纤钡锂石 BaMg2LiAl3[Si2O6]2（OH,F）8及其晶体结构

1974年在一水晶矿石英脉晶洞中,发现了一种含Ba、Li的硅酸盐新矿物--纤钡锂石。我们对纤钡锂石进行了光性研究、比重测定、差热及热失重分析、红外光谱分析、X射线单晶结构分析等工作,现分述如下。

BALIPHOLITE AND ITS CRYSTAL STRUCTURE

Balipholite is a species of barium-and lithium-bearing silicate mineral occurring in zinnwaldite quartz vein geode. It is white with silky luster, and forms acicular, fibrous, radiating or parallel sheaf-like aggregates, the fibre being up" to 1 cm. long.This mineral includes two varieties of different composition: BaMg2LiAl3(Si2O5)2 (OH, F)8 and BaMgLi1.5Al3.5(Si2O6)2(OH, F)8. It belongs to the orthorhombic system, with a space group Ccca, in which a = 13.60A, 6 = 20.24A, c = 5.16A. The strongest diffraction line of the powdered patterns: 10.12A (100), 4.05A (78), 3.39A (91), 2.605A (31), 3.390A (28). It is characterized by perfect cleavage parallel to {010} and distinct cleavages parallel to {100} and {110}, colourless under the microscope. Ng = 1.5984 (a-axis), Nm = 1.5954 (c-axis, parallel to the fibre), Np = 1.5807 (b-axis). Specific gravity = 3.32-3.33.Its infrared absorption spectrum is similar to that of carpholite group and ortho-rhombic pyroxenes. The difference of OH stretching frequency and intensity between the two varieties is caused by the type and the amount of the cation connected with the OH ion in the structure.A big endothermic effect appears near 700℃ on the differential thermal curves, and small ones are at 870°, 900°, 985° and 1030℃. A sharp fall in weight appears at 600°-700 ℃ on the dehydration curves.The samples heated at different temperatures (500°-1000℃) have been subjected to quantitative analysis on H2O and F, X-ray analysis by diffractometer on powdered samples and infra-red study, from which it is ascertained that thermal changes have brought about the appearance of different minerals, namely tainolite (K, Ba) LiMg2Si4O10F2, beta-spodumene (β-LiAlSi2O6), and cymrite (BaAl2Si2O8H2O) at 620℃, hexagonal celsian (BaAl2Si2O8) at 900℃ and monoclinic celsian (BaAl2 Si2O8) at 1600℃. The differentia] thermal curves have been satisfactorily explained. Diffraction points hk0, 0kl have been collected with weissenbergs equi-inclination multi-film technique. The crystal structure has been analysed and the various atomic co-ordinates have been determined. B(0kl) is 11.12%, and B(hk0) is 9.76%.Determination of the crystal structure indicates that, balipholite has the similar structure as that of the carpholite group, that is, having single chains of pyroxene (SiO2O6)104-. Such a mineral from the angle of the crystallochemistry for silicate is characterized by having both double chains and sheet-like structure, from which the various aspects such as the crystal habit, the cleavage and the optical characters can be properly explained.