福建溪源头伟晶岩及围岩中的云母特征

Characteristics of Micas from the Xiyuantou Pegmatite and Wall Rocks,Fujian Province

作者对福建溪源头稀有金属花岗伟晶岩及其围岩中的40余个云母样品进行了成分、物性、结构及热性质的综合研究。溪源头伟晶岩之不同组合带中产出的云母均为2M_1型含铷白云母,云母中含铷、铯而贫锂、贫氟。云母斜长变粒岩中的白云母为含铁白云母。其中钛、镁含量偏高。石英云母片岩中之3T铝黑鳞云母的八面体阳离子占位介于二八面体和三八面体之间。该云母以富稀碱元素及挥发分为其特征。溪源头云母的成分及种属反映了形成它的地质环境特点,与围岩性质及成矿类型有关,可作为稀有金属伟晶岩矿床的标型矿物,可以用作找矿标志。

The Xiyuantou rare metal granite pegmatite of Fujian province is a raremetal deposit dominated by Ta and containing various sorts of associated ele-ments. The pegmatite were found into mica-plagioclase granulite and quartzmicaschist of Sinian metamorphic series. Micas from different assemblage zones of pegmatitic veins all belong to 2M_1type Rb-bearing muscovite, which has quite stable chemical composition at thepositions of tetrahedra and octahedra, varying within the compositional rangeof normal muscovite. Micas contain Rb (Rb_2O=0.45-1.78%) and Cs (Cs_2O=0.02%-0.06%) but are deficient in Li and F. The variation in Rb_2O and Cs_2Ocontent of different assemblage zones is connected to certain extent with modesof occurrence, mineral assemblages, grain sizes and colors of micas.Ferriferous muscovite in mica-plagioclase leptynite differs from Rb-bearingmuscovite in that the former is a bit higher in Ti, Mg apd Fe content andspecific gravity (G=2.83--2.88), but contains extremely low Li_2O, Rb_2O andCs_2O. The site occupancy of cations (Li^1+ +R^2+ +R^2+ +Ti^4+ =4.372) in the octahe-dron of 3T type alumo-protolithionite from quartz-mica schist is between thedioctahedron and the trioctahedron, belonging to the transitional type. Besides ??containing abundant Ti, Mg and Fe, this mica is characterized by rich rarealkali elements (Rb_2O=1.42, Cs_2O= 1.36% and Li_2O= 1.90%) and volatiles (F-=3.34%). Data of infrared absorption spectrum and differential thermal ana-lyses also suggest the transitional characteristics of this mica. Its unit cell para-meters are aS follows: ao=5.19A, bo=5.19A, co=30.05A, a=90°, β=90°, γ=120°, Nm=Ng= 1.601, 2V= 0-3°, and G= 2.91. Chemical composition and species of micas reflect geologi-cal environmentsfor their formation, thus playing an indicating role in prospecting for minerali-zation. Mica rich in rare alkali formed in pegmatite and quartz-mica schist is.related to properties of wall rocks and rich rare alkali elements in pegmatitemelt-solution. It is therefore known that mica can serve as a typomorphic mine-ral in the rare metal pegmatite deposit.