94 0.71 0.16 (± 0.21)
musc− fluid Table 24 Experimentally determined constant partition coefficients D NH4 for the fluid muscovite-fluid exchange series in the range of X K > 0.95 to 1.0
Experimental conditions musc− fluid D NH4
400-600 °C / 400 MPa 0.19 ± 0.10
400-600 °C / 1500 MPa 0.26 ± 0.08
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6.3 The NH4 – K partitioning between feldspars and muscovites
On basis of the feldspar-fluid as well as the muscovite-fluid exchange data, the K-NH4 fractionation behaviour between feldspar and muscovite is determined. Figure 25 shows the obtained K-NH4 partitioning for 400 and 1500 MPa at varying temperatures. A positive deviation from equal distribution is observed at 400 MPa indicating that (K-NH4)-muscovites + + under these conditions preferentially incorporate K instead of NH4 . This corresponds to the
K-NH4 fractionation obtained from coexisting feldspars and muscovites within the feldspar-
fsp musc fluid exchange series. Here, the comparison between the mole fractions X K and X K of each sample shows quite clearly that feldspars contain less K+ than coexisting muscovites (Figure 26; Table 9 a-d). At 1500 MPa K+ also fractionates preferentially into tobelite- muscovite solid solutions. However, the K-NH4 distribution deviates less from equal partitioning than at 400 MPa demonstrating the influence of increasing pressure on the K-NH4 fractionation between feldspars and muscovites. A temperature dependence in the range of 400 to 600 °C at 400 and 1500 MPa, respectively is not observed. The distribution coefficients of the feldspar-muscovite exchange are determined according to:
(19) K-feldspar + Tobelite = Buddingtonite + Muscovite
fsp musc − XX⋅ (20) K fsp musc = NH4 K . D fsp ⋅ musc XXK NH4
fsp− musc In Table 25 the resulting K D values are listed. They vary between 1.39 to 2.75 for 400- 600 °C / 400 MPa and between 1.18 to 2.47 for 400-600 °C / 1500 MPa. For natural + environments with generally low NH4 concentration in the bulk composition, a constant
fsp− musc fsp musc partition coefficient D NH4 = X NH4 / X NH4 can be derived to describe the fractionation fsp− musc ± behaviour. The obtained D NH4 values are 1.26 ( 0.08) for 400-600 °C / 400 MPa and 1.11 ± fsp ( 0.10) for 500-600 °C / 1500 MPa, respectively. They are valid in the range of X K > 0.8 and indicate only minor deviation from equal distribution for the K-NH4 exchange between + feldspar and muscovite at low NH4 concentration in the bulk composition (Figure 25).
80
1 0,9 400 MPa 0,8 0,7 400 °C 0,6 500 °C 0,5
musc 600 °C
K 0,4
X 0,3 0,2 0,1 0 0 0,2 0,4 0,6 0,8 1 fsp XK
Figure 25a K-NH4 distribution between feldspars and muscovites derived from thermodynamic fits of experimental data of the feldspar-fluid and muscovite-fluid exchange series at 400-600 °C / 400 MPa
1 0,9 1500 MPa 0,8 0,7 400 °C 0,6 500 °C 0,5
musc 600 °C
K 0,4 X 0,3 0,2 0,1 0 0 0,2 0,4 0,6 0,8 1 fsp XK
Figure 25b K-NH4 distribution between feldspars and muscovites derived from thermodynamic fits of experimental data of the feldspar-fluid and muscovite-fluid exchange series at 500-600 °C / 1500 MPa
81
fsp− musc Table 25 Distribution coeffcients K D determined from feldspar-fluid and muscovite-fluid exchange data in the range of 400-600 °C at 400 and 1500 MPa (calculated in steps of 0.1 X fsp ; K values given in steps of 0.5 X fsp ) K
400 °C 500 °C 600 °C 400 °C 500 °C 600 °C 400 MPa 400 MPa 400 MPa 1500 MPa 1500 MPa 1500 MPa − − − − − − X fsp fsp musc fsp musc fsp musc fsp musc fsp musc fsp musc K K D KD KD KD KD KD 0.01 2.75 2.75 2.74 2.40 2.44 2.47 0.05 1.69 1.68 1.67 1.46 1.48 1.49 0.10 1.59 1.58 1.56 1.35 1.37 1.38 0.15 1.59 1.56 1.54 1.32 1.34 1.36 0.20 1.61 1.57 1.54 1.31 1.33 1.35 0.25 1.63 1.58 1.55 1.31 1.33 1.34 0.30 1.65 1.59 1.55 1.31 1.33 1.34 0.35 1.65 1.59 1.55 1.30 1.32 1.33 0.40 1.64 1.59 1.55 1.30 1.32 1.33 0.45 1.63 1.57 1.54 1.28 1.31 1.32 0.50 1.60 1.55 1.52 1.27 1.29 1.31 0.55 1.57 1.53 1.50 1.25 1.28 1.30 0.60 1.53 1.50 1.48 1.23 1.26 1.28 0.65 1.49 1.47 1.46 1.22 1.25 1.27 0.70 1.46 1.45 1.44 1.20 1.23 1.26 0.75 1.43 1.42 1.42 1.19 1.22 1.25 0.80 1.40 1.40 1.41 1.18 1.22 1.24 0.85 1.39 1.40 1.40 1.18 1.22 1.25 0.90 1.40 1.41 1.43 1.21 1.25 1.28 0.95 1.54 1.56 1.57 1.34 1.38 1.42 0.96 1.63 1.66 1.67 1.43 1.47 1.51 0.97 1.83 1.85 1.87 1.60 1.65 1.69 0.98 2.43 2.46 2.49 2.13 2.20 2.25
82
1 400 °C 500 °C 0,8 600 °C
0,6 musc K
X 0,4
0,2
0 00,20,40,60,81 fsp XK
+ + Figure 26 Derived distribution behaviour of K and NH4 between coexisting muscovite and feldspar solid solutions obtained from the feldspar-fluid exchange series at 400-600 °C and 400 MPa (error limits only given for 500 °C / 400 MPa); similar K-NH4 partitioning between feldspar and muscovite as derived from thermodynamic fits of the experimental data of the feldspar-fluid and muscovite-fluid exchange (cf. Figure 25a); strong indication that equilibrium conditions were achieved within the experimental runs
