Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities
Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities
复制标题
天然方镁石和不同氧逸度合成的钛钙铁矿的晶体化学
DOI：
10.1007/bf00209694
复制
发表时间：
1989
影响因子：
1.4
通讯作者：
Georg Amthauer
中科院分区：
文献类型：
--
作者：
A. Kühberger;Th. Fehr;H. Huckenholz;Georg Amthauer
在P(H2O)=3kbar，温度700-800℃的条件下合成了钛钙钛矿，氧逸度由固态缓冲液(Ni/NiO；SiO_2+Fe/Fe_2SiO_4)控制。用低温穆斯堡尔谱测定了Fe2+和Fe3+的分布。水分含量由固体水分分析仪测量。用电子探针对合成样品和天然样品的化学成分进行了测定。在高氧逸度条件下生成的钛氧化物具有八面体和四面体配位的Fe3+，在低氧逸度条件下生成的钛氧化物还具有四面体和八面体配位的Fe2+。这些“还原”的石榴石还必须在八面体位置上含有Ti3+。由于O2−被(OH)−取代，通过两种机制保持电荷平衡：(SiO4)4−⇌(O4H4)4−和(Fe3+O6)9−⇌(Fe2+O5OH)9−。合成样品的傅里叶变换红外光谱表明存在结构键合的(OH)−。在天然样品中，观察到四面体和八面体配位的Fe3+和Fe2+在石榴石结构的所有三个阳离子位置上。
Ti-andradites were synthesized at a pressure of P(H2O)=3 kbar and temperatures of 700–800° C. Oxygen fugacities were controlled by solid state buffers (Ni/NiO; SiO2 + Fe/Fe2SiO4). The Fe2+-and Fe3+-distribution was determined by low temperature Mössbauer spectroscopy. The water content was measured by a solid's moisture analyzer. The chemical composition of the synthetic and the natural sample has been determined by electron microprobe. Ti-andradites from runs at high oxygen fugacities have Fe3+ on octahedral and tetrahedral sites; Ti-andradites from runs at low oxygen fugacities have tetrahedrally and octahedrally coordinated Fe2+ as well. These “reduced” garnets must also contain Ti3+ on octahedral sites. Charge balance is maintained due to substitution of O2− by (OH)− by two mechanisms: (SiO4)4− ⇌ (O4H4)4− and (Fe3+O6)9− ⇌ (Fe2+O5OH)9−. FTIR spectra of the synthetic samples do show the presence of structurally bound (OH)−. In a natural sample tetrahedrally and octahedrally coordinated Fe3+ are observed together with Fe2+ on all three cation sites of the garnet structure.