Dmisteinbergite, CaAl2Si2O8, a Metastable Polymorph of Anorthite: Crystal-Structure and Raman Spectroscopic Study of the Holotype Specimen

The crystal structure of dmisteinbergite has been determined using crystals from the type locality in Kopeisk city, Chelyabinsk area, Southern Urals, Russia. The mineral is trigonal, with the following structure: P312, a = 5.1123(2), c = 14.7420(7) angstrom, V = 333.67(3) angstrom(3), R-1 = 0.045, for 762 unique observed reflections. The most intense bands of the Raman spectra at 327s, 439s, 892s, and 912s cm (-1) correspond to different types of tetrahedral stretching vibrations: Si-O, Al-O, O-Si-O, and O-Al-O. The weak bands at 487w, 503w, and 801w cm(-1) can be attributed to the valence and deformation modes of Si-O and Al-O bond vibrations in tetrahedra. The weak bands in the range of 70-200 cm(-1) can be attributed to Ca-O bond vibrations or lattice modes. The crystal structure of dmisteinbergite is based upon double layers of six-membered rings of corner-sharing AlO4 and SiO4 tetrahedra. The obtained model shows an ordering of Al and Si over four distinct crystallographic sites with tetrahedral coordination, which is evident from the average <T-O> bond lengths (T = Al, Si), equal to 1.666, 1.713, 1.611, and 1.748 angstrom for T1, T2, T3, and T4, respectively. One of the oxygen sites (O4) is split, suggesting the existence of two possible conformations of the [Al2Si2O8](2-) layers, with different systems of ditrigonal distortions in the adjacent single layers. The observed disorder has a direct influence upon the geometry of the interlayer space and the coordination of the Ca2 site. Whereas the coordination of the Ca1 site is not influenced by the disorder and is trigonal antiprismatic (distorted octahedral), the coordination environment of the Ca2 site includes disordered O atoms and is either trigonal prismatic or trigonal antiprismatic. The observed structural features suggest the possible existence of different varieties of dmisteinbergite that may differ in: (i) degree of disorder of the Al/Si tetrahedral sites, with completely disordered structure having the P6(3)/mcm symmetry; (ii) degree of disorder of the O sites, which may have a direct influence on the coordination features of the Ca2+ cations; (iii) polytypic variations (different stacking sequences and layer shifts). The formation of dmisteinbergite is usually associated with metastable crystallization in both natural and synthetic systems, indicating the kinetic nature of this phase. Information-based complexity calculations indicate that the crystal structures of metastable CaAl2Si2O8 polymorphs dmisteinbergite and svyatoslavite are structurally and topologically simpler than that of their stable counterpart, anorthite, which is in good agreement with Goldsmith's simplexity principle and similar previous observations.