title: Crystal structure of Cr-bearing Mg3BeAl8O16, a new polytype of magnesiotaaffeite-2N'2S.
abstract: The crystal structure of a new polytype of magnesiotaaffeite-2N'2S, ideally Mg3BeAl8O16 (trimagnesium beryllium octa-aluminium hexa-deca-oxide), is described in space-group symmetry P-3m1. It has been identified in a fragment of a mineral sample from Burma (Myanmar). The new polytype is composed of two Mg2Al4O8 (S)- and two BeMgAl4O8 (N')-modules in a stacking sequence N'SSN'' which differs from the N'SN'S-stacking sequence of the known magnesiotaaffeite-2N'2S polytype. The crystal structure can be derived from a close-packed arrangement of O atoms and is discussed with regard to its polytypism and its Cr(3+) chromophore content.
authors.affiliation: Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindelallee 48, 20146 Hamburg, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Malcherek
authors.affiliation: Centrum für Naturkunde (CeNak), Mineralogisches Museum, Universität Hamburg, Grindelallee 48, 20146 Hamburg, Germany.
authors.forename: Jochen
authors.initials: J
authors.lastname: Schlüter
keywords: crystal structure; gemstones; magnesiotaaffeite; modular structure; polysomatism; polytypism