83
7. Discussion
7.1 Equilibrium conditions in “exchange synthesis experiments“
7.1.1 The system buddingtonite – K-feldspar – NH4Cl – KCl
Precise K-NH4 distribution coefficients can be derived from “exchange synthesis experiments“ in case equilibrium conditions were attained during the synthesis runs. The reciprocal ternaries buddingtonite – K-feldspar – NH4Cl - KCl strongly suggest that (K-NH4)-feldspars grew in equilibrium with the coexisting (K-NH4)Cl fluid phase. This is because all phase relations in the reciprocal ternaries are internally consistent since the determined tie-lines between feldspar solid solutions and fluids are subparallel within each P-T series and show no cross-cutting (Figure 21a-e). Additionally, in all runs yielding only feldspar and no muscovite, the tie-lines are close to the corresponding bulk compositions. There are moreover no hints indicating that fractional crystallisation occurred during the “exchange syntheses“ and reproducibility tests yield an excellent agreement between runs performed under the same conditions (Table 9 a-d). + However, with increasing NH4 content in the bulk composition and increasing temperature, the tie-lines between feldspar solid solutions and fluids miss the corresponding starting mixture. This is especially seen in runs performed at 500 and 600 °C (Figure 21 a-e). It is easily explained since (K-NH4)-muscovite and quartz volunteered in all runs which had high + NH4 concentration in the bulk composition (Table 9 a-d). The formation of muscovite solid solutions results from the instability of NH4-rich feldspars under these conditions. Hallam and Eugster (1976) investigated the equilibria between buddingtonite, tobelite, sillimanite and quartz at 500-700 °C / 2000 MPa and varying NH3 fugacities. They showed that above 600 °C -4 buddingtonite requires a high level of NH3 fugacity in the vapour phase (fNH3 ≥ 10 bars) whereas tobelite is stable even at extremely low levels of fNH3. Thus, instead of the assumed
“exchange synthesis“ (21) reaction (22) probably took place leading to (K-NH4)-muscovite formation from feldspar stoichiometry.
aq aq (21) 2 (NH4,K)OH + 6 SiO2 + Al2O3 + n (NH4,K)Cl =
aq 2 (NH4,K)AlSi3O8 + n (NH4,K)Cl + H2O
84
aq aq (22) 2 (NH4,K)OH + 6 SiO2 + Al2O3 + n (NH4,K)Cl + 1/3 H2O =
aq + - 2/3 (NH4,K)Al2AlSi3O10(OH)2 + 4 SiO2 + n (NH4,K)Cl + 4/3 (NH4,K) + 4/3 OH
Reaction (22) demonstrates that in case of (K-NH4)-muscovite synthesis from the feldspar bulk + + - the amount of K and NH4 exceeds the amount of Cl present in the fluid phase and most - + + likely is balanced by OH . The K and NH4 concentrations measured within the product fluids of these experiments indeed yield up to 25 % higher Cl- molalities than expected from “exchange synthesis“ according to (21) (Table 9a-d). This is a strong argument for reaction (22). However, the strongest evidence would have been pH values > 7 for the obtained fluid phases but unfortunately pH measurements were not performed within this study. The formation of additional (K-NH4)-muscovite in feldspar-fluid exchange experiments generally
bulk leads to an offset of the feldspar-fluid tie-lines to higher X K values with respect to the corresponding starting mixtures. One possibility for this is that the (K-NH4)-muscovites + incorporated more NH4 than the coexisting (K-NH4)-feldspars. However, this is not the case since muscovite solid solutions contain slightly more K+ than coexisting feldspar solid
bulk solutions. Another possibility for the shift of the tie-lines to higher X K values is a partial loss + of NH4 during the experimental run. Taking the determined (K-NH4)Cl, (K-NH4)-feldspar and
(K-NH4)-muscovite compositions in account along with the obtained phase proportions + + determined by Rietveld analysis of XRD spectra, a mass balance for K and NH4 can be + + derived (Table 26). The K and NH4 concentrations in the solid run products are calculated assuming that Al3+ and Si4+ within the starting compositions are completely incorporated into crystalline phases. For the feldspar ss resp. muscovite ss Al:Si ratios of 1:3 resp. 1:1 were used since EMP analyses on some of the synthesised (K-NH4)-feldspars and -muscovites show only slight deviations from the ideal composition. The solubility of Al3+ and Si4+ within the fluid phase and the formation of an amorphous phase after quenching is neglected. However, for + + most of the feldspar-fluid exchange experiments at 400 and 500 °C / 400 MPa the K and NH4 concentrations in starting mixtures and corresponding run products deviate only slightly demonstrating that no loss of ammonium occurred during the experimental runs. In contrast, experiments performed at 600 °C / 400 MPa as well as at 500 and 600 °C / 1500 MPa and low
bulk + X K in the starting mixture exhibit definitively lower NH4 concentrations within the run products indicating a partial loss of ammonium during the exchange syntheses.
