Sokolova (2006) developed general structural principles and established the relation between structure topology and chemical composition for 24 Ti disilicate minerals containing the TS (titanium-silicate) block, which is composed of a central trioctahedral (O) sheet and two adjacent (H) sheets of [5-7]-coordinated polyhedra and Si2O7 groups. 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, Mn, Ca + REE, Ca; MO = Ti, Zr, Nb, Fe2+, Mg, Mn, Ca, Na; AP and BP = cations at the peripheral (P) sites = Na, Ca + REE, Ca, Ba, Sr, K; X = anions = O, OH, F, H2O; X4+n = XO4 + XPn, n = 0, 1, 2, 1.5, 4. The stoichiometry of core part of the TS block, MH2MO4(Si2O7)2XO4, is invariant. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. The I block consists of alkali and alkaline-earth cations, oxyanions (PO4)3-, (SO4)2- and (CO3)2-, and H2O groups. Structures of TS-block minerals 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. Here we introduce the concept of basic and derivative structures for 32 TS-block minerals. 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 a number of 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, possible atomic arrangements have been predicted for (1) 2 basic structures (Groups III and IV) and (2) 12 derivative structures (Groups II and III). Each derivative structure has been derived by adding structural fragments (which represent related basic structures) via sharing the central O sheet of the TS blocks of adjacent structural fragments.
From chemical composition to structure topology in Ti silicates: Prediction of new structure topologies
CAMARA ARTIGAS, Fernando;
2013-01-01
Abstract
Sokolova (2006) developed general structural principles and established the relation between structure topology and chemical composition for 24 Ti disilicate minerals containing the TS (titanium-silicate) block, which is composed of a central trioctahedral (O) sheet and two adjacent (H) sheets of [5-7]-coordinated polyhedra and Si2O7 groups. 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, Mn, Ca + REE, Ca; MO = Ti, Zr, Nb, Fe2+, Mg, Mn, Ca, Na; AP and BP = cations at the peripheral (P) sites = Na, Ca + REE, Ca, Ba, Sr, K; X = anions = O, OH, F, H2O; X4+n = XO4 + XPn, n = 0, 1, 2, 1.5, 4. The stoichiometry of core part of the TS block, MH2MO4(Si2O7)2XO4, is invariant. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. The I block consists of alkali and alkaline-earth cations, oxyanions (PO4)3-, (SO4)2- and (CO3)2-, and H2O groups. Structures of TS-block minerals 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. Here we introduce the concept of basic and derivative structures for 32 TS-block minerals. 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 a number of 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, possible atomic arrangements have been predicted for (1) 2 basic structures (Groups III and IV) and (2) 12 derivative structures (Groups II and III). Each derivative structure has been derived by adding structural fragments (which represent related basic structures) via sharing the central O sheet of the TS blocks of adjacent structural fragments.
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