Experimental stability relations of the hornblende marrnesiohasti ngsite :
p' Departmen; of Earth and Space Sciences, University of California, Los Angles, California 90024 W. ERYST S. ) "Present address: lnstltut de Physique de Globe, UniVersite.de Paris, 75230 Paris Cedex 05, France,
Geological Society of America Bulletin, Part II, V. 92, p., 274-358,8 figs., 16 tables, February 1981, Doc. no. M10203
are present in the clinopyroxene and AB sTRACT the olivine, respectively, with in-
Phase_ relations for thd bulk composi- 'creasing fo ._ Magnesiohastings,ite 2 t ion Na 0. 8Mg0 Fe203*12Si02 2A1203 breakdown the presence of 2 Ma00 in excess + excess H20 havewbeendetermined as a fluid occurs under the following' condi-
function of fluid pressure (P), tempera- tions (see Tables 1 and 2 for abbrevia-
ture (2'); and,oxygen' fugacity (f ). tions and fo equations of oxygen 02 .. 2 using conventional hydrothermal appara- buffer equilibria):
tus and the oxygen bufter method. Mag- Fluid Ebr&ssdre in Bars nesiogastingsite, NaCa2Mg4Fe3+ Si6Al2OZ2 Buffer- 300 500 1000
(OH)2, is .stab16 over a wide range of , IQF 692 OC 785 OC 944 OC FMQ ' 860 OC 925 OC 1011 OC p and Fo conditions. Al-clinopyroxene + HM 968'$C 1018 OC -
/ 2 olivine + neph'ellne + spine1'"Wmagnetite
.ime4t & fluid,"all0 of variable komposi- The F-T curve for the bq@nning of
tionsd, afe produced upon dehydratiqn of melting intkrsects the amphibole reaction
thee amphibole at high temperature. curve at a minimum fluid pressure of 420
Physical properties of the anhydrous bars at 1002 OC (HM buffer). Nepheline
phases indicate that increasing amounts probably dissolves in the silicate liquid
of aeg rine and forsterite components at temperatures and pressures slightly 274
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in excess of the investigated range. potential sources of 2 great deal of
Magnesiohastingsite is thus stable well information regarding physico-chemical c within the magmatic range. characteristics attending their forma-
Standard enthalpy and entropy of re- tion. Moreover, amphiboles occur as
act,ion for the equilibria Plh' = Cpx + 01 major or minor constituents in most-meta-
+ Ne + Sp f Mt + F range from 50 to 30 morphic and igneous rock types. Their
kca'llmol and 37 to 35 cal/K/mol, re- compositions may be dekcribed in terms of
spectively, from low to high fo (see hypothetical end members such as described' 2 Table 3 for abbreviations for synthetic, by Leake (1978). In general, natural
phases). specimens are complex comkiinations of
Because of their extensive pressure- several end members. The paragenesis
temperature stability fields, Mh-rich of amphiboles may be attacked in several
hornblendes should occur in igneous and ways. One, pioneered by Boyd (1959), is
highrgrade metamorphic rocks whose norms to study the phase relations of selected
contain major magnesian Cpx + 01 + end members under carefully contro:led
aluminous phases. The rarity of amphi- laboratory conditions. Valuable informa- .. boles from such rocks indicates that low tion applicable, either directly or in-
H20 fugacities (50 to 500 bars,, approxi- directly through thermodynamic reasoning,
mately) and reducing conditions attended to natural occurrences may thus be ob-
their crystallization. tained (for a review, see Cameron and
Papike, 1979) . INTRODUCTION The generalized amphibole formula
Because of the large number of may be written as: AM(4)211(1)2M(3)M(2)2
crystallographically different sites T(l)4T(2)i022(OH)2; A-is- a large 10- to
present in the amphiboles, most major 12-fold coordinated site and contains Na,
elements of the Earth's crust may enter K, or is partially or completely vacant;
their structure. As such, they are M(4) is a smaller site enriched in 8-fold
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M(1), M(2), and M(3) are octahedral sites physico-chemical properties of magnesio-
of slightly different sizes and accom- hastingsite with conditions of synthesis
modate Mg, Fe, Al, .Ti, Mn, . . . ; T(l) , have be'en previously presented by Semet
+ T(2) are tetrahedral sites containing (1973).
Si and Al; and OH may be partly replaced In Semet's (1970) infrared and
by F,-Cl, or 0 in natural specimens. Site MGssbauer study of synthetic Mh, it was
nomenclature follows Papike.and others shown,that, in accord with theoretical
(1969). predictions (Whittaker, 1960; Chose, 19651,
This study 'reports on the phase re- Fe3+. is concentrated in M(2)3 whereas
lations of the end-member magnesiohasting- Fe2+*prefers, although less strongly,
site, NaCa2MgiFe*Si6A12022(OH)2 (Semet, M(1) and M(3). Cal;cium is confined to I 1970). Along with pargasite, NaCa MG A1 2. 4 the 8-fold M(4) site, and Na resides Si6A12022(OH)2 (Boyd, 1959; Holloway, in the larger A site. This'scheme is
expected to hold for more complex njdtural. 1973; Holloway and Ford, 1975), and 2+ pargasites containing minor amounts of ferropargasite, NaCa2Fe4 A1Si6A12022 additional cations (Ti, Mn, Co, Ni, and' (OH)2 (Gilbert, 1966), Mh,is che third so forth), as well as for actinolite composftion. to be investigated experi- -. (A site vacant) and .sodic-amphiboleI (Na mentally in the "hornblende quadfi- i'n M(4)') solid-solutions. Iron-rich lateral." The last member of the quadri- varieties (f erropargasite and hastingsite) lateral; hastingsite, NaCa2FeyFe3+Si6 \I require more -significht changes, par- A1202* (OH) 2, recently has been studied ticularly in the geometry of .the 'octahedral experimentally by Thomas (1977) ,and ribbons due to the larger Fe2+ ion. Charles (1978). Magnesiohastingsite Earlier studies (C. C. Addison and has been synthesized (previously others, 1962; W. W. Addiion and others, (Colville and others, 1966) and its 1962; Addison and Sharp, 1962; Ernst unit-cell parameters and optical
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and Wai, 1970; Semet, 1973) indicated is in large part because of the rarity
that H+ and electrons are particularly of appropriate rock bulk' compositions.
mobile in the amphibole structure and The widespread occurrence of inter-
account for rapid solid-state oxidation- mediate members in contrasting geologic *2+ reduc,tion reaction rates (mainly Fe : settings reflects the ability of'the
Fe3+ exchange). Synthetic magnesio- structure to accommodate most major
hastingsite and possibly some other cations.
natural hornblendes may contain hydrogen Mh-rich hornblendes typically occur
in amounts greater than the ideal two- in mafic and ultramafic igneous rocks,
per-formu1.a unit (as much as -2.85). marbles, skams, and amphibolites, and
The "extra" protons are presumed to be they contain appreciable Fe3+ in addition
bonded to tetrahedral chain oxygens to ofher substitutions. The purpose of T in a positionafly' diserd&ad- fashion .this investigation was to ascertain the
.i Although analyzed hornblendes takeq influence of octahedral Fe3+ substitution
from a variety. of rocks (for example, for A1 in pargasite structure and phase 3+ see Leake, 1968) show wide variation of relations. Enrst (1968) noted that Fe
OH contents, this may not mean that'they substitution for A1 increase's to.;%
crystallized as such. The status of minpr extent the maximum temperature a*t
oxy- or H-rich amphiboles as stable which amphibole is stable. This has been
species must still be considered as largely confirmed for the- pargasite-
unrosolired. magnesiohas t ingsite pair.
Solid solutJon among members of the
pargasite-has tingsite series is 2+ virtually complete and involves Mg-Fe
and A1-Fe3+ octahedral substitution.
The comparatively rare natural occur-
rence ferropargasite (Gilbert, 1966) - of
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higher pressures. Horizontal, nichrome- EXPERIMENTAL PROCEDW wound tubular electric ?urnaces were AND ANALYT'ICAL TECHNIQUES' used as heating elements for the pressure
Pressure Measurements vessels. Temperature regularion was ob-
Synthesis and the study of phase re- tained by applying a constant voltage
lations for the Nh bulk composition were to the furnaces employing electronic
carried out with conventional hydrother- temperature regulators or variable
mal equipment at pressures greater than transformers in series with a voltage
1 atm..- Cold-seal pressure vessels modi- regulator. fied after a design by Tuttle (1949) Ex.-crimcnts at atmospheric pressure
were fabricated from Re& 41.or Haynes were carried out in a small platinum-
25 alloys. H20 fluid pressure was ap- 'wound res,istance furnace. Teyerature
plied through an oil-operated hydraulic regulation was attained by applying a
pump and an oil-water separator-intensi- constant voltage to the furnace through
fier with a maximum pressure capability the same equipment as described above.
of 3,000 bars. Pressures were measured Temperature Measurements on a variety of gauges indexed against
14 in., 4 kb, and against 10 in., 1 kb I; the 1-atm runs, temperatures yere -. factory-calibrated 'Heise gauges. . The me'asured with calibrated Pt-PtgORh10
certified accuracy is 4 bars for the thermocouples. For each high-pressure
4-kb gauge and 2 1 bar"fpt the 1-kb .apparatus, a chromel-alumel thermo- '* gauge. Fluctuations were pQintained' to couple was placed in a small external
within 3 bars at low pressures' (to well. in the pressure vessel near the
1,000 bars), and within 10 bars at charge. Emcloying an ice-water bath as
reference,-temperature readings were 'Detailed descriptions, of experimental methods, complete tables of run data, and taken on a Leeds and Northrup Type K-3 physical properties of the phases have been presented by Semet (1972). potentiometer as often 'a%very 15 minutes .
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for short runs. A 24-station strip-chart 2 OC but reaches 8 Co, at 1000 OC, for
recorder (f2 Co accuracy) was used to a 2.5-cm-long capsule. Because of un-
monitor longer runs in conjunction with certainties in sample location in the
I regular potentio'metric measurements. pressure vessel, and *pressure vessel
The chromel-alumel thermocouples were position in the furnace, temperature
calibrated against the melting point of gradients, and the precision of.measure-
NaCl (800.6 OC) by a thermal pause ments, the over-all accuracy is thought
method; both increasing and decreasing to be about f6 Co. Consistent results
temperature paths were followed. Re- in repetitive runs under the same condi-
peated measurements on at least ten ttons, however , show that the precision
samples per' wi're spool have shown that may be better than +3 OC.
the precision ior 'a single set of .I Starting )la t erials thermocouple wires is bdttgr than -fO. 5
Co at 800 OC. Syntl.escs were carried out using
Tempcraturcs at the chargc location oxide mixtures, whcrcas thc phasc rclcltion
in. the pressure vessel and temperature study was done using exclusiveTy synthetic
gradients near the charge were measured mineral mixtures. Starting material for
with calibrated thermocouple probes both syntheses of magnesiohastingsite or the
at atmospheric and at higher pressures assemblages of equivalent bulk composi-
as a function of the externaf thermo- tion fH20 consisted of mixtures of
couple reading. Relatively large vari- analyzed reagent-grade oxides, hydroxides,
ations (ti 'cO for a single ty~eoi and carbonates in the stoichiometric
pressure vessel) were observed for cation .proportion Na20=4Ca0*8Mg0~Fe2030
di,fferent bombs and furnace assemblies 12Si02*2A1203 (+ excess H20).
and different furnace configurations, 6eterminations of equilibria involving
The measur'ed temperature gradient along Mh were cazried out by running mixtures
a 1-cm charge capsule is approximately of synthetic reslctant and product phase
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assemblages in 10:90 and 9O:lO weight inferred to be a linear function of
ratios. This provides a sensitive temperature, and the AV of solids par-
method for determining the direction of ticipating in the reaction is presumed
a reaction by X-ray diffraction methods, to be independent of pressure and
Reaction equilibria where fo was the temperature. These appear to be valid 2 variable of interest were studied using approximations for relatively low
only pure reactants. pressures (<4 kb) and a wide range of
temperatures. Knowledge of the oxygen Control of Oxygen Fugacity fugacity allows the calculation of the
The fluid phase composition yields fugacities of other gas species in the
l a set of intensive variables which, system El-0-H when considering the addi- I together with pressure and temperature, tional equilibrium H2 + p2= H20.
influences redox and dehydroxilation Buffering of gas species obtains as long
reactions of the type studied here. as both MOx and MO coexist. Specific - xt2y Hydrogen osmosis (Eugster, 1957, 1959; buffer reactions and abbreviations are'
Eugstcq and Wone,s, 1962) provides a presented in Table 1.
way of controlling the gas phase compo- A FORTRAN IV computer program was used
sition in the 0-H system. Experimental to generate fugacities of the gas species
configurations were described by Huebner as a function OZ temperature and pressure.
(1971). Empirical equations were fitted by a
The equilibrium oXygen fugacity for least-squares method to the data of Holser
buffering reactions of the type NOx + (1954), Shaw and \Jones (1964), the JANAF
0 - may be expressed as tables (1965), Robie and Waldbaum (1968), yo2 - M0x+2y and Burnham and others (1969) for the A C(P - 1) log f02 = r + B + T temperature and pressure ranges of 650 to
for a wide range of conditions. The 1000 OC and 100 to 1,100 bars, respec-
change of free energy for the reaction is tively. Fugacities were then obtained
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TABLE 1. BUFFER REACTIONS AND SYMBOLS
IQF 2Fe + Si02 t O2 2 Fe2Si04 iron quartz fayal ite
w M 6Fe0 + O2 ;! 2Fe304 wus ti te magnetite
FMQ 3Fe2Si04 .+ O2 2 2Fej04 + 3Si02 . fayal ite magnetite quartz.
