Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Seryotkin, Y. V. Articles by Likhacheva, A. Y. GeoRef GeoRef Citation

Pyrometamorphic osumilite: occurrence, paragenesis, and crystal structure as compared to cordierite

¹ Institute of Geology and Mineralogy, Russian Academy of Sciences, Koptyuga 3, 630090 Novosibirsk, Russia
² Novosibirsk State University, Pirogova 2, 630090 Novosibirsk, Russia
³ Institute of Inorganic Chemistry, Russian Academy of Sciences, Lavrentieva 3, 630090 Novosibirsk, Russia

^* Corresponding author, e-mail: yuvs{at}uiggm.nsc.ru

A detailed mineralogical study and single-crystal X-ray analysis were carried out on K-Mg osumilite from high temperature pyrometamorphic rocks. These rocks were generated during spontaneous combustion of coal-bearing spoil-heaps in the South Urals, Russia. The osumilite-bearing metapelitic rocks – clinkers – contain K-Na- and K-Na-Ca feldspars, tridymite, mullite as main phases and magnesian K-bearing cordierite, corundum, native iron, pyrrhotite, cohenite, graphite, black carbon, and iron phosphide as minor phases. The composition of pyrometamorphic osumilites approaches the synthetic compound KMg₂Al₃(Al₂Si₁₀)O₃₀.

Pyrometamorphic alterations of sedimentary protolith during coal combustion proceeded at atmospheric pressure. The osumilite-bearing parageneses are formed under the conditions of low f_O2 and water activity, at temperatures above 900 °C and below the melting point for the osumilite-bearing associations. The crystal structure of osumilite was refined from X-ray single-crystal data. Its crystal-chemical formula is K_0.83^CNa_0.10^B'(Mg_1.78Fe_0.16Mn_0.03Ti_0.03)^A(Al_2.88Fe_0.12)^T2(Al_1.91Si_10.09)^T1O₃₀. K cations occupy the standard 12-coordinated C site, whereas Na cations populate the B'position, located between three 6-membered double rings above the A position. The comparative crystal-chemical analysis shows that in the structure of osumilite, in contrast to cordierite, potassium not only compensates for the framework charge, but also stabilizes the structure. The role of sodium in cordierite and osumilite is similar and consists in a charge compensating function.

Key-words: osumilite, crystal structure, crystal chemistry, paragenesis, pyrometamorphism.