56 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
When clinochlore or leuchtenbergite is strongly ignited, it yields, 
like xanthophyllite, a product insoluble in hydrochloric acid, having 
the composition of spinel. This reaction establishes still more defi- 
nitely the relationship between the chlorites and the clintonite group, 
aud it is readily intelligible in the light of the structural expressions. 
The splitting up, under influence of heat, of mixtures containing such 
groups of atoms as MgOH, AlOH, and A10 2 H 2 , ought to generate 
spinel, an<J the appearance of a compound of this character is evidence 
in favor of the formulae. 
VI. THE TOURMALINE GROUP. 
Although tourmaline, in its several varieties, is apparently quite 
complex, the evidence for its interpretation is abundant and ample. Its 
variations in composition are shown by numerous good analyses, its 
associations are well known, and its alteration products have been 
observed in a sufficient number of cases. From the minerals which 
have been discussed in the preceding chapters, it differs essentially in 
that it contains boron, and the part played by this element is a new 
question to be interpreted. 
When tourmaline undergoes alteration, the commonest product is a 
mica, and between the micas and the tourmalines there are very strik- 
ing analogies. With the lithia micas, lithia tourmalines are generally 
associated; with muscovite and biotite, iron tourmalines occur, and 
magnesian tourmalines accompany phlogopite. In each case the com- 
position of the tourmaline seems to bear a relation to that of the asso- 
ciated mica. Furthermore, the varieties of tourmaline shade one into 
another through an unbroken series of gradations, and this may happen 
to some extent in one and the same crystal. The genus tourmaline, in 
short, represents a series of compounds, and these are parallel to the 
normal mica series. These considerations lead directly to a system of 
formula?, which, like those of the micas, are derivable from molecules 
of the normal aluminum orthosilicate, and which express completely 
all the known chemical relations of tourmaline. 
On studying carefully all the more recent and more trustworthy 
analyses of tourmaline, a constant ratio appears connecting silicon, 
boron, and oxygen, which is represented by the figures Si 6 B 3 3 i. The 
variations from constancy are almost all within the limits of accuracy 
of the analyses, but occasionally a small amount of fluorine is noted, 
which appears to replace the univalent acid group B0 2 . Aluminum, 
with respect to silicon, varies widely, the ratio ranging from Al 8 Si 6 to 
Al f) Si 6 , while the other bases vary reciprocally with aluminum and in 
the opposite direction. The tourmalines high in aluminum approach, 
like muscovite, most nearly to the normal ortho-salt; those lowest in 
aluminum are high in magnesium, like phlogopite; and, like biotite in 
the mica group, the iron tourmalines lie between these extremes. 
