Original paper

Crystal structure refinement of synthetic osumilite-type phases: BaMg2Al6Si9O30, SrMg2Al6Si9O30 and Mg2Al4Si11O30

Winter, Wolfgang; Armbruster, Thomas; Lengauer, Christian

European Journal of Mineralogy Volume 7 Number 2 (1995), p. 277 - 286

38 references

published: Mar 29, 1995
manuscript accepted: Nov 14, 1994
manuscript received: Aug 24, 1994

DOI: 10.1127/ejm/7/2/0277

BibTeX file

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Abstract

Abstract Three synthetic osumilite-type double-ring silicates of technical interest as crystalline basis of glass-ceramic materials with compositions BaMg2Al6Si9O30, SrMg2Al6Si9O30 and Mg2Al4SinO30 (MAS) were synthesized by crystallization of glasses and their crystal structures were subsequently refined. The structure of BaMg2Al6Si9O30 (P6/mcc, a = 10.129(2), c = 14.340(2) Å) was refined to Rw = 2.46 % for 1020 single-crystal reflections with MoKa X-radiation. The structures of SrMg2Al6Si9O30 (a = 10.1273(5), c = 14.2274(8) Å) and Mg2Al4Si11O30 (a = 10.058(2), c = 14.336(3) Å) were refined in space group P6/mcc from CuKα X-ray powder data using multiphase Rietveld methods. The C site between consecutive double-ring units is occupied by Ba2+ and Sr2+ in BaMg2Al6Si9O30 and SrMg2Al6Si9O30, respectively, while it is empty in MAS osumilite. This leads to increased C-O2 distances in MAS osumilite (3.18(1) Å) compared to 3.041(1) and 3.025(9) Å for the Ba- and Sr-bearing compounds, respectively. The longer C-O2 distance is compensated by a stronger distortion of the neighbouring T1 double-rings and T2 tetrahedra. In BaMg2Al6Si9O30 and SrMg2Al6Si9O30, Mg is constrained to the octahedral A site whereas in MAS osumilite the A octahedron and the T2 tetrahedron have a mixed Mg, Al occupation. Osumilite-type compounds with partial Al in the six-membered double-rings show a different T1-O-T1 angular distortion compared to double-ring silicates with pure Si12O30 units. The reason for this difference is not primarily the Al concentration on T1 but the higher ionic valences on A and T2 which become necessary for charge balance

Keywords

synthetic osumilitescrystal structuresingle-crystalRietveld analysis