An experimental study of glaucophanic amphiboles in the system Na2O-MgO-Al2O3-SiO2-SiF4(NMSF); some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures

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Authors:Welch, Mark D.; Graham, Colin M.
Author Affiliations:Primary:
Edinburgh Univ., Dep. Geol. Geophys., Edinburgh, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 111(2), p.248-259. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1992
Note:In English. 34 refs.; illus. incl. 5 tables
Summary:The phase relations of glaucophanic amphiboles were studied at 18- 31 kbar/680-950 degrees C in the system, using the bulk composition of fluor-glaucophane, Na2Mg3Al2Si8O22F2. Previous studies of glaucophane in the water-bearing system (NMASH) have been hampered by problems of fine grain size, and consequently good compositional data are lacking. Fluor-amphiboles, on the other hand, generally have much higher thermal stabilities than their hydrous counterparts. By using the F-analogue system NMASF, amphibole crystals sufficiently coarse for microprobe analysis have been obtained. Furthermore, NMASH amphibole phase relations are directly analogous to those of the NMASF system because SiF4 fills the role of H2O as the fluid species. High-P NMASF amphibole parageneses are comparable to those obtained for NMASH amphiboles under similar P- T conditions, except that the NMASF solidus was not encountered. In the P-T range of the NMASF experiments, fluor-glaucophane is unstable relative to glaucophane-nyboite-Mg-magnesio-katophorite amphiboles. Variations of P-T in synthetic fluor-amphibole composition with P and T are discussed. The most glaucophanic amphiboles analysed have aGp = 0.50-0.60 and coexist with jadeite and coesite at 30 kbar/800 degrees C. Amphiboles become increasingly nyboitic with decreasing P through the NaAlSi-1 exchange, which is the principal variation observed. The most nyboitic amphiboles have aNy = 0.65-0.70 and coexist with fluor-sodium-phlogopite and quartz at 21-34 kbar/800-850 degrees C. At 800 degrees C amphiboles are essentially glaucophane-nyboite solid solutions. At 850 degrees C there is some minor displacement along MgMgSi-1, but Mg-magnesio-katophorite activities are very low. Activities of eight other NMASF amphibole end-members are < 0.001, except for eckermannite activity which varies from 0.01 to 0.11. The results indicate that: 1) synthetic amphiboles mimic the stoichiometries observed in blueschist amphiboles; 2) synthetic studies should be relevant to petrologically important high-P parageneses and reactions involving glaucophane amphiboles, sodic pyroxenes, albite and talc; 3) the high-P stability limit of fluor-glaucophane lies at P higher than those reached in this study (31 kbar), and 4) in natural systems an approach to glaucophane stoichiometry should be favoured by high water activities as well as high P. [P.Br.]
Subjects:Alkali metals; Alkaline earth metals; Aluminum; Amphibole group; Chain silicates; Clinoamphibole; Correlation; Experimental studies; Fluorides; Glaucophane; Halides; High pressure; High temperature; Magnesium; Metals; Oxides; Phase equilibria; Pressure; Silicates; Silicon; Sodium; Stability; Temperature
Abstract Numbers:93M/589
Record ID:1993040341
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe
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