400 J. D. Dana on the Feldspar group of minerals. 
2. The oxygen ratio in Anorthite is that of a Unisilicate, 
forl +3:4= 1:1; and the formula may be, on the old and 
Old (483+3:41)2Si3 New SifO,f($R,4+26Al? — 
In the other species, the oxygen ratios vary from that of 
Anorthite, through an increase in the proportion of silica, they 
being as follows : 
Bases Silica Bases Silica 
Labradorite ae Oligoclase tS oe 
Andesine foes Albite 5 Ba eS 
Hyalophane Orthoclase i roe 
3. The silica of a Unisilicate is.enough for the making of 
a feldspar ; that is, to produce a species having that assem- 
blage of crystalline, physical and other qualities which charac- 
terize the group. The type is therefore essentially Unisilicate. 
But while Unisilicate, it admits of increase in the propor- 
tion of silica to produce its variety of species. ; 
An excess of silica above that of a Unisilicate may exist 
under the Unