clarke.] META- AND DIMETASILICATES. 91 
id place of Na 2 . Potassium may also partly replace sodium. Another 
alkaline amphibole of doubtful character, waldheimite, approximates to 
E 
II 
/Si 3 CV 
N3i 3 o/ 
II 
Na 2 
which is the formula of a trisilicate pure and simple, with R=Ca, Fe, 
Mg. The existence of this compound is strong evidence in favor of 
the pseudometasilicate theory; and, as will be seen later, it does not 
stand alone. The formula) as written suggest a close analogy between 
the pyroxene-amphibole minerals and the olivine group, which may 
have some future significance. 
Among the amphiboles, as among the pyroxenes, aluminous and 
ferric compounds are common, and with these the minerals approach 
to orthosilicate ratios. Tschermak's interpretation of these ratios is 
practically the same as in the pyroxene series, namely, by the assump- 
tion of molecules of the form Al 2 IiSi0 6 or Al 4 R.>Si 2 Oi 2 . An alterna- 
tive to this view is offered by Scharizer,* who shows that the horn- 
blendes can be explained as mixtures of actinolite, R 4 Si 4 12 , with an 
orthosilicate called syntagmatite (E / 2 R // ) 3 Al 2 (Si0 4 ) 3 , whose ratios are 
similar to those of garnet. An amphibole from Jan May en's Island 
approaches very nearly to syntagmatite in composition. If a com- 
pound of this type is present in the amphiboles it would explain at 
once their alterability into epidote, micas, and chlorites; but so far as 
the composition of the group is concerned neither Tschermak's view 
nor Scharizer's is absolutely necessary. The Tschermakian molecule, 
however, can be written either as 
AlO, Mg (AlO), 
I II 
>9i<V ^SiO 
AW >A1 or as Mg< >Mg 
\SiO/ " \SiO/ 
I II 
A1G 2 Mg (AK)) 2 
the latter form resembling that of tremolite, and also connecting the 
group still more closely with the olivines. It is also parallel to the 
last formula suggested for the corresponding pyroxene compound, being 
one-half of the latter and identical with it in type. 
No amphibole is yet known which, by itself, corresponds precisely in 
its constitution to acmite and spodumene. In glaucophane we find a 
species, which, as a metasilicate, may be written ALNaSi 2 6 + (MgFe) 
Si0 3 ; and in crocidolite another similar salt, Fe /// N"aSi 2 6 4-FeSi0 3 . 
Crocidolite alters easily; and one of the products of alteration, which 
*Nenes Jahrbuch, 1884, (2), p. 143. 
