Penkvilksite, a new kind of silicate structure: OD character, X-ray single crystal (1M), and powder Rietveld (2O) refinements of two MDO polytypes

The nomenclature of the mineral species penkvilksite has been revised, after the second finding in nature of the mineral, which has been demonstrated to occur in two poltypic modifications: the earlier-described orthorhombic (penkvilksite-2O) and the present monoclinic (penkvilksite-1M). 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.