Crystal chemistry of double-ring silicates: structure of natural and dehydrated milarite at 100 K
Armbruster, Thomas; Bermanec, Vladimir; Wenger, Marc; Oberhänsli, Roland
European Journal of Mineralogy Volume 1 Number 3 (1989), p. 353 - 362
published: Jul 27, 1989
manuscript accepted: Feb 1, 1989
manuscript received: Aug 8, 1988
ArtNo. ESP147050103007, Price: 29.00 €
Abstract The crystal structure of milarite (space group P6/mcc, a = 10.398(2), c = 13.771(4) Å, Z = 2), D(K,Ca)0.01B(K,Ca)0.14CK ACa2T2(Be2.31 Al0.69) T1Si12 O30 [0.6 B'H2O], from Val Giuf (Graubunden, Switzerland), was refined at 100 K from single-crystal X-ray data. Difference-Fourier maps showed an electron density triplet of 3 e/Å3 along the 6̅-axis at 1/3, 2/3, ± 0.046 (B' site) and 1/3, 2/3, 0 (B site). A weak peak of 0.7 e/Å3 occurred at 0, 0, 0 (D site). A second crystal was dehydrated at 1100 K, and an additional X-ray data set was collected at 100 K. Heating changed the cell dimensions to a = 10.363(2) Å and c = 13.827(3) Å. The weak peak at the D site (K) remained with one peak of the triplet at 1/3, 2/3, 0, (Na,K,Ca), whereas the peak at 1/3, 2/3, ± 0.046 vanished ; thus the latter represents H2O in natural milarite. The change in cell dimensions as well as the variation in fractional coordinates and displacement parameters upon heating is explained in terms of strong interactions between the CaO6 coordination polyhedron and the adjacent H2O molecule. Refractive indices decrease due to dehydration from n̄ =1.542 to n̄ = 1.526.