西藏南迦巴瓦峰地区发现的星叶石

Astrophyllite from the Namjabarwa Area, Eastern Tibet

在西藏境内喜马拉雅山脉的东端的南迦巴瓦峰地区发现了星叶石。该矿物产出于火成碳酸岩脉中，与白云石、方解石、镁橄榄石、镁铝尖晶石、金云母、韭闪石、磷灰石、镁钛矿、石榴石以及三种未定名副矿物共生。该矿物的晶胞参数为：ａ０＝５．３５６（１），ｂ０＝１１．６０７（４），ｃ０＝１１．８５１（６），α＝６４．５７°（３），β＝７６．９８°（４），γ＝８５．４５°（３）。化学成分（％）为：ＳｉＯ２３５．２２，ＴｉＯ２１１．４７，Ａｌ２Ｏ３１．１８，ＦｅＯ２８．９０，ＭｎＯ５．１５，ＭｇＯ１．５１，ＣａＯ１．７５，Ｋ２Ｏ５．９０，Ｎａ２Ｏ２．４６，总和９３．５４％，化学式为：（Ｋ，Ｎａ）３（Ｆｅ，Ｍｎ，Ｍｇ，Ｃａ）７Ｔｉ２Ｓｉ８Ｏ２４（Ｏ，ＯＨ）７。

The Namjabarwa area, the core of the eastern Himalayan syntaxis, is the leastknown part of the Himalayas. Recent research has revealed that this area consists of three tectonic units, viz. the Gangdise unit, the Yarlung Zangbo unit, and the Himalayan unit, separated from each other by faults. The Himalayan unit consists of amphibolite facies rocks in the south and granulite facies rocks in the north, with the latter rocks displaced southward over the former along the Namula thrust system. As an accessory minral in a carbonate vein that intruded the granulite facies rocks, astrophyllite is associated with dolomite, calcite, olivine, spinel, phlogopite, pargasite, apatite, garnet (Gr24.5Alm49.6Pyr18.6And6.7), geikielite and three unknown minerals. One of these unknown minerals is composed of B, C, Mg, Ti, Fe and Al, being probably a new mineral. The unit cell parameters of astrophyllite are a0 = 5.356(1), b0 = 11.607(4) , c0 = 11.851(6) , α = 64.57°(3),β = 76.98°(4), γ= 85.45°(3). Electron microprobe analysis (average of three samples)gave SiO2 35.22, TiO2 11.47, Al2O3 1.18, FeO 28.90, MnO 5.15, MgO 1.51, CaO 1.75, K2O 5.90, and Na2O 2.46, totally 93.54%,corresponding to the formula of (K, Na)3(Fe, Mn, Mg, Ca)7Ti2Si8O24(O, OH)7, which indicates that the astrophyllite is of a new mode of occurrence.