GENESIS AND OCCURRENCE OF GROUP OF GARNET GEM MINERAL

According to its features of crystal chemistry, garnet clan gem mineral can be divided into two series: calcareous and aluminous garnets. Hessonite and Tsavorite in calc-skarn for the genesis of contact metasomatism, and topazolite, domantoid as the metasomatic altered minerals of ultrabasic rock body by gasification-hydrothermal activity, are mainly the mineral species of calcareous garnet. Because of the genesis of regional metamorphism, aluminous garnet formed in crystalline schist would transform from spessartine in greenschist face to almandine in amphibolite face, or to rhodolite, or to pyrope in granulite face. As the product of a primary mineral in rock mass by magmatic crystallization, pyrope might occur as porphyritic crystal in ultrabasic intrusive rocks and extrusive rocks.
Almandine might occur in phenocryst in intermediate-acid extrusive rocks or subvolcanic rocks. Aluminous garnet can also be found in granitic pegmatite by pegmatization, and in almandine formed at the early stage of pegmatite by crystallization. Spessartine occured at the late stage of pegmatite by metasomatism. Calcareous garnet is formed at shallow depth in altered minerals of skarn or ultrabasic rocks. Garnet formed in crystalline schist belongs to aluminous garnets. Magmatic and pegmatitic aluminous garnets vary from pyrope to almandine and to spessartine with the decrease in depth. This change can be explained by different cation radiuses in the octahedral coordination site of garnet lattice. Under high pressure, cation with small coordination radius tend to enter lattice easily and form stable octahedral coordination. However, under low pressure, the octahedral coordination by cations with large coordinaton radius entering the crystal lattice is more stable than that by cations with small coordination radius.