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Structural and Thermodynamic Properties of Ca-Al-Bearing Amphiboles

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Structural and Thermodynamic Properties of Ca-Al-Bearing Amphiboles Structural and thermodynamic properties of Ca-Al-bearing amphiboles vorgelegt von Diplom-Mineraloge Jens Najorka Vom Fachbereich 9 - 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. Gerhard Franz Berichter: PD Dr. Matthias Gottschalk Berichter: Prof. Dr. Wilhelm Heinrich Tag der wissenschaftlichen Aussprache: 16. Februar 2001 Berlin 2001 D83 EIDESSTATTLICHE ERKLÄRUNG Hiermit erkläre ich an Eides statt, dass ich die vorliegende Arbeit selbständig und nur mit den angegebenen Mitteln angefertigt habe. Berlin, 12.12.2000 1 DANKSAGUNG Am erfolgreichen Abschluß dieser Arbeit waren viele Personen beteiligt. Ohne ihre Hilfe wäre es mir viel schwerer gefallen, sie fertigzustellen. Deshalb spreche ich allen an dieser Stelle meinen herzlichsten Dank aus. Die Anregungen zu dieser Arbeit und die Möglichkeiten zu ihrer Verwirklichung verdanke ich Prof. W. Heinrich und Dr. M. Gottschalk. Ihr stetes Interesse am Fortgang der Arbeit war mir sehr wertvoll und hat zum Gelingen maßgeblich beigetragen. Für die Zusammenarbeit an der TU-Berlin bin ich Prof. G. Franz sehr verbunden. Besonderer Dank gilt Dr. M. Gottschalk für die stimulierenden Diskussionen über Thermodynamik und Röntgendiffraktometrie, Dr. M. Andrut für die Hilfe bei der Infrarotspektroskopie, Dr. R. Wirth für die Anleitung und Zusammenarbeit am Transmissionselektronenmikroskop, Frau O. Appelt und Dr. D. Rhede für ihre Hilfe an der Mikrosonde und Frau U. Glenz für ihren Beistand am Rasterelektronenmikroskop. Ein recht herzliches Dankeschön gilt allen Kollegen des Projektbereiches 4.1 fuer die angenehme Arbeitsatmosphäre am GFZ in Potsdam. - besten Dank. 2 Jens Najorka ZUSAMMENFASSUNG „Strukturelle und thermodynamische Eigenschaften von Ca-Al-haltigen Amphibolen“ In dieser Arbeit wurden die Ca-Sr-Substitution und die Mg-Tschermaks Substitution (MgSi ↔ AlAl) in dem Amphibol Tremolit sowie die Ca-Sr-Substitution in dem Pyroxen Diopsid experimentell untersucht. Die Amphibol- und Pyroxensynthesen wurden in Anwesenheit einer halogenidischen Lösung durchgeführt. Die Experimente zur Ca-Sr Substitution erfolgten bei 750°C und 200 MPa und die Experimente zur Mg-Tschermaks-Substitution bei 600-850°C und 200-2000 MPa. Sowohl Tremolit- als auch Diopsidstruktur erwiesen sich als flexibel für den vollständigen Ersatz des Ca durch Sr. Zwischen Tremolit und Sr-Tremolit existiert eine kontinuierliche Mischkristallreihe. (SrCa)-Diopside konnten in den Bereichen von XSr = 0.00-0.31 und 0.90-1.00 synthetisiert werden. Mischkristalle entlang des Mg-Tschermaksvektors konnten zwischen Tremolit und Magnesiohornblende beobachtet werden. Die Gitterkonstanten von Tremolit und Diopsid verändern sich mit zunehmenden Sr-Einbau linear. Zunehmender Sr-Einbau führt zur Zunahme der Zellparameter a, b und ß im Tremolit sowie a, b im Diopsid, wobei sich ß im Diopsid verringert. Eine lineare Änderung konnte auch mit zunehmender Mg-Tschermakskomponente im Tremolit verzeichnet werden, wobei die Zellparameter a und b abnehmen und c und ß zunehmen. IR-Untersuchungen zeigten, daß der Einbau von Sr und Al im Tremolit eine komplexe Feinstruktur der OH-Streckschwingungsbande bewirkt. Die Feinstruktur konnte hierbei im (Sr,Ca)- Tremolit der Substitution von Sr, Ca, und Mg auf der M4-Position zugeordnet werden. Im Al- Tremolit wird die Feinstruktur durch Mg-Al-Substitution auf M2- und M3-Positionen sowie Si-Al- Substitution auf T1-Positionen verursacht. Quantitative Berechnungen zu den Bandenzuordnungen deuten hierbei auf eine statistische Verteilung von Sr, Ca, und Mg auf der M4-Position sowie von Mg und Al auf M2- und M3-Positionen im Tremolit hin. Das Verteilungsverhalten von Sr und Ca zwischen Tremolit, Diopsid und chloridischer Lösung wurde bei 750°C und 200 MPa untersucht. Sr fraktioniert hierbei stark in die Fluidphase. Amphibol/Fluid Pyroxen/ Fluid Die Mineral/Fluid-Verteilungskoeffizienten für Sr von DSr = 0.045 und DSr = 0.082 wurden abgeleitet. Die Mischungsenergien in (Sr,Ca)-Tremolit- und (SrCa)-Diopsid-Serie wurde Amph mit einem regulären Mischungsmodell berechnet, wobei die Wechselwirkungsparameter von WCaSr Pyr = 9.8 kJ und WCaSr = 11.7 kJ ermittelt wurden. Verschiedene ideale Mischungsmodelle wurden für die Mg-Tschermaks-Substitution entlang der Tremolit-Tschermakit-Mischkristallreihe getestet. Der beste Fit für die thermodynamischen Daten des Tschermakit wurde mit der Verwendung des “two-site coupled” Modells erzielt. Für ∆ o Tschermakit konnte eine Enthalpie von f H ts = -12528.3 ± 11.7 kJ/mol und eine Standardentropie o von Sts = 556.5 ± 12.0 J/K/mol extrahiert werden. 