NNO ' 2tji t '02 $ 2Ni0 nickel bunseni te
MMO 6Mn0 + O2 +-+ ZMn304 manganosj te hausmannite
cco 4cu t o2 2 2cu20 copper cuprite
HM 4Fe304 t O2 +3 6Fe203
magnetite hematite
CT 2cu20 + o2 : pcuo cuprite tenorite
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by iterative computation (gas mixing particularly useful in experiment plan-
considered ideal) : ning and extrapolations of equilibrium
curves. - 0.2361 (2.125 x 10-4)P logyi~20 - T - '273 - Thermodynamic data for a.number of (present study) ; I chemical reactions involving oxidation- 13288 log Kw = -3.055 + - keduction of metal compopnds are known to T +13 / (present study); a high degree of accuracy. Use of these
buffers allows step-wise variation of log YII = e~p(-3.8402T'/~ + 0.5410)P - I 2 oxygen fugacity. Furthermore, the avail- exp(-O. 1263P1/2 - 15.980)P2 + 300 l ability of accurate?- 7- T data and e~p(-O.O11901T - 5.941)e~p(~300 -1) equilibrium constants €or gases in the O-H (Shaw and Wones, 1964); system make the use of the oxygen buffer
technique a reliable tool in the investiga-
(Table 2). tion of redox and dehydroxilation
Corresponding values for H20 and H2 equilibria. Pertinent data are presented
fugacity may be obtained from the Cables in Table 2.
recognizing the relationships Procedure
The standard welded two-capsule assembly
(Eugster and Wones, 1962; Eugster and
Skippen, 1967) was used throughout this
study. The inner sealed charge contdner
is made of an Ag 70.Pd30 alloy in an at-
The accuracy of the computations is tempt to minimize iron loss -(Muan, 1963).
better than 3% compared, to single point However,,it was necessary to use Pt cap-
calculations of Eugster and Skippen sules in conjunction with the NNO buffer
(1967). These computations have proved because Ni alloys with the Ag-Pd, thereby
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IQF 29,382 7.51 0.050 Eugster and Wones (1962) W M 32,730 13.12 0.083 Eugster and Wones (1962)
FM Q 25,660 8.92 0.0937 Wones and Gilbert (1969)
NNO 24,810 9.31 0.046 Huebner and Sat0 (1970) MMO 25,680 13.38 0.0807 Huebner and Sat0 (1970)
c co 17,050 6.85 0.096 This work, after the data of
Robie and Waldbaum (1968)
, HM 24,912 14.41 0.01 9 Eugster and Wones (1962)
CT 13,928 10.34 0.0105 This work, after the data of
Kel 1ey (1 960)
CT 14,028 10.43 0.0105 This work, after the data of
Robie and Waldbaum (1968)
-A Note: log .fo2 - - t B t - ') , where is the"temperature in K; - T T P is the total pressure in bars.
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no longer affording protection against complete determination of the optical
contamination. The outer sealed capsule properties of the phase present. It
consists of a noble metal through was, however, possible in some cases to
which H2 diffuses only slowly (Au or obtain approximate values of refractive
Ag). Diffusion of H2 through the outer indices and/or extinction angles of
capsule was significant at the high prismatic minerals by the oil-immersion
temperatures encomtered in,this study, technique. Refractive index oils of 0.002 --
and the buffer eventually. equilibrated interval between 1.450 and 1.700, and
with the pressure vessel walls + fluid 0.005 above 1.700 were used.
medium (f, is close to the NNO buffer Identification of phases was primarily 2 for new bombs and is h.igher for older done by X-ray powder diffraction. A
ones). Run-up times were on the order Norelco powder diffractometer equipped
of 45 to 60 min, whereas quenching to with a single crystal Tonochromator was
temperatures lower than 500 OC was ef- used in conjunction with CuK radiation. a fected in 4 or 5 min. For precise measurements of cell parameters, 0 Si (a = 5.43054 A; Parrish, 1960; Beu Identification of Phases and others, 1962; Burnham, 1965) or
+) Run products were examined employing calibrated AgCl (a = 5.5487A) was used 1 a stereoscopic micfascope after con- as an internal standard.
clusion of €he experinlent to check for Single crystal intensity data were
persi-stence of ar. a4.1eous phase and collected for a natural specimen of
to examine large-scale .textures. After magnesiohastingsite (collected at Iron
air drying at 110 OC for 15 to 20 min, Hill, Gunnison, Colorado, from the same
an' aliquot of the sample was dispersed outcrop as sample U-1236 of Larsen, 1942)
in an index oil and studied under the from Wissenberg photographs of the hkO
polarizing,,microscope. The generally through hk6 levels. The data were then
fine grain size (- 5 pm) precluded used in conjunction with a computer-
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~
generated listing of the possible re- limitations of petrographic. observa'-
fleckions for the group C' 2/m (Evans tion' (<1 volume pefcent of the ,sample),
and others, ;963) employing trial minor or trace phases may have been
parameters from Colville and others overlooked. Material balance calcula-
(1966) to ascertain an unambiguous tions discussed below do not, however, identification of the reflections for ,leave significant. amounts of material the synthetic amphiboles. A Fortran,IV unaccounted for. Microscopic observation
least-squares refinement program (Burn- of the run products has not shown any
ham, 1962) was used tb' obtain'the cell growth of quench phases (except in runs
edges from the index 20 values .(Setnet, where hydrated glass was-present) . ;'The
1973, Table 5). lack of quench products is thought to be
The large number of synthetic phases due to the low pressures and sluggish
in the breakdown products.of blh led to rates of reaction attending this 'study.
severe interference of many reflections Detection of Reaction anci of one ,phase with those of other phases. Demonstration of Equilibrium However, it proved possible to find a
sufficient number of isolated. rzfler- In general, because of.the fine grain
tions for each major phase to obtain a sizes, it was not possible to determine
reasonably .reliable statistical sample.' the directjon of reaction by microscopic
On the other hand, when amphibole.was observation. However, reactions involving
present with abundant breakdown products, the appearance of an opaque phase
the diffractometer trace was too complex (magnetite-rich spinel, or hematite), or
to be usable forcell-paramter deter- the formation of a glass generally were
mination. only possible to bracket by optical means
Due to the finite resolution of the X- because of the small amounts involved or
ray method (approximately 3 to 5 volume, because of the noncrystalline nature of
percent of the sample) and to the the phase. Host interpretations were
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based on X-ray diffractograms of the run fluid pressures (but also farther away
products. Growth or:disappeaknce. of from the reaction surface), complete
amphibole or its breakdown products was conversion of the breakdown products to
determined chiefly by comparing dif- synthetic amphibole was readily achieved.
fractograms of starting material and run Because of the discontinuous nature
products. In many experiments, 10:90 Of .r"2 control with the buffer method, and 9O:lO mixtures of,reactants and pro- certain r'eactions could not be investi- ducts were run side by side in the same gated experimentally. In those cases, buffer. capsule, thus affording unam- however, it was possible to bracket the v L biguous indication of the direction of equixibria reLdtively closely by con- react ion. trolling fo with'that of the pressere. 2 Although the synthesis of akphfbole vessel + pressure medium itself (un- or its breakdown products from an un-' buffered runs) and by careful choice of stable oxide mix is relatively rapid the vessel. (possibly < 5 hr in the range 850 to 950 Electron Microprobe Analyses 0C and 400J.o 2,000 bars), reaction
rates near equilibrium are very sluggish. An ARL-EEM electron microprobe was case was conversion of the.break- In no employed to obtain chemical compositions .. down products to an amphibole completed of synthetic and natural Mh and the in the Mh stability field near the anhydrous phases of equivalent bulk equilibrium curve at pressures <1,000 I composition. An accelerating potential bars. Longer run. time did, however, 1 of 12 kV and a sample current of 0.15 promote very slow amphibole growth. or 0.501 microamperes on brass' were the Therefore, it was not possible to usual settings. At least 20 points (or ascertain the existence or non-existence grains) were analyzed with 40 s counting of reaction zone instead of a well- a times. 'Background readings Mere taken defined P-T-fo surface. .At higher on the samples and standards with a total 2
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counting time of 500 s or more for .each characterization of the minerals pro-
eiement on both low and high wavelsgth duced.' Unfortunately, because of the
sides. Analyzed hornblrndes, clino- generally very fine grain sizes, and
pyroxene, plivines, and Amelia County, the invariable clustering of grains,'
Virginia, albite were used as 'itandards. optical methods were seldom definitive - Data reduction was performed with a in routine examination of the run
. computer program following the Bence products. Cell parameters, MEssbauer arid and. Albee (1968) scheme.. The accuracy infrared spectra (for Mh only), and of the analyses is consiaered to be electron-microprobe analyses of the
better than 3% of the amount present major phases were, however, detained
for sanple UT3.236, the natural Mi, for a few selected runs. For those,
whereas for synthetic amphiboles it is special care was taken (1) to minimize
no better than 7% of the amount present contamination of the charge when several
owing to the small grain sizes and sub- reruns were required; (2) to recognize
microscopic inclusions (Semet, 1973, the possibility of metastable persistence'
Table 6). of early formed phases or, more 'impor-
tantly, metastable compositional relation- SYNTHETIC PHASES ships between phases; and (3) to grow
Table 3 presents the ideal composi- giains large enough for positive identifi- ,. tions and abbreviations for the phases cation and reliable data collection.
considered in this study. Because Magnesiohastingsite elucidation of phase relations for the
Fih bulk composition involves the chemical Physical properties and crystal-chemical
'-- compositio'ns of the phases produced as characteristics of synthetic Mh have been
well as P, T, and fugacities of species studied mostly at 850 OC, 2,000 bars
in the gas phase, special attention was as a function of f, and were previously 2 given to the crystal-chemical presented (Semet, 1973). The refractive
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Ak Akermani te: Ca2MgSi20q /-
CPX Clinopyroxene: (Ca,Na)0,5-1 (Mg,Fe,Al )l -1 .5 (Si ,A1 )206 Di Diops i de : CaMgSi 206 , F. Fluid phase: in general H20 t-H2 ri,ch
Fa Fayal i te : Fe2Si O4 Fo Forsteri te: Mg2Si04 2+ . FPg Ferro pargasi t e : Na Ca2 Fe4 A1 Si 6A1 2022 ( OH)2
Hbl e Hornblende: (Na,K)O-l .5 Ca .5-2 ( Mg 9 Fe ,A1 9 etc- 5- 5 ..5 (Si ,A1 )8022(OH)2 Hc Hercyni te : FeAl 204 2+ 3+ Hs t Hastingsi te: NaCa2Fe4 Fe Si6A1 2022(OH)2 L Silicate me1 t
Me1 i1 ite : (Ca ,Na)2 (!Ig,Al ,Si )307 *'-.. 3t Mf Magnesioferri te: MgFe O4
Mh Ma gnes i ohas ti ngsi te :-, NaCa2Mg4 Fe 3+Si 6A1 2022(OH)
Mo Monticell ite: CaMgSi04
. rdt Magnetite : Fe304 Na-Phl Sodium phlogopi te: NaMg3SijA~1010(OH)2 L Ne Nephel i ne : NaAl Si O4
OPX Orthopyroxene: (Mg,Fe)Si03 Pc P1 agioclase: (Na,Ca) (Si ,A1 )408
pg Pargasi te: NaCa2Mg4A1 Si6A12022(0H)2 Qtz Quartz: Si02
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indices of this phase increase . the M(1), M(2), and M(3) octahedral
systematically with oxidation state sites presumably is a function of
(Semet, 1973, Fig. 5). Because the temperasure (and pressure), although,
geometry of the amphibole structure as was pointed out by Semet (1973), it
along the c axis is essentially un- may not change rapidly over the in- 2+ 3+ ‘changed by variable Fe /Fe. pro- vestigated range. No data‘are ayailable Q portions, as indicated by the con- on these effects, bpcause it is viktually
stancy of the e parameter, and also impossible to separate Mh produced at
because the electric vector for the high temperature from accompanyi.ng meta-
y index is subparallel to the octa- stable phases in order to obtain-even hedral cation strip, the y index re- qual.itative results.
lates most closely to changes in the \ Clinopyroxene oxidation ratio. Observation of the .. relative amounts of breakdown phases An augitic clinopyroxene is one of t_he
from X-ray diffratcograms indicates major phases produced in the assemblage
that cation (other than H+.) propor- corresponding to.the Nh bulk composition
tions in coexisting Mh do not differ at temperatures in excess of tlie amphibole
appreciably from the ,;md-member stability field. It typically occurs
formula. The data of Table 4 thus as.aggregates of subhedral grains or as
c suggest that the refractive index (or stout isolated prisms, 5 to 10 pm-in
the oxidation ratio) is primarily a dimension, in many cases poikilitically
function.of fo and it is not greatly enclosed in nepheline grains. The color 2’ affected by pressure and temperature. ranges from colorless.to pale straw-yellow - Similar results were obtained by Ernst at low to high fo , respectively. Where 2 3+ it coexists with melt, it is generally (1960) for magnesioriebegkite, Na 232Mg Fe Sig022(OH) 2. coarser (10 to 20 pm) and has a deeper 2+ 3+ Ordering of Fe , Fe , and Mg among color. Optical properties do not vary
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TABLE 4. 64 .REFRACTIVE INDEX FOR SYNTHETIC MAGNESIOHASTINGSITE
m .n Observed L- -109 702 Y refractive ( .'c 1 (bars) index
765 500 20.77 1 .652
904 1,000 17 :4'1 1.656
81 7 750 19.d ' 1.656
979 750 11.51 1.664
809 200 14.78 1'.660
894 200 8.61 1.669
994 400 5.24 1 .672
960 300 5.79 1.670
944 500 1.09 1.672
961 400' 0.93 1.674
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presents refractive. indices of the .electron rricroprobe work. For these,
-synthetic Cpx? Due to the complex relative y long run times (up to 1,000
cation substitution possibilities, hours at 950 OC, 300 bars) were ad-
refractive indices are an imperfect vantaFeous. Substantially coarser
measure of the chemical composition grain size and less clustering could be
(Deer and others, 1963a, p. 131-134), obtained by running synthetic Mh at
34- 4- particularly where Fe -+ A1 substitu- temperatures in excess of its stability
tions occur. field until all of the amphibole disap-
Cell parameters of the clinopyroxenes peared. Compositions of the Cpx and the
obtained in iepresentative runs are other major phalses are presented in Table
summarized in Table 6 and compared to 7. Althbugh more data are desirable,
.'J natural diopside (Cnleman, 1962) Qnd to the analyses provide a working basis for
the aluminian clinopyroxene formed in material balance computations.
