西藏罗布莎蛇绿岩豆荚状铬铁矿中镁橄榄石的晶体结构及其意义

在西藏罗布莎蛇绿岩块的豆荚状铬铁矿中，存在包裹体矿物镁橄榄石，这些镁橄榄石为无色透明多晶面形成的自形晶，Fo值可达到97－98，具富镁端员组分，选取一代表性颗粒的单晶进行X单晶和衍射分析，表明其为斜方晶系，空间群Pbnm，镁橄榄石晶胞参数,a,b,c以及M1－0和M2－0键长均小于地幔包裹体和一般地幔岩中橄缆中的对应值，测试出的晶胞参数与人工合成镁橄榄石的一致，镁橄榄石具有橄榄石族中已知最小的晶胞参数，据此推断它是在超高压环境下结晶的。

Crystal Structure ofForsterite from Podiform Chromitite in Luobusa Ophiolite of Tibet And Its Implications

The forsterite mineral inclusions in chromite of podiformchromitite occur in mantle peridotite of Luobusa ophiolite in the Indo_Yarlung Zangbo tectonic unit. The forsterite inclusions from chromite grains have Fo number of 97～99 and assume the form of colorless, transparent and euhedral grains with abundant small faces. The crystal of forsterite used for X_ray analysis was measured to be approximately 0.15 mm×0.15 mm×0.15 mm. Crystal system of this forsterite is orthorhombic, space group Pbnm, α, β and γ is 90°, a=4.7597Å, b=10.205Å, c=5.9831Å. The unit cell parameters of forsterite, a, b, c as well as M1—O and M2—O bond lengths are smaller than those of some olivines from mantle xenolith and mantle peridotites. The unit cell parameters of the forsterite in the study area are the same as those of the synthetic forsterite. According to experimental study cited in the literature, the small unit cell andcation_oxygen bond distances of olivine suggest a high pressure of its crystallization.