Enthalpy of mixing synthetic almandine-grossular and almandine-pyrope garnets from high-temperature solution calorimetry

Journal Article · Mon Jun 01 00:00:00 EDT 1987 · Geochim. Cosmochim. Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(87)90353-X· OSTI ID:6992568

Geiger, C A; Newton, R C; Kleppa, O J

Enthalpy of solution measurements of synthetic garnet solid solutions on the joins Fe/sub 3/Al/sub 2/Si/sub 3/O/sub 12/ (almandine)-Ca/sub 3/Al/sub 2/Si/sub 3/O/sub 12/ (grossular) and Fe/sub 3/Al/sub 2/Si/sub 3/O/sub 12/-Mg/sub 3/Al/sub 2/Si/sub 3/O/sub 12/ (pyrope) have been made in eutectic (Li, Na)BO/sub 2/ at 760/sup 0/C. Garnets were prepared by high pressure, high temperature crystallization of homogeneous glasses and mechanical mixtures of end-member glasses and characterized by X-ray diffraction, microprobe analysis and Moessbauer resonance spectroscopy. Less than one percent of the total iron is ferric. The calorimetry shows that Ca, Fe/sup 2 +/ enthalpy of mixing in garnet is virtually zero, and that Mg, Fe/sup 2 +/ mixing is substantially non-ideal, with largest positive deviations near the almandine composition. These calorimetric results are not predictable from ionic size considerations, which would suggest ideality for Mg, Fe/sup 2 +/ mixing and considerable non-ideality for Ca, Fe/sup 2 +/ mixing. They are, however, consistent with predictions based on analysis of natural garnet occurrences. Apparently, subtle effects of X-site volume and distortion in garnet solid solutions produce important energetic effects not currently predictable from crystal chemistry. The present calorimetry, in conjunction with experimental phase equilibrium work, indicates that excess entropy in both solid solutions is very small.

• Research Organization: Univ. of Chicago, IL (USA)
• OSTI ID: 6992568
• Journal Information: Geochim. Cosmochim. Acta; (United States), Journal Name: Geochim. Cosmochim. Acta; (United States) Vol. 51:6; ISSN GCACA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
CALORIMETRY
CHEMICAL COMPOSITION
CHEMISTRY
COHERENT SCATTERING
CRYSTALLIZATION
DIFFRACTION
DISPERSIONS
ENTHALPY
GARNETS
GEOCHEMISTRY
MASS SPECTROSCOPY
METAMORPHISM
MINERALS
MIXING
MIXTURES
OXYGEN COMPOUNDS
PETROGENESIS
PHASE STUDIES
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
SCATTERING
SILICATES
SILICON COMPOUNDS
SOLID SOLUTIONS
SOLUTIONS
SPECTROSCOPY
THERMODYNAMIC PROPERTIES
THERMODYNAMICS
X-RAY DIFFRACTION