the breakdown of synthetic pargasite Finally, in an effort to determine
(see Boyd, 1959). The parameters are oxidation ratios in the synthetic clino-
consistent with results obtained for pyroxenes, MGssbauer spectra of the high-
synthetic diopside solid-solutions temperature assemblages for seleCted
(Sakata, 1957; Coleman, 1962; Clark and oxygen buffers were obtained. Although
others, 1962) and natural clinopyroxenes the spectra for mineral mixtures can
of .corresponding composition (Winchell yield only semiquantitative results due
and Tilling, 1960; Viswapathan, 1966; to the general overlap of the quadrupole-
Lewis, 1967). split doublets, two important conclusions
Because refractive indices, combined were derived: (1) the amount of
with cell parameters, do not afford octahedrally coordinated Fe3+ present,
much accuracy in determination of chemi- presumably almost exclusively in Cpx
cal composition, efforts were made to . but also in minor amounts of Mt,,varies
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TABLE 5. REFRACTIVE 1NDICES.OF SYNlHETIC CLINOPYROXENE
P (bars) a Y
881 750 1 7 . 92 _'' 1.675 1.698
92 5 800 16.98 1.679 1.704
809 200 14.65 1.680 1.707
92 9 500 12.38 1.679 1.706
944 300 12.14 1.678 1.706 957, 300 11.95 1.675 1 .707
948 200 10.5 .1 .ti90 1.715
990 192 8.0 1.686 1.711
958 300 7.46 1.679 1.708
978 300 7.13 1.680 1.705
1,040 200 6.16 1.685 1.710
934 200 6.22 1.689 1.710
94 9 200 5.97 1.687 1.709
968 200 5.66. 1.688 131
955 200 0.99 1 .691 . 1.718
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TABLE 6. CELL-PARAMETERS Of SYNTHETIC CLINOPY ROXENES
-a -b -C B V a sin 6 T E: ("C) (bars) a ii A i ii ii
925 800 8.875 5.271 105.92,' 437.4 9.350 . (2 1 (2) .,- (3) * (2) ' .. -
.9b5 765 8.879 5.270 105.91 437.7 9.353 ' (4 1 (8) (1 0) (1 3) 944 300 8.893 5.271 105.89 439.4, -9.375 (2) (2) (2) ' (2 1
. 957 300 8.900 5.270 105.92 439.4 9.37'8 (2) (1 ) (1 1' (2) 955 250 8.91 4. 5.277 105.94 441 .2 9.380 (5) (8) . (5) (6)
94 8 200 8.903 , 5.273 105.87 440.5 9.380 (5) (5) (8). (8) 949 2 00 8.892 5.287 105.98 441-.O 9.379 (3) (7) (5) (5) 960 400* 8.875 5.265 105.89 437.4 9.361 (6) (7) (10) (10) Natural Di' 9.746 8.924 5.247 105.92 438.9 9.373 .
*A1 -Di synthesized from par asite bul k composition .. 'The assemblage consists o'f Al-DitFotNetPctSp ?Boyd, 1959). +See Coleman (1 962).
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TABLE 7. CHEMICAL COMPOSITION OF ANHYDROUS PH&+K . PRODUCED FROM THE MAGNESIOHASTINGSITE BULK COMP0?SITION
Oxidation States NNO , MNO Oxides CPX 01 Ne CPX 01 Ne ..
Si O2 52.4 36.9 39.8 51 .6 39.1 41 '3 10.1 - 32.5 5.2 3t.3 2'3 0.7 FeO* 5.2 13.4 0.5 4.6 4.6 1.1
HgO 15.7 47.9 0.09 14.8 30.8 0.13
Ca 0 23.9 1.7 3.0 24.5 1 .o 5.2 Na20 0.46 - 17.3 0.61 0.03 16.6
Cations 1.77 0.87 4.04 1.86 0.98 4.17
A1 I" 0.2-3 - 3.89 0.14 0.02 3.73
A1 "I 0.1 7 - - 0.08 - - Fe3+ 0.09 - 0.04 0.. 11 - 0.09
Fez' 0.06 0.27 - 0.03 0.09 -
M 9 0.79 1.69 0.01 0.79 1 .i9 0.02 Ca 0.87 0.04 0.33 0.95 0.03 0.56 Na 0.03 - 3.41 0.04 0.00 3 :28
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TABLE 7 (continued)
Oxidation States HM Ai r Oxides CPX 01 Ne CPX 01 Sp Melt
sio2 49.6 40.0 42.0 53.6 41.3 0.1 45.6
2'3 8.9 0.5 33.0 11.9 1.4 45.7 18.6 FeO* . 7.2 .l . 7 2.5 7.8 1.5 36.0 5.7 MgO - 10.9 54.7 '0.1 11.6 55.1. 16.9 7.4
CaO .. 21.1 0.9 - 2.4 22.6 1.3 - 13.1
Na20 2.8 0.0 * 16.8 1.1 0.09 - 8.2
Cations ' 1.80 ' 0.95 4'.ll 1.79 0.97 - 4.51
A1 I" . 0.20 0.01 3.81 0.21 0.03 0.60 2.16
A1 "I 0.18 0.26 -
Fe3'. 0.22 - 0.20 0.22 - - 0.47 ie2+ - 0.03 - - 0.03 l:,O 1 14 9 * 0:59 1.94 0.01 0.63 1.94 1.10 1.09
Ca 0.82 0.02 0.25 0.81 0.03 - 1.39
Na 0.20 0.00 3.19 0.07 0.00 - 1.57
*All iron assume8 as FeO.
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2+ from close to zero percent to virtually orthopyroxenes [(Mg, Fe ) Si 0 1, 2 26 100% of the total Fe from low to high fo ; jadeite (NaAlSi26 0 ), aegirine (NaFe3+Si2 2 3+ tetrahedrally coordinated Fe as in 0 ) Ca or Mg Tschermak's end member 6.' synthetic ferri-diopside (Hafner and [ (Ca,Mg) A1SiA106], and the ferri-Tschermak's 3+ Huckenholz, 1971) has not been detected i end member [ (Ca,Mg)Fe SiA106] (Huckenholz
the silicate phases; (2) the relative and others, 1968, 1969). Variation of Cpx
amount of total Fe in Cpx increases with compositlon with pressure, temperature
increasi!g f , whereas it decreases in 01 , and the composition of the other O2 All synthetic clifiopyroxenes produced phases will be discussed under the section
from Mh breakdown may be represented by thi dealing with phase relations.
general formula: 2+ 2+ 3+ Olivfne (NaxCa Fe ) (MgsFet Feu Alv) (SiiAln)06, YZ where 0.02 --< x < 0.18 An olivine solid-solution is the 0.87 --< y < 0.91 second major phase encountered at tempera- tures in excess of Mh s,tability. It oc- d 0.59 --< s < 0.84 curs as very fine (in general,<- 5 pm) 0.00.-- < t < 0.11 colorless granules or short prisms, but 0.00 < u < 0.20 it is easily recognized under the micro- 0.02 --< v < 0.15 scope becausmf its relatively high '1.73 --< m < 1.83 birefringence. In contrast to the clino- 0.17 --.< n < 0.27 pyroxenes, its chemistry is rather simple. All olivines produced are in the
x + y + 2 = 1.00 range Fo to Fo Fa with minor 70Fa30 100 0 s + t + u + v = 1.00- amounts (up to -5 mol percent) of
M + n = 2.90 monticellite (CaMgSiO ) (see 01 analyses 4 These represent diopside solid-solutions in Table 7). The small amount of
toward hedenbergit; (CaFe2+Si206) ,. monticellite in the 91, even at a
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wlth the experimentally determined sition is a K-free nepheline solid-
forsterite-monticellite solvus of colution. It occurs as relatively large
Biggar and O'Hara (1969) and with ex- anhedral grains (15 to 30 pm) or hexa-
perimental work of Warner and Luth gonal platelets, invariably containing
(1973), who report from 6 to 7 mol small inclusions of other phases. It ig
percent of monticellite In Fo ss coexisting of very low birefringence (- .022 to .003) with Mo at 1000 1200 'C. Composi- ss to and the average refractive index (1.535 tions of the olivincs were nhtajned mainly to 1.540) is characteristic (Tahle 9).
by the method of Yoder and Sahama (1957) Cell parameters and cell volume do not
and are in good agreement with composi- significantly change with run conditions.
tion-molar volume relationships However, owing to the difficulties in
(Jahanbagloo, 1969; Fisher and Medaris, obtaining resolved X-ray reflections st
1969) and optical determinations (Deer relatively high 28 in the diffractogram,
and others, 1962, p. 22). A summary s nall hut systematic trends would be
of the compositional data is presented lost. K-free nepheline may show a
in Table 8. In the pressure-temperature variety of solid-solutions (Donnay and
range coveted, olivine composition is others, 1959; Dollase and Thomas, 1978),
essentially a function of fo : de- mainly toward CaA1204, CaA12Si208, and 2 creasing amounts of fayalite are present NaA1Si308. Significant Fe3+ $: A1 sub-
with increasiGg f because of the stitution is also possible in oxidizing O2 inability of 01 to 4,qcorporate signifi- environments (Deer and others, 1963b,
cant amounts of Few 'ions (Williams, p. 242-244; Brown, 1970; Anderson, 1971).
1971; Obata and others, 1974). As is the case with Cpx, the synthetic Ne
produced at temperatures exceeding the Nephel ine Mh stability field contain detectable
The third major phase produced at amounts of alrcomponents present in the
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TABLE 8. CELL PARAMETERS, CELL' VOLUME, dl 30, AND REFRACTIVE INDICES OF SYNTHETIC OLIVINES
881 750 17.92 898 750 17.55 4.81 5
905 750 17.40 906 750 17.38
920 750 17.07 -
92 5 800 16.98 '4.81 3
81 5 200 14.65
869 500 13.51
899 497 .12..93
903 400 12.86
92 9 500 12.38
944 300 .12.14
959 300 11.95
1,000" ? ?
948 200 10.5
955 250 10.3
990 192 8.0
1,030 200 9.3
1,035 2 00 9.20
751 100 11 ,69
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T P dl30 X t ("C) (bars) -log f02 ib a Y (Z Fo)
881 k750 17.92 2.787 1.695 1.732 69 ~, 898 750 17.55 2.783 1.682 1.723 74
905 750 1'7.40 2.785 - - 72
906 750 17.38 2.783 - - 74
92 0' 750 ~ 1'7.07 2.789 1 .700 1.738 65
92 5 800 16.98 2.785 1.683 1 ~.728 72 i' 81 5 200- 14.65 2.785 - - 72 a
869, ~1 500 13.51 2.786 - - 69
898 497 12.93 2.781 1.673 1.712 78
903 400 12.86 2.786 - - 69
92 9 500 12.38 2.782 1.682 1 .721 75
944 300 12.14 2.783 1 .680 1 .720 74
959 300 11.25 2: 783 1.679 1 .721 74
1 ,ooo* ? ? 2.783 - - 74
948 200 10.5 2.778 1.677 1 :71 7 80
955 250 10.3 2.781 - - 77
990 192 8.0_- 2.780 1.673 1 .710 78 1,030 200 9.3 2.779 1 .670 1.712 80,
1,035 200 9.20 2.777 - - 83
751 100 11.69 2.773. 1.652 1.691 89
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TABLE 8 (continued) -
923 200 8.08
958 300 7.46
1,016 200 6.53
1,027 200 6.36
906 200 6.71
9 34 2 oa 6.22 94 9 2100 5.97
.950 200 5.95
968 200 5.68
985 3 98 5.39
934 200 1.19
955 ,200' ' 0.99
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TABLE 8 (continued)
.1. T P d130 . X (oC) (bars) -log f02 i 0 Y (% fo)
923 200 8.08 2.772 1'.650 1.690 91
958 300 7.46 2.779 1 .659 1.698' 85 1,016 200 6.53 2 .'7 78 - - 82 1,027 200 6.36 2.775 - - 87 906 200 6.71 2.772 1.649 1.692 91
9 34 200 6.22 2.775 - - 87
949 200 5.97 2.772 1 .656 - 1.693 91 950 200 5.95 2.775 - - 87 968 200 5.66 2.771 1 .654 1.693 92 .. 985 398 5.39 2.775 i1 .658 1.696 87
934 2 00 1.19 2.772 1.642 1.680 9l.
955 200 0.99 2.770. 1 .640 1 .679 94
*Accuracy i's 0.018 i,0.918 i,0.011 i,0.6 i for a, b, c, and V, respectively; 0.003 A for d130; 0.004 for the-refractive i nd.i ces . 'A small percentage bf the monticell i te ( CaMgSi04) component has not been taken into account. When Mo is taken into account
this raises the Fo content by approximately 4%. P
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TABLE 9. CELL PARAMETERS, CELL VOLUME, AND AVERAGE REFRACTIVE INDEX OF SYNTHETIC NEPHELINES
-a -C il A n
9.988( 8) 8.376( 7) 723.7( 1 1 ) 1.535
9.984( 4) 81 370( 5) 722.6 (6) 1.536
9.996( 3) 8.366(4) 724.0( 5 ) 1.538
9.988( 3) 8.369( 2) 723.0 (4) 1.537
9.987(8) 8.379( 16) 723.7( 17) 1.540
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formulae recalculated on the basis of 16 MgA1204 (Table 10).
