From Structure Topology to Chemical Composition. I. Structural Hierarchy and Stereochemistry in Titanium Disilicate Minerals

Abstract

The structural hierarchy and stereochemistry have been considered for 24 titanium disilicate minerals that contain the TS (Titanium Silicate) block, a central trioctahedral (O) sheet, and two adjacent (H) sheets of [5]-and [6]-coordinated polyhedra and (Si 2 O 7 ) groups. The TS block is characterized by a planar cell based on translation vectors t 1 and t 2 , with t 1 ≈ 5.5 and t 2 ≈ 7 Å, and t 1 ٙ t 2 close to 90°. The general formula of the TS block is A P 2 B P 2 M H 2 M O 4 (Si 2 O 7 ) 2 X 4+n , where M H 2 and M O 4 are cations of the H and O sheets; M H represents Ti (= Ti + Nb), Zr, Mn 2+ , Ca; M O represents Ti, Zr, Mn 2+ , Ca, Na; A P and B P represent cations at the peripheral (P) sites, i.e., Na, Ca, Ba; X represents the anions O, OH, F and H 2 O; n = 0, 2, 4. Cations in each sheet of the TS block form a close-packed layer, and the three layers are cubic close-packed. There are three topologically distinct TS blocks, depending on the type of linkage of two H sheets and the central O sheet. The H sheets of one TS block are invariably identical and attach to the O sheet in the same way. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, (H 2 O) groups and oxyanions (PO 4 ) 3-, (SO 4 ) 2-and (CO 3 ) 2-. These structures naturally fall into four groups, based on differences in topology and stereochemistry of the TS block. In Group I, Ti = 1 apfu, Ti occurs in the O sheet, and (Si 2 O 7 ) groups link to a Na polyhedron of the O sheet (linkage 1). In Group II, Ti = 2 apfu, Ti occurs in the H sheet, and (Si 2 O 7 ) groups link to two M 2+ octahedra of the O sheet adjacent along t 2 (linkage 2). In Group III, Ti = 3 apfu, Ti occurs in the O and H sheets, and (Si 2 O 7 ) groups link to the Ti octahedron of the O sheet (linkage 1). In Group IV, Ti = 4 apfu (the maximum possible content of Ti in the TS block), Ti occurs in the O and H sheets, and (Si 2 O 7 ) groups link to two Ti octahedra of the O sheet adjacent along t 1 (linkage 3). The stability of the TS block is due to an extremely wide range in Ti(Nb)-O bond lengths, 1.68-2.30 Å, which allows the chemical composition of the TS block to vary widely. In a specifi c structure, only one type of TS block and only one type of I block occur. The TS block propagates close-packing of cations into the I block. General structural principles have been developed for these 24 titanium disilicates, and the relation between structure topology and chemical composition has been established for minerals based on the TS block. SOMMAIRE La hiérarchie structurale et les relations stéréochimiques ont été évaluées pour 24 minéraux disilicatés de titane contenant le bloc TS (silicate de titanium), un feuillet central trioctaédrique (O), et deux feuillets adjacents (H) contenant des polyèdres à coordinence [5] et [6] et des groupes (Si 2 O 7 ). Le bloc TS contient une maille planaire fondée sur les vecteurs de translation t 1 et t 2 , avec t 1 ≈ 5.5 et t ...

Shkatulkalite, a Rare Mineral from the Lovozero Massif, Kola Peninsula: A Re-Investigation

“…In agreement with the suggestions by Németh et al [25] and Sokolova and Cámara [6], shkatulkalite belongs to the seidozerite supergroup, since its structure is based upon the HOH [22] or TS [17] blocks. More precisely, shatulkalite is a member of the lamprophyllite group, having Ti + (Nb + Mn) = 3 apfu, from which the H and O sheets has 2 and 1 (Ti + Nb + Mn) apfu, respectively.…”

“…Belov and Organova [14] were the first who considered the crystal chemistry of several minerals of the current murmanite group of seidozerite supergroup [6]. Belov [15] and Pyatenko et al [16] made further generalizations and called minerals with a "seidozerite block" (=TS block, [17]) and astrophyllite-group minerals titanosilicate analogues of micas. The modular approach to these minerals has been developed by Egorov-Tismenko and Sokolova [18,19], who described a homologous series of Ti-analogues of micas, and Ferraris [20][21][22], who named those minerals heterophyllosilicates and described them as members of a single polysomatic series.…”

“…The modular approach to these minerals has been developed by Egorov-Tismenko and Sokolova [18,19], who described a homologous series of Ti-analogues of micas, and Ferraris [20][21][22], who named those minerals heterophyllosilicates and described them as members of a single polysomatic series. Sokolova [17] quantitatively divided TS-block minerals into four groups based on the content of Ti, structural topology and stereochemistry of the TS block.…”

Petrology of nepheline syenite pegmatites in the Oslo Rift, Norway: Zr and Ti mineral assemblages in miaskitic and agpaitic pegmatites in the Larvik Plutonic Complex

“…For the compositions of individual species, see Table 1. The structures of the two groups differ mainly in the mode of stacking of polyhedral sheets (Sokolova 2006). …”

From structure topology to chemical composition. X. Titanium silicates: the crystal structure and crystal chemistry of nechelyustovite, a group III Ti-disilicate mineral

“…Structures with the TS block have a characteristic two-dimensional minimal cell in the plane of the O sheet, with t 1 & 5.5 and t 2 & 7.1 Å , t 1^t2~9 0º. Sokolova (2006) established the relation between structure topology and chemical composition for those minerals and divided them into four groups, characterized by different topology and stereochemistry of the TS block. The topology and stereochemistry of the TS block is strongly related to the stereochemistry of Ti.…”