Abstract
The effects of Al3+–Fe3+ substitution on 10 synthesized garnet samples along the grossular–andradite binary solid solution were investigated using both powder and single-crystal X-ray diffraction. Results showed that cell volume increased with andradite content. Small negative excess volume was observed in the Al-rich samples. By measuring the bond length, polyhedral volume, octahedral distortion parameter (σ), and tetrahedral rotation angle (α), we determined that the distortion occurred on the dodecahedral site. The width of the diffraction peaks was obviously related to the composition of the solid solution. Full width at half maximum of diffraction peaks was used to calculate the microstrain, which may have a relationship with enthalpy of mixing.
Highlights
Garnet (X3 Y2 Si3 O12 ) is a significant phase in the upper mantle and transition zone because it can be stable over a broad temperature and pressure range [1,2]
We investigated the grossular–andradite binary system to examine the effects of changing octahedral cations
A few bright-colored minerals between the garnet grains in the backscattered electron (BSE) images might have been a Ca-rich quench product formed from the melt, which was always less than 1%
Summary
Garnet (X3 Y2 Si3 O12 ) is a significant phase in the upper mantle and transition zone because it can be stable over a broad temperature and pressure range [1,2]. Natural garnets always exhibit extensive substitution and exist as complex solid solutions. Previous studies on the mixing behavior of binary garnet solid solutions have mainly focused on the effects of divalent cation substitutions (Mg2+ –Fe2+ –Ca2+ ) [5,8,9,10,11,12,13]. It has been shown that the larger the size difference between the cations that mix on the X sites, the larger will be the deviation from the ideal mixing solid solution [13]. The nonlinear variation of thermal expansion, bulk modulus, and thermodynamic properties (e.g., H and S) for pyrope–grossular solid solution is considered to be related to the nonideal behavior in structure [5,14,15,16]
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