\ oxygens show that the analyzed Minor amounts of an opaque phase were
specimens contain significant Ca and observed in runs at fo higher than "0. 2 depart from the idealized NaAlSi04 Under the microscope it shows up as
formula. relatively lasgc (5 to 10 m) octahedra
or clusters of cuhedral grains that Spinel, Ma netite, Hematite B were identified as a magnetite solid-
Optical examination of the run products solution [ (Pe,Elg) (Fe,A1)204J. Semi-
revealed that minute (<1- um) grains of quantitative compositional data for this
an isotropic(?) phase with high refrac- phase could only be obtained from micro-
tive index (>1.75)- were present in probe analysis of a sin$le sample where
prolonged runs at low f (IQF buffer) not more than five or six measurable O2 at temperatures in excess of Mh grains were present (identified by the
stability. The same phase was also high reflectivity); this indicates that
detected in most runs yielding the it is a magnesio-aluminous magnetite.
assembla . (Cpx + 01 + Ne 2 Pit f (Mh). All X-ray reflections for magnetite
Examinatian of X-ray diffractograms solid-solution are strongly interfered
at slow scanning speeds showed one with by other phases and were not
or two ref lectionryot attributable' positively identified. At relatively
to the above .phases but consistent with high temperatures (1100 to 1200 OC, 1
a spinelss. Tfiese, however, were not barr ma'gnetite and spinel are mutually
observed in diffractograms for runs at soluble (see below) ; the resulting c f higher than NNO due-to compositional spinel was analyzed (Table .7). O2 shifts in peak positions for the major In runs at fo higher'than HM; up'to 2 phases. Cell parameters obtained for 960 OC at 200 bars, hematite replaces
two samples from the (311) reflec.tion It occurs in minor @aunts as fine Mtss.
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TABLE 10. CELL PARAMETER OF SPINEL IN THE ANHYDROUS CONDENSED ASSEMBLAGE CORRESPONDING TO THE MAG%ESIOHASTI NGSITE BULK COMPOSITION
-a Buffer i
IQF 8.1 19( 7) Bomb buffer 8.11 2( 7)
8.080*
*From the data of Robie and Waldbaum (1968).
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hexagonal platelets, typically ruby to be similar to that produced at 1 atm
red in transmitted light. It was ob- except for a small amount of H20. This
served only macroscopically, giving a would be consistent with the slightly
brownish-pink color to the run products lower refractive index of the, former
or under the microscope. (Table 11). Quartz-normative melts
coexisting with Nh for the particular Me 1t (Glass ) bulk composition studied have been pro-
A' few runs at pressure-temperature duced at fluid p.ressures higher than the
conditions where the pressure vessel investigated range where liquids are
deforms significantly after a few hours richer in H20 (Ernst, 1972; Mcrrill and
(-1050 OC, 200 bars; -1020 OC, 500 Wyllie, 1975; Cawthorn, 1976).
bars) showed that the solidus for the Mh Other Synthetic Phases bulk composition was attained. A
greenish glass containing a'large At low ? (IQF buffer) and tempera- " O2 amount of minute crystallites-unidenti- tures higher than Mh stability, a melilite / ss fied quench(?) phases-was observed under appears on initial. synthetjs from an
the microscope. The soli&&gas also oxide mix. It decreases in amount, as
reached in 1-atm experiments where a seen on X-ray diffractograms, and dis-
brown glass, free of inclusions, was appears(?) when the charge is rerun at
produced (see Table 7). Normative the same conditions for longer durations.
composition of t>e melt based on the Pfelilite was thus interpreted as a
subtraction of Ne and Di gives the metastable phase. Optically, it is repre-
following: NaAlSi04, 50.3%; CaMgSi206, sented by very fine (-1 to 2 pm) elongated
34.9%; CaA12Si208, 9.6%; and Fe304, 5.1%. prisms with parallel extinction and
Although the composition of the melt refractive indices bracketed by 1.660 and
produced under hydrothermal conditions 1.640. The melilite foped must be a
could not be obtained, it is presumed solid-solution of akermanite (Ca2MgSi207)
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TABLE 11. REFRACTIVE INDEX OF SILICATE GLASSES (MELTS) PRODUCED FOR MAGNES IOHASTI NGS I TE BULK COMPOSITION
Sample Buffer n*
~ HA9- 7A1 FMQ 1.568
HA9-23A FMQ 1.574 HIH6-1 U2 - 1.576 H10-4D MMO 1 .570
HA8-1783 HM 1.570
HA8-14B HM I .575
HA9-25B HM 1.572
PT24-9A A tmo s p he re 1.584
PT12-11 Atmoi phe re 1.580 *Accuracy i s 20.005.
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and soda-melilite (NaCaA1Si2O7) with appearance of amphibole from mixtures
minor gehlenite (Ca2AlSiA1O7) (Shairer of M3 + breakdown phases + H20 (10:90)
and others, 1967). on the high-temperature side of the re-
At low temperatures (500 to 600 OC, action curve; and (2) the growth of
'\ 2,000 to 3,000 bars), layer silicates amphibole from a similar mixture on the
are produced on initial synthesis from low-temperature side of the reaction
an oxide mix. The assemblage consists curve. .As was noted previously, comp1.ete
of Na-phlogopitess Cpx minor conversion of breakdown products H 0 + + + 2 * plagioclase +- amphibole. The presence to Mh at temperatures close to but be1.ow
of Na-phlogopite was identified in the the reactioncurve has not been achieved.
X-ray diffractograms as the Na-phlogopitc This phenomenon has been observed repcated- -. hydrates I and I1 (Carman, 1974). The ly in amphibole-stability studies (Boyd,
Mh bulk composition can be recalculated 1959; Emst, 1960; Gilbert, 1966) and is
almost completely in terms of thought to arise from a competitive
Ka-phlogopite + Cpx. Prolonged runs (2 growth rate (and/or nucleation) effect
mo) under the same conditibrps,r&ith 10:90 superimposed on the possible multivariant
and 90:lO mixtu'ies of Na-phl + Cpx + Mh nature of the reaction(s); at low
demonstrated slow but increased growth pressure (100 to 1,000 bars) the growth
of tih, so that the bphlogopite pro- rate (andlor nucleation) for anhydrous
duced is evidently metastable. The very phases is larger than it is for the
fine grain size and clustering pre- amphibole. A compromise, essentially
vented optical measurements. kinetic in nature, between the deh$dration
of metastable amphibole supplying material EXPERIfiIENTN, RESULTS for the growth of anhydrous phases and
Experimental brackets of magnesio- the growth of amphibole in its stability
hastingsite'dehydratibn have been field from the unstable breakdown
located on the basis of (1) the dis- products is eventually reached. This
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phenomenon precludes any direct observa- re-rence to a particular fo buffer a&'d ~2 tion of 'the nature of the equilibri- de- then summarized in an f -T isobaric O2 hydration process, which may occur along P section. - fluid a pressure-temperature zone and would Diagrams 'fluid -T thus'involve amphibole solid solution.
(Most investigated reactions are of vari- Figures 1 through 4 present the
ance higher than two. The experimental dehydration and beginning of melting
curves represent,the appearance or curves for Plh bulk composition + excess
disappearance of a phase, and are fluid under oxygen fugacities defined -. . projections from compositional space by the iron-quartz-fayalite, fayalite-
into theP-T-J"' volume.) Optical and magnetite-quartz, manganosite-hausmanite, O2 X-ray evidence, however, 'suggest that and hematite-mgnetite buffer assemblages
solid solution in magnesiohasthgsite respectively. Critical run data for
in this system is not large. The these buffers are summarized in Tables
breakdown curve locations .thus apply 12-15, respectively (brackets enclose
for the disappearance of an *amphibole very phases interpreted as metastable) ; addi-
close to. the Mh composition. tional experiments are presented in Semet
Considering the high temperatures (1972, Tables 13 to 21). The diagrams
involved, z'eaction rates for amphibole are of almost deceiving simplicity for a
growth or decomposition are f-elatively system containing seven oxide components,
sluggish, and run times of 50 or more and present obvious similarities to
' than 100 hours at fluid pressures phase relations for pargasite (Boyd,
lower than 500 bars are required to 1959), ferropargasite (Gilbert, 1966) , and
ascertain unambiguously the direction hastingsite (Thomas, 1977, 1981). I
of reaction. At oxygen fugacities defined by the
The results of hydrothermal experi- IQF .buffer (Fig. l), Mh stability is
ments will be presented, sequentially with restricted to relatively low temperaJure.
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800 - - Magnesiohastingsite
c0 + n .-c 600- - 2 3 u) cn 2 a .- 400- - -3 LL
-
0 I I I I I 1 1 600 650 700 750 800 850. 900 950
Figure 1. Pfluid - 7' diagram for Mh + fluid bulk composition with fo defined by the 2 iron-quartz-fnyalite buffer. Squares represent reversal runs where reactants represent
a synthetic mineral mixture. Circles indicate runs where an oxide mix was used as the
starting material; diamonds..runs where pure synthetic Mh was used; and triangles, runs
