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THE CRYSTAL STRUCTURE OF STRONALSITE AND A REDETERMINATION OF THE STRUCTURE OF BANALSITE

Ruslan P. Liferovich¹, Andrew J. Locock², Roger H. Mitchell^3, and Arkadiy K. Shpachenko⁴

¹ Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada
² Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
³ Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada
⁴ Geological Institute, Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, Apatity 184200, Russia

E-mail address: rmitchel{at}lakeheadu.ca

The structures of stronalsite, SrNa₂Al₄Si₄O₁₆, and banalsite, BaNa₂Al₄Si₄O₁₆, have been determined by direct methods using single-crystal X-ray diffractometry with a CCD area detector. The structures are topologically identical and adopt the non-centrosymmetric space-group Iba2 rather than the centrosymmetric space-group Ibam previously ascribed to banalsite. Unit-cell parameters are: a 8.4080(9), b 9.8699(11), c 16.7083(18) Å, V 1386.6(3) ų, Z = 4, D_calc. 2.92 g/cm³ for stronalsite (Khibina massif, Kola, Russia); a 8.5400(7), b 10.0127(9), c 16.7897(14) Å, V 1435.7(2) ų, D_calc. 3.05 g/cm³ for banalsite from Wales (Benallt mine, UK), and a 8.5068(16), b 9.9811(18), c 16.7485(31) Å, V 1422.1(5) ų, D_calc. 3.08 g/cm³ for banalsite from Långban (Sweden). The structures are built on an infinite framework of corner-sharing tetrahedra, with Si and Al being completely ordered at four independent sites. Alternately pointing up (U) and down (D) Si(1)–Al(1) and Si(2)–Al(2) tetrahedra form four- and eight-fold rings parallel to the (001) plane, resulting in a –UDUD– framework, which differs from the –UUDD– framework of feldspars. The stacking results in six-fold rings parallel to the (100) plane. The interstices of the framework are occupied by large Sr or Ba atoms (^XA²⁺), and smaller ^VINa⁺, ordered at alternate levels parallel to (001), and separated by c. The isostructural character of banalsite and stronalsite accords well with the complete solid-solution Ba_1–xSr_xNa₂Al₄Si₄O₁₆ occurring in nature.

Keywords: tectosilicate, stronalsite, banalsite, crystal structure, lisetite, solid solution, feldspar.

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