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 (Si2O7) 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 A, and t 1 ∧ t 2 close to 90°. The general formula of the TS block is A2  P B2  P M2 H M4 O (Si2O7)2 X4+n, where M2 H and M4 O are cations of the H and O sheets; MH represents Ti (= Ti + Nb), Zr, Mn2+, Ca; MO represents Ti, Zr, Mn2+, 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 H2O; 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, (H2O) groups and oxyanions (PO4)3−, (SO4)2− and (CO3)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 (Si2O7) 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 (Si2O7) groups link to two M2+ 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 (Si2O7) 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 (Si2O7) 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 A, which allows the chemical composition of the TS block to vary widely. In a specific 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.