THE CRYSTAL STRUCTURE OF NANPINGITE-2M2, THE CS END-MEMBER OF MOSCOVITE

Abstract

Nanpingite, ideally CsA12(AlSi3)OIO(OH)2, is a Cs end-member dioctahedral mica, crys- tallizing in C21c, with a = 9.076(3), b = 5.226(2), c = 21.41(5) A, and (3= 99.48(6)°. The crystal structure of type nanpingite was refined to R = 0.058 using single-crystal X-ray diffraction methods. Nanpingite is a 2M2 polytype dioctahedral mica rather than a 2Ml 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 dimensions. The existence of the rare 2M2 polytype is attributed to this large interlayer separation, which minimizes the repulsion of the super- imposed (along (001)) 0 anions in the basal planes of neighboring tetrahedral layers. The 2M2 polytype involves ditrigonal prismatic coordination of the interlayer Cs+, in contrast to the octahedral coordination of interlayer K + and Na + in muscovite-2Ml and paragonite- 2Mj. The tetrahedral rotation in nanpingite is the smallest among dioctahedral micas and is also caused by the incorporation of the large interlayer cation Cs + .