Refined structure of afwillite from the northern Baikal region

The crystal structure of the mineral afwillite [Ca-12(H2O)(8)][SiO4](4)[SiO2(OH)(2)](4) from the northern Baikal region was refined in a triclinic unit cell with the following parameters: a = 16.330(2) A..., b = 5.6389(6) A..., c = 11.685(1) A..., alpha = 90.08(1)A degrees, beta = 126.446(2)A degrees, gamma = 89.95(1)A degrees, and sp. gr. P1. The triclinic unit cell is related to the monoclinic unit cell determined earlier (sp. gr. Cc, a = 16.278(1) A..., b = 5.6321(4) A..., c = 13.236(1) A..., and beta = 134.898(3)A degrees) by the transformation matrix [-100/010/101]. The structural model was determined using the phase-correction procedure and refined anisotropically to R = 2.8% based on 3270 independent reflections with F > 3 sigma(F). The crystal-chemical formula of afwillite was revised. The characteristic features of the IR spectrum of afwillite were explained based on the structural data.