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

We have synthesized 11 garnet compositions in the pyrope-grossular join 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. Syntheses of homogeneous crystals were also successful within the compositional range Gr₅₀–Gr₈₀, which represented a conspicuous gap in earlier studies on synthetic garnets.

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 $WCaMgV=0.36±0.23$ and $WMgCaV=1.73±0.3cm3/mol$ (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, determined from our data and those of Geiger et al. (1989), varies smoothly and similarly between the terminal compositions and exceeds the molar volume by <1%, which has an effect of no more than 1.5% on the calculation of equilibrium pressure of the assemblage grossular + aluminosilicate + plagioclase + quartz. The partial molar volumes of pyrope and almandine change very similarly as a function of the Ca content of garnet. The V-X relation of the pyrope-grossular join determined in this work has been used to calculate the effect of lattice strain on the enthalpy of mixing. We have also extended the formulation of Ferreira et al. to derive a simple but general expression to account for the effect of multiatom interaction on lattice strain.