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

Abstract

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) Å, V = 333.67(3) Å3, R1 = 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 Å 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 P63/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.

Mechanical Properties of CaO–Al2O3–SiO2 Glass-Ceramics Precipitating Hexagonal CaAl2Si2O8 Crystals

“…The mineral with a hexagonal structure is known as "dmisteinbergite" [37]. Its crystal structure and phase diagram have been investigated by several researchers [38][39][40][41][42]. The mineral is also known to have a cleavage behavior [37].…”

Structural and chemical complexity of minerals: an update

“…Krivovichev (2013) provided an overview of relations between structural complexity and metastable crystallisation based upon the Goldsmith's simplexity principle that states that simple structures crystallise more easily. This idea was supported by many subsequent observations, including metastable feldspar polymorphs (Zolotarev et al, 2019;Krivovichev, 2020, and references therein), perovskite-group minerals (Zaitsev et al, 2017), uranyl minerals (Plášil et al, 2018b(Plášil et al, , 2018c, sulfates (Plášil et al, 2017;Majzlan et al, 2018), phosphates and arsenates (Krivovichev et al, 2016;Krivovichev, 2017;Kolitsch et al, 2020), Cu 2 (OH) 3 Cl polymorphs (Krivovichev et al, 2017), borates (Grew et al, 2016), borosilicates (Cempírek et al, 2016), oxalates (Huskić et al, 2019), etc. In his review on metastable-mineral formation in oxidation zones and mine wastes, Majzlan (2020) recently noted that "a quantitative link…" between structural complexity and metastability "…is missing".…”

Rapidcreekite of anthropogenic origin - ’korkinoite’ from burnt mine dump in the Chelyabinsk coal basin, South Urals, Russia: crystal structure refinement, thermal behavior and spectroscopic characterization

“…The current work is a continuation of the series of our recent publications about the anthropogenic mineralogy of burnt dumps of coal mines in the Chelyabinsk coal basin (Zolotarev et al 2019a(Zolotarev et al , 2019b(Zolotarev et al , 2020(Zolotarev et al , 2021Avdontceva et al 2021) and is devoted to the study of crystal structure, chemical composition, thermal behavior, Raman and IR spectra of 'korkinoite', the mineral phase that was described by Chesnokov (1995) from the huge dump of the Korkinskiy open pit near the city of Korkino and the dumps of mines Nos. 45, 47 and 204 in the city of Kopeisk as a potentially new mineral with the formula Ca 4 (SO 4 ) 2 (CO 3 ) 2 •9H 2 O.…”