where the assemblage Cpx + 01 + Ne + Sp f Mt was used. Filled symbols indicate growth
of the anhydrous solid assemblage; open symbols indicate Elh growth; half-filled symbols
indicate that the phases Plh + Cpx + 01 + Ne + Sp + F persisted.
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TABLE 12. RUN DATA FOR MAGNESIOHASTINGSITE BULK COMPOSITION t EXCESS H20 WITH OXYGEN FUGACITY DEFINED BY THE I RON -QUARTZ- FAYAL I TE B U FFE R
P T m Starti n g Ma teria 1 (bars) ("C) (hr) Results
CpxtOl +Ne+Sp+Mh 300 660 5 72 Mh+( Cpx+Ol +NetSp)
Cpx+Ol +Ne+SptMh 300 681 577 MhtCpxtOl tNetSp
Cpx+Ol +Ne+Sp+Mh 300 71 0 603 Cpx+Ol tNe+Sp+( Mh)
Cpx+Ol +Ne+Sp+Mh 300 740 488 Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp 500 647 137 Mh+( Cpx+Ol +Ne+Sp)
Mh 500 746 137 Mh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 500 775 259 Mh+( Cpx+Ol +Ne+Sp)
CpxtOl +Ne+SptMh 500 78 3 120, MhtCpxtOl tNetSp
CpxtOl tNetSptMh 500 800 170 Cpx+Ql +Ne+Sp
CpxtOl +Ne+Sp+Mh 600 81 5 103 Mh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 600 838 77 Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp+Mel 700 934 144 Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp+Mh 750 801 7 Mh+( Cpx+Ol tNetSp)
CpxtOl +Ne+SptMh 750 81 7 7 Mh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 750 852 52 Mh+( Cpx+Ol +Ne+Sp)
CpxtOl +Ne+Sp+Mh 750 865 149 Mh+( CpxtOl +NetSp)
CpxtOl +NetSp+Mh' 750 881 %o, 'CpxtOl tNe+Sp
Cpx+Ol +Ne+Sp+Mh 750 891 100 Cpx+Ol +Ne+Sp Cpx+Ol +Ne+Sp+Mel 750 898 48 Cpx+Ol +Ne+Sp Cpx+Ol +Ne+Sp+Mel 750 906 72 Cpx+Ol +Ne+Sp Cpx+Ol +Ne+Sp+Mh 1,000 925 76 Mh+( Cpx+Ol +Ne+Sp) C p x +O 1+N e +S p +M h 1,000 939 20 Mh+( CpxtOl +Ne+Sp) Cpx+Ol +Ne+Sp+Mh 1,000 949 18 Cpx+Ol +Ne+Sp MhtCpxtOl +Ne+SptMd 2,000 801 120 Mh CpxtOl tNetSp 2,000 84 9 13 Mh+( Cpx+Ol +Ne+Sp)
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P T Time Starting Materia1 (bats) ("C) (hr) Results
Cpx+Ol tNe+Sp 2 00 658 263 CpxtOl +Ne+Sp
CpxtOl +Ne+SptMt 200 660 263 Cpx+Ol +Ne+Sp
Cpx+Ol +Ple+Sp+Mh 200 780 142 Mh+( Cpx+Ol tNetSp)
Cpx+Ol +Ne+SptMh 2 00 80 9 94 Nh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+SptMh 2 00 82 0 139 Cpx+Ol +Ne+Sp
Mh+Cpx+Ol +Ne+Sp 300 81 2 21'6 Mh+( Cpx+Ol +Ne+Sp)
Cpx+Ol tNe+Sp+Mh 300 81 8 32 9 Mh+( Cpx+Ol tNe+Sp) '
Cpx+Ol +Ne+Sp+Mh 300 822 212 Mh+( Cpx+Ol +Ne+Sp)
Mh 300 840 21 6 Mh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+Sp 300 840 100 Mh+( Cpx+Ol +Ne+Sp)
Mh+Cpx+Ol +Ne+Sp 300 842 32 9 Mh+( Cpx+Ol +Ne+Sp)
CpxtOl +Ne+Sp+Mh 300 85 3 124 Mh+Cpx+Ol +Ne+Sp
Mh 300 857 21 2 Mh+Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp+MttMh 300 86 3 31 2 Cpx+Ol +Ne+Sp
CpxtOl +Ne+Sp+Mt 300 902 36 Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp 300 987 12 Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp 300 1,005 16 Cpx+Ol +Ne_+Sp
CpxtOl +Ne+Sp 300 1,020 14 Cpx+Ol +Ne+Sp+L
Cpx+Ol +Ne+Sp+Mt 350 893 95 Cpx+Ol tNe+Sp
CpxtOl +Ne+Sp+Mh 400 879 84 Mh+( Cpx+Ol+NetSp)
Mh+Cpx+Ol +Ne+Sp 400 886 54 Mh+( CpxtOl +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 400 890 140 Mh t(Cpx+Ol tNetSp)
Cpx+Ol +Ne+Sp+Mh 400 903 152 Cpx+Ol +Ne+Sp+(Mh)
Cpx+Ol +Ne+Sp+Mh 400 91 0 72 Cp?+Ol +Ne+Sp
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P T Time Sta rti ng Ma teria1 (bars) ("C) (hr) Results
Cpx+Ol +Ne+Sp 500 856 335 Mh + ( Cpx+O 1+Ne +S p )
Mh+Cpx+Ol +Ne+Sp 500 85 9 142 Mh+( Cpx+Ol +Ne+Sp)
Mh+Cpx+Ol +Ne+Sp 500 866 104 Mh+( Cpx+Ol +Ne+Sp)
Mh+Cpx+Ol +Ne+Sp 500 870 142 M h+ ( C p x+O 1 +Ne f S p )
Mh+Cpx+Ol +Ne+Sp 500 876 104 Mh+( Cpx+Ol +Ke+Sp)
Mh 500 876' 335 Mh+( Cpx+Ol +Ne+Sp)
M h + C p x + 0 1'+N e +S p 500 882 112 Mh+( Cpx+Ol +Ke+Sp)
Mh+Cpx+Ol +Ne+Sp 500 889 96 Mhk( Cpx+Ol +Ke+Sp )
Mh+Cpx+Ol +Ne+Sp 500 896 148 Mn+( Cpx+Ol +Ne+Sp)
I1h 500 904 330 Nh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 51 5 91 9 94 Mh+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Nesp+Mh 500 924 140 Mb+( Cpx+Ol +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 500 92 6 90 Mh+Cpx+Ol +NetSp
Mh 500 929 31 2 Mh+Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp+Mh 520 92 9 51 Mh+Cpx+Ol +Ne+Sp
Cpx+Ol +Ne+Sp+Mh 500 932 96 CpxtOl +Ne+Sp+( Mh)
Mh 500 936 137 CpxtOl +Ne+Sp+Mh
Mh 497 938 111 Cpx+Ol +Ne+Sp+( Mh)
Cpx+Ol +Ne+Sp 500 975 6 CpxtOl +Ne+Sp
Cpx+Ol +Ne+Sp 500 987 6 Cpx+Ol,+Ne+Sp
Cpx+Ol +Ne+Sp+Mt 500 998 7 Cpx+Ol +Ne+Sp+L
Cpx+Ol +Ne+Sp+Mt 500 1,026 4 Cpx+Ol +Ne+Sp+L
Cpx+Ol+Ne+Sp+Mh 610 94 7 37 Mht( CpxtOl +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 61 0 957 37 cpx+ol +tie+Sp
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P' T . Time Starti n g Ma teria1 (bars) (,OC) (hr) Results
Cpx+Ol +Ne+Sp 750 901 3 32 Mh+( Cpx+Ol +Ne+Sp)
Mh-eCpx+Ol +Ne+Sp 750 902 2 06 Yh
Mh 750 '911 3 32 ' Mh
MhtCpxtOl +Ne+Sp 750 91 3 103 Mh+( Cpx+Ol +NetSp)
Mh+Cpx+Ol tNe+S'p 750. . 91 9 118 r.lh+(-Cpx+Ol +Ne+Sp-)
Mh+Cpx+Ol +Ne+Sp 750 92 6 4a Mh+( Cpx+Ol tNe+Sp)
Mh+Cpx+Ol +Ne+Sp 750 936 74 Mht( cpx+ol +Ne+Sp j
Cpx+Ol +Ne+Sp+Mh 750 979- 20 Mh+( CpxtOI +Ne+Sp)
Cpx+Ol +Ne+Sp+Mh 750 989 20 Cpx+Ol +Ne+Sp+L+qMh
Cpx+Ol +Ne+Sp+Mh 750 1,000 17 Cpx+Ol +Ne+SptLtqMh
Cpx+Ol +Ne+Sp+Mh 750 1,000 1 Cpx+Ol +Ne+Sp+qMh
Mh 1,000 866 164. Mh
Mh 1,000 872 306 Mh
Mh: 1,000 ,887 473 Mh
Cpx+Ol +Ne+Sp+Mh . 1,000 1,002 2.5 Mh + ( C p x +O 1+Net S p+ L.) Cpx+Ol +Ne+Sp+Mh 1,000 1,020 2.5 Cpx+Ol TNe+Sp+L+qMh
Cpx+Ol +NetSptMh 1,000 1,029 2.5 C px +01 +Ne +S p+i+$l h
Cpx+ol +Ne+SF 2,000 a03 120 Mh
Mh 2,000 830 67 Mh
Mh 2,000 a40 144 Mh
Na-Phl +Cpx+Mh 2,000 500 1,028 Mh+( Na-Phl+Cpx)
Mh+Na-Phl +Cpx 2,000 500 1 ,028 Mh
G1 ass 2,000 ' 505 1,582 Mh+Na-Phl +Cpx
Na-Phl +Cpx+Mh 3,000 505 867 Mh+( Na-Phl +Cpx)
Mh+Na-Phl +Cpx 3,000 505 86 7 Mh
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TABLE 14. RUN DATA FOR MAGNESIOHASTINGSITE BULK cori OSITION t EXCESS H20 WITH OXYGEN FUGACITY DEFINED BY THa MANGANOSITE- HAUStlANNITE BUFFER
\ ? T Ti me Starting Clateri a1 (bars) (.C) (tir) Results
CpxtOl tNetSptMt 100 660 250 CpxtOl tNe+Sp+Mt
CpxtOl tNetSp 100 661 250 C p x to1 t Ne +S p tMt
CpxtOl tNetSp 100 691 21 0 C pxto1 tNe+S p t Mt
CpxtOl tNetSp 100 82 8 196 ( CpxtOl tNetSptMt)tMh
CpxtOl tNetSp 100 858 144 C px to1 t Ne t S p +!It.
C px t 0 1 t N e t S p tM t t M h 200 880 94 M h t ( C p x to1Wet S p t:1 t ) \ C p x t 41 t N e t S p t M t t Mh 200 894 94 tlht( CpxtOl tNetSptMt)
C p x to1 t N e tSp tM t tM h 200 91 5 95 M h +'( C p x t 01 t Ne t S p tYt )
CpxtOl tNetSptMttt1h 2 00 91 7 100 Mht( CpxtOl +Ne+Sp+Mt)
C p x tO1 t N e tSp tMt t tjh 200 92 3 96 CpxtOl +Ne+Sp+Mt+( Mh)
C p x t 01 t Ne t S p t Mt tr1 h 200 92 7 103 c p x t 01 t N e t S;, t
CpxtOl tNetSp 200 1,016 10 C;lx+Ol tNe+Sp+tlt
CpxtOl tNetS p 200 1,027 37 C px +O 1 +N e tSp +M t
CpxtOl tNetSp 200 1,040 18 Cpx to1tNe tSptMt t L
Cp x t 01 t N e tSp tMt 249 94 9 60 C px t 0 1 tNe tSp tM t.
C px +O 1+N e tSp t M t t M h 300 94 6 32 CpxtDl tNetSptMttMh
C p xt01t Ne tSp t M t t Mh 300 950 50 C p x t 01 t Ne tSp tMt t t.1h
C px +O 1 +N e tSp +t4 t +M h 300 952 150 Mht( CpxtOl +Ne+Sp+Mt)
CpxtOl +Ne+Sp+Mt+Mh 300 958 188 Mht( CpxtOl tIVetSptMt)
CpxtOl tNetSptMttPlh 300 959 10 CpxtOl +Ne+Sp+Mt+Mh
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TABLE 14
P T Time S t a rf.i n g Mate r ia 1 (bars) (OC) (hr)' Res ults
CpxtOl tNetSptMttMh 31 5 963 166 C pxt 0 1 tNe tSp tMt tM h
9 CpxtOl tNetSptMttMh 300 963 15 CpxtOl tNetSptMttMh
CpxtOl +Ne+Sp+Mt+Mh 300 965 188 CpxtOl +Ne+Sp+Mt+Mh , J CpxtOl tNetSptMttMh 31 5 968 .1 66 Cpxtol +Ne+Sp+Mt
CpxtOl tNe+Sp+Mt+Mh 300 968 172 CpxtOl +Ne+Sp+Mt
Mh 300, 978 376 C p x t 0 1 t Ne t S pt M t t t r Mh CpxtOl tNetSptMt 350 933 98 CpxtOl tNetSptMtttrMh
Cpx+Ol tNe+Sp+Mt+Nh 400 987 80 Mht ( CpxtOl tNetSptAt) '.. CpxtOl +NetSp+Mt+Mh 40q, 999 87 CpxtOl +Ne+Sp+Mt
CpxtOl +Ne+Sp+Mt ' 400 1,010 27 CpxtOl tNetSptMttLtqMh?
Mh 500 944 104 , ', Mh
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TABLE 15. RUN DATA FOR MAGNESIOHASTINGSITE BULK COMPOSITION + EXCESS a20 WITH OXYGEN FUGACITY DEFINED BY THE HEMATITE-MAGNETITE BUFFER -
P T Time Starting Material (bars) (oC) (hr) Results
CpxtOl +Ne+Sp+Mt+Mh 201) 830 190 Mht( CpxtOl tNetSptMt )
Cpx+Ol +Ne+S p+Mt tMh 195 870 89 Mh+( Cpx+Ol +Ne+Sp+Mt)
Cpx+Ol +Ne+SptMt+Mh 2 00 879 3a Cpx+Ol +Ne+Sp+Mt+Mh
Cpx+Ol +Ne+Sp+Mt+Mh 200 90 6 91 M h + ( C p x to1 tNe tSp +M t )
Cpx+Ol +Nets p+Mt+Mh 200 91 0 190 Mh+( Cpx+Ol +Ne+Sp+Mt )
C px +01 t N@+ S p t Mt +I1h 200 92 4 190 Mh+( CpxtOl +Ne+Sp+Nt )
Cpx+Ol +NetSp+Mt tMh 200 , 934 192 CpxtOl +Ne+Sp+Mt
CpxtOl +Ne+Sp 200 939 .24 C px +O 1 +Ne + S p +M t
CpxtOl +Ne+Sp 200 949 180 C px to1 +NetS p t M t
C px + 01 t Ne +S p + Ne tM t 200 1,005 28 Cpx+Ol +ktSp+Mt
C px +O 1 t Ne + S p +N e +M t 200 1,025 17 Cpx+Ol +Ne+Sp+Mt
Cpx+Ol +ide+Sp+Mt 200 1,037 17 CpxtOl +NetSp+tlt+L’
CpxtOl +Ne+Sp 200 1,048 35 C p x to1 +Ne +S p +M t +L Iz CpxtOl +Ne+Sp+Mt 300 92 5 140 CpxtOl tNe+Sp+Mt+trMh
C px+O 1+Net S p tMt +M h 300 92 9 100. Mh+( Cpx+Ol +Ne+Sp+Mt ) CpxtOl tNekp+Mt+Mh 300 946 100 blh+( Cpx+Ol +Ne+SptMt )
CpxtOl +Ne tSptM t +Mh 300 951 32 Cpx+Ol +NetSp+Mt+Mh
Cpx+Ol +Ne+Sp+Mt+Mh 300 960 109 Mh+( CpxtOl +Ne+Sp+Mt )
CpxtOl +Ne+Sp+Mt+Mh 300 962 80 Mh t ( CpxtOl +Ne+Sp+Mt )
CpxtOl +Ne+Sp+Mt+MK 300 965 125 Cpx+Ol +Ne+Sp+Mt+(Mh)?
