交代实验形成的硅镁石族矿物特征和物理化学条件

Characteristics of Humite Group Minerals Formed in Metasomatic Experimentation and Physico-chemical Conditions for Their Formation

本文概括了1939—1988年以来硅镁石族矿物研究的进展;叙述了以橄辉岩和结晶石灰岩或黑云母石英片岩和白云质大理(或白云)岩作试料,在450—700℃和(500—1400)×105Pa的温度和压力下的近中性卤素化合物水溶液中,交代作用出现的硅镁石族矿物的特征:探讨了它们形成时的物质组分及其活度,反应溶液及其pH值,氧化还原环境,压力和温度等物理化学条件;并用拓朴学的原理剖释了硅镁石族矿物和其最密切共生的矿物——镁橄榄石、水镁石和叶蛇纹石依赖于的变化和它们之间的内在联系;揭示了硅镁石族矿物形成的机理。

Using natural olivine pyroxenite and crystalline limestone or biotite-quartz schist and dolomitic marble as starting materials, the authors performed metasomatic experimentation in 0.5M KCl+0.5M NaF solution with pH 6.0-8.0 at 450-700℃ and under (500-1400)×10~5Pa; as a result, humite group minerals were formed. Humite group minerals formed by metasomatism are colorless (some with light yellow tone) and transparent. Crystal morphology is perfectly developed with crystal sizes being generally 0. 005-0. 02mm; biaxial positive. Their optical properties such as refractive indices and their X-ray diffraction patterns indicate that they are chondrodite, humite and clinohumite. The principal conditions for the formation of humite group materials are that the mole number ratio between Mg and Si must be higher than 3:1 in the starting materials which, in addition, should contain saturated H_2O. Under suitable temperature and pressure, neutral-weakly alkaline solution of K, Na halogenides and oxidation environment are favorable for their growth and development. With topologic graphical method, the authors have reached the conclusion that, at constant chemical potential of SiO_2, the formation of clinohumite (one of the humite group minerals)and its major paragenetic minera s-forsterite, brucite and antigorite is dependent on fairly high μ_(MgO) and μ_(H_2O). It is also proved that the coexistence of these minerals is an inherent law in their interaction.