FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. XVI. NEW DEVELOPMENTS IN THE CRYSTAL CHEMISTRY AND PREDICTION OF NEW STRUCTURE TOPOLOGIES FOR TITANIUM DISILICATE MINERALS WITH THE TS BLOCK

Abstract

New developments in crystal chemistry have been considered for 34 titanium disilicate minerals that contain the TS (Titanium-Silicate) block, a central trioctahedral (O) sheet, and two adjacent heteropolyhedral (H) sheets of [5–7]-coordinated polyhedra and Si 2 O 7 groups. 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 = cations of the H and O sheets; M H = Ti, Nb, Zr, Mn, Ca + REE, Ca; M O = Ti, Zr, Nb, Fe 2+ , Mg, Mn, Ca, Na; A P and B P = cations at the peripheral ( P ) sites = Na, Ca + REE, Ca, Ba, Sr, K; X = anions, O, OH, F, and H 2 O groups; X 4+n = X O 4 + X P n , n = 0, 1, 1.5, 2, 4. There are three topologically distinct TS blocks based on three types of linkage of H and O sheets. In the crystal structures of TS-block minerals, TS blocks either link directly or alternate with intermediate ( I ) blocks. The I block consists of alkali and alkaline-earth cations, oxyanions (PO 4 ), (SO 4 ) and (CO 3 ), and H 2 O groups. There are four groups of TS-block structures, based on the topology and stereochemistry of the TS block: Groups I, II, III, and IV, where Ti (+ Nb + Zr + Mg + Mn) = 1, 2, 3, and 4 apfu , respectively. In a TS-block structure, four types of self-linkage between adjacent TS blocks occur. The concept of basic and derivative structures has been introduced for TS-block minerals. A basic structure has the following four characteristics: (1) there is only one type of TS block; (2) the two H sheets of the TS block are identical; (3) there is only one type of I block, or it is absent; and (4) there is only one type of self-linkage of TS blocks. Basic structures obey the general structural principles of Sokolova (2006). A derivative structure has one or more of the three following characteristics: (1) there is more than one type of TS block; (2) there is more than one type of I block; (3) there is more than one type of self-linkage of TS blocks. A derivative structure is related to two or more basic structures of the same Group: it can be derived by adding these structures via sharing the central O sheet of the TS blocks of adjacent structural fragments which represent basic structures . There are 30 basic TS-block structures and four derivative TS-block structures. Based on established relations between basic and derivative structures, possible atomic arrangements and chemical formulae have been predicted for 12 derivative structures and two basic structures.