The crystal-chemistry of holmquistites:: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles
The crystal-chemistry of holmquistites:: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles
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DOI：
10.2138/am.2005.1793
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发表时间：
2005-07-01
影响因子：
3.1
通讯作者：
Oberti, R
中科院分区：
文献类型：
--
作者：
Cámara, F;Oberti, R
对斜方晶系斜方辉石进行了系统的晶体化学研究，以确定其成分变化有限的原因。Li-B角闪石的相对稳定性的结构限制也建议的铁和铁-铁斜长辉橄榄岩的发生，以及缺乏斜长辉橄榄岩。详细的晶体化学分析表明：（1）在组成上，无论是在电荷排列方面还是在有限的同价方面，都具有显着的恒定性（M1，3）。（Mg-1Fe 2+），（M2）（Al-1 Fe ~（3+））和（O ~ 3）（OH-1 F）交换;（2）晶胞尺寸具有显著的恒定性，其中Fe ~（3+）在M_2位的含量是影响B边的唯一因素;（3）Li在M_4位完全有序，而在斜闪石中，Li在M_4和M_3位之间的分配是完全有序的，但这与A位的部分占据有关：（4）三价阳离子在M_2位完全有序;（5）Fe ~（2+）和Fe ~（3+）含量呈反比关系，这是保持八面体条带尺寸不变的一种方法;（6）八面体位置的强烈扭曲，无论是在角度变化还是二次伸长方面。（A）正方形（B）Li（2）（C）（Mg3 Al 2）（Si 8 O22）-Si-T（OH）（2）是由最小可能的结构模量组成的角闪石组合物。在Pnma对称中结晶，其中两个双链的四面体可以呈现不同的构象，可能是需要获得更合适的Li-B的[5 + 1]-配位，并缩小阳离子-阳离子距离。在这项工作中，一个Fe 2+含量略高于Mg但明显高于Mg的样品被表征，然后被IMA-CNMMN（2004-030）识别为矿物物种。全型铁钾镁矾a = 18.287（1），B = 17.680（1），c = 5.278（1）埃，V = 1706.6（1）埃（3）。其晶体化学式为（K0.01Na0.01B）-K-A（Li1.88Mg0.08Na0.03Fe0.012+）（C）（Al1.89Fe1.702+Mg1.39Mn0.022+）（Si8.00O22）-Si-T（OH1.97F0.03）。Feroholmquistite以细长的黑色至蓝紫色棱柱状晶体出现;它是双轴负的，α = 1.628，β = 1.646，γ = 1.651（λ = 589 nm），2 V（x）（计算值）。= 55.1度。它是弱多色性的，α =无色，=淡紫蓝色，γ =蓝色到深紫色;计算密度为3.145 g/cm（3）。正模标本属于Renato和Adriana Pagano（意大利）的矿物收藏，来自Greenbushes伟晶岩（西澳大利亚）。已分析的样品以代码2004-01保存在帕维亚大学（意大利）的地球科学学院博物馆。
A systematic crystal-chemical investigation of orthorhombic holmquistites has been done to determine the reasons for their limited compositional variations. Structural constraints to the relative stability of Li-B amphiboles are also suggested by the occurrence of ferro- and ferri-ferroclinoholmquistites, and the lack of clincholmquistite. Detailed crystal-chemical analysis shows: (1) a remarkable constancy in composition, both in terms of charge arrangement and of limited homovalent (M1,3)(Mg-1Fe2+), (M2)(Al-1Fe3+), and (O3)(OH-1F) exchanges, (2) a remarkable constancy in the unit-cell dimensions, with the Fe3+ content at the M2 site being the only factor affecting the b edge; (3) complete ordering of Li at the M4 site, in contrast with the common partitioning between the M4 and M3 sites in clinoamphiboles, which however couples with partial A-site occupancy; (4) complete ordering of trivalent cations at the M2 site; (5) an inverse relationship between the Fe2+ and the Fe3+ contents, which is interpreted as a way to keep the size of the octahedral strip constant; (6) a strong distortion of the octahedral sites, both in terms of angular variance and quadratic elongation.(A)square(B)Li(2)(C)(Mg3Al2)(Si8O22)-Si-T(OH)(2) is the amphibole composition composed of the smallest possible structural moduli. Crystallization in Pnma symmetry, where the two double-chains of tetrahedra can assume different conformations, is probably required by the need to obtain a more suitable [5 + 1]-coordination for Li-B, and to shrink the cation-cation distances. This arrangement does not allow for extensive incorporation of larger homovalent substituents, which are hosted via mechanisms implying distortion of the octahedral sites.During this work, a sample with Fe2+ slightly but significantly higher than Mg was characterized, and then recognized as a mineral species by the IMA-CNMMN (2004-030). Holotype ferroholmquistite has a = 18.287 (1), b = 17.680 (1), and c = 5.278 (1) angstrom, and V = 1706.6(1) angstrom(3). Its crystal-chemical formula is (K0.01Na0.01B)-K-A(Li1.88Mg0.08Na0.03Fe0.012+)(C)(Al1.89Fe1.702+Mg1.39Mn0.022+)(Si8.00O22)-Si-T(OH1.97F0.03). Ferroholmquistite occurs as elongated black to bluish-violet prismatic crystals; it is biaxial negative, with alpha = 1.628, beta = 1.646, and gamma = 1.651 (lambda = 589 nm), 2V(x) (calc.) = 55.1 degrees. It is weakly pleochroic, with alpha = colorless, = pale violet-blue, and gamma = blue to deep violet; the calculated density is 3.145 g/cm(3). The holotype specimen belongs to the mineral collection of Renato and Adriana Pagano (Italy), and comes from the Greenbushes pegmatite (Western Australia). The analyzed sample has been deposited at the Museum of the Dipartimento di Scienze della Terra, Universita di Pavia (Italy) under the code 2004-01.