Bernalite from the Clara mine, Germany, and the incorporation of tungsten in minerals containing ferric iron

The very rare mineral bernalite is a ferric iron hydroxide with a structure related to that of perovskite. The present description of only the second known occurrence focusses on two samples from the Clara mine, central Black Forest, Germany. X-ray powder diffraction, scanning electron microscopy, a nd optical microscopy are used to characterize the mineral. The bernalite i s polysynthetically twinned, with the twinning seen both microscopically, i n the case of apparently "single" crystals, and macroscopically, in crystal lographically oriented intergrowths of crystals. The samples investigated c ontain small amounts of tungsten and trace amounts of silicon. Crystal-chem ical considerations suggest that W6+ ions in bernalite and in other ferric iron minerals substitute for Fe3+ ions.