Syntheses, volume, and structural changes of garnets in the pyrope- grossular join: implications for stability and mixing properties

Eleven garnet compositions in the pyrope-grossular join were synthesized from glass starting materials by a combination of hydrothermal synthesis and recycling of most of the products at ~40 kbar, 1400°C, in graphite capsules. The lattice parameters of the synthetic garnets have been determined with a precision of 0.0002-0.0004 Å. The results show a slightly positive excess volume of mixing, which is asymmetric toward the grossular end, and can be described by a subregular or Margules formulation with W^V _CaMg = 0.36 ± 0.23 and W^V _MgCa = 1.73 ± 0.3 cm³/mol (12 O atom basis). Powder X-ray diffraction (XRD) structural refinements show linear dependence of the O positional parameters and cation to O distance in dodecahedral, octahedral, and tetrahedral sites on the Ca content of garnet. The partial molar volume of grossular in pyrope-grossular and almandine-grossular joins varies smoothly and similarly between the terminal compositions and exceeds the molar volume by <1%. The partial molar volumes of pyrope and almandine change very similarly as a function of the Ca content of garnet. -from Authors