85
+ + Table 26a (K-NH4)- feldspar-fluid exchange series: K and NH4 concentrations in starting mixtures and obtained solid and fluid run products at 400-600 °C / 400 MPa; formation of additional (K-NH4)- muscovite >20 wt% in runs 1-99, 4-98, 9-98, 2-99, 7-99, 6-98, 8-99, 9-99, 7-98
400°C / 400 MPa Starting mixture Run products Deviation [mmol] [mmol] [%] 38-99 K + 0.069 0.068 - 1.5 + NH4 0.206 0.191 - 7.3 39-99 K + 0.025 0.022 -13.6 + NH4 0.064 0.038 - 9.8 40-99 K + 0.067 0.063 - 6.6 + NH4 0.096 0.090 - 6.5 43-99 K + 0.133 0.131 - 1.7 + NH4 0.084 0.078 - 6.9 44-99 K + 0.170 0.167 - 1.8 + NH4 0.059 0.058 - 2.2 500°C / 400 MPa 1-99 K + 0.035 0.036 3.8 + NH4 0.221 0.200 - 9.5 4-98 K + 0.064 0.061 - 5.1 + NH4 0.197 0.174 -11.5 9-98 K + 0.053 0.054 1.4 + NH4 0.177 0.169 -4.5 2-99 K + 0.079 0.080 0.9 + NH4 0.169 0.172 2.1 3-99 K + 0.105 0.106 1.1 + NH4 0.146 0.149 1.9 2a-98 K + 0.119 0.118 -1.0 + NH4 0.119 0.100 -15.9 10-98 K + 0.116 0.115 -1.2 + NH4 0.112 0.113 0.8 4-99 K + 0.146 0.149 1.2 + NH4 0.099 0.100 1.9 5-99 K + 0.171 0.179 4.6 + NH4 0.074 0.077 4.5 5-98 K + 0.192 0.191 -0.3 + NH4 0.055 0.054 -1.5 11-98 K + 0.169 0.174 3.0 + NH4 0.047 0.053 13.1 6-99 K + 0.213 0.223 4.5 + NH4 0.027 0.029 5.9 600°C / 400 MPa 7-99 K + 0.032 0.032 0 + NH4 0.222 0.186 -16.2 6-98 K + 0.064 0.064 0 + NH4 0.198 0.156 -21.1 8-99 K + 0.078 0.080 3.9 + NH4 0.166 0.158 -5.1 9-99 K + 0.106 0.106 0 + NH4 0.138 0.130 -5.6 7-98 K + 0.124 0.120 -3.2 + NH4 0.120 0.091 -24.2 10-99 K + 0.146 0.146 0 + NH4 0.098 0.082 -16.5 8-98 K + 0.190 0.202 6.3 + NH4 0.056 0.052 -6.8 12-99 K + 0.212 0.216 2.0 + NH4 0.025 0.024 -2.7
86
+ + Table 26b (K-NH4)- feldspar-fluid exchange series: K and NH4 concentrations in starting mixtures and obtained solid and fluid run products at 500-600 °C / 1500 MPa; formation of additional (K-NH4)- muscovite > 20 wt% in runs 17-00, 19-00, 41-99, 42-99, 13-00, 14-00, 15-00
500°C / 1500 MPa Starting mixture Run products Deviation [mmol] [mmol] [%] 17-00 K + 0.014 0.019 35.0 + NH4 0.083 0.059 -29.3 19-00 K + 0.016 0.021 31.8 + NH4 0.079 0.055 -29.8 41-99 K + 0.028 0.030 5.4 + NH4 0.064 0.052 -18.3 42-99 K + 0.035 0.042 21.2 + NH4 0.051 0.037 -28.2 28-99 K + 0.042 0.045 6.2 + NH4 0.039 0.029 -24.9 45-99 K + 0.056 0.057 2.1 + NH4 0.039 0.027 -31.2 46-99 K + 0.066 0.068 2.3 + NH4 0.027 0.017 -38.8 600°C / 1500 MPa 13-00 K + 0.026 0.027 2.0 + NH4 0.066 0.040 -39.4 14-00 K + 0.040 0.047 18.0 + NH4 0.051 0.039 -22.9 15-00 K + 0.056 0.066 17.5 + NH4 0.038 0.026 -30.5 16-00 K + 0.069 0.074 7.2 + NH4 0.022 0.015 -33.6
fluid fsp In general, the relationship between the cation ratios X K and X K is independent of the total amount of cations in the fluid. Thus, reliable K-NH4 distribution coefficients within the feldspar-fluid series can be derived as long as equilibrium conditions were attained during the exchange syntheses. The strongest argument for equilibrium is that the K-NH4 partitioning between muscovite solid solutions and fluids observed within the feldspar-fluid exchange is the same as observed within the muscovite-fluid exchange series. This is evident from Figures 25a and 26 demonstrating a similar K-NH4 partitioning between feldspars and muscovites derived from feldspar-fluid and muscovite-fluid exchange experiments as well as from the feldspar- fluid series alone.
