| Summary: | The nomenclature of the mineral species penkvilksite, Na4Ti2Si8O22.4H2O, has been revised, after the second finding in nature of the mineral, which has been demonstrated to occur in two polytypic modifications: the earlier-described orthorhombic (penkvilksite-2O) and the present monoclinic (penkvilksite-1M). Penkvilksite-2O is found in the Lovo-zero alkaline massif, Kola Peninsula, Russia, with space group Pnca, a = 16.3721(5), b = 8.7492(3), c = 7.4020(2) angstrom; its structure was refined by X-ray powder diffraction with the use of full-profile Rietveld technique, R(p) = 0.076, R(wp) = 0.098. Penkvilksite-1M is found in the Khibina alkaline massif, Kola Peninsula, Russia, with space group P2(1)/c, a = 8.956(4), b = 8.727(3), c = 7.387(3) angstrom, beta = 112.74(3)-degrees; its structure was refined by single-crystal X-ray diffraction on 1174 independent reflections, R = 0.045, R(w) = 0.047. Penkvilksite represents a novel kind of silicate structure, characterized by the occurrence of alternating clockwise- and counterclockwise-growing spirals of corner-sharing SiO4 tetrahedra, whose periodicity corresponds to six tetrahedral units. The 2O and 1M polytypes of penkvilksite are also described, according to the OD theory, as two out of the four possible maximum degree of order (MDO) polytypes within a family of OD structures formed by two layers.
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