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 fO2 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 K0.83CNa0.10B′(Mg1.78Fe0.16Mn0.03Ti0.03)A(Al2.88Fe0.12)T2(Al1.91Si10.09)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.

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