Mineral equilibria in the MgO--SiO/sub 2/--H/sub 2/O system. II. Talc--antigorite--forsterite--anthophyllite--enstatite stability relations and some geologic implications in the system

Journal Article · Fri Apr 01 00:00:00 EST 1977 · Am. J. Sci.; (United States)

DOI:https://doi.org/10.2475/ajs.277.4.353· OSTI ID:5401268

Hemley, J J ^[1]; Montoya, J W; Shaw, D R; Luce, R W

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1. Geological Survey, Reston, VA

Mineral equilibria in the MgO--SiO/sub 2/--H/sub 2/O system were investigated, with emphasis on the stability relations of talc, antigorite, forsterite, anthophyllite, and enstatite. Thermal dehydration temperatures at 1 kb H/sub 2/O are at 514/sup 0/ +- 12/sup 0/C (Ta--Antig--Fo), 627/sup 0/ +- 7/sup 0/C (Ta--Fo--Anth), 670/sup 0/ +- 7/sup 0/C (Anth--Fo--En), 675/sup 0/ +- 7/sup 0/C (Ta--Anth--Q), 643/sup 0/ +- 8/sup 0/C (Ta--Fo--En), 690/sup 0/ +- 8/sup 0/C (Ta--En--Q), 729/sup 0/ +- 8/sup 0/C (Anth--En--Q). Derived G/sup 0//sub f/ values in kcal/gf are -928.756 +- 1.1 (Antig, based on Mg/sub 2/./sub 825/Si/sub 2/O/sub 3/ (OH)/sub 3/./sub 65/), -2711.320 +- 1.7 (Anth) and -348.955 +- 0.6 (EN). Anthophyllite has a relatively narrow temperature stability range at 1 kb, but it increases as P/sub H/sub 2/O/ increases. The Ta--Fo--En and Ta--En--Q reactions are low-pressure changes, metastable at 1 kb. The (Q) and (Fo) invariant points are at approximately 603/sup 0/C, 500 bars, and 601/sup 0/C, 175 bars, P/sub H/sub 2/O/, respectively. Higher P/sub total/ raises and spreads the P/sub H/sub 2/O/ values of the invariant points. The log SiO/sub 2/ /sub aq/ versus 1/T diagrams utilized in this study are applied to metasomatic and isochemical processes involving this system. Aspects of contact metamorphism and metasomatism, the process of serpentinization, and compositional controls and relative stabilities of the serpentine phases are discussed. Isochemical cooling in the subsolidus at finite, though not necessarily constant, P/sub H/sub 2/O/ results in a mineral reaction series analogous to that of the classical reaction series of silicate melts. The chemical constraints revealed in the various equilibria studied help to clarify and more rigorously to define the isochemical PT relationships of the system. The geological application of activity-activity or chemical potential diagrams is also discussed, and the necessity of considering reactions that correspond to or approximate the true process involved is illustrated.

• OSTI ID: 5401268
• Journal Information: Am. J. Sci.; (United States), Vol. 277:4
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
15 GEOTHERMAL ENERGY
ENSTATITE
EQUILIBRIUM
MAGNESIUM OXIDES
MAGNESIUM SILICATES
SILICA
CHEMICAL ANALYSIS
FREE ENTHALPY
GEOCHEMISTRY
METAMORPHISM
PRESSURE DEPENDENCE
STABILITY
TEMPERATURE DEPENDENCE
THERMODYNAMIC PROPERTIES
WATER
ALKALINE EARTH METAL COMPOUNDS
CHALCOGENIDES
CHEMISTRY
ENERGY
HYDROGEN COMPOUNDS
MAGNESIUM COMPOUNDS
MINERALS
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
PYROXENES
SILICATES
SILICON COMPOUNDS
SILICON OXIDES
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