62 
MUSEUM BULLETIN NO. 27. 
to in dealing with the crystallography, viz., the faces of the prism (100), 
even when free from vertical strise, are in general not simple, but each 
is made up of two parts inclined to one another at an angle varying up 
to half a degree or more, their edge being parallel to the principal axis 
of the crystal. It is possible also that the variable values found for the 
refractive indices may be partly due to a slight non-homogeneity in the 
material of the crystals, such as has been frequently observed in the 
case of vesuvianite. 
The crystals are always optically negative, and the double refraction 
is weak. The average of ten fairly good determinations on the lilac 
crystals gave for the refractive indices: 
o) = 1*708, € = 1*705, and e-a) = -0*003 
As an indication of the limits which were observed, the eight possible 
45-degree prisms in one crystal examined, and on which the first and 
second order tetragonal prisms were about equally developed, gave 
values ranging from 
= 1*705, e = 1*702 to ci) = 1*711, e = 1*709. 
The indices of the emerald-green and yellowish-green crystals also 
fall within these limits, and approach closely to the above mean values, 
from which it would appear that the effect on the refractive power of 
their slight differences in chemical composition is not very pronounced. 
Although it has not been possible to determine the refractive indices 
very exactly, the results are sufficient to show that the crystals have an 
appreciably lower refractive power than is usual in vesuvianite, and 
this is no doubt owing to their relative freedom from iron. The highest 
observed limits for these crystals (o) = 1*711, e = 1*709) are lower 
than the generally recorded values for vesuvianite, which range from 
(t) — 1*712 to 1*732 and e = 1*7108 to 1*726; although Iddings 1 quotes 
one exceptional case in which green-brown crystals from Sandford, Maine, 
were found by Hlawatsch to have the values o) — 1*705, e =1*701. In 
each case the refractive indices given above refer to yellow (sodium) 
light. 
Examined with the dichroscope, the crystals exhibit a noticeable 
pleochroism, which, although weak, is fairly well marked in the stouter 
crystals ; this is as follows : 
Colour of crystals. 
For light vibrating along 
horizontal axis (&>). 
For light vibrating along 
vertical axis (e). 
Lilac 
pink 
colourless 
Emerald-green 
bottle-green 
bluish-green 
Clove-brown 
deep brown 
paler brown 
1 Iddings, J. P., "Hock minerals,” 1906, p. 378. 