3 Jens Najorka ABSTRACT „Structural and thermodynamic properties of Ca-Al bearing amphiboles“ The Ca-Sr and Mg-tschermaks (MgSi ↔ AlAl) substitutions in tremolite, and the Ca-Sr substitution in diopside were experimentally investigated. Syntheses were performed in the presence of a halogenidic fluid. Experiments about Ca-Sr substitution were carried out at 750°C and 200 MPa and experiments about Mg-tschermaks substitution were carried out at 600-850°C and 200-2000 MPa. Both tremolite and diopside structures are flexible for a complete substitution of Sr for Ca. A continuous solid solution series exists between tremolite and Sr-tremolite. (Ca,Sr)-diopside solid solutions could be synthesized between XSr = 0.00-0.31 and 0.90-1.00. Along the Mg-tschermaks vector solid solutions between tremolite and magnesiohornblende were observed. The lattice parameters of tremolite and diopside are a linear function of the Sr-content. For (Ca,Sr)-tremolites the lattice parameters a, b and ß increase with increasing Sr-content. For (Ca,Sr)-diopsides a and b increase and ß decreases with rising Sr-content. Increasing Mg-tschermaks content in tremolite also changes the lattice parameters linearly, a and b decrease whereas c and ß increase. Complex fine structures of the OH stretching bands can be observed in IR spectra of (Ca,Sr)- tremolites and Al-tremolites. Fine structures were assigned to substitutions of Sr, Ca and Mg on M4 sites in (Ca,Sr)-tremolite, and to Mg-Al substitution on M2 and M3 sites and to Si-Al substitution on T1 sites in Al-tremolite. The assignments were supported by quantitative calculations, which indicate a statistical distribution of Sr, Ca and Mg on M4 sites and Mg and Al on M2 and M3 sites in tremolite. The distribution of Sr and Ca between tremolite, diopside and chloridic solution were investigated at 750°C and 200 MPa. Sr fractionates strongly into the fluid. amphibole/ fluid pyroxene/ fluid The mineral/fluid partition coefficients for Sr are DSr = 0.045 and DSr = 0.082. The mixing energies of (Ca,Sr)-tremolite and (Ca,Sr)-diopside series were calculated using a regular amph pyr solution model. Interaction parameters of WCaSr = 9.8 kJ and WCaSr = 11.7 kJ were derived. The thermodynamic properties of the tschermakite endmember and the mixing properties along the tremolite-tschermakite join were extracted from six exchange reactions. Ideal mixing models were tested for Mg-tschermaks substitution along the tremolite-tschermakite join. Best fits ∆ o were obtained for a two-site coupled model, resulting in an enthalpy of formation of f H ts = o -12528.3 ± 11.7 kJ/mol and a standard entropy of Sts = 556.5 ± 12.0 J/mol/K for tschermakite endmember. 4 INHALTSVERZEICHNIS Einleitung 7 Kapitel 1 Structural and compositional characterization of synthetic (Ca,Sr)- tremolite and (Ca,Sr)-diopside solid solutions by EMP, HRTEM, XRD and OH-valence vibrational spectroscopy 11 Abstract 12 Introduction 13 Experimental and analytical techniques 14 Results 17 Phases and proportions 17 SEM 18 HRTEM 18 Electron microprobe 21 X-ray diffraction 24 FTIR 29 Interpretation and discussion 34 Conclusions 39 Acknowledgments 40 References cited 41 Zusammenfassung 43 Kapitel 2 Ca-Sr distribution between amphibole, clinopyroxene and chloride- bearing solutions 44 Abstract 45 Introduction 46 Experimental and analytical techniques 47 Results 49 Interpretation and discussion 55 Thermodynamic evaluation 61 Petrogenetic implications 66 Acknowledgments 68 References cited 69 Zusammenfassung 71 Kapitel 3 Crystal chemistry of tremolite-tschermakite solid solutions 72 Abstract 73 Introduction 74 Experimental and analytical techniques 77 Experimental Methods 77 Analytical Methods 79 Results 80 REM 80 HRTEM 80 5 EMP 81 XRD 84 IR 87 Discussion 87 Phase composition and XRD 87 IR 93 OH-stretching vibration and occupancy of sites 93 Band positions 95 Local configurations 95 Band assignment 101 Quantification of site occupancies 103 Acknowledgments 106 References cited 107 Zusammenfassung 110 Kapitel 4 Composition of synthetic tremolite-tschermakite solid solutions in amphibole-anorthite and amphibole-zoisite bearing assemblages 111 Abstract 112 Introduction 113 Experimental and analytical techniques 116 Results 119 Phase characterization 119 Compositions of amphiboles in the P-T-X-space 126 Bulk composition amphibole-anorthite-quartz-diopside 126 Bulk composition amphibole-anorthite-quartz-talc 128 Bulk composition amphibole-anorthite-quartz-enstatite 128 Bulk composition amphibole-anorthite-clinochlore-talc 128 Bulk composition amphibole-zoisite-talc-quartz 128 Bulk composition amphibole-zoisite-talc-clinochlore 128 Bulk composition amphibole-zoisite-kyanite-clinochlore 129 Discussion 132 Phase assemblages 132 Comparison with earlier results 134 Thermodynamic evaluation 137 Thermodynamic
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