MINERALOGY OF BLACK LAKE AREA. 
61 
iron, and ferric iron may at the same time take the place of some of the 
aluminum. Water of constitution appears to be an essential component 
of the mineral, and in thirty-six analyses quoted in Dana's Mineralogy, 
the percentage of water ranges from 0-55 to 3- 41. 
Clarke 1 regards vesuvianite as an orthosilicate with composition 
essentially CarCOlH^zAbCSiO^e, and he assigns to the mineral the constitut- 
ional formula reproduced below. The percentage composition calcul- 
ated for this formula is given in column 1 of the preceding table. 
SiO*: 
/ 
Al— SiO* 1 
\ 
SiO*- 
1 
:Ca tiz 
:Ca*= 
-Ca- 
;SiO* 
\ 
:SiO*~Al 
/ 
-SiO* 
A1(0H) 
, 
Al(uH) 
SiO*==Ca »===. SiO* 
/ \ 
k\ -SiO* == Ca, = SiO*— A1 
\ / 
SiO. = Ah = SiO. 
Vesuvianite Garnet 
Constitutional formulas of vesuvianite and garnet (after Clarke). 
In the occurrences here described, garnet is frequently associated 
with vesuvianite, but at many localities in the area it is also found alone, 
forming dykes under conditions which suggest that its mode of origin 
must have been similar to that of the vesuvianite in the dykes of this 
mineral noted at the Montreal chrome pit and elsewhere. A comparison 
of the graphic formulae of the two minerals (after Clarke) is thus instruc- 
tive, as indicating that their molecules are very similar, both as regards 
composition and constitution; from this it might be inferred that the 
genetic relation between these two species would be very close. 
Optical Characters. 
The crystals always exhibit both the first and second order prisms; 
and, although one of these, usually (100), generally has a large develop- 
ment relatively to the other, it is possible to select crystals in which the 
faces of both forms have an appreciable width, and to use these as prisms, 
with internal angle of 45 degrees, for the determination of the refractive 
indices by the minimum deviation method. A series of trials, however, 
showed that no great degree of accuracy could be attained. This is in 
part owing to the fact that perfect prism faces are found only on very 
small crystals ; the prisms used consist of two faces, of which one belongs 
to the form (100) and the other to (110), and it usually happens that one 
or other of these is extremely narrow, so that the light from a sodium 
flame refracted through the prism is so feeble that the images cannot be 
very accurately adjusted on the cross hairs. On the other hand, if larger 
crystals are employed, the images, while being sufficiently bright, are 
multiple; this is the result of a feature which has already been referred 
1 Clarke, F. W., “The constitution of the natural silicates, ’ U. S. Geol. Surv,, Bull. 588, p. 30. 
