86 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
by the foregoing expressions is to find a set of structural formula 
which shall represent all of the available evidence. Now, wollastonite 
is commonly classed with the pyroxenes, on crystallographic grounds ; 
and so too is pectolite. But both species are very easily decomposed 
by even dilute and weak acids, while the normal pyroxenes are quite 
refractory ; and furthermore, wollastonite has a lower density than any 
pyroxene which approaches it in composition. Chemically, then, these 
species are dissimilar, and it is very doubtful whether they can properly 
be grouped together. 
Empirically, however, the nonaluminous pyroxenes resemble wol- 
lastonite in their ratios. Thus we have, according to the commonly 
accepted formulae developed by Tschermak, Doelter, and others: 
Enstatite, orthorhombic Mg 2 Si 2 6 
Diopside, monoclinic 0aMgSi 2 O 6 
Hedenbergite, monoclinic 0aFeSi 2 O 6 
Rhodonite, triclinic Mn 2 Si 2 6 
There is also a great variety of other intermediate species or isomor- 
phous mixtures in the pyroxene series, such as bronzite, hypersthene, 
schefferite, sahlite, jeffersonite, and fowlerite, in which we find, variously 
replacing one another, salts of magnesium, calcium, iron, manganese, and 
zinc. All of these minerals, however, conform to the general formula 
ESi0 3 , or R 2 Si 2 6 , which adequately expresses their constitution so far 
as they alone are concerned. This, structurally, can be written 
y Si0 3x 
R< >R 
X Si(V 
which would be satisfactory if the pyroxene series ended here and if 
the amphiboles were unknown. 
Going a step farther we find in augite a pyroxene containing alu- 
minum, and having an oxygen ratio greater than Si : G 3 . In place of 
aluminum, ferric iron also occurs, and alkalies are sometimes present. 
Leaving these variations out of account, for consideration later, we 
have in augite, as interpreted by Tschermak, together with the normal 
compound R 2 Si 2 6 , the basic salt EAlAlSiO c ; which, as shown by 
Groth, is empirically analogous to kornerupine. It is also similar in 
composition to the silicate which is characteristic of the clmtonite 
micas 5 but the two are probably not identical. A metasilicate isomer 
of the clintonite molecule might be written either as 
y Si0 3 -Al=0 /SiOs-A^O 
Mg< or as Al— O 
\0— A1=G \q >M 2 
having in the first form some resemblance in structure to the metasiii- 
cate formula for the nonaluminous pyroxenes. The compound, however, 
unless represented by kornerupine, is not known by itself, but is j 
assumed as existent in mixtures ; its reality, therefore, is still question- 
