Acta Musei Moraviae

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.