Acta Musei Moraviae

New occurrence of grandidierite, ominelite, boralsilite and Fe-rich werdingite in Horní Bory, Czech Republic – overview

A new occurrence of four anhydrous borosilicates (grandidierite, ominelite, boralsilite, Fe-rich werdingite) was found in Horní Bory quarry, Bory Granulite Massif, Moldanubicum, Czech Republic. The borosilicates were found associated with dumortierite, tourmaline and corundum. They occur in a thin veinlet (Kfs + Qtz ± Pl) cross-cutting a leucocratic, garnet- and kyanite-bearing granulite. Primary grandidierite to ominelite is enclosed in quartz; its crystals are locally overgrown by prismatic aggregates of boralsilite and Fe-rich werdingite. Boralsilite also occurs as separate radial aggregates with Fe-rich werdingite in quartz. Grandidierite-ominelite is commonly rimmed by a narrow zone of secondary tourmaline or is partially replaced by the assemblage tourmaline + corundum ± hercynite. Other accessory minerals in the veinlet involve monazite-(Ce), ilmenite, rutile to Nb-rich rutile, ferberite to Nb,Sc-rich ferberite, srilankite, löllingite, arsenopyrite and apatite.