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The Canadian Mineralogist; June 2004; v. 42; no. 3; p. 723-729; DOI: 10.2113/gscanmin.42.3.723
© 2004 Mineralogical Association of Canada
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Articles

THE CRYSTAL STRUCTURE OF STURMANITE

Dmitry Yu. Pushcharovsky1,§, Yulia S. Lebedeva1, Natalia V. Zubkova1, Marco Pasero2,¶, Marco Bellezza2, Stefano Merlino2 and Nikita V. Chukanov3

1 Faculty of Geology, Moscow State University, Vorobievy Gory, RU–119899 Moscow, Russia
2 Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via S. Maria 53, I–56126 Pisa, Italy
3 Institute of Chemical Physics Problems, RU–142432 Chernogolovka, Moscow Oblast, Russia

§ E-mail address: dmitp{at}geol.msu.ru

Sturmanite, ideally Ca6Fe2 3+ (SO4)2.5[B(OH)4](OH)12·25H2O, is trigonal, P31c, Z = 2, with a 11.188(9), c 21.91(7) Å, V 2375.1(5) Å3. The crystal structure has been refined to an R index of 0.079. As in the case of ettringite, two coaxial elements parallel to [001] can be considered as the most distinctive features of sturmanite. The main one is a column of polyhedra formed by Fe-octahedra and Ca-polyhedra. The Fe(OH)6 octahedra are located along the three-fold axes that pass through the origin of the cell. They are linked to three VIIICa-polyhedra, located around the three-fold axes, in between the Fe(OH)6 octahedra. The second structural element parallel to [001] contains [SO4] and B(OH)4 tetrahedra; it is located along three-fold axes at (1/3, 2/3, z) and (2/3, 1/3, z). Both structural modules are linked to each other by a complex system of hydrogen bonds. On the basis of a complete determination of the structure, the chemical composition of sturmanite is close to Ca6[Fe0.6 3+Al0.2Mn0.2 4+]2 [SO4]2.7[B(OH)3]0.3 [B(OH)4](OH)12·24H2O.

Keywords: sturmanite, crystal structure, boron coordination, Black Rock mine, South Africa.






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