BASSANITE FROM THE CHAERHAN SALT LAKE,QINGHAI AND ITS GENESIS
察尔汉盐湖的半水石膏及其成因分析

Bassanite is a sulfate mineral which is rarely found and poorly studied. However, in the salt formation of the Chaerhan Salt Lake, Qinghai Province, bassanite is of common occurrence. The bassanite is characterized by its association with chloride minerals, such as halite, sylvite and carnallite. Chaerhan Salt Lake belongs to a hydrochemical area characterized by sulfate type and chloride type in transition. The bassanite in the salt lake is the product of the metasomatism caused by the brine mixture at ordinary temperatures. It is characterized by its widespread distribution, yet not occurring in definite horizons, and close association with chloride minerals, thus indicating the existence of the process of brine mixture. The presence of interstices among the fibrous crystal aggregates of bassanite pseudomorph after larger tabular gypsums seen under the scanning electron microscope shows that when the original gypsum becomes bassanite, the volume reduces because of the decrease of crystal water and thus the formation of interstices appeal's. This process can also be used to prove the existence of the metasomatism from gypsum to bassanite.