Chromium-bearing spinel – illite mineral assemblage in quartz rock from Radňoves near Křižanov (Gföhl Unit, Moldanubian Zone, Bohemian Massif)

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Acta Mus. Moraviae, Sci. Geol. 105 (2020), issue 1, pages 65-72
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Abstract:

Chromium-bearing spinel – illite mineral assemblage in quartz rock from Radňoves near Křižanov (Gföhl Unit, Moldanubian Zone, Bohemian Massif)

Fine-grained quartz rock was found near the southern contact of the garnet-spinel-orthopyroxene serpentinized peridotite body enclosing in the granulites and gneisses of the Gföhl unit (Moldanubian Zone). It is regularly grained, consisting of nearly isometric grains of quartz with accessory chromium-rich spinel. Irregular veinlets with small quartz crystals are also common. Spinel is almost completely replaced by tiny aggregates of chromium mica, which gives the rock a bright green color. The chemical composition of mica (6.53–7.01 wt. % K2O; 0.542–0.587 apfu K; 5.67–6.16 wt. % Cr2O3; 0.292–0.317 apfu Cr) corresponds to illite with higher chromium content. No relicts of the original rock have been preserved, but according to the occurrence of chromium-rich spinel it can be assumed that the protolith of the quartz rock was peridotite. Whether the silicification is of hydrothermal origin or is associated with the existence of a highly mature lateritic-type weathering is not proven.

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Stanislav Houzar, Pavla Hršelová*, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic, *e-mail: phrselova@mzm.cz

Citation:
Houzar, S., Hršelová, P., 2020: Chromem bohatá minerální asociace spinelu s illitem v křemenné hornině z Radňovsi u Křižanova (gföhlská jednotka, moldanubikum, Český masiv). – Acta Mus. Morav., Sci. Geol., 105, 1, 65–72 (with English summary)
ISSN: 1211–8796