High temperature crystal chemistry of K and Na fluor-richterites

Crystal structure parameters have been determined for synthetic potassium richterite K(NaCa)MgsSL"O22F2at24o,4(X)",600o, and 800'C, and synthetic sodium richterite Na(NaCa) Mg5Si6O22F2 at24",400",600',800', and 900'C. Anisotropic refinements in space group I2lm using I 150-1200 reflections resulted in R factors ranging from 0.034 to 0.063. The K atom in the A site of potassium richterite was refined at all temperatures using a half atom model with K randomly occupying two special positions (4i) within the (010) mirror plane. The Na atom in the A site of sodium richterite was refined using a quarter atom model in which Na randomly occupies four general positions (8.1) offboth the (010) mirror plane and the twofold axis parallel to b. As observed in other high-temperature studies of silicate minerals, the polyhedra in these structures expand differentially. Over the temperature intervals studied, the tetrahedral distances remain statistically identical, but all other mean polyhedral distances increase significantly. Mean thermal expansion coefficients (MTEC) for mean bond lengths increase as follows: Tl = T2 << M3 < Ml < M2 44 vIIt144 < xA (K richterite) and Tl: T2 << Ml < M2 < M3 qq VIII144 < xA 1Na richterite). This differential polyhedral expansion is accompanied by straightening of the tetrahedral chains and by increased displacement of the double chains relative to each other. Expansion of three of the four M polyhedra in K richterite is slightly greater than for Na richterite, and the MTEC of its unit cell volume is larger (3.39 vs. 3.10/"C x l0-5). The behavior of the richterite structures at elevated temperatures is generally similar to that of tremolite. The MTEC's for structural and thermal parameters are often identical within three standard deviations. The largest difference among the three structures involves the coordination polyhedron about the A site, where the rate of expansion of the mean l0-coordinated A-O distance is significantly greater in the richterites than in tremolite