(Ga,Ge)-Analogue of Tourmaline: Crystal Structure and Composition
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(Ga,Ge)-Analogue of Tourmaline: Crystal Structure and Composition D. Yu. Pushcharovskya,*, N. V. Zubkovaa, T. V. Setkovab, V. S. Balitskiib, A. N. Nekrasovb, and V. A. Nesterovaa a Geology
b Institute
Department, Moscow State University, Moscow, 119899 Russia of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *e-mail: [email protected] Received April 22, 2020; revised May 3, 2020; accepted May 7, 2020
Abstract—Crystals of a (Ga,Ge)-analogue of tourmaline have been synthesized by hydrothermal method in the temperature range of 600–650°C under a pressure of 100 MPa. The composition is analyzed, and the structure of single crystals is determined by X-ray diffraction (XRD) analysis. The rhombohedral-cell parameters are found to be a = 16.0102(4) Å, c = 7.18423(19) Å, sp. gr. R3m, and Z = 3. The distribution of cations over nonequivalent sites in the structure under study is determined to be as follows: + 2+ Na0.57(Al1.20Ga1.20Fe20.60 )(Al4.728Fe1.20 Ti0.072)((Si4.80Ge0.78Al0.42)O18)(BO3)3(OH)3(O0.42ОН0.58). The results obtained confirm possibility for obtaining Ge-substituted crystals with tourmaline structure, which (by analogy with the Ge-analogue of quartz) exhibit high piezo- and pyroelectric properties. DOI: 10.1134/S1063774520060279
INTRODUCTION Representatives of the tourmaline supergroup belong to the superfamily of borosilicates, comprising about 130 mineral forms [1], among which 40 belong to the largest tourmaline group. This is one of the most chemically complex groups of the class of silicates [2] with the general crystallochemical formula XY3Z6(T6O18)[BO3]3V3W, where X are nine-vertex polyhedra with large Na+, Ca2+, and K+ cations or O6 Z
Y O2
O8
X O7 T
O5
O4
O1 c b
a
O3
Fig. 1. Structure of tourmaline with indicated atomic sites. Black triangles correspond to [BO3] anions.
vacancies (□); Y are octahedra with Li+, Mg2+, Fe2+, Mn2+, Al3+, Cr3+, V3+, Fe3+, Ti4+, etc. cations; Z are deformed octahedra with Al3+, Mg2+, Fe2+, Fe3+, Mn3+, V3+, Cr3+, Ti4+, etc.; T are tetrahedra with Si4+, Al3+, B3+, Be2+; V are (OH)–, O2–; and W are (OH)–, F–, and O2– cations. The polyhedral framework of the tourmaline structure with indicated atomic sites is shown in Fig. 1. Its most characteristic feature is the “antigorite” fragment, formed by six-membered rings of TO4 tetrahedra, based on triads of YO6 octahedra. Several crystallochemical reviews were devoted to wide variations in the tourmaline composition and their influence on the tourmaline structural transformations [2–4]. It is fairly difficult to discuss the specific features of the composition, structure, crystallization conditions, and properties of this important mineral (playing the role of indicator in many geological processes). In this context, we only note that 1125 publications were devoted to tourmaline in the period from 1766 to 1997, whereas the results of various studies of this mineral in the next 20 years were reported in 1130 publications [5]. Possible structural
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