花岗伟晶岩及其接触带含铍宝石矿物的成因和产状

ORIGIN AND OCCURRENCE OF BE-BEARING GEM MINERALS OF GRANITIC PEGMATITE AND THE CONTACT ZONE

伟晶岩及其接触带的主要含铍宝石矿物有祖母绿、海蓝宝石、黄色绿柱石、铯绿柱石、金绿宝石和硅铍石等。伟晶岩在其形成过程中由于物理化学体系的平衡条件──—温度、压力、组分浓度等──—不断变化而从早期结晶作用阶段发展至后期交代作用阶段，早期形成的绿柱石一般透明度差，呈很弱的色彩，难能作为宝石矿物利用。随着伟晶岩成岩过程中交代作用的发展．相继出现海蓝宝石、黄色绿柱石、铯绿柱石，形成绿柱石族宝石矿物的演化系列。当伟晶岩与其成分相差较大的围岩如超基性岩接触时，可在伟晶岩晶洞中产出祖母绿，其成分中铬元素来源于超基性岩。同时在伟晶岩接触带视去硅作用程度不同而形成金绿宝石或硅铍石。这是由于伟晶岩与围岩之间化学组分的差异而发生组分相互交换反应，影响Be，Si和Al的对比，导致不同的含铁宝石矿物在不同部位的出现。

Be-bearing Sem minerals occuring in the pegmatite and its contact zone are mainly emerald,aquamarine, heliodor, morSanite (vorobyerite), chrysoberyl and phenakite.According to equilibrium parameters of the system, temperatures, pressures and concentrationsetc, the formation of the pegmatite can be divided into two stases: the early crystallization stage and thelate metasomatism stage. Beryl formed in the early staSe is usually poor in transparency with dull colorand tends to be valueless. However, an evolutionary series of beryl minerals is developed in the order ofaquamarine, heliodor and morganite with a progress of metasomatism.Emerald might occur in the pegmatite vug when the pegmatite extrudes into a wall rock which hasgreat different composition, such as ultrabasic rocks. Chrome in emerald is derived from the ultrabasicrocks. Chrysoberyl or phenakite can form depending on desilication in this process. occurrence of Bebearing minerals in different rocks or positions is caused by Be, Si and Al variations, which in turn resulted by a composition exchange in the contact zone of pegmatite and wall rocks due to the great difference in composition.