MINERALOGY OF BLACK LAKE AREA. 
59 
It was at first thought that slight variations might be observed in the 
angles of the differently coloured crystals, due to a difference in chemical 
composition, and with a view to testing this, the means of the angles 
of the severally coloured varieties have been assembled together for 
comparison. For the three principal pyramids on the lilac, emerald, 
and pale yellow crystals the mean values for <p and p were as follows: 
Colour of 
No. of 
P(lll) 
No. of 
s(311) 
No. of 
i(312) 
crystal 
angles 
9 
P 
angles 
9 
P 
angles 
9 
9 
Lilac. . . . 
31 
45°01' 
37° 15^ 
37 
18° 25*' 
59° 35' 
22 
182° 7' 
40° 23 V 
Emerald. 
63 
44 59f 
37 15* 
112 
18 23* 
59 34| 
15 
18 29 1 
40 23 
Yellow... 
Gdt. 
19 
45 02 
37 16 
46 
18 26* 
59 33 
— 
— — • 
value. . 
' 1 
45 00 
37 14 
— 
18 26 
59 32 
18 26 
40 22 
The results are of a negative character, since the differences observed 
between the corresponding angles for the severally coloured varieties are 
only of, the same order as those met with in a number of crystals of the 
same colour. As will have appeared from the foregoing description, 
however, it would seem possible that the slight differences in composition, 
which have given rise to the several colours, have at least been sufficient 
to influence the habit of the crystals; in other words, the habit is, in 
general, constant in crystals of the same colour and distinctly different 
in crystals of different colours. 
Chemical Composition. 
Heating at 105 degrees C. produces no apparent change in the 
mineral, and there is practically no loss in weight (about 0*05 per cent); 
in one instance the finely powdered material was kept at 165 degrees C. 
for an hour, with the same result. At a higher temperature fusion with 
intumescence takes place, and if the material is in the form of a powder 
it cakes and turns a bath-brick colour, losing in weight at the same time 
to the extent of about 3 per cent. The melt obtained after fusion with 
sodium carbonate is green, indicating manganese, which is always 
present, though only in very small amount. The mineral is not acted 
on by acids. A qualitative test for chlorine and fluorine gave negative 
results. 
