• journal_title：Mineralogical Magazine
• Contributor：T. Armbruster ; B. Lazic ; I. O. Galuskina ; E. V. Galuskin ; E. Gnos ; K. M. Marzec ; V. M. Gazeev
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2012.076.3.02
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：76
• issue：3
• firstpage：455
• section：Articles

The crystal structure of the rare skarn mineral trabzonite, Ca₄[Si₃O₉(OH)]OH, from the type locality near Ikizdere, Turkey and from the Upper Chegem caldera, Northern Caucasus, Kabardino-Balkaria, Russia has been solved and refined using single-crystal X-ray data. This shows that the chemical formula should be modified from Ca₄(Si₃O₁₀)·2H₂O, reported in the original trabzonite description, to an OH-bearing composition. The crystal structure, which contains Si₃O₁₀ trimers embedded in a framework of CaO_6–8 polyhedra, has orthorhombic symmetry, space group m>Amam>2, m>am> = 20.6, m>bm> = 9.1, m>cm> = 10.3 Å. The orthorhombic m>Am>-centred cell is easily obtained by transformation from the original monoclinic cell of m>Pm>2₁/m>mm> symmetry. The revised formula means that trabzonite and foshagite are polymorphs; foshagite has wollastonite-like silicate chains and in contrast to trabzonite it does not contain silanol groups.

The structure and composition of killalaite from both localities was also studied. Single-crystal X-ray structure refinement of killalaite from the Northern Caucasus confirmed it to be non-stoichiometric with a composition between Ca₆[Si₂O₆(OH)]₂(OH)₂ and Ca₇[Si₂O₇]₂(OH)₂ (m>Zm> = 2). Trabzonite, killalaite and dellaite form a series of modular structures which differ mainly in the degree of condensation of the SiO₄ units.