87
7.1.2 The system tobelite – muscovite – NH4Cl – KCl
The reciprocal ternaries tobelite – muscovite – NH4Cl – KCl strongly suggest that (K-NH4)- muscovites grew in equilibrium with the coexisting (K-NH4)Cl fluid phase. This is because the tie-lines between muscovite solid solutions and fluids are subparallel within each P-T series and show no cross-cutting. Additionally, in nearly all runs, the tie-lines are close to the corresponding bulk composition or even coincide with them (Figures 23a-d). There is moreover a good agreement between the expected and estimated Cl- molalities (Table 10a-b) indicating that the starting materials indeed reacted according to the equation
aq aq (23) 2 (NH4,K)OH + 6 SiO2 + 3 Al2O3 + n (NH4,K)Cl + H2O =
aq 2 (NH4,K) Al2AlSi3O10(OH)2 + n (NH4,K)Cl .
+ + Due to the experimental set-up the K and NH4 concentrations in the fluid in situ are the same - + + as measured at 1 bar and 25 °C since Cl balanced the amount of K and NH4 present in the fluid phase. Taking the determined (K-NH4)Cl fluid and (K-NH4)-muscovite compositions in account along with the obtained phase proportions determined by Rietveld analysis of XRD + + spectra, a mass balance for K and NH4 can be derived (Table 27). As for the feldspar-fluid + + exchange, the K and NH4 concentrations in the solid run products are calculated assuming that Al3+ and Si4+ within the starting compositions are completely incorporated into crystalline + + phases. For most of the muscovite-fluid exchange experiments the K and NH4 concentrations in starting mixtures and corresponding run products deviate only slightly demonstrating that no loss of ammonium occurred during the experimental procedure. At hydrothermal conditions + + the deviations between K and NH4 in starting compositions and run products generally do not + exceed ± 15%. The one exception is run 15-99 at 500 °C / 400 MPa yielding ~ 30% more NH4 within the runs products. However, this only leads to a small off-set between the (K-NH4)- muscovite – (K-NH4)Cl tie-line and the corresponding bulk composition (Figure 23b). Experiments performed at 500 °C and 1500 MPa show larger deviations up to 21% for K+ and + –25% for NH4 in run 20-00. This is due to lower absolute concentrations used within the starting compositions. For example, run 20-00 yields 0.017 instead of 0.014 mmol K+ and + 0.042 instead of 0.056 mmol NH4 .
88
+ + Table 27 (K-NH4)- muscovite-fluid exchange series: K and NH4 concentrations in starting mixtures and obtained solid and fluid run products at 400-600 °C / 400 MPa and 500 °C / 1500 MPa
400°C / 400 MPa Starting mixture Run products Deviation [mmol] [mmol] [%] 7-00 K + 0.141 0.139 -1.4 + NH4 0.463 0.432 -6.7 8-00 K + 0.244 0.240 -1.6 + NH4 0.311 0.299 -3.9 9-00 K + 0.367 0.370 0.8 + NH4 0.208 0.199 -4.3 10-00 K + 0.477 0.489 2.5 + NH4 0.106 0.102 -3.8 500°C / 400 MPa 13-99 K + 0.000 0.000 0 + NH4 0.145 0.149 2.8 14-99 K + 0.026 0.028 7.7 + NH4 0.125 0.123 -1.6 15-99 K + 0.042 0.045 7.1 + NH4 0.111 0.144 29.7 16-99 K + 0.059 0.064 8.5 + NH4 0.100 0.101 1.0 16a-99 K + 0.059 0.068 15.3 + NH4 0.100 0.104 4.0 17-99 K + 0.071 0.078 9.9 + NH4 0.086 0.087 1.2 18-99 K + 0.086 0.094 9.3 + NH4 0.074 0.075 1.4 19-99 K + 0.103 0.112 8.7 + NH4 0.060 0.060 0 20-99 K + 0.118 0.127 7.6 + NH4 0.049 0.047 -4.1 21-99 K + 0.131 0.138 5.3 + NH4 0.036 0.035 -2.8 22-99 K + 0.144 0.151 4.9 + NH4 0.023 0.022 -4.3 23-99 K + 0.157 0.161 2.5 + NH4 0.000 0.000 0 600°C / 400 MPa 1-00 K + 0.183 0.196 7.1 + NH4 0.625 0.518 -17.1 2-00 K + 0.360 0.381 5.8 + NH4 0.460 0.395 -14.1 3-00 K + 0.530 0.556 4.9 + NH4 0.305 0.277 -9.2 4-00 K + 0.676 0.700 3.6 + NH4 0.156 0.139 -10.9 500°C / 1500 MPa 20-00 K + 0.014 0.017 21.4 + NH4 0.056 0.042 -25.0 5-00 K + 0.020 0.019 -5.0 + NH4 0.050 0.041 -18.0 6-00 K + 0.033 0.034 3.0 + NH4 0.037 0.032 -13.5 11-00 K + 0.048 0.049 2.1 + NH4 0.027 0.024 -11.1 12-00 K + 0.058 0.059 1.7 + NH4 0.015 0.015 0
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7.2 Thermodynamic evaluation of the buddingtonite – K-feldspar and tobelite – muscovite solid solution series
Equilibrium conditions allow to use the experimentally determined K-NH4 fractionation between feldspars, muscovites and fluids to derive mixing parameters for both the (K-NH4)- feldspar as well as the (K-NH4)-muscovite solid solutions. For the exchange reactions (aq) (aq) (24) K-feldspar + NH4Cl = buddingtonite + KCl (aq) (aq) (25) muscovite + NH4Cl = tobelite + KCl the difference in the standard state chemical potential ∆µ 0 is expressed by the equations
∆µ 0 µ 0 µ 0 µ 0 µ 0 (26) = budd + KCl - Kfsp - NH4 Cl
∆µ 0 µ 0 µ 0 µ 0 µ 0 (27) = tob + KCl - musc - NH4 Cl The following thermodynamical formulations are used to describe equilibrium conditions
aafsp ⋅ fluid (28) ∆µ 0 + RT ln budd KCl = 0 fsp ⋅ fluid aaKfsp NH4 Cl
aamusc ⋅ fluid (29) ∆µ 0 + RT ln tob KCl = 0 musc ⋅ fluid aamusc NH4 Cl with a representing the activity of each component. Because the mixing in both the (K-NH4)- feldspar as well as the (K-NH4)-muscovites occurs only on one site within the crystal structure, respectively, the activities are expressed as
(30) ai = Xi γi · where Xi stands for the mole fractions determined for all solid and fluid products and γi for the activity coefficients. Using this expression for the activities and the definition of the distribution coefficient KD,
XXfsp ⋅ fluid (31) K = NH4 K D fsp ⋅ fluid XXK NH4
XXmusc ⋅ fluid (32) K = NH4 K D musc ⋅ fluid XXK NH4 equations (28) and (29) become (33) and (34) γγfsp ⋅ fluid 0 NH4 K (33) ∆µ + RT ln + RT lnKD = 0 γγfsp ⋅ fluid K NH4
90
γγmusc ⋅ fluid 0 NH4 K (34) ∆µ + RT ln + RT lnKD = 0 γγmusc ⋅ fluid K NH4
It is assumed that the components KCl and NH4Cl in (K-NH4)Cl solutions behave equally under P and T. Consequently, equations (33) and (34) are simplified to γ fsp 0 NH4 (35) ∆µ + RT ln + RT ln KD = 0 γ fsp K
γ musc (36) ∆µ 0 + RT ln NH4 + RT ln K = 0 γ musc D K where the second term refers to deviations from ideal mixing within the (K-NH4)-feldspar and -muscovite solid solutions. Assuming a regular solution as the simplest activity model, the deviation from ideal mixing can be expressed using the interaction parameter W which is:
γ fsp fsp 2 γ fsp fsp 2 (37) RT ln NH4 = W * (X K ) and RT ln K = W * (X NH4 ) γ musc musc 2 γ musc musc 2 (38) RT ln NH4 = W * (X K ) and RT ln K = W * (X NH4 ) This reduces equations (35) and (36) to ∆µ 0 fsp fsp− fluid (39) + W (2X K - 1) + RT ln K D = 0
∆µ 0 musc musc− fluid (40) + W (2X K - 1) + RT ln K D = 0
fsp− fluid fsp musc− fluid musc and thus plots of (-RT ln K D ) versus (2X K -1) and (-RT ln K D ) versus (2X K - 1) should yield a linear relationship if the regular solution model is appropriate. In Figures 27a- b and Figure 28a-b the obtained plots are presented for the (K-NH4)-feldspar and (K-NH4)- muscovite solid solutions. They show that the derived lnKD values for both series do indeed obey a linear relationship. The intercept is ∆µ 0 and the slope is the binary interaction parameter W. From linear regression the differences in molar enthalpy ∆h 0 , entropy ∆s 0 and volume ∆v 0 are obtained (Table 28) using the relationship
(41) ∆µ 0 = ∆h 0 -T∆s 0 + ∆v 0 (P-0.1).
The data given in Table 28 is used to calculate the equilibrium distribution curves shown in Figures 22a-b and 24a-b, respectively and indicates that there is no change in ∆h 0 for the K-
0 0 NH4 distribution between feldspars–fluids and muscovites–fluids. Only differences ∆s , ∆v
91
+ + and the mixing behaviour of K and NH4 on the A site in feldspar and the interlayer site in muscovite are responsible for the observed fractionation behaviour.
35000
400 / 400 30000 500 / 400
600 / 400 25000
D 20000
15000 -RT lnK
10000
5000
0 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 fsp [2 XK - 1]
Figure 27a Plot of [–RT lnK ] versus [2X fsp -1] for the feldspar-fluid exchange series at 400 to 600 °C D K / 400 MPa. A linear relationsship (regular solution model) is consistent with the experimentally derived KD values; lines calculated from thermodynamic data given in Table 28
35000
500 / 1500 30000
600 / 1500 25000
D 20000
15000 -RT lnK
10000
5000
0 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 fsp [2 XK - 1]
Figure 27b Plot of [–RT lnK ] versus [2X fsp -1] for the feldspar-fluid exchange series at 500 to 600 °C D K / 1500 MPa. A linear relationsship (regular solution model) is consistent with the experimentally derived KD values; lines calculated from thermodynamic data given in Table 28
92
25000 400 / 400 500 / 400 20000 600 / 400
D 15000
-RT lnK 10000
5000
0 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 musc [2 XK - 1]
musc Figure 28a Plot of [–RT lnKD] versus [2X K -1] for the muscovite-fluid exchange series at 400 to 600 °C / 400 MPa. A linear relationsship (regular solution model) is consistent with the experimentally derived KD values; lines calculated from thermodynamic data given in Table 28
25000
20000
D 15000
-RT lnK 10000
5000
0 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 musc [2 XK - 1]
musc Figure 28b Plot of [–RT lnKD] versus [2X K -1] for the muscovite-fluid exchange series at 500 °C / 1500 MPa. A linear relationsship (regular solution model) is consistent with the experimentally derived KD values; lines calculated from thermodynamic data given in Table 28
93
Table 28 Results from linear regression for feldspar-fluid and muscovite-fluid exchange experiments feldspar-fluid muscovite-fluid exchange reaction exchange reaction ∆h0 0 0
∆s0 -8.8 (0.6) J / (mol·K) -11.2 (0.8) J / (mol·K)
∆v0 -1.2 (0.4) J / (mol·MPa) -1.8 (0.6) J / (mol·MPa)
W 5.4 (0.6) kJ / mol 5.1 (0.7) kJ / mol
In order to crosscheck the data obtained from linear regression, the values given in Table 28
fsp musc fsp− fluid musc− fluid along with the X K , X K and K D , K D values at P and T are put into equation 39 and 40, respectively. The results should equal zero. However, for the feldspar-fluid exchange
fsp experiments in the range of 0.2 musc J/mol and for the muscovite-fluid exchange series in the range of 0.2 94 7.3 Comparison to other K-NH4 exchange data The exchange reaction K-feldspar + NH4Cl = NH4-feldspar + KCl was previously investigated by Lorch (1978) using the so-called rotating tie-line technique. Synthetic K-feldspar and aqueous (K-NH4)Cl solutions of varying concentrations served as starting materials. Experiments were performed between 400 to 600 °C and 200 MPa. All product fluids lay fluid within the range of X K 0.29 to 0.35. Since corresponding feldspars reached mole fractions fsp from X K = 0.65 to 0.71, Lorch (1978) observed a K-NH4 fractionation behaviour between feldspars and fluids similar to the experimental data presented here. However, in contrast to + this study, Lorch (1978) additionally observed an increasing NH4 incorporation into the feldspars with increasing temperature. This is questionable, because all his experiments plot in a very narrow compositional range probably within the error limits of his measurements. + + Moreover, only K and NH4 concentrations in the fluids were analysed and used to estimate fsp the mole fractions X K according to the linear correlationship between bulk-, fluid- and feldspar- composition in the reciprocal ternary. Assuming that equilibrium was attained during his experiments and knowing the K-NH4 contents of starting mixture and product fluid, Lorch fsp (1978) could calculate the X K values within a very narrow range. The distribution coefficients derived from Lorch’s exchange data are similar to those determined for equal feldspar concentrations from the present study. However, this only holds for experiments performed with K-feldspar as starting material. Reversals with NH4-feldspar instead of K- feldspar as solid phase did not show the same results due to the formation of additional muscovite during these experiments. Since Lorch (1978) calculated the feldspar composition of fsp the reversals on the assumption that pure NH4-muscovite was synthesised, the determined X K values are not accurate. Moreover, the amount of muscovite was only estimated from powder XRD spectra without Rietveld refinements. The formation of muscovite from NH4-rich bulk compositions of feldspar stoichiometry was also observed in the present study. However, in contrast to Lorch’s study (1978), the feldspar compositions were measured directly within this study and therefore reliable distribution coefficients were derived in case of muscovite syntheses during the feldspar-fluid runs. Studies on the K-NH4 fractionation between micas and fluid were done on phlogopite compositions (Bos et al., 1988). The rotating tie-line technique was used for investigation, starting with phlogopite and NH4-phlogopite, respectively and an aqueous KCl-NH4Cl solution 95 + + of a varibale K /NH4 ratio. Experiments were performed at 550 and 650 °C / 200 MPa and + showed that NH4 is preferentially incorporated into phlogopite using NH4-phlogopite as + + starting material. Assuming ideal mixing of K and NH4 among phlopopite and fluid, constant distribution coefficients for the K-NH4 partitioning between phlogopite and fluid were derived for 550 and 650 °C, respectively. However, these values have to be reviewed critical. Bos et al.’s (1988) data on the K-NH4 exchange between phlogopite and fluid strongly suggests that equilibrium could not be attained during the experimental runs. The obtained distribution coefficients seem to depend upon whether phlogopite or NH4-phlogopite was used as starting + material. For example, two runs of similar NH4 concentration in the initial fluid yielded phl− fluid phl− fluid K D = 0.87 for phlogopite-fluid exchange and K D = 1.69 for NH4-phlogopite-fluid exchange. Moreover, additionally performed time experiments on the rate of the exchange reaction at 550 °C predicted equilibration times of serveral months for their exchange experiments. phl− fluid Moine et al. (1994) reevaluated Bos et al.’s (1988) experiments and emphasised that K D is not constant but should vary at given P and T with the bulk XNH4/XK ratio in phlogopite. phl− fluid phl They showed that K D increases with (XNH4/XK) , which is important especially in the solid fluid range of low NH4 concentrations. In Figure 29 plots of (XNH4/XK) versus (XNH4/XK) are presented for the phlogopite-fluid exchange series at 500 °C / 200 MPa (Moine et al., 1994) as well as for the feldspar-fluid and muscovite-fluid exchange series at 500 °C / 400 MPa from this study. Distribution coefficients KD for the K-NH4 partitioning between phlogopite and + phl fluid are <1 for low NH4 concentrations and >1 for (XNH4/XK) > 0.35. This indicates a + + change of the distribution behaviour from NH4 fractionation into the fluid at low NH4 content + + to NH4 fractionation into phlogopite at high NH4 concentrations. However, for geological phl− fluid phl applications Moine et al. (1994) determined K D = 0.53 for (XNH4/XK) < 0.10 (≈ 3200 phl− fluid ppm). The corresponding partition coefficient D NH4 is 0.57 and allows to calculate the fsp− phl fsp− fluid phl− fluid musc− phl musc− fluid phl− fluid partition coefficients D NH4 = D NH4 /D NH4 and D NH4 = D NH4 /D NH4 to 0.38 and 0.33, respectively. They are only valid for 500 °C since the K-NH4 exchange between phlogopite and fluid is dependent on temperature (Bos et al., 1988; Moine et al., 1994). The pressure difference of 200 MPa between the experimental series should only have a minor influence on the derived KD values. 96 7,0 KD = 1.0 6,0 Musc Fsp 5,0 fluid 4,0 ) Fsp 400/400 K /X Fsp 500/400 3,0 NH4 Fsp 600/400 (X 2,0 Musc 400/400 Musc 500/400 1,0 Phl Musc 600/400 Phl 500/200 * 0,0 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 solid (XNH4/XK) 2,0 K = 1.0 1,8 D 1,6 1,4 KD < 1.0 1,2 fluid ) K 1,0 /X Fsp 400/400 NH4 0,8 (X Fsp 500/400 0,6 Fsp 600/400 Musc 400/400 0,4 Musc 500/400 0,2 KD > 1.0 Musc 600/400 Phl 500/200 0,0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 solid (XNH4/XK) fluid solid Figure 29 (XNH4/XK) versus (XNH4/XK) plot. Slope of the curves are distribution coefficients KD for the feldspar-fluid, muscovite-fluid and phlogopite-fluid exchange series. K-NH4 fractionation for the + + + phlogopite-fluid series changes from NH4 partitioning into fluid at low NH4 concentrations to NH4 solid partitioning into phlogopite at (XNH4/XK) > 0.3. (* data from Moine et al., 1994) 97 7.4 Geological applications + + Nitrogen incorporated as NH4 in K -bearing minerals is not only released in form of N2 during + prograde metamorphism but also as NH3 and/or NH4 . This was shown by Mingram & Bräuer (2001) in a study on Erzgebirge schists ranging from C-bearing slates (T ~ 300 °C / P < 200 MPa) to garnet-phengite-schists (T ~ 730 °C / P > 1.2 GPa). The natural observation of ammonia release fits very well experimental results obtained within this study on the + + distribution of NH4 and K between aqueous chloride fluids and solid solutions of both the (K-NH4)-feldspar and (K-NH4)-muscovite join, respectively. In all exchange synthesis + + reactions NH4 was found to fractionate preferentially into the fluid phase. For low NH4 fsp− fluid concentrations, relevant for most natural rock environments, the partition coefficients D NH4 musc− fluid and D NH4 were determined to 0.22 and 0.19 at 400-600 °C / 400 MPa and to 0.25 and 0.26 at 400-600 °C / 1500 MP, respectively. These results show clearly that loss of