Cpx+Ol +‘Ne+Sp+Mt+Mh 295 971 i25 Cpx+Ol tNetSp+Mt
Cpx+Ol +NetSp+Mt+tlh 300 980 86 CpxtOl tNetSptMt
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P m Time Starting Material (bars) (O'C) (hr) Results
Cpx+Ol +Ne+Sp+Mt+Mh 350 964 124 Mh+( Cpx+Ol +Ne+Sp+Mt)
Cpx+Ol +Ne+Sp 400 954 190 Mh+( Cpx+Ol +Ne+Sp+Mt )
.l$x+oJ +Ne +S p +M t + tlh 398 985 35 Mh+( Cpx+Ol tNe+SptMt)
Cpx+Ol +k+Sp+Mt+Mh 400 994 140 tlh+( Cpx+Ol +Ne+Sp+Mt )
Cpx+Ol +Ne+Sp+Mt+Mh 400 1,002 140 Cpx+Ol +Ne+Sp+Mt
Cp x+O1 +Ne +S p+M t +M h 398 1,007 35 Cpx+Ol +Ne+Sp+Mt+L Cpx+Ol +Ne+Sp+Mt+Mh 400 1,021 70 Cpx+Ol +Ne+SptMt+L . 3 C p x +O 1t Ne +S p t Mt +M h 500 1 ,0017 18 Mh+( Cpx+Ol +Ne+Sp+Mt)
Cpx+Ol +Ne+SptMt+Mh 500 1,018. 18 Mh+( Cpx+Ol +Ne+Sp+Mt+L)
Cpx+Ol +Ne+Sp+Mt+Mh 500 1,025 14 C p x + 0 1 +N e +S p +M t + L Cpx+Ol +Ne+Sp+Mt+Mh 500 1,033 18 Cpx+Ol +Ne+Sp+Mt+L+qMh
Cpx+Ol +NetSp+Mt+rlh 500 1,044 10 Cpx+Ol +Ne+Sp+Mt+LtqMh
C p x + 0 1 +N e +S p +M t +I1h 600 1,030 5 Mh+ ( Cpx+Ol +Ne+SptMt+trL)
Cpx+Ol +Ne+Sp+Mt+Mh 600 1,042 5 Cpx+Ol +Ne+SptMt+L+tr( Mh)
Cp x+O1 +Ne+S p+Mt +Mh 600 1,048 5 Cpx+Ol +Ne+Sp+Mt+L+( Mh)
G1 ass 1,000 901 72 Mh
Mh 2 ,oBQ 84 0 46 Mh
Na-Phl +Cpx+MR 2,000 51)2 1,023 Mh+( Na-PhA +Cpx)
Mh+Na-Phl +Cpx 2,000 503 1,024 Mh
Na-Phl +Cpx+Mh 3,000 505 867 Mh+( Na-Phl +Cpx)
Mh+Na-Phl +Cpx 3,000 505 867 Mh
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/2_Part_II/274/3429774/i0016-7606-92-2-274.pdf by guest on 24 September 2021 Decomposition of the amphibole melilitess.
occurs at 785 OC and 500 bars, 875 OC Restricted Nh stability at low fo 2 and' 750 bars, and 943. OC and 1,000 results from the reaction to produce
bars fluid pressure. The anhydrous the more favorable reduced &n!-.ydrous
p,hascs of corresponding bulk composi- assemblage, axid from t.lw decreased H 0 2 t-ion are all solid solutions. They in- fugacity as well. Great difficulty was
clude an aluminous clinopyroxene experienced in obtaining valid run data
containing significant jadeite and heden- with this buffer owing to rapid depletion
bergite components, a forsteritic of the buffeeing capacity through H2
olivine, a nepheline and a spinel- diffusion out of the cohtainer. Runs
hercynite. Although the exact chemical at temperatures higher than 900 0 C could
composition of the phases have not not be prolonged more than approximately
been obtained for this buffer, measure- five hours before the buffer had to be
ments referred to above are consistent renewed.
with the following approximations: At relatively higher f (FMQ buffer), O2 2+ magnesiohastingsite dehydration occurs Cpx = Na.05Ca, 95P*g.90Fe.05A1.05- 2+ Si1.87A1.1306; O1 = (Mg.73Fe.27)2Si04; at higher temperatures (Fig. 2). It is
Ne = Na SiA104; and Sp = Mg stable at 860 OC at 300 bars fluid .85Ca. 05 .63 2+ Fe A1204. The fluid composition in pressure, 900 OC at 500 bars, and 1012 .37 equilibrium with this assemblage is ? at 1,000 bars. At pressures higher
H2-rich "80 to 70 mol percent from low than 740 bars (982 0 C),% Mh melts in-
to high pressure) and may explain congruently to aluminous clinopyroxene,
greatly diminished reaction rates (H20 olivine, nepheline, spinel, H20-saturated
does not form a polar fluid and thus does melt,. and fluid. Chemical compositions
not participate in a dissolution/ of the anhydrous phasqs obtained by the
precipitation mechanism); this accounts same tiethod as above are as follows: 2+ 3- for the occurrence of metastable Cpx = Na .04Ca. 88"1$. 86Fe. 04Fe. 07*l. 1lSil.83
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Fa yo Ii t e - quartz - magnetite ' IOOO~P~I I I I
800 - Magnesiohastingsite !!? !!? 0 + n Fluid cpx,+ 01
0 V +L+Fluid
Cpx+OI + Ne+ Sp+Fluid
0 I I 1 I 750 800 8 50 900 950 1000 1050 I100 Temperature in "C
Figure 2. - 1' diagram f& Mh + fluid composition with f defined by the 'fluid O2 fayalite-magnetite-qyartz buffer. Symbols follow usage of Figure 1, except as fqllows:
triangles indicate runs where the assemblage Cpx +Wl + Ne + Sp 5 Mt was analyzed;
inverted triangles indicate runs where melt das present after reaction.
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2+ A1,1706; 01 = (Mg decreased slightly, although this may .75Fe. 25)2si04 (+ minor montioellite); Ne = Na.85 not be significant. 3+ CaaO5SiA1O4; and Sp = big At still higher oxygen fugacities 1.00(Fe.25A1. 75'2' (may contain signficane amounts of Ht or (MMO and ,HM buffers, Figs. 3, 4), the
Hc). The melt composition has not stability of Mh is expanded to still
..been obtained but is thought to be higher temperatures. Breakdown occurs-
similar to the analysis of glass pre- at ,.930 ?C and 200 bars fluid pressure,
seqted in Table 7, although it is 968 OC and 300 bars, 998 OC and 400
prmbably richer in Xa20 and Si02 and bars for the HY buffer, and 5 to 9 OC
poorer in Fe. As is expected, a small below those values for the "0 buffer.
but finite change to higher din/& , The minimum melting curve is reached
exists in the slope of the amphibole on the amphibole reaction curve at
dehydration curve at the pressure- 1002 OC, 420 bars and 1003 'C, 430
temperature "invariant point" (piercing . bars for the PC.10 and ID1 buffers, respec-
point of a P, T, fo line) where Mh, tively. At subsolidus temperatures, blh 2 Cpx, 01, Ne, Sp, L, and I: coexist. decomposes to an assemblage similar to
In Figure? 2 through 4, the begin- that encountered at lower f but where O2 ning of melting curve is extrapolated magnetite appears as an additional SS toward the temperature found in l-atm stable phase. The electron microprobe 3+ experiments. Small amounts of Fe analysis of Cpx, 01, and Ne presented
are present in the breakdown products. in Table 7 show that the'aegirine
as indicated by the appearance of qn component in clinopyroxene increases
aegirine(?) component in the clino- significantly at higher f, , and that 2 pyroxene and a magnesioferrite-or, small amounts of Fe3+ enter the
more probably, magnetite-component nepheline, whereas the fayalite
in the spinel. . On the average, the content of the olivine decreases with
fayalite content of olivine has increasing f, . The sharp increase 2
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0
Magnesiol lasting site + +,,Cpx+Ol n Fluid .-g400tc . +Ne+Sp +Mt + L \ +Fluid \ 0
+Mt+Fluid /, Q Q 'A 4 / / -I
I I I I O 740 ' . sbo Temperature in "C
Figure 3. - T diagram for 'Mh + fluid bulk composition with f defined.by the
'fluid O2 ~ manganosite-hausmannite buffer. Symbols follow usage of Figures 1 and 2 except that Mt has
joined the high-temperature assemblages. Half-filled symbols indicate that the assemblage
Mh + Cpx + 01 + Ne + Sp + Mt + F persisted. Runs at 100 bars do not represent equilibrium
with respect to amphibole.
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2+ of 'the Fe3+/(Fe3+ + Fe ) ratio in the lower than observed by Boyd (the
anhydrous phases is also well illus- anhydrous assemblage corresponding to
trated by Mgssbauer spectra (Semet, 'pargasite is aluminous diopside +
1972). forsterite + nepheline + plagioclase +
Because of equipment limitations, spinel). On the other hand, the addition
the reaction relationship between of A1-Di and/or Pc to the Ne-Fo join
nepheline and melt could not be investi- could produce Foss + Ness, thereby
gated in detail. The disappearance of causing melting to occur at higher
Ne as a solidus phase is expected to temperature. It is also possible that
occ'ur near or at the highest tempera- the formation of quench Ne has been
tures investigated. For the pargasite missed or misinterpreted by either
bulk.cornposition, the Ne-Pc-H20 melting author. Several runs in the present
curve is attained at -1060 OC at 300 investigation were performed at tempera-
bars fluid pressure, -1045 'OC at 500 tures in excess 'of the curves mentioned,
bars, and 1030 OC at 600 bars ac- yet do not show the disappearance of
cording to Boyd (1959). Carman (19741, ngpheline, nor textures (Carman, 19x4)
however, reported that the melting attributable to quench nepheline.. This
curve for forsterite + nepheline dilemma is not resolved and must await
bulk compositions [reaction Fo + Ne + further experimentation in the simpler
H20 = Sp + L + (?)I is situated at system Fo-Di-Ne-H20.
990 OC and 1,000 bars fluid pressure, Isobaric fo -T Diagrams 975 OC and 2,000 bars. Extrapolation 2
of the latter to lower pressu_re The phase relations for the magnesio-
toward the eutectic Ne-Fo-Si02 bastingsite bulk composition + H20 are - (Shairer and Yoder, 1961) indicates summarized in the isobaric (Ptotal that melting for the pargasite bulk = 400 bars) f, , T section of 'f 'f iuid 2 composition may occur at temperatures Figure 5. Points A, B, and D represent
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600
500 -
Magnesiohastingsite v) 400 + 0 - n Fluid .-t 2 300 - ln 2 .-U 200- 0
- Temperature in "C
Figure 4. - T diagram for M~I+ fluid bulk composition with.fo defined by the 'fluid 2 hematite-magnetite buffer. Symbols follow usage of Figure, 1.
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the experimentally determined or interpo- experimentally. Runs on "bomb buffer"
lated temperatures for the disappearance in the field of anhydrous condensed
of Mh according to the reaction Mh = phases supplemented by low-pressure
Cpx + 01 + Ne + Sp + F for oxygen experiments on t%e FMQ and MMO buffers
fugacities defined by the IQF, WM, FMQ, where Mh fails to nucleate indicate
and "bomb buffer,",respectively. In the that the surface lies approximgtely
type of diagram used [log fo versus midway between the NNO and MMO P-T-fo 2 2 l/T (K)1, those four points closely surfaces and does not cross them in the
approximate a straight line. L and M range' covered. This intersection is
delimit the reaction Mh = Cpx + 01 + represented by line EF and its extension
Ne + Sp + Mt + F for the HM and NMO Curve GKJFI is the intersection of the
buffers. The comparatively steep slope minimum melting surface for the assemblage inditates that relative to the first Cpx + 01 + Ne + Sp +- Mt + - F with the reaction, less oxygen is involved. log fo -1/T section. The steep slope 2 Ex'trapolated by a draight line ap- at intermediate to high fo indicates 2, proximation, the two reaction curves that very little oxygen is involved in
intersect at point E where Mh, Cpx, 01, 1 this reaction. The negative slope'below i Ne, Sp, Mt, and F coexist. E is thus ' the FMQ buffer, however, reflects
the piercing point of an isobaric di- decreased H20 fugacity at low fo - 2- variant equilibrium in an eight- Inasmuch as H20 is the principal solute
component system. in the silicate melt, this decrease causes
The intersection of the surface for melting to occur at higher temperature.
Point G (not determined experimentally) the appearance of Mtss in the anhydrous .- assemblage of Mh bulk composition (or is the piercing point of the divariant.
its metastable extension in the amphi- equflibrium involving the eight phases
bole field) with the isobaric log fo -T Mh, Cpx, 01, Ne, Sp, Mt, L, and F. 2 section has also been located The area bound by EFJKGLM (labeled I in
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Temperature in "C
- = 400 bars) logs f -T diagram for.M.1 + fluid bulk Figure 5. Isobaric (Pfluid 'total O2 composition [the abscissa is linear for 1/T (K)]. Field I is for the assemblage Cpx +
01 + Ne + Sp + Mt +,F. Field I1 is the high fo area where Cpx + 01 + Ne + Sp + L + F 2 is the stabel assemblage. See text for discussion.
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for the assemblage Cp;+ 01 + Ne + Sp . equilibrium involves the reaction of
+ Pit + F. Phase relations at f higher Cpx + 01 + Ne + Sp to form melt, with O2 than the hematite-magnetite buffer have the ultimate disappearance of nepheliness not been studied in detail because of - (all phases are complex solutions). The
experimental difficulties (melting of beginning of melting occurs at 1069 5
the buffer,. capsule). The anhydrous phase 15 OC and was detected by fritting of assemblage consists of Cpx + 01 + Ne + the charge. At higher temperature, Ness Sp + Hm at temperatures lower than.955 0C reacts with Cpx + 01 + Sp + L to form ,-. (pressures lower than -250 bars, above the assemblage Cpx + 01 + Sp + L at
which Pih is the stable phase) and Cpx + 1160 OC.. This temperature is in
01 + Ne + Sp +_. L at higher temperatures accordance with 'melting relationship in
(field I1 in Fie 5). the system Di-Alc-Ne (Onuma and Yagi,
At lower fluid, pressure, the stability 1967). However, no melilite was observed
fields for the assemblages Cpx + 01 + in the run products. Analyses of co- '
Ne + Sp + O2 = Cpx + 01 + Ne + Sp 'k Elt existing phases in the latter assemblage
is apparently little affected. suggest that the reaction may be written
At higher'fluid pressure, both Mh + in simplified form as Cpx + Ne = L + Sp.
F and the assemblages containing melt This reaction is also consistent with.
expand to higher and lower temperature, the normative composition of L (see .
respectively. This causes incongruent melt analysis in Table 7).
melting of Mh to occur at successively Thermal Stability Limit of, lower fo . 3 L Magnesiohastingsite + Quartz (5 One-Atmosphere Phase Relations Reconnaissance runs were made on the
A few experiments were carried out bulk composition Mtl + excess silica in an
at atmospheric pressure in an effort to attempt,to delimit the stability of Mh +
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Qtz. As is the case for pargasite eight phases. All curves (except GEE,
(Boyd, 1959) and both fcrropargasite Fig. 5) are projections of hypcr-
(Gilbert, 1966) and hastingsite surfacesJbounding faceted tri- .or
(Thomas, 1977, 1981) at low oxidation quadri-variant phase volumes in multi-
D states, pure Mh is not stable in the dimensional P-T-fo composition space. 2 presence of quartz; Mh + 3Qtz charges Variance analysis requires that composi-
recrystallize instead of Hble + Pc + tional changes of one or more phases
4 Qtz or Cpx + Opx 4- Pc + Qtz. Judging occur along any particular experimentally I from its X-ray powder pattern and from determined curve. However, there seem
its optical properties, the synthetic to be only minor compositional changes
hornblende is an aluminous actinolite along the intersections of the.reaction
(blue-green amphibole) substantially hyper-surfaces with the P-T-fO surfaces 2 richer in Si02 than Mh. Dehydration defined,by the cxygen buffer reactions.
