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

Be-bearing gem minerals occuring in the pegmatite and its contact zone are mainly emerald, aquamarine, heliodor, morganite (vorobyerite), chrysoberyl and phenakite.
According to equilibrium parameters of the system, temperatures, pressures and concentrations etc, the formation of the pegmatite can be divided into two stages: the early crystallization stage and the late metasomatism stage. Beryl formed in the early stage is usually poor in transparency with dull color and tends to be valueless. However, an evolutionary series of beryl minerals is developed in the order of aquamarine, heliodor and morganite with a progress of metas omatism.
Emerald might occur in the pegmatite vug when the pegmatite extrudes into a wall rock which has great different composition, such as ultrabasic rocks. Chrome in emerald is derived from the ultrabasic rocks. 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.