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Thermo-compression of pyrope-grossular garnet solid solutions; non-linear compositional dependence

Wei Du, Simon Martin Clark and David Walker
Thermo-compression of pyrope-grossular garnet solid solutions; non-linear compositional dependence
American Mineralogist (January 2015) 100 (1): 215-222


Unit-cell parameters of a series of synthetic garnets with the pyrope, grossular, and four intermediate compositions were measured up to about 900 K and to 10 GPa using synchrotron X-ray powder diffraction. Coefficients of thermal expansion of pyrope-grossular garnets are in the range 2.10-2.74 X 10 (super -5) K (super -1) and uniformly increase with temperature. Values for the two end-members pyrope and grossular are identical within experimental error 2.74 + or - 0.05 X 10 (super -5) K (super -1) and 2.73 + or - 0.01 X 10 (super -5) K (super -1) , respectively. Coefficients of thermal expansion for intermediate compositions are smaller than those of end-members and are not linearly dependent on composition. Bulk modulus of grossular is K (sub 0) = 164.3(1) GPa (with K (sub 0) ' the pressure derivative of the bulk modulus fixed to 5.92) and bulk modulus of pyrope is K 0 = 169.2(2) GPa (with K (sub 0) ' fixed to 4.4) using a third-order Birch-Murnaghan equation of state, which are consistent with previously reported values. The bulk moduli of garnets of intermediate composition are between approximately 155 and DF0 GPa, smaller than those of the end-members no matter which K (sub 0) ' is chosen. The compositional dependence of bulk modulus resembles the compositional dependence of thermal expansion. Intermediate garnets on this binary have large positive excess volume, which makes them more compressible. We find that excess volumes in the pyrope-grossular series remain relatively large even at high pressure ( approximately 6 GPa) and temperature ( approximately 800 K), supporting the observation of crystal exsolution on this garnet join. The curiously "W"-shaped compositional variation of thermal expansion and bulk modulus is anti-correlated with the compositional dependence of microstrain documented in our companion paper (Du et al. in preparation) on the excess volumes in this series of garnets. Minimum thermal expansions and bulk moduli go with maximum microstrains.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 1
Title: Thermo-compression of pyrope-grossular garnet solid solutions; non-linear compositional dependence
Affiliation: Lamont-Doherty Earth Observatory, Palisades, NY, United States
Pages: 215-222
Published: 201501
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 43
Accession Number: 2015-007714
Categories: Mineralogy of silicatesGeophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
Secondary Affiliation: Macquarie University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201504
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