Almandine-Rich Garnets from Payalite-Perrosilite- Quartz-Bearing Ferrosyenites in the Prakasam District of Andhra Pradesh, India

The fayalite-bearing ferrosyenites in the Prakasam district of Andhra Pradesh are distinctly unique as they strangely contain not only the ferrosilitic orthopyroxene but also Iate aIrnandine-rich garnet, the two minerals which are totally alien to similar rocks occurring elsewhere in the world. Symplectic, vermicular, granular and coronal garnet (with intergrowth of quartz) occurs in insignificant amounts with sporadic distribution around ore, orthopyroxene, clinopyroxene, hornblende and fayalitic olivine when they are in contact with feldspar (occasionally with intervening and incomplete rim of quartz), All the garnet-forming reactions might have resulted due to falling temperature. High and variable silica activity, local variations in f₀₂ and rate of cooling may explain many of the observed textural features.

The electron microprobe analyses of almandine-rich garnets suggest two independent substitutions have taken place, one involving Ca and Mn, and the other involving (Ca+Mn) and Fe. The minor chemical variations in garnet are significantly controlled by its habit. Textural features, and inter- and intra-granular compositional variations in garnet even within a single specimen reflect its disequilibrium formation.