220 | J.D. Dana's Mineralogical Contributions. — 
- Oxygen ratio. Old formula. algae 
8. Monometric. Leucite, 1:3:8 K3 Si2+341 Si2 (K++) Sis 
- = eo. = 2. Ww o- 4 1 
Sodalite,1:3:4 Na3 S$i+341Si+NaCl (Na+) Si3[+pNa ll 
rs eS ae 
Hauyne, 1:3:4 Nas S$itskSiteCaS (Na+) Sisp+gaS 
Rg te . | eee 
Nosean, 1:3:4 Nas SitsAlSi-NaS (Na+aAi)Sie[+pNad 
Lazulite. 
=" ‘esi ied 
4, Dimetric. Nepheline,1:3:4 R2 Sit+-2A1 Si (Na+X))Si2 
ne er eee 
Cancrinite, 1:3:44 R2 Si,2A1 8i+ RG (Na+) Siz 4gR6 
The formulas of the feldspars in the last column, show what is 
equally plain in the oxygen ratios, that the species differ in the 
amount of silica, and this is the great essential difference. It is 
least in anorthite; and from this species it increases to 4Si 1n al- 
bite or orthoclase. This increase takes place without any change 
of crystallization, there being only very small variations in the 
angles. In anorthite, the oxygen ratio for the bases and silica 
is 1:1; and as anorthite is common in good crystallization, 
and is every way a well characterized species, it shows us that 1 
of oxygen of the silica to 1 for the bases is all that the feldspar 
type requires. Moreover, the relation of the species to Scapolite, 
which has the ratio 1: 1, also favors this view. Hence 1:1 may 
be considered the ¢ype-ratio, upon which, variations take place 
according to definite proportions. When silica abounds 10 the 
rock material in process of crystallization, and the other ingredi- 
ents are at hand, the species holding the largest proportions of 
silica would be formed. : 
The isomorphism of Sodalite, Hauyne, and Nosean, and di- 
morphism with Anorthite, parallel with the dimorphism of Leucite 
and Andesine, show that the ratio 1: 3:4, is their type ratio. 
modifying the form, and although chemically included, are wnes- 
sential to the type. ‘The same is true of RG in Cancrinite, which 
has the crystallization of Nepheline. 
The facts above exhibited appear to show that a type admits 
of some variation in the amount of silica without changing the 
character of the species. In Meionite, having the ratio of 1 122% 
the scapolite type is exhibited ; and Bischof and Rose take this 
as the only ratio of the species. But the ratio 1 : 2: 4 appears 
be required by most analyses of Scapolite, in which there 3§ ” 
addition to the silica. In the same manner R25i2, with an ad 
tion of 485i, becomes Hornblende.* oa 
Among the feldspars, Andesine and Leucite have essentially ~ 
oxygen ratio of Angite,1+3:8=1:2, and the formula mig 
* The amount of silica p t may be on e leading to the formation of Hort- 
blende in place of Augite. But in pseudomorphic changes, the same proportion® 
ey of the bases, or an addition of magnesi@ 
may result, by a removal of 
Bium and Bischof have suggest: 