, of the amphibole produced occurs 'at a ' Chemical compositions of all of the
0 temperature lower than 855 C at 1,000 phases involved are only known semi-
bars fluid pressure; this illustrates quantitatively, and thermodynamic data
the profound influence of silica activi- for those phases are imprecise or lacking.
ty on the equilibria studied. In addition, this study is concerned with
a fixed bulk composition, exclusive of THEmlODYNAMIC CONSIDERATIONS 112, so that many equilibria which have
The phases encountered in this study a bearing on magnesiohastingsite
lie in. the eight-component system stability and phase relations have not
, , Na20-Ca0-Mg0-Fe0-Fe 230 -A1203-Si02-H20 . been investigated. Therefore, a
The curves of Figures 1 th'rough 5 cannot systematic 'approach to generalizing
therefore represent divariant eqduilibria the findings of this scgdy (Zen, 1966)
(if no undetected phase were present) is not possible. Thermodynamic.. data because in most cases they do not involve may nevertheless be obtained for certain
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reactions and pfovide a test for in- this will affect equilibrium constant
ternal consistency of the experiments. computations more substantially at
values below those defined by the Thermochemical Calculations NNO buffer. Equilibrium constants for four Reaction Mh = Cpx + 01 + Ne + Sp + critical reactions have been computed i F; “fo defined b’y the IQF Buffer. Using 2- from thermochemical data for the 0-H the phase compositions as previously-’ and the fo buffer systems (see section 2 determined, the following equation may on control of oxygen fugacity), and the be written (molar volumes as computed derived or determined phase composi- from the data of Semet, 1973, and this tions and molar Volumes. The thermo- paper) : dynamic formalism employed is well
known (Ofville and Greenwood, 1965: NaCa2Wg4Fe3+Si6A12022 (OH) =
Anderson, 1970, 1977; Zen, 1973). (V = 274.17 cm3/mol)
Stoichiometric coefficients for the 24- 2*20 Na.07Ca.87Ng.93Fe.llA1.02Sil.73A1.2706 reactions envisaged were derived by (65.87 cm 3/rnol)* successive approximations alternatively + 1.25 (Mgs73Fe.27)2Si04 varying the coefficients and the phase .(44.49 cm 3 moll compositions within analytical uncer-
tainties. The coefficients are + 0.95 Na.85Ca.05SiA104 3 estimated to be accurate to approxi- (54.47 cm /moll
mately 20% of stated values. The I
largest probable errok will affect
the O2 stoichiometric coefficients
because of error accumulation. Owing + 1.00 H2D + 0.25 O2 (1)
to the very low fo values involved and The equilibrium constant for this 2 to the relatively larger coefficients, reaction is:
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2.20. 1.25. 0. 95.a0. 21. 0.25 buffer), and of acpx aO1 %e sp aH20'"o2 pure O2 at T and ref- % :,.I erence pressure, or respectively. log KMh = 2.20 log aCpx + 1.25 log aO1 IQF Provided that phase compositions do not + 0.95 log %,e + 0.21 log a SP change appreciably with temperature and
I pressure along the fo -F-T surface defined
fH20 2 +log?- + log by 'the buffer (the data of Table 8 seem H2° to warrant such an approximation), then: - 1% % 9
V(P log a -PO) = 2.303RT where a the 'activity solid is of the solid at the where V is the molar volume of the .- par,ticular composition, solid phase, P and Po are the pressure P, and T, considered and the reference pressure,
0 c are the fugacities of . respectively, and T is'the temperature ' H20' fH20 H20 in the fluid phase considered in K. With the standard
at the particular P state defined as the pure gaseous and T (as somputed species and the solid phases with
for the IQF buffer), compositions as in equation 1, all at
and the fugacitiy of one bar and T, the equilibrium constant
pure H20 at the same may be rewritten as:
T but at reference AVs(P - Po) pressure, respectively, log = log 'fH + 0.25 KMh 2.203RT + IQF 2 are the fugacities of fo2, c2 I O2 in the fluid indeed, we have = LQ = 1.00. f"H2@ J02 phase at P and T . (Lomputed for the IQF .I Here AVs is the'change in volume for the
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computed for the 1,000, 750, 600, 500, point has usually been omitted from
and 300 bars fluid pressure curve similar treatments in the literature.
bracketing runs. From the relation, Reaction IWZ Cpx + OZ +We f Sp' +
AGO = mo - TASO = - RT .In K, F; f, defiiied by the FNQ Bzrffer?. The 2 converting to base 10 logarithms and magnesiohastingsite breakdown reaction
rearranging for this buffer has 3een estimated by
mo the same nethods as dbove: log K = +- as" - 2.303RT 2.30313 '
where, AGO, and lylo, and ASo are the NaCa2Mg4Fe3+Si6Al2OzZ(OH) =
Gibbs free energy, enthalpy, and entropy . (V = 275.17 cm3 mol)
of reaction, respectively, for the 2+ substances in their standard states. 2'10 Na.04Ca.88Mg.86Fe.04Fe?~7A1.11si1.83 LHo and ASo do not usually vary ap- (66.16 cm3/mo1)
preciably for relatively small ranges *l.17'6 of temperature. Therefore, a linear
fit of' log Km versus 1/T yields IQF .5 myQF and ASo (Table 16). Note that IQF if AHo and ASo do vary over the range , considered and the data still are + 0.99 Na,gOCa.05SiA104, (54.50 cm 3 /mol) compatible with a linear relation for
log K - 1/T , then AHo and ASo no 3+ longer truly represent the standard + 30 Mgl.OO(Fe. 25*l. 75'2'4 (40.2 cm 3/mol, estimated) enthalpy and entropy of reaction. I? this case, the temperature dependence + 1.00 H20 + 0.17 O2 T cp of So = .T dT is combined with AHo, Here T=l
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TABLE 16. STANDARD ENTHALPY. AND ENTROPY OF REACTION FOR MAGNESIOHASTINGSITE DEHYDRATION
AH' A so Buffer (kcal /mol ) (cal/K/mol )
IQF 54 (2) 370)
FMQ 40.3( 2) 36.'0(1 )
HM 30.3(4) 35.5(2)
Note: The'standard states are defined as one bar pressure, T of reaction, the solid phases as they occur in the breakdown equa- tions (1), (Z), and (3) as indicated in the text, and the pure fluid components H20 and 0, at one bar and T. Due to compositional changes in the breakdown products, the defi- nition is not equivalent for the three buf- fers. This accounts for the discprepancy in the values of AH0 and ASo of reaction.
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2.10 .1.25 0.99 .0,.30 0.171 3+ a 2'38 Na.20Ca.82Mg.59Fe.22A1.18si1.80A1.2006 cpx 01 %e. sp 9120 a~2 (66.40 cm'/mol) %%.I. 2+ + 1.12 Mg1.94Fe.03Ca.02Si04 and for the same standard state as for (43.95 cr?3/mo1) the ' IQF:
AVs(P - Po)' Mh log%Q = 2.303 RT + 1% fH20+
0.17 log fo . . 2 (41.0 cm3/mol; Robie and Waldbaurn, Note that aCpx, aO1, aNe, a are not 1968) SP directly comparable to the' activities + 0.17 E1gFe2O4 of the same pahses for the IQF buffer be- (53.0 cm 3 /mol; Robie and Waldbaum, - cause of compositional differences; if 1968)
thermodynamic data on the complex solid- + 0.03 Fe3Q4 + 1.00 H20 + 0.008\0, - . (3) solutions were available, a more complete
treatment would .be possible. A linear Following the above procedure, one obtains:
regression of log GQversus I/T (K) AVs(P - Po) ' log = . 2.303RT + log fH 0 + for the 750, 600, 500, 400, 300, and $ 2
200 bars fluid pressure brackets yields 0.008 log fo . 2' AHklQand @So (Table 16). FMQ
Reaction Mh 7 Cpx +' 02 + Ne + Sp + Using the 500, 400, 300, and 200 bars c Mt f F; fo Defined by' the HM Buffe2~. fluid 'pressure bracket gives the AH& 2 0 Based on the chemical analyses of the and ASm of Table 16.
appropriate phases, the following 'The AHo values obtained in Table 16
equation may be written: are of the same order of magnitude as
those observed for other dehydration NaCa,2Mg4Fe3+si6A12022 (OH) = equilibria ' (Emst, 1960). In reactions (V = 275.17 cm3/mol)
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is very close to the latter value ” T’ and fO 2 o’f the run may, however, be = 34.42 entropy unit [e.u.] ; different. Because changes in this (%p Robie and Waldbaum, 1968). The standard ratio occur rapidly (Ernst and Wai,
entropy of reaction for the other buf- 1970; Semet, 1973), this latter point
fers is higher due to evolution of cannot be confirmed. When thermodynamic
additional 02. calculations of the breakdown equilibria
In the preceding equilibria (equa- are carried out with a-varying Fe3+/ 2+ tions l, 2, and 3), magnesiohastingsite (Fe3+ +.Fe ) ratio for Mh according
was assumed to have the ideal composi- to the equation
NaCa’2Plg4Feq~xFe~Si6A12022(OH) 2H1-x = era1 substitutions can alter the anhydrous phases composition of amphibole coexisting + LF H2e with anhydrous phases along the 4a+x-1 -T breakdown surface. 02-’f luid +4 O2 As noted earlier, metal/cation
ratios do not seem to vary signifi- where a is the O2 coefficlent of equations
cantly. On the other hand, the actual 1, 2, or 3, the values of and ASo
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are different from those obtainebh following assumptions. (1) Fe3+ dis-
Table 16 and are not consistent with tribution in Mh (Fe2+ is negligible
a dehydration and an oxidation-reducr for the HM buffer) considered in
tion reaction (AHo = 17.2 kcal/m%,l, , equation 3 is the same as for sample niQ ASo = -3.34 e.u.1. Although this CT-A2 (Semet, 197.3, Table 11); (2) Only FMQ does not constitute a proof that the configurational entropy difference
Mh coexisting stably with anhydrous for the M(1), M(2), and M(3) sites
phases + fluid is close to the ideal between partly disordered and "ordered"
end-member composition, it does show MH [one ~e~+cat-ion for two ~(2)sites
that the former interpretation is on the average] is considered; an more consistent with the experimental - order-disorder relationship in the data. breakdown phases is neglected; and '. The value of configurational entropy (3) the ordering process does not
due to an order-disorder. relationship involve an enthalpy change. 5 in Mh (and the anhydrous breakdown With these assumptions, an uppek
phase$) is another factor-that influ- limit for breakdown temperature differ-
ences the stability reections. Because ence will be obtained. Yhe ef- of
synthetic phases do not, in general, adding S(disordered Mh) - S(ordered MH) =
2 0 crystallize with ordeked cation 2.08 e.u to ASm (Table 12) is to'lower
distributions, the partitioning of the breakdown temperature computed from
Fe and Mg among the' M(1), M(2), and 2This. figure is obtained by considering ideal mixing of Mg and Fe3+ at each amphi- M(3) sites in synthetic Mh (see Semet, bole site. For ordered Mh, M(l) and M(3) are filled only with Mg, and M(2) is ran- 1973) may not reflect.true equilibrium. domly filled with 0.5 Fe9 and 0.5 Mg, whereas in disordered Mh (CT-A2, Semet, To test the magnitude of this effect, 1973, Table ll), M(1) and M(3) are randomly occupied by 0.15 Fe3+ and 0.85 Mg, and M(2) the breakdown temperature at a fluid with 0.27 Fe3+ and 0.73 Mg. Then, = .-R(Cx 1nxi)=-N3(0.15 In 0.15 + 0.84 In 0.85) pressure of 500 bars and fo defined 2 +2(0.27 In 0.27 + 0.73 In 0.73) - 2(0.50 In 0.50 + 0.50 In by the HM buffer was calculated with the 0.50)] = 2.08 e.u.
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the relation and kcfor the reaction amount to 15.1
AH~~)+-.-- ASo - kcal and 5.5 e.u., respectively. Direct log = - 2.303RT 2.303R calculation of this equilibrium from
Ho - H& = 10.0 kcal, and So - S& = FMQ FMQ 0.5 cal/OK (Table 12) resuits in a curve
located between the MMO and NNO buffers + 0.08 log fo (at 500 bars 2 for temperatures between 675 OC, where fluid pressure) its metastable extension intersects the
from 1021 to 908 OC, a difference of MMO buffer curve, and 1020 OC, where
I 113 Co! Synthetic Mh crystals with it would cross the NNO buffer curve. The
different non-equilibrium Fe-Mg distribu- agrezment between the observed curve
tions may thus coexist isobarically with for the reaction and the curve computed
the same anhydrous phases over a signi- from amphibole breakdown equilibria is
ficant temperature interval. This may, a positive test of the interal con-
in part, explain why experiments at sistency of the experimental data.
temperatures well into the field of Equilibria Involving a Silicate Melt first amphibole appearance do not show
recrystallization to pure Mh in ex- Partial melting curves for the
periments of limited duration. assemblage Cpx + 01 + Ne + Sp 2 Mt + F,
Reaction Cpx + OZ + Ne + Sp + 02 = and magnesiohastingsite'incongruent Cpx + 02 + Ne + Sp + Mt. The same melting occur at the highest temperatures
formal treatment may also be applied to and pressures attainable with the
experimental data for the equilibrium equipment and technique used. According-
between Mt absent and Mt present ly, experimental data on these
anhydrous assemblages (curve EF in Fig. equilibria'are incomplete and do not
5). Using all experimental data bearing include the phase relations where Ne is
on the location of this equilibrium, AHo no longer a stable phase in this system.