ammonia in metasediments during prograde metamorphism and continuous dehydration can be explained + + by the observed fluid-solid distribution behaviour of K and NH4 . Generally, the partition coefficients derived for the K-NH4 exchange between feldspar-fluid and muscovite-fluid can be used for mass balance calculations of fluid-rock interactions in crustal rocks. The obtained fractionation data also indicates that synthesised (K-NH4)-feldspars incorporate + fsp more NH4 in comparison to (K-NH4)-muscovites. This is especially seen in the range of X K between 0.1 and 0.9 and could explain, why natural buddingtonites found so far are richer in + NH4 than natural tobelites. For example, several localities with buddingtonites containing + approximately 0.80 pfu NH4 [ ≈ 7.8 wt% (NH4)2O ] are known (Erd et al., 1964; Gulbrandsen, 1974; Ramseyer et al., 1993) whereas the maximum (NH4)2O content in tobelite found so far is + 3.6 wt% [≈ 0.53 pfu NH4 ] (Higashi, 1982). Investigations on coexisting natural buddingtonites and tobelites are necessary to clarify this observation. + For low NH4 concentrations constant partition coefficients are derived for the K-NH4 distribution between feldspar, muscovite and phlogopite. In Table 29 the corresponding solid12− solid + D NH4 values are compared to those obtained from coexisting NH4 -bearing minerals found in granitic and metamorphic rocks. Investigations on the K-NH4 partitioning between natural muscovites and phlogopites are numerous (Honma and Itihara, 1981; Duit et al., 1986; Boyd and Philippot, 1998; Sadofsky and Bebout, 2000) whereas only few data exists for the feldspar-mucovite and feldspar-phlogopite K-NH4 exchange in nature so far (Honma and 98 solid12− solid Itihara, 1981). The agreement between D NH4 observed in experiment and nature is reasonable, especially for the K-NH4 partitioning among feldspar-phlogopite and muscovite- phlogopite indicating that equilibrium conditions can be achieved in natural K-NH4 exchange fsp− musc processes. For feldspar-muscovite a slight difference in D NH4 derived from experimental + data and nature is observed. Muscovites in natural rocks of low NH4 content exhibit more + fsp− musc NH4 than coexisting feldspars as shown by D NH4 < 1 whereas the experimentally fsp− musc determined D NH4 is slightly > 1 indicating that (K-NH4)- feldspar solid solutions + incorporate NH4 preferentially. However, the difference is only minor and since both fsp− musc D NH4 values are close to 1, the K-NH4 partitioning among feldspars and muscovites may be regarded as nearly equal. fsp− musc fsp− phl musc− phl Table 29 Comparison between partition coefficients D NH4 , D NH4 and D NH4 derived from + experimental data (400-600 °C / 400 MPa) and from analyses of coexisting natural NH4 -bearing phases solid12− solid solid12− solid K-NH4 partitioning D NH4 (exp) DNH4 (nat) feldspar - muscovite 1.16 0.68 – 0.97 d feldspar - phlogopite 0.38 0.20 – 0.48 d muscovite - phlogopite 0.33 0.31 - 0.43 a ∼ 0.36 b 0.39 – 0.46 c 0.32 – 0.65 d a Boyd and Philippot (1998); b Duit et al. 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(1992) Characterization of hydrothermal tobelite veins from black shale, Oquirrh Mountains, Utah. Clays and Clay Minerals, 40 (4), 405-420. Zimmermann R., Gottschalk M., Heinrich W. and Franz G. (1997b) Experimental Na-K distribution between amphiboles and aqueous solutions, and a mixing model along the richterite – K-richterite join. Contributions to Mineralogy and Petrology, 126, 252-264. Zimmermann R., Knop E., Heinrich W., Pehlke I. and Franz G. (1997a) Disequilibrium in cation exchange experiments between Na-richterite – K-richterite and aqueous solutions: effects of fractional crystallization. European Journal of Mineralogy, 9, 97-114. 105 Eidestattliche Versicherung Hiermit versichere ich eidesstattlich, dass ich die von mir vorgelegte Dissertation mit dem Titel “Experimentally determined K-NH4 partitioning between feldspars, muscovites and aqueous chloride solutions” eigenständig verfasst habe. Außer den in der Arbeit selbst angegebenen Hilfsmitteln und Quellen sowie der unten aufgeführten Zusammenarbeit mit anderen Wissenschaftlern habe ich keine weiteren Hilfsmittel und Quellen verwendet. Dortmund, 22.11.2002 Zusammenarbeit mit anderen Wissenschaftlern: 1) Die Charakterisierung synthetischer Buddingtonite und Tobelite aus Kapitel 3 basiert auf Proben, die von Herrn Dr. Daniel E. Harlov am GeoForschungsZentrum Potsdam hergestellt und mir zur Untersuchung überlassen wurden. Die Auswertung der infrarotspektroskopischen Daten an diesen Proben erfolgte durch Herrn Dr. Michael Andrut. 2) Messungen des Ammoniumgehaltes von insgesamt fünf (K-NH4)-Feldspat- Mischkristallen aus Kapitel 5 wurden von Herrn Dr. Andreas Kronz an der Universität Göttingen durchgeführt und von mir in enger Absprache mit Herrn Dr. Andreas Kronz ausgewertet.