Schiavinatoite
Schiavinatoite is a very rare mineral, a natural niobium borate of the formula (Nb,Ta)BO4. Schiavinatoite is classified as monoborate. It contains tetrahedral borate anion instead of planar BO3 group, which is more common among minerals. Schiavinatoite is one of the most simple niobium minerals. It forms a solid solution with its tantalum-analogue, béhierite. Both minerals possess zircon-type structure (tetragonal, space group I41/amd) and occur in pegmatites. Schiavinatoite and nioboholtite are minerals with essential niobium and boron.