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Therefore, thermodynamic calculations presented in Table 7; because of oxida-
similar to those carried out above have tion-reduction reactions involving Fe.
not been attempted. Qualitative deduc- Comparison of Pargasite and tions may nevertheless be derived from -7 Magnesiohastingsite Stability experimwts at atmospheric pressure
combined with experimental studies on Figure 6 presents the maximum stability
systems having similar chemical composi- curves for Mh with oxygen fugacities de-
tions (Boyd, 1959; Yoder and Tilley, fined by the IQF, FMQ, and HM buffer
1962; Hamilton and Anderson, 1967; and pargasite (Boyd, 1959). Direct
Holloway, 1971; Holloway and Burnham, comparison of the two end members is'
1972; Eggler, 1972; Carman, 1974; possible at high fo (HM buffer), where 2 Merrill and Wyllie, 1975; Cawthorn, little or no oxygen is involved in Mh
1976). Incongruent melting is presumed breakdown reaction. As was noted by Ernst
to be of the type Mh = Cpx + 01 + Ne + (1968, p. 104; Thomas, 1977) for other Sp +- Mt + L +- F and occurs at a minimum amphibole .end members, Fe3+ * A: replace-
-fluid pressure of 420 bars (HM buffer). ment in octahedral coordination increases
Minor amounts of a peralkaline (?) the amphibole stability field, to hydrouS melt- is prdduced in this reac- slighbly higher temperatures. This tion. At temperatures exceeding probably reflects the greater ease with
approximately 1050 OC for a fluid which A1 assumes tetrahedral coordination
pressure of 1 kb, the Ne-Cpx in the anhydrous phases when compared
"eutectic" will be reached, thus pro- to Fe%. At lower fo , however, the 2 ducing significant amounts of liquid. increased entropy of reaction for Mh
0 Oxygen fugacity (in addition to due to the release of additional O2
temperature and pressure) influences 'causes breakdown to occur at lower tempera-
the latter equilibrium, assuming a melt ture than pargasite.
composition similar to the glass analysis
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800 -
2 0' -0 .-C 600- c
Temperature in "C
Figure 6. Magnesiohastingsite and pargasite dehydration curves: (1) Mh, IQF buffer;
(2) Mh, FMQ buffer; (3) MI, tC.i Buffer; (4) pargasite (Boyd, 1959).
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and type of amphiPoless produced. ’’ GEOLOGIC APPLICATIONS Our purpose, here, is to estimate the
General Statement relative sign5ficance of these variables.
More than 65% of some 100 analyzed Because the problem involves consideration
amphiboles (Leake, 1968) closely of a large number of factors and experi-
approximating the formula N&a2 (Mg,Fe2+I4 mental data on amphibole-producing systems
(Fe3+,Al)Si6A120;2(0H)2, where Mg/(Mg + at high pressures is scanty,, only a cursory
Fe2+) > 0.50 (hereafter called Mh for treatment is possible.
sjmpliflcty), occur in igneous focks Metamorphic Rocks and Skarns of ultramafic or mafic affinity. The
remaining are evenly distributed among The relativge scarcity of amphiboles
amphibolitcs, skarns, and maryes. This approximating MI comp-osition in meta-
distribution is thought to reflect morphic rocks and skarns stems from the
fairly accurately the relative abundance rarity of appropriate rock bulk compo-
of members of tiis series in natural sitions much more than the attendance of
rocks. Such hornblendes coexist with physical conditions exceeding the amphibole
a variety of mineral species with rela- pressure-temperature stability field.
tively large ranges of chemical vari- Amphibolites and greenschists are in
ability and attest to their wide pres- general poorer in alkalis and richer in
sure-temperature stability field. The,’ silica and iron than Mh. The parageneses
presence of free quartz is never re- att-,nolite+ epidote + chlorite +
ported, however, in agreement with the plagioclase, and hornblende intermediate
experimental observatin of Boyd (19591, between actinolites and pargasites +
Gilbert (1-966), and this study. Rock plagioclase + pyroxene (s) are typical.
bulk composition, pressure, temperature, Metamorphosed ultramafic bodies do
and the composition of the fluid phase provide bulk compositions suitable for
(if present) all influence the presence Mh occurrence, provided H20 fugacity was
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high enough; the other variables (P, oxide) has been dehydrated over a
T, and f ) are of much less importance, distance of 200 to 1,000 m around the O2 inasmuch as the Mh stability field peridotite and now consists of hypersthene
enconppasses the near totality of their + augite + plagioclase granulites. Ap-
common ranges in metamorphic rocks. parently a net transfer of H20 from the
A hot, presumably anhydrous, es- hydrous amphibolite to the "dry"
sentially solid, relatively high- peridotite occurred, resulting in the
pressure spinel peridotite body in- observed assemblages. At the elevated
truded metamorphic rocks of the temperatures of peridotite emplacement,
amphibolite facies in the Lizard area magnesiohastingsite ..was Ltabilized in
of Cornwall (Green, 1964). Concentric the plutonic margin, whereas less ,re-
zonation in the'pluton resulted from fractory hastingsite amphiboles were
contact and contamination effects. destabilized in the adjacent country
The core of the body represents the rock.
11primary" anhydrous peridotite as- Typical occurrences of Mh in skarns
semblage, Al-rich Cpx + 01 + Opx + Sp, and marbles have been described by Larseri
in which only minor recrystallization (1942) and Suwa and Tatsumi (1969). In
occurred. An outer zone is regarded to the first case, an amphibole close to the
have partially readjusted to the lower Plh end member (sample U-1236 of this
pressures prevai1ing"after ihtrtision study) crystallized in veins and pockets
and consists now of Al-poor Cpx + Pc + of an "altered uncompahgrite. It The
Opx +- 01 +.Sp. Toward the contact with paragenesis is complex and consists of
the amphibolite country rock, hydration diopsidic augite, phloeopite, magnesio-
of the ultramafic assemblage to hastingsite, andradite, calcite,
Mh serpentinized 01 + + minor Opx and+- veswianite, apatite, orthoclase, Sp prevails. Concurrently, the intruded perovskite, and titaniferous magnetite.
amphibolite (richer-in silica and iron The assemblage, occurring in a
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~ ~~
carbonatite aureole (Temple and Grogan, metamorphic rocks of appropriate bulk
\ 1965), indicates recrystallization u-t composition (Cpx, -01, fNe, f Sp, -+ Opx
at fairly high temperatures in the nonnative), attending H20 fugacities . .. presence of volatiles. Andradite and must have been'very low-most likely on
titaniferous magnetite suggest a .* the order of 50 to 100 bars in most
fairly oxidizing environment. In cases.
addition, textural &elations between Igxieous Rocks augite and calcite point to a fluid
phase relatively impoverished in H20 Compos!tion variatian of pargasite-
but rich in C02. The occurrence of Flh hastingsite reflecting the 'parental ig-
in such an environment is in accord neous rock compbsition was recognized .
with the elevated maximum stability by Billings (1928); He deduced that, .. temperature even where H20 fugacity is with increasing differentiation of a mafic L. low (approximately 850 OC at ,200 bars magma accompinyiiig falling temperatures, .. . F'MQ buffer). . 'H20; amphibole composition shifted progres- Pargasite (with significant sively from pargasite-magnesiohastingsitess
magnesiohastingsite and ferropargasite typical of the gabbrodiorite clan, to .. solid solution) occurs with scapolite, hastings$te-richss occurring in syenites
phlogopite, apatite, and dolomite in and granites. His observations have been
marbles of the Skallen district, largely verified by subsequpt petro-
Antarctica (Suwa and Tatsumi, J969), logical studies (see Frish, 1970). Figure
metamorphosed in the pyroxene granu- 7 presents a summary of analytical data
lite facies. Here again the paragene- for 65 pargasite-hastingsitess occurring
sis suggests high temperatures and a in crustal igneods rocks (Leake, 1968).
fluid phase where H20 may not have been Large composition'variations clearly
a major component. It is concluded that exist. Hastingsitic and Agnesiohasting-
when M3 is absent from high-temperature sitic amphiboles are typical of andesites
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0 0 .8C e $ r" e + 0 C' a z :70 4 0 + - 00 0
vCS 2 .60 0 \ Pargasite 0' Magnesiohastingsite .r" 0
50 0 0 0
.40 0 Ferropargasite Hastingsite { 1 .30
Figure 7. Composition of natural amphibolesglosely approximating Mh and occurring in
igneous rccks. Analyses are from Leake (1968); nomenclature is after'Ernst (1968). Open
circles are analyses from dioritic and andesitic rocks; filled circles represent amphiboles
.from gabbroic and basaltic rocks.
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.. I + diorites, whereas more magnesian magnesiohastingsite series are stable
pargasitess occurs in basalts and at hypersolidus temperatures fo? gabbros. This is thought to result magnesian Cpx + 01+ Ne + Sp f Pc +Mt
primarily from rock bulk composition normative compositiocs. The relations 3+ effects. Large Fe (Mh component) between amphibole, anhydrous phases, and
concentrations are relatively rare, re- silicate melt do not vary considerably
~ -. flecting the fact that plutonic and for-wide ranges of pressure-temperature
hypabyssal igneous oxygen fugacities conditions even at values of P much H2° rarely exceed those of the NNO buffer lower than total pressure (Holloway and i . curve (see Fudali?, 1965; Carmichael Burnham, 1972; Eggler, 1972; Holloway,
and Nicholls, 1967; Anderson, 1968; 1973; Holloway and Ford, 1975; see also
Lewis, 1970) ; Fe2+-rich amphiboles Yoder and Tilley, 4962; Green and therefore are favored. Of course, in Ringwood, 1967a, 1967b) and probably also- many basaltic and*andesitic rocks, . , apply at pressures and temperatures
Mh- (or rkaersutite-) rich amphiboles thought to exist in the uppermki mantle
show extensive secondary resportion (Gilbert, 1969). Curves for the dis-
due to dehydration and oxidation (see appearance of amphibole, however, 'develop L Deer and other&, 1963a). The latter negative pressure-temperature slopes __ , effects undoubtedly occur at a late where pyropic garnet becomes a stable
stage in the eruptive process. phase (the high temperature condensed
Recently, attention has been assemblage also becomes the low-volume
focused on amphiboles occurring in assemblage at high pressure dde to I rocks of upper-mantle affinities and increasing solubility of,H20 in the
their possible role in calc-alkaline melt). According to Gilbert (19691,
magma generation (Oxburgh, 1964).. pargasite sensu stgcto is unstable I7 - > 25 kb at 900 OC, Boyt&(1959) and this study have shown at 'H20 'total 0 ! 0 that amphiboles of the pargasite- Higher stability temperatures may
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nevertheless obtain when P < Occurrences of MI (and titaniferous H20 'total 0 (Eggler, 1972). A 'pressure maximum varieties syh as kaersutite) in lherzolites
of 20 to 30 kb is evident for various and pther peridotite nodules from basaltic
hornblende compositions (Lambert and rocks'have been desczibed in a number of
Wyllie, 1968, 1970, 1972; Essene and papers (Flason, 1966; Melson and others,
others, 1970; Mysen and Boettcher, 1967; Kuno, 1967; Best, 1970, 1974, 1975;
1975a, 1975b; Allen and others, 1975; Binns and others, 1924; Conqugrg, 1970, 1971,
Allen aqoettcher, 1978). 1977; V?me, 1970, Bdey and others,
J Enrichment of the hornblendes in Ti and 1971; Green and others, 1974; Wilkinson,
F may, however, sigpificantly eleva-te 1975; Merrill and Wyllie, 1975).
'. their upper thermal stability Qanges. Chemical compositions have been plotted I Holloway and Ford (1975) and Merrill in Figure 8. The diagram is somewhat
and Wyllie (1975) have reported expcri- misleading inaamuch as some of the
mental data a;t high pressures that plotted amphiboles have cdmpositions
suggest that the breakdown temperatures slightly displaced from the pargasite-
0 at 25 kb may be raised by 150 to 200 C hastingsite-magnesiohastingsite plane. , '. over that of pure end-member hydroxyl- Except for variations in Ti, the
pargasite (Gilbert, 1969) at the same amphiboles have a rather narrow composi-
pressure. Such experiments dkmonstrate, tional range, presumably reflecting
that Ti- and F- bearing pargasite- a common origin. The parageneses-are
magnesiohastingsite solid solutions may similar, being Cpx + 01 + Sp -f Pc -f Opx
participate in crysral fractionation -f. garnef. Although a few of the
of magmas in the deep crust and' afnphiboles described may not be primary
uppermost mantle, and that refractory (Wilshire and Trask, 1971, give an
hornblendes may be staSle residual example of a similar occurrence where
phases during the fractional fusion the ho rnb1 end c .un doub t e d 1y postdate s
of the upper mantle. crystallization of the primary peridotfte
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10 .20 30 .40 .50 .60 .70 .80 .90 . 100 I ooo I I I 1 I I I I a..
0 0 0 90 - e , - e. 4
0 >a 80 - 8 -
ob O8 r" 0 0 + 0 70 - - + c5+ N v a Q) 60 - - LL \ -g Pa rgasite Magnesiohastingsite 50
40 - -
Fer r op a rga s it e Hastingsite
30
Figure 8. Composition of amphiboles occurring a5 primary minerals in peridotite
nodules from basaltic rocks and in rocks of the lherzolite clan. Compare with Figure 7.
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assemblage), there is substantial Thomas. We thank the above insti'tutions
evidence that others crystallized and'individuals for their help.
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WSCRIPT RECEIVED BY THE SOCIETY
DECEMBER 26, 1979
REVISED ElANlJSCRIPT RECEIVED JUNE 6, 1980
MANUSCRIPT ACCEPTED JULY 28, 1980
', ~
1 Printed' in U.S.A.
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