Substitution of [sup [6,4]]Al in phlogopite: High-temperature solution calorimetry, heat capacities, and thermodynamic properties of the phlogopite-eastonite join
- Princeton Univ., NJ (United States)
Enthalpies of solution of synthetic magnesium aluminum phlogopite samples K(Mg[sub 3[minus]x]Al[sub x])(Al[sub 1+x]Si[sub 3[minus]x])O[sub 10](OH)[sub 2] with 0.00 [le] X[sub East] [le] 0.92 were measured in molten 2PbO-B[sub 2]O[sub 3] at 977 K. The enthalpy of mixing for the phlogopite-eastonite join is large, asymmetric, and endothermic. The solid solution is destabilized more by [sup [6]]Mg,[sup [4]]Si substitution into eastonite than by [sup [6]]Al,[sup [4]]Al substitution into phlogopite, reflecting the increased strain in the mica structure at high [sup [6,4]]Al content that arises from increased lateral misfit between the smaller octahedral and larger tetrahedral sheets. The entropy of mixing has been calculated assuming a random distribution of Mg and Al on one octahedral site and a short-range ordered distribution of Al and Si on four tetrahedral sites. The calculated activity-composition relations predict a large region of immiscibility up to [approximately] 1,500 K that is consistent with observed limits of [sup [6,4]]Al substitution in naturally occurring Fe-poor phlogopite. The thermodynamic data for phlogopite and eastonite are assessed using the metastable reaction Phl + 3 Qtz = San + 3 En + H[sub 2]O, in which the breakdown of magnesium aluminum phlogopite also produces spinel. A standard enthalpy of formation of phlogopite from the elements at 298 K of [minus]6215.0 [plus minus] 3.5 kJ/mol is consistent with both data from high-temperature solution calorimetry and results from phase equilibria experiments. The standard enthalpy of formation of eastonite from the elements at 298 K is between [minus]6352 and [minus]6363 kJ/mol; the large uncertainty arises from estimating the heat capacity of eastonite. The P,T stability of phlogopite increases with increasing [sup [6,4]]Al content., 7 figs., 5 tabs.
- DOE Contract Number:
- FG02-85ER13437
- OSTI ID:
- 6789435
- Journal Information:
- American Mineralogist; (United States), Vol. 77:11-12; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
MINERALS
SPECIFIC HEAT
THERMODYNAMIC PROPERTIES
ALUMINIUM SILICATES
CALORIMETRY
ENTHALPY
ENTROPY
IGNEOUS ROCKS
MAGNESIUM SILICATES
MELTING
MELTING POINTS
METAMORPHIC ROCKS
MICA
SILICATES
THERMOCHEMICAL PROCESSES
ALKALINE EARTH METAL COMPOUNDS
ALUMINIUM COMPOUNDS
MAGNESIUM COMPOUNDS
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
ROCKS
SILICATE MINERALS
SILICON COMPOUNDS
TRANSITION TEMPERATURE
580000* - Geosciences