Acta Musei Moraviae

Hydroxylherderite and associated beryllophosphates from pegmatite Rožná-Borovina

In lepidolite pegmatite Rožná, hydroxylherderite was known since 1950 in pseudomorphs after unknown mineral of pseudocubic shape and the original mineral remained unknown up to now. New samples available for this study confirmed rather monoclinic symmetry. Recent study of this material (powder X-ray diffraction, electron microprobe, cathodoluminiscence) revealed that several beryllium minerals occur within pseudomorphs in two distinct assemblages involving hydroxylherderite as a dominant mineral. The assemblage I consists of beryllonite + hurlbutite + hydroxylherderite + fluorapatite, whereas assemblage II contains bertrandite + quartz + hydroxylherderite + fluorapatite. In both assemblages, small inclusions of unknown Ba- or Sr-phosphates occur; stoichiometrically they best correspond to Ba- and Sr- equivalents of hurlbutite. Beryllonite was very likely the primary mineral of pseudomorphs with the assemblage I, whereas in the case of the assemblage II the primary mineral might be beryllonite or beryl. The herderite pseudomorphs from Rožná are a good example of complicated polyphase replacement of Be-dominant primary mineral (beryllonite and/or beryl) in highly variable activities of Ca, P, F and Si, producing various beryllophosphates versus Be-silicates.