THE CRYSTAL-STRUCTURE OF NANPINGITE-2M(2), THE CS END-MEMBER OF MUSCOVITE

Nanpingite, ideally CsAl2(AlSi3)O-10(OH)(2), is a Cs end-member diocta hedral mica, crystallizing in C2/c, with a = 9.076(3), b = 5.226(2), c = 21.41(5) Angstrom, and beta = 99.48(6)degrees. The crystal structur e of type nanpingite was refined to R = 0.058 using single-crystal X-r ay diffraction methods. Nanpingite is a 2M(2) polytype dioctahedral mi ca rather than a 2M(1) polytype as reported originally. In comparison with the interlayer cations in common muscovite and paragonite, Cs+ in nanpingite increases the [001] interlayer separation between adjacent 2:1 layers but has little effect on the lateral (a, b) cell dimension s. The existence of the rare 2M, polytype is attributed to this large interlayer separation, which minimizes the repulsion of the superimpos ed (along [001]) O anions in the basal planes of neighboring tetrahedr al layers. The 2M, polytype involves ditrigonal prismatic coordination of the interlayer Cs+, in contrast to the octahedral coordination of interlayer K+ and Na+ in muscovite-2M(1) and paragonite-2M(1). The tet rahedral rotation in nanpingite is the smallest among dioctahedral mic as and is also caused by the incorporation of the large interlayer cat ion Cs+.