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The crystal chemistry of holtite

机译：锂铁矿的晶体化学

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Holtite, approximately (Al,Ta,square)Al_6(BO_3)(Si,Sb~(3+),As~(3+))_(sum 3)O_(12)(O,OH,square)_(sum 3), is a member of the dumortierite group that has been found in pegmatite, or alluvial deposits derived from pegmatite, at three localities: Greenbushes, Western Australia; Voron'i Tundry, Kola Peninsula, Russia; and Szklary, Lower Silesia, Poland. Holtite can contain >30 wt.% Sb_2O_3, As_2O_3, Ta_2O_5, Nb_2O_5, and TiO_2 (taken together), but none of these constituents is dominant at a crystallographic site, which raises the question whether this mineral is distinct from dumortierite. The crystal structures of four samples from the three localities have been refined to R_1 = 0.02—0.05. The results show dominantly: Al, Ta, and vacancies at the Al(l) position; Al and vacancies at the Al(2), (3) and (4) sites; Si and vacancies at the Si positions; and Sb, As and vacancies at the Sb sites for both Sb-poor (holtite I) and Sb-rich (holtite II) specimens. Although charge-balance calculations based on our single-crystal structure refinements suggest that essentially no water is present, Fourier transform infrared spectra confirm that some OH is present in the three samples that could be measured. By analogy with dumortierite, the largest peak at 3505—3490 cm~(-1) is identified with OH at the O(2) and O(7) positions. The single-crystal X-ray refinements and FTIR results suggest the following general formula for holtite: Al_(7-[5x+y+z]/3) (Ta,Nb)_x square_([2x+y+z]/3)BSi_(3-y)(Sb,As)_yO_(18-y-z)(OH)_z, where x is the total number of pentavalent cations, y is the total amount of Sb + As, and z < = y is the total amount of OH. Comparison with the electron microprobe compositions suggests the following approximate general formulae Al_(5.83)(Ta,Nb)_(0.50) square_(0.67)BSi_(2.50)(Sb,As)_(0.50)O_(17.00)(OH)_(0.50) and Al_(5.92)(Ta,Nb)_(0.25) square_(0.83)BSi_(2.00)(Sb,As)_(1.00) O_(16.00)(OH)_(1.00) for holtite I and holtite II respectively. However, the crystal structure refinements do not indicate a fundamental difference in cation ordering that might serve as a criterion for recognizing the two holtites as distinct species, and anion compositions are also not sufficiently different. Moreover, available analyses suggest the possibility of a continuum in the Si/(Sb + As) ratio between holtite I and dumortierite, and at least a partial continuum between holtite I and holtite II. We recommend that use of the terms holtite I and holtite II be discontinued.
机译：菱铁矿，大约（Al，Ta，square）Al_6（BO_3）（Si，Sb〜（3 +），As〜（3 +））_（总和3）O_（12）（O，OH，平方）_（总和3）是在三个地方的伟晶岩或衍生自伟晶岩的冲积矿床中发现的硬蒙脱石组的成员：西澳大利亚州的Greenbushes;俄罗斯科拉半岛的沃龙尼·滕德里（Voron'i Tundry）;波兰下西里西亚的斯克拉斯（Szklary）。锂铁矿可以包含> 30 wt。％的Sb_2O_3，As_2O_3，Ta_2O_5，Nb_2O_5和TiO_2（合计），但是这些成分在晶体学位点中都不占主导地位，这引发了该矿物是否不同于硬蒙脱石的问题。来自三个位置的四个样本的晶体结构已精炼为R_1 = 0.02-0.05。结果主要显示：Al，Ta和Al（l）位置的空位; Al和Al（2），（3）和（4）站点的空位;硅和硅职位的空缺;贫Sb（软质I）和富Sb（软质II）样品的Sb，As和空位。尽管基于我们的单晶结构改进进行的电荷平衡计算表明，实际上不存在水，但傅立叶变换红外光谱证实，在可以测量的三个样品中存在一些OH。通过与硬蒙脱石类似，在O（2）和O（7）位置的OH鉴定出最大峰在3505-3490 cm〜（-1）。 X射线单晶细化和FTIR结果表明，该白云母具有以下通式：Al_（7- [5x + y + z] / 3）（Ta，Nb）_x square _（[2x + y + z] / 3 BSi_（3-y）（Sb，As）_yO_（18-yz）（OH）_z，其中x是五价阳离子的总数，y是Sb + As的总量，z <= y是OH总量。与电子探针组成的比较表明以下近似通式Al_（5.83）（Ta，Nb）_（0.50）square_（0.67）BSi_（2.50）（Sb，As）_（0.50）O_（17.00）（OH）_ （0.50）和Al_（5.92）（Ta，Nb）_（0.25）square_（0.83）BSi_（2.00）（Sb，As）_（1.00）O_（16.00）（OH）_（1.00）对于I二。但是，晶体结构的细化并不能说明阳离子顺序的根本差异，而可以将其作为识别两种辉石岩为不同种类的标准，并且阴离子组成也没有足够的差异。此外，可用的分析表明，在辉石I和硬蒙脱石之间的Si /（Sb + As）比连续的可能性，以及在辉石I和辉石II之间至少部分连续的可能性。我们建议停止使用术语“辉石I”和“辉石II”。

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