石榴石族宝石矿物的产状和成因

GENESIS AND OCCURRENCE OF GROUP OF GARNET GEM MINERAL

石榴石族宝石矿物中的钙系石榴石或由接触交代作用产于钙质夕卡岩中 ,如桂榴石、沙弗来石 ;或由气化热液对超基性岩体交代以蚀变产物出现 ,如黄榴石、翠榴石。铝系石榴石或由区域变质作用产于结晶片岩中 ,随变质程度的加深由绿片岩相中的锰铝榴石向角闪岩相中的铁铝榴石至麻粒岩相中的红榴石、镁铝榴石过渡 ;或由岩浆结晶作用作为岩体原始矿物产出 ,如镁铝榴石呈斑晶见于超基性侵入岩和喷出岩中 ,铁铝榴石呈斑晶见于中、酸性喷出岩和次火山岩中 ;或由伟晶作用而产于花岗伟晶岩内 ,其中早期以结晶作用为主的伟晶岩中为铁铝榴石 ,晚期以交代作用为主的伟晶岩中为锰铝榴石。依据其产状 ,钙系石榴石产于深度不大的条件下 ,不论是作为夕卡岩矿物还是作为超基性岩的蚀变产物 ,形成深度大于夕卡岩的结晶片岩中产出的均为铝系石榴石。岩浆成因和伟晶作用成因的铝系石榴石 ,随形成时深度由深至浅的变化 ,相应地由镁铝榴石向铁铝榴石至锰铝榴石转变。这种规律性的变化可由石榴石晶格中处于八次配位时不同阳离子的配位半径不同作出解释。因为在高压下有利于小配位半径阳离子进入晶格 ,并呈稳定的八次配位 。

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.