On the occurrence of ilvaite in the pegmatitic facies of the Whin Sill, Teesdale, North Pennines, England

lvaite, a rare calcium-iron silicate, is present in the dolerite-pegmatite facies of the Whin Sill at Forcegarth Quarry near High Force in Teesdale, Co. Durham. It has a near stoichiometric composition in the range Ca0.98-1.00Fe2+1.95-2.00Fe3+Si1.96-1.98O8(OH). Ilvaite forms part of an association of opaque minerals that includes ilmenite, altered titanomagnetite, titanite and minor sulphides, notably pyrrhotite, chalcopyrite and sphalerite that is characteristic of ilvaite in mafic igneous rocks. Other than the presence of ilvaite and trace amounts of molybdenite there is nothing unusual about the sampled dolerite-pegmatite suggesting that ilvaite may be present in dolerite-pegmatites elsewhere in the Whin Sill. This is the first authenticated occurrence of ilvaite in the British Isles. The opaque mineralogy of pink aplite from the Whin Sill at Ratcheugh Quarry near Alnwick comprises ilmenite, titanite and pyrite with minor amounts of pyrrhotite, chalcopyrite, iron-rich sphalerite and galena. Much of the sulphide assemblage and associated barite is later than the crystallisation of the aplite.