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From structure topology to chemical composition. XIV. Titanium silicates: refinement of the crystal structure and revision of the chemical formula of mosandrite, (Ca3 REE)[(H2O)2Ca0.5□0.5]Ti(Si2O7)2(OH)2(H2O)2, a Group-I mineral from the Saga mine, Morje, Porsgrunn, Norway

Published online by Cambridge University Press:  05 July 2018

E. Sokolova  and
F. C. Hawthorne
E. Sokolova*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
F. C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
*
* E-mail:elena_sokolova@umanitoba.ca
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Abstract

The crystal structure of mosandrite, ideally (Ca3 REE)[(H2O)2Ca0.5☐0.5]Ti(Si2O7)2(OH)2(H2O)2, from the Saga mine, Morje, Porsgrunn, Norway, has been refinedas two components related by the TWIN matrix ( 0 0, 0 0, 1 0 1): a 7.4222(3), b 5.6178(2), c 18.7232(7) Å, β 101.4226(6)°, V = 765.23(9) Å3, space group P21 /c, Dcalc. = 3.361 g.cm–3,R 1 = 3.69% using 1347 observed (F o > 4σF) reflections. The empirical formula of mosandrite (EMPA) was calculated on the basis of 4 Si a.p.f.u., with H2O determined from structure refinement: [(Ca2.89Ba0.01)Σ2.90(Ce0.39La0.18Nd0.14Sm0.02Gd0.03Y0.16Th0.03)Σ1.01Zr0.09]Σ4[(H2O)2.00Ca0.32Na0.17Al0.10Mn0.04Fe2+ 0.02☐0.35]Σ3(Ti0.87Nb0.09Zr0.04)Σ1(Si2O7)2[(OH)1.54F0.46]Σ2[(H2O)1.50F0.50]Σ2,Z = 2. The crystal structure of mosandrite is a framework of TS (titanium silicate) blocks; each TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). In the TS block, there are five fully occupied cation sites, two [4]-coordinated Si sites with <Si–O>1.623 Å , [7]-coordinated M H and A P sites occupied by Ca and REE in the ratio ∼3:1, and one [6]-coordinated Ti-dominant M O(1) site. There are two H2O-dominant H2O-alkali-cation sites. The partly occupiedM O(2) site has composition [(H2O)0.5☐0.33Na0.17], ideally [(H2O)0.5☐0.5] p.f.u. The M O(3) site has ideal composition [(H2O)1.5Ca0.5] p.f.u.In the O sheet, the X O M and X O A anion sites have compositions [(OH)1.54F0.46] (X O M) and [(H2O)1.50F0.50] (X O A), ideally(OH)2 and (H2O)2 p.f.u. The MH and AP polyhedra and Si2O7 groups constitute the H sheet that is completely ordered. In the O sheet, M O(1) octahedra are long-range ordered whereas H2O andOH groups and alkali cations Na and Ca are long-range disordered. Two SRO (short-range ordered) arrangements have been proposed for the O sheet: (1) Na [MO(2)], Ca2 [MO(3)] and F4[X O M and X O Aanion sites]; (2) 2 H2O [MO(2)] and M O(3)] and (OH)2 and (H2O)2 [X O M and X O A]. Linkage of H and O sheets occurs mainly via common vertices of MHpolyhedra and Si2O7 groups and M O(1) octahedra. Two adjacent TS blocks are related by the glide plane c y. Mosandrite is an H2O- and OH-bearing Na- and Ca-depleted analogue of rinkite, ideally (Ca3 REE)Na(NaCa) Ti(Si2O7)2(OF)F2.Mosandrite and rinkite are related by the following substitution at the M O(2,3) and X O (M,A) sites in the O sheet: M [(H2O)2 + ☐0.5] + X [(OH)– 2+ (H2O)2] ↔ M [Na+ 2 + Ca2+ 0.5] + X [(OF)3– + (F2)2–].

Keywords

mosandritecrystal-structure refinementEMP analysischemical formulaTi-disilicate mineralsrinkiteGroup I

Information

Type
Research Article
Information
Mineralogical Magazine , Volume 77 , Issue 6 , August 2013 , pp. 2753 - 2771
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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