Sokolova 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. The TS block 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 AP 2BP 2MH 2MO 4(Si2O7)2X4+n, where MH 2 and MO 4 are cations of the H and O sheets; MH = Ti (= Ti + Nb), Zr, Mn2+, Ca; MO = Ti, Zr, Fe2+, Mn2+, Ca, Na; AP and BP = cations at the peripheral (P) sites = Na, Ca, Ba, K; X = anions = O, OH, F; n = 0, 2, 4. 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 the oxyanions (PO4)3–, (SO4)2– and (CO3)2–. Structures of Ti disilicate minerals naturally fall into four groups, each characterized by the topology and stereochemistry of the TS block. In Groups I, II, III and IV, Ti = 1, 2, 3 and 4 apfu, respectively. General structural principles of Sokolova [1] have been developed for structures that contain one type of a TS block, one type of an I block and exhibit one type of self-linkage of TS blocks. We define them as basic structures. Here we consider 32 Ti disilicates. Of those 24 minerals [1], structure refinement was done for rinkite, barytolamprophyllite, nabalamprophyllite, murmanite and lomonosovite. Basic structures have been reported for mosandrite [2], nacareniobsite-(Ce) and jinshajiangite. Our recent work on 5 titanium disilicates, bornemanite, nechelyustovite and cámaraite, and two possible new mineral species, with preliminary ideal formulae Na4Ba Ti2Nb (Si2O7)2 O3 (OH) (H2O)2 (mineral A) and Na3KBa Ti2Nb (Si2O7)2 O3 (OH) (H2O)6 (mineral B), has resulted in discovery of a new type of TS-block structure. These new structures have more than one type of I block and one or more types of self-linkage of TS blocks. We define them as derivative structures. A derivative structure is usually related to two basic structures of the same Group, and all three structures have the same topology of the TS block and content of Ti. All derivative structures occur in Groups II and III (Ti = 2 and 3 apfu) and contain Ba in at least one I block. Discovery of derivative structures allows us to predict possible new structure topologies for Groups II and III. Those structure models will be developed in our talk. Here, we also address another problem: different topologies for Ti and Fe3+ disilicates. The crystal structure of orthoericssonite, ideally BaMn2Fe3+(Si2O7)O(OH), does not obey the general topological principles for Ti disilicates with the TS block. The substitution Ti4+ Fe3+ is quite common because of the similar size of the cations: 0.58 for [5]Fe3+ and 0.51 for [5]Ti4+. The O sheet of the HOH block in orthoericssonite is identical to the O sheet in the structures of Group-II Ti disilicates: MO = Fe2+, Mn2+, but the topology of the HOH block is different from the structures of Group II. We explain how bond-valence requirements of anions shared by the O and H sheets dictate different topologies for Ti and Fe3+ disilicates. Based on new structural information, revised structural principles for Ti disilicates are outlined.
From chemical composition to structure topology in Ti silicates.
CAMARA ARTIGAS, Fernando
2010-01-01
Abstract
Sokolova 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. The TS block 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 AP 2BP 2MH 2MO 4(Si2O7)2X4+n, where MH 2 and MO 4 are cations of the H and O sheets; MH = Ti (= Ti + Nb), Zr, Mn2+, Ca; MO = Ti, Zr, Fe2+, Mn2+, Ca, Na; AP and BP = cations at the peripheral (P) sites = Na, Ca, Ba, K; X = anions = O, OH, F; n = 0, 2, 4. 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 the oxyanions (PO4)3–, (SO4)2– and (CO3)2–. Structures of Ti disilicate minerals naturally fall into four groups, each characterized by the topology and stereochemistry of the TS block. In Groups I, II, III and IV, Ti = 1, 2, 3 and 4 apfu, respectively. General structural principles of Sokolova [1] have been developed for structures that contain one type of a TS block, one type of an I block and exhibit one type of self-linkage of TS blocks. We define them as basic structures. Here we consider 32 Ti disilicates. Of those 24 minerals [1], structure refinement was done for rinkite, barytolamprophyllite, nabalamprophyllite, murmanite and lomonosovite. Basic structures have been reported for mosandrite [2], nacareniobsite-(Ce) and jinshajiangite. Our recent work on 5 titanium disilicates, bornemanite, nechelyustovite and cámaraite, and two possible new mineral species, with preliminary ideal formulae Na4Ba Ti2Nb (Si2O7)2 O3 (OH) (H2O)2 (mineral A) and Na3KBa Ti2Nb (Si2O7)2 O3 (OH) (H2O)6 (mineral B), has resulted in discovery of a new type of TS-block structure. These new structures have more than one type of I block and one or more types of self-linkage of TS blocks. We define them as derivative structures. A derivative structure is usually related to two basic structures of the same Group, and all three structures have the same topology of the TS block and content of Ti. All derivative structures occur in Groups II and III (Ti = 2 and 3 apfu) and contain Ba in at least one I block. Discovery of derivative structures allows us to predict possible new structure topologies for Groups II and III. Those structure models will be developed in our talk. Here, we also address another problem: different topologies for Ti and Fe3+ disilicates. The crystal structure of orthoericssonite, ideally BaMn2Fe3+(Si2O7)O(OH), does not obey the general topological principles for Ti disilicates with the TS block. The substitution Ti4+ Fe3+ is quite common because of the similar size of the cations: 0.58 for [5]Fe3+ and 0.51 for [5]Ti4+. The O sheet of the HOH block in orthoericssonite is identical to the O sheet in the structures of Group-II Ti disilicates: MO = Fe2+, Mn2+, but the topology of the HOH block is different from the structures of Group II. We explain how bond-valence requirements of anions shared by the O and H sheets dictate different topologies for Ti and Fe3+ disilicates. Based on new structural information, revised structural principles for Ti disilicates are outlined.
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