The structure of spurrite, tilleyite and scawtite, and relationships to other silicate-carbonate minerals

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doi: 10.2113/gscanmin.43.5.1489
Authors:Grice, J. D.
Author Affiliations:Primary:
Canadian Museum of Nature, Research Division, Ottawa, ON, Canada
University of Saskatchewan, Canada
McGill University, Canada
Volume Title:S<sup>3</sup>; sulfides, structures, and synchrotron light; a tribute to Michael E. Fleet
Volume Authors:Henderson, Grant S., editor; Pan, Yuanming; Martin, Robert F.
Source:S>3`; sulfides, structures, and synchrotron light; a tribute to Michael E. Fleet, edited by Grant S. Henderson, Yuanming Pan and Robert F. Martin. The Canadian Mineralogist, Vol.43(Part 5), p.1489-1500. Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada. ISSN: 0008-4476
Publication Date:2005
Note:In English. 18 refs.; illus., incl. 4 tables
Summary:Crystals of spurrite and tilleyite from Cornet Hill, Metaliferi Massif, Apuseni Mountains, Romania, and scawtite from the type locality at Scawt Hill, Northern Ireland, were used to refine the cystal 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, b 6.712, c14.156 Å, β 101.27°, V 977.1Å3; tilleyite, space group P21/a, a 15.082, b 10.236, c 7.572 Å, β 105.17°, V 1128.3 Å3 and scawtite, space group Im, a 6.631, b 15.195, c 10.121 Å, β 100.59°, V 1002.4 Å3. The calcium silicate-carbonate structures are layered: the spurrite structure has two layers, CaO6 polyhedra adjoin CO3 groups in one layer, and CaO7 polyhedra adjoin isolated [SiO4] tetrahedra in the second layer. The structure of tilleyite has CaO8 polyhedra adjoining CO3 groups in one layer while 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 CaOn polyhedra (n between 6 and 8); the second layer has space to accomodate 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 over-bonded. 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 REE have low Lewis-acid strengths, thus forming large polyhedra that act as connectors between the carbonate and silicate groups. [R.A.H.]
Sections:Crystal structure
Subsections:Non-metallic deposits
Mineral Groups:Ortho-and ring silicates
Subjects:Carbonates; Cell dimensions; Chemical composition; Classification; Crystal chemistry; Crystal structure; Formula; Lattice parameters; Metals; Nesosilicates; Orthosilicates; Polyhedra; Rare earths; Ring silicates; Scawtite; Silicates; Spurrite; X-ray diffraction data; Apuseni Mountains; Europe; Ireland; Romania; Southern Europe; Western Europe; Cornet Hill; Metaliferi Massif; Scawt Hill; Tilleyite
Abstract Numbers:06M/888
Record ID:2006039685
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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