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THE STRUCTURE OF SPURRITE, TILLEYITE AND SCAWTITE, AND RELATIONSHIPS TO OTHER SILICATE–CARBONATE MINERALS

Joel D. Grice

Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada

E-mail address: jgrice{at}mus-nature.ca

Spurrite, Ca₅(SiO₄)₂(CO₃), tilleyite, Ca₅(Si₂O₇)(CO₃)₂, and scawtite, Ca₇(Si₆O₁₈)(CO₃)·2H₂O, are the only calcium carbonate–silicate minerals known to date; all three form in high-temperature skarns. Crystals of spurrite and tilleyite from Cornet Hill, Metaliferi Massif, Apuseni Mountains, Romania, and scawtite from the type locality in Scawt Hill, Ireland, were used to refine the structures to R values of 0.036, 0.021 and 0.018, respectively. All three structures are monoclinic: spurrite, space group P2₁/a, a 10.484(1), b 6.712(1), c 14.156(2) Å, ß 101.27(1)°, V 977,1(2) ų; tilleyite, space group P2₁/a, a 15.082(3), b 10.236(2), c 7.572(1) Å, ß 105.17°, V 1128.3(3) ų, and scawtite, space group Im, a 6.631(1), b 15.195(3), c 10.121, ß 100.59(3)°, V 1002.4(1) ų. The calcium silicate–carbonate structures are layered. The structure of spurrite has two layers; [CaO₈] polyhedra adjoin [CO₃] groups in one layer, whereas [CaO₇] polyhedra adjoin isolated [SiO₄] tetrahedra in the second layer. The structure of tilleyite has [CaO₈] polyhedra adjoining [CO₃] groups in one layer, whereas [CaO₈] polyhedra intertwine [Si₂O₇] pairs of tetrahedra in the other layer. The structure of scawtite has [CO₃] triangles sharing a layer with [Si₆O₁₈] rings and a second layer with [CaO_n] polyhedra (n between 6 and 8). The second layer has space to accommodate the H atoms of the H₂O molecule. The 13 described structures in the silicate–carbonate chemical class are compared. The high bond-strength and rigid nature of the carbonate group prevent carbonate–carbonate bonds and carbonate–silicate bonds, as the "bridging oxygen" would be overbonded. Polymerization of the silicate polyhedra is possible; the degree of polymerization depends on the cation-to-anion ratio and Lewis-base strength of the silicate group. The alkali, alkaline-earth and rare-earth elements have low Lewis-acid strengths, thus forming large polyhedra that act as connectors between the carbonate and silicate groups. As the degree of polymerization of the silicate groups increases, the Lewis base-strength decreases from 0.33 vu for nesosilicates to 0.06 vu for a double-sheet silicate. In general, all these structures are layered, with high-coordination polyhedra and carbonate groups in one slab and the silicate groups with lower-coordination polyhedra in the second layer. As the Lewis base-strength decreases in the silicate layer, there is an increase in the number of large cations within the layer.

Keywords: crystal structure, silicate–carbonate, Apuseni Mountains, Romania, Scawt Hill, Ireland, classification of structures.

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