DUMORTIERITE, a mineral described in 1881 by M.F. Gonnard, who named it after Eugène Dumortier, a palaeontologist of Lyons, France. It is essentially a basic aluminium borosilicate, belonging to the orthorhombic system; it occurs usually in fibrous forms, of smalt-blue, greenish-blue, lavender or almost black colour, and exhibits strong pleochroism. According to W.T. Schaller (Amer. Journ. Sci., 1905 (iv.), 19, p. 211) a purple colour may be due to the presence of titanium. Analyses of some specimens point to the formula (SiO4)3Al(AlO)7(BO)H, which, written in this form, explains the analogy with andalusite and the alteration into muscovite. Dumortierite occurs in gneiss at Chaponost, near Lyons, and at a few other European localities; it is found also in the United States, being known from near New York City, from Riverside and San Diego counties, California, and from Yuma county, Arizona. The last-named locality yields the mineral in some quantity in the form of dense fibres embedded in quartz, to which it imparts a blue colour. The mineral aggregate is polished as an ornamental stone, rather resembling lapis lazuli.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)