DUMORTIERITE  , a

mineral described in 1881 by M . F . Gonnard, who named it after Eugene 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 .