clarke] META- AND DIMETASILICATES. 87 
able. Its simplest representation would be as a basic orthosilicate, 
Mg(Si0 4 )(A10) 2 , and it will be seen later that a polymer of this form is 
far more satisfactory than any metasilicate expression. 
Still another series of silicates containing triad bases and also alka- 
lies are classed with the pyroxenes, as follows : 
Spodumene ■ . AlLiSi 2 6 
Jadeite AlNaSi 2 6 
Acmite Fe /// NaSi 2 6 
and their empirical formulae are fairly satisfactory. Structurally, these 
expressions become, as metasilicates, 
\si0 3 — R' 
and babingtonite, which contains no alkalies, is similar, thus: 
.Si0 3 
Fc'"f 
SiO 
I /SiOgv 
Fe" + 3 R"< >R" 
I N3iO/ 
/Si0 3 
Fe'% 
^Si0 3 
R" being = Oa, Fe", and Mn. The ferric molecule is evidently equiva- 
lent to two acmite molecules, with Na 2 replaced by a linking atom of 
iron. 
So far, except partially in the comparison between wollastonite and 
pectolite, the formulae cited for the pyroxenes express composition and 
composition only. But spodumene, as shown by the elaborate research 
of Brush and Dana, splits upon alteration into a mixture of eucryptite, 
an orthosalt, and albite, a trisilicate. This observation suggests two 
alternatives: either that spodumene is derived from a polymetasilicic 
acid, or else that it is a pseudoinetasilieate, a mixed ortho- and tri-salt, 
like some of the species which have already been explained. An anal- 
ogy with leucite, for example, will at once be inferred, and that species, 
empirically, is strikingly like spodumene, thus : 
Leucite AlKSi 2 O e 
Spodumene AlXaSi>0 6 
Like spodumene, leucite alters into a feldspar and a member of the 
nepheline group; but it differs from spoduinene inform and in density. 
The specific gravity of the isometric leucite is 2.5, that of the mcno- 
clinic spodumene is nearly 3.2, and hence Ave may reasonably infer that 
the latter species has the larger and more condensed molecule. In 
order to explain the relations of leucite, its empirical formula was 
