1911 Encyclopædia Britannica/Dumortierite

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 boro-silicate, 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.