PCHALLER.] DUMORTIERITE. 113 
The axial angle on the California dumortierite is small. The fol- 
lowing measurements were made with the microscope and micrometer 
ocular, and represent only approximate values: 
2E Li =33° 
2E Na =37 
2E Cu =42 
The dispersion thus is p<.v. 
INDICES OF REFRACTION. 
Linck obtained for the dumortierite from Germany 
*=1.678 
p = 1.686 
^=1.689 
a — y— .011 
Michel Levy and Lacroix give (y—a) = . 010 with the dispersion 
(strong) p>£\ while Bertrand in his original description gives p<v as 
the dispersion. The value (y — a) for quartz is .009 and for andalusite 
.011. The birefringence for dumortierite is always a little higher than 
that of the quartz, and in the Washington mineral very nearly that of 
the andalusite. 
PYROGNOSTICS. 
The mineral is infusible before the blowpipe, but loses its color, 
becoming white. It gives a blue color on heating with cobalt nitrate. 
If the mineral be very finely powdered and intimately mixed with 
potassium bisulphate and calcium fluoride and the powder be carefully 
introduced into the flame, a green color, due to the boric acid, may be 
momentarily seen, but it is an exceedingly difficult reaction to obtain. 
CHTCMICAT, COMPOSITION. 
In 1881, after the first announcement of the discovery of the mineral 
had been made by Gonnard, Damour gave the following analysis of 
the mineral: 
SiO, 20. 85 
A1 2 3 66. 02 
Fe 2 G 3 1.01 
MgO 45 
Ign 2.25 
99. 58 
Specific gravity 3. .'>0 
From this analysis the formula 4Al 2 3 ,3Si0 2 was calculated. 
Damour did not suspect the presence of boric acid in the mineral, and 
what was weighed as alumina probably contained several per cent 
Bull. 202—05 8 
