DUMORTIERITE, a mineral first recorded from pegma tite in gneiss at Chaponost, near Lyons, France, and named after E. Dumortier, a French palaeontologist. It is essentially a basic aluminium borosilicate, most reliable analyses conforming to the formula Crystallizing in the rhombic system, it occurs usually in fibrous or columnar aggregates of blue, lavender or almost black colour. These commonly observed colours are due to the oxides of iron and titanium always present in small amounts. The mineral is strongly pleochroic in blue and violet tints. On heating to 800° C the colour rapidly disappears. At higher temperatures is volatilized and decomposition oc curs with formation of mullite liquid appearing first at a temperature of 1,550°C.
On account of its high alumina content dumortierite possesses considerable advantages as a basis for refractory bodies. Dumor tierite occurs as a rare constituent of pegmatites and gneisses. The best known locality is at Clip, Ariz., where it is found as dense fibres embedded in quartz. Among European localities its presence in pegmatite at Ellon (Scotland), in cordierite-gneiss at Tvedestrand (Norway), with corundum in pegmatite at Wolf shan (Silesia) may be mentioned. In company with lazulite and kyanite it occurs in quartzites at Chari (French Central Africa) and Svarina (Madagascar) . (C. E. T.)