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首页> 外文期刊>Contributions to Mineralogy and Petrology >Compositional limits and analogs of monoclinic triple-chain silicates
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Compositional limits and analogs of monoclinic triple-chain silicates

机译：单斜三链硅酸盐的组成限制和类似物

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Growing recognition of triple-chain silicates in nature has prompted experimental research into the conditions under which they can form and the extent of solid solution that is feasible for some key chemical substitutions. Experiments were done primarily in the range of 0.1–0.5 GPa and 200–850 °C for durations of 18–1,034 h. A wide range of bulk compositions were explored in this study that can be classified broadly into two groups: those that are Na free and involve various possible chemical substitutions into jimthompsonite (Mg10Si12O32(OH)4), and those that are Na bearing and involve chemical substitutions into the ideal end-member Na4Mg8Si12O32(OH)4. Numerous attempts to synthesize jimthompsonite or clinojimthompsonite were unsuccessful despite the type of starting material used (reagent oxides, magnesite + SiO2, talc + enstatite, or anthophyllite). Similarly, the chemical substitutions of F− for OH−, Mn2+, Ca2+, or Fe2+ for Mg2+, and 2Li+ for Mg2+ and a vacancy were unsuccessful at nucleating triple-chain silicates. Conversely, nearly pure yields of monoclinic triple-chain silicate could be made at temperatures of 440–630 °C and 0.2 GPa from the composition Na4Mg8Si12O32(OH)4, as found in previous studies, though its composition is most likely depleted in Na as evidenced by electron microprobe and FTIR analysis. Pure yields of triple-chain silicate were also obtained for the F-analog composition Na4Mg8Si12O32F4 at 550–750 °C and 0.2–0.5 GPa if a flux consisting of Na-halide salt and water in a 2:1 ratio by weight was used. In addition, limited chemical substitution could be documented for the substitutions of 2 Na+ for Na+ + H+ and of Mg2+ + vacancy for 2Na+. For the former, the Na content appears to be limited to 2.5 cations giving the ideal composition of Na2.5Mg8Si12O30.5(OH)5.5, while for the latter substitution the Na content may go as low as 1.1 cations giving the composition Na1.1Mg9.4Si12O31.9(OH)4.1 based on a fixed number of Si cations. Further investigation involving Mg for Na cation exchange may provide a pathway for the synthesis of Na-free clinojimthompsonite. Fairly extensive solid solution was also observed for triple-chain silicates made along the compositional join Na4Mg8Si12O32(OH)4–Ca2Mg8Si12O32(OH)4 where the limit of Ca substitution at 450 °C and 0.2 GPa corresponds to Na0.7Ca1.8Mg7.8Si12O31.9(OH)4.1 (with the OH content adjusted to achieve charge balance). Aside from the Na content, this composition is similar to that observed as wide-chain lamellae in host actinolite. The relative ease with which Na-rich triple chains can be made experimentally suggests that these phases might exist in nature; this study provides additional insights into the range of compositions and formation conditions at which they might occur.
机译：在自然界中，对三链硅酸盐的认识日益提高，已经促使人们对它们形成条件和固溶程度进行一些关键化学取代的实验研究。主要在0.1–0.5 GPa和200–850°C的温度范围内进行了18–1,034 h的实验。在这项研究中，探索了各种各样的散装成分，这些散装成分可以大致分为两类：不含钠且涉及到各种可能的化学取代成金缕石（Mg 10 Si 12 O 32 （OH） 4 ），以及那些含Na且涉及化学取代成理想端基Na 4 Mg的元素 8 Si 12 O 32 （OH） 4 。尽管使用了起始原料类型（试剂氧化物，菱镁矿+ SiO 2 ，滑石+顽辉石或直闪石），但合成金刚磷石或克林诺克次硒酸盐的许多尝试均未成功。同样，用F -代替OH -，Mn 2 + ，Ca 2 + 或Fe Ms 2 + 的 2 + 和Mg 2 + 的2Li + 且空位未能成功成核-链状硅酸盐。相反，在440–630°C和0.2 GPa的温度下，Na 4 Mg 8 Si 的成分可制得近乎纯的单斜晶三链硅酸盐。如先前的研究中所发现的，12 O 32 （OH） 4 ，尽管通过电子探针和FTIR分析证明其组成很可能在Na中耗尽。 F-类似物Na 4 Mg 8 Si 12 O 32 的纯收率也得到了三链硅酸盐的纯收率。如果使用的助熔剂是由卤化钠盐和水组成的重量比为2：1，则在550-750°C和0.2-0.5 GPa的温度下获得sub> F 4 。此外，可以证明有限的化学取代可以替代2 Na + 替代Na + + H + 和Mg 2 + +空缺为2Na + 。对于前者，Na含量似乎被限制为2.5个阳离子，从而给出理想的Na 2.5 Mg 8 Si 12 O 组成30.5 （OH） 5.5 ，而对于后者的取代，Na含量可能低至1.1个阳离子，组成为Na 1.1 Mg 9.4 < / sub> Si 12 O 31.9 （OH） 4.1 （基于固定数量的Si阳离子）。涉及Mg的Na阳离子交换的进一步研究可能为合成无Na的斜长石次氯酸盐提供一条途径。沿组成连接Na 4 Mg 8 Si 12 O 32 <的三链硅酸盐也观察到相当广泛的固溶体/ sub>（OH） 4 –Ca 2 Mg 8 Si 12 O 32 （OH） 4 ，其中在450°C和0.2 GPa下Ca的置换极限对应于Na 0.7 Ca 1.8 Mg 7.8 Si 12 O 31.9 （OH） 4.1 （已调整OH含量以实现电荷平衡）。除了Na含量外，该组成与宿主阳起石中的宽链片状结构相似。富钠三链的实验相对容易，表明这些相可能存在于自然界中。这项研究为可能发生的成分范围和形成条件提供了更多的见解。

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