The Xiyuantou rare metal granite pegmatite of Fujian province is a rare metal deposit dominated by Ta and containing various soxts of associated ele- meets. The pegmatite were found into mica-plagioclase granulite and quartzmica schist of Sinian metamorphic series. Micas from different assemblage zones of pegmatitic veins all belong to 2M1 type Rb-bearing muscovite, which has quite stable chemical composition at the positions of tetrahedxa arnd octahedra, varying within the compositiohal range of normal muscovite .Micas contain Rb (Rb2O=0.45-1.78%)and Cs (Cs2O= 0.02%-0.06%)but are deficient in Li and F.The variation in Rb2O and Cs2O content of different assemblage zones is connected to certain extent with modes of occurrence, mineral assemblages, grain sizes and colors of micas. Ferriferous muscovite in mica-plagioclase leptynite differs from Rb-bearing muscovite in that the former is a bit higher in Ti, Mg and Fe content and specific gravity (G= 2.83-2.88)，but contains extremely low Li2O, Rb2O and Cs2O. The site occupancy of cations (Li1+ +R2+ +R2+ +Ti4+=4.372) in the octahe- drop of 3T type alumo-protolithionite from quartz-mica schist is between the dioctahedron and the trioctahedron, belonging to the transitional type. Besides conitaining abundant Ti, Mg and Fe, this mica is characterized by rich rare alkali elements (Rb2O=1.42, Cs2O=1.36% and Li2O=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 parameters are as follows: a0=5.19A，bo=5.19A c0=30.05A，β=90º，a=90º，γ= 120º，Nm=Ng=1.601, 2V= 0一3º，and G= 2.91. Chemical composition and species of micas reflect ge}logi-cal environments for 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 pegmatite melt-solution. It is therefore kniown that mica can serve as a typomorphic mine- ral in the rare metal pegmatite deposit.