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自由SiO_2系指石英及其同质多型物(polymorphs)柯石英、斯石英等。石英广泛分布于地壳中的各种岩石中,柯石英和斯石英只存在于超高压岩石和陨石坑中。由于石英和非饱和SiO_2的橄榄石不能共生,因此在地幔橄榄岩和超镁铁岩中不存在原生石英。最近笔者在西藏罗布莎蛇绿岩的地幔岩(方辉橄榄岩)的豆荚状铬铁矿中发现了自由SiO_2和柯石英相。根据高温高压相平衡实验资料,橄榄石、辉石这样的硅酸盐矿物在地幔深部的压力条件下可以分解成简单氧化物,如FeO(方铁矿)、MgO(方镁石)以及SiO_2(斯石英)等。由此推测,西藏蛇绿岩地幔岩中自由SiO_2可能是来自于下地幔的矿物,是地幔柱作用将其搬运到上地幔浅部。
Free SiO2 refers to quartz and its polymorphs, such as coesite and quartz. Quartz is widely distributed in a variety of rocks in the crust, coesite and quartz only exist in the ultra-high pressure rocks and craters. Since quartz and olivine of unsaturated SiO 2 can not coexist, no primary quartz exists in the mantle peridotites and ultramafic rocks. Recently, I found free SiO 2 and coesite phases in podiform chromite of the mantle rock (hibergest) of the Luobusa ophiolite in Tibet. According to the experimental data of high temperature and high pressure phase equilibrium, silicate minerals such as olivine and pyroxene can be decomposed into simple oxides under the conditions of deep mantle pressure such as FeO (iron ore), MgO (periclase) and SiO 2 Sri Lanka quartz) and so on. Therefore, it is speculated that the free SiO_2 in the mantle rock of the ophiolite in Tibet may be the minerals from the lower mantle, which are transported by the mantle plume to the upper part of the upper mantle.