An electron microscope study of the stability field and degree of nonstoichiometry in olivine
An electron microscope study of the stability field and degree of nonstoichiometry in olivine
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橄榄石稳定场和非化学计量程度的电子显微镜研究
DOI：
10.1029/jb091ib05p04711
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发表时间：
1986
影响因子：
--
通讯作者：
A. Duba
中科院分区：
文献类型：
--
作者：
J. Boland;A. Duba
通过使用透射和分析电子显微镜研究单晶的微观结构变化，确定了圣卡洛斯橄榄石 (Fo 90.5) 相对于 1300 /sup 0/C 下氧逸度变化的稳定性。第一个可检测金属相沿 (100) 和 (001) 亚晶界内的位错异质成核，其成分为 Ni/sub 65/Fe/sub 35/，与氧逸度 f/sub 02/ 为 10/sup -5.6/ Pa 的 CO/CO/sub 2/ 气体混合物平衡。尽管 (100) 亚晶界包含具有 Burgers 矢量的高密度位错(100)，成核位点仅限于(010)和(001)位错：这一结果与沿位错的异质成核理论一致。在f2为10/sup -12/ Pa的还原条件下，在气体-矿物界面处生成镁橄榄石，并在橄榄石中均匀成核的高密度金属沉淀物（Fe/sub 96/Ni/sub 4/）。橄榄石的非化学计量是根据不存在任何额外的富含二氧化硅的相来推断的。根据可能的晶体缺陷结构，特别是空位，分析了非化学计量，假设除了氧交换反应之外，表面没有蒸发损失。根据所提出的模型，高度还原的橄榄石从表面到几微米的深度将含有过量的 SiO/sub 2/（x2more » = 0.355 与化学计量值 0.333 相比）。
The stability of San Carlos olivine (Fo 90.5) relative to changes in oxygen fugacity at 1300 /sup 0/C has been determined by studying the microstructural changes in single crystals using transmission and analytical electron microscopy. The first detectable metallic phase, heterogeneously nucleated along dislocations within (100) and (001) subboundaries, had a composition Ni/sub 65/Fe/sub 35/ equilibrated with a CO/CO/sub 2/ gas mixture having an oxygen fugacity f/sub 02/ of 10/sup -5.6/ Pa. Although the (100) subboundaries contained a high density of dislocations with Burgers vector (100), the nucleation sites were restricted to (010) and (001) dislocations: a result consistent with the theory of heterogeneous nucleation along dislocations. Under the reducing conditions of an f2 of 10/sup -12/ Pa, a forsteritic olivine is produced at the gas-mineral interface with a high density of metallic precipitates (Fe/sub 96/Ni/sub 4/) homogeneously nucleated in the olivine. Nonstoichiometry of the olivine was inferred from the absence of any additional silica-rich phases. The nonstoichiometry was analyzed in terms of possible crystalline defect structures, especially vacancies, assuming no evaporative losses from the surface except an oxygen exchange reaction. On the basis of the model proposed, the highly reduced olivine would contain an excess of SiO/sub 2/ (x2more » = 0.355 compared with a stoichiometric value of 0.333) from the surface to a depth of several micrometers.« less