Metamorphic Na- and OH-rich disordered dravite with tetrahedral boron, associated with omphacite, from Syros, Greece: chemistry and structure
Metamorphic Na- and OH-rich disordered dravite with tetrahedral boron, associated with omphacite, from Syros, Greece: chemistry and structure
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产自希腊锡罗斯岛的富含 Na 和 OH 的无序镁镁石，与四面体硼相关，与绿辉石相关：化学和结构
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发表时间：
2004
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通讯作者：
C. McCammon
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作者：
H. Marschall;A. Ertl;John M. Hughes;C. McCammon
通过化学分析（EMPA、西姆斯、穆斯堡尔谱研究）和晶体结构测定，对来自希腊锡罗斯蓝闪石片岩的变质含铁镁铁闪石进行了表征。优化配方，根据化学和结构数据计算（包括来自穆斯堡尔光谱学）是x（Na 0.96 Ca 0.02 □ 0.02）Y（Mg 1.29 Al 0.99 Fe 2+ 0.44 Fe 3+ 0.19 Ti 0.05□ 0.04）Z（Al 4.90 Mg 1.10）T（Si 5.83 B 0.17）B 3 O 27 [（OH）3.93 F 0.07 ]，a = 15.9443（3），c = 7.2094（3）A，R = 0.017。OH-含量接近4 apfu，因此在V和W位没有明显的O 2−。与大多数天然碧玺相反，X位点几乎完全被Na填充。我们的结论是，来自镁黄长石-黑云母系列的电气石样品，其O 1位（W位）主要被OH-和/或F占据，而不是被O2 −占据，有利于Na占据X位。镁和铝是强烈的无序在这个电气石样品。这种富含镁的电气石是铝镁无序的一个不寻常的例子，它不是由O 1位的O 2−的短程需求驱动的。令人惊讶的是，我们发现少量的[4] B在这个富镁橄榄石。这通过化学分析（包括轻元素）、精细化（δ 0.26 [4] B apfu）以及相对较小的T-O>距离1.6179 A得到证实。这是第一个已知的富含镁的电气石的例子，它含有大量的[4] B。镁铁云母的形成（PT条件为1.6 ~ 7 kbar/1.400 °C）发生在SiO2不饱和条件下的俯冲-折返环境中。
Metamorphic Fe-bearing dravite from a glaucophane schist from Syros, Greece, associated with omphacite, has been characterized by chemical analyses (EMPA, SIMS, Mossbauer study) and by crystal structure determination. The optimized formula, calculated using chemical and structural data (including from Mossbauer spectroscopy) is x (Na 0.96 Ca 0.02 □ 0.02 ) Y (Mg 1.29 Al 0.99 Fe 2+ 0.44 Fe 3+ 0.19 Ti 0.05□ 0.04 ) Z (Al 4.90 Mg 1.10 ) T (Si 5.83 B 0.17 ) B 3 O 27 [(OH) 3.93 F 0.07 ], with a = 15.9443(3), c = 7.2094(3) A, R = 0.017. The OH- content is nearly 4 apfu , thus there is no significant O 2− at the V and W site. The X site is nearly completely filled with Na, contrary to most natural tourmalines. We conclude that tourmaline samples from the dravite-schorl series where the O1 site ( W site) is mainly occupied by OH- and/or F, but not by O 2− , favour an X -site occupation with Na. Mg and Al are strongly disordered in this tourmaline sample. This Mg-rich tourmaline is an unusual example of Al-Mg disorder which is not driven by the short-range requirements of O 2− at the O 1 site. Surprisingly, we found small amounts of [4] B in this Mg-rich dravite. This is confirmed by the chemical analysis (including light elements), by the refinement (∼ 0.26 [4] B apfu ), as well as by the relatively small T -O> distance of 1.6179 A. This is the first known example of a Mg-rich tourmaline which contains significant amounts of [4] B. The formation of this dravite (at PT conditions of ∼ 6 to 7 kbar/∼ 400°C) took place in a subduction-exhumation environment at undersaturated SiO 2 -conditions.