Prediction of New Structural Arrangements and Chemical Compositions in Ti Silicates.

Sokolova [1] developed general structural principles and established the relation between structure topology and chemical composition for Ti disilicate minerals containing the block, which is composed of a central trioctahedral (O) sheet and two adjacent (H) sheets of [5-7]-coordinated polyhedra and Si2O7 groups. All structures consist of a TS block and may or not have 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 the oxyanions (PO4)3-, (SO4)2-and (CO3)2-. Structures of Ti-disilicate minerals with the TSblock fall into four groups, each characterized by the topology and stereochemistry of the TS block: Groups I, II, III and IV, with Ti = 1, 2, 3 and 4 apfu, respectively. The general formula of the TS block is AP2BP2MH2MO4(Si2O7)2X4+n, where MH2 and MO4 = cations of the H and O sheets; MH= Ti, Nb, Zr, Mn2+, Mg, Ca; MO= Ti, Zr, Mg, Mn2+, Ca, Na; APand BP= cations at the peripheral (P) sites = Na, Ca, K, Ba, Sr; X = anions = O, OH, F, H2O; X4+n = XO4 + XPn, n = 0, 1, 2, 1.5, 4. The stoichiometry HO Oof core part of the TS block, MH2MO4(Si2O7)2XO4, is invariant. There are two types of structures with the TS block. Basic TS-block structure contains one type of TS and I blocks (or TS blocks link directly, additional cations do not occur and the I block is absent) and is characterized by one type of self-linkage of TS blocks; the two H sheets of one TS block are invariably identical. Derivative TS-block structure contains one or more types of TS and I blocks, is characterized by one or more types of self-linkage of TS blocks and is related to several basic structures of the same group. Basic and derivative structures occur in 28 and 4 TS-block minerals (Groups II and III), respectively. Based on established relationships between derivative and basic structures, atomic arrangements and chemical formulae have been predicted for 2 basic structures (Groups III and IV) and 13 derivative structures (Groups II and III). [1] Sokolova, E. (2006) Can. Min. 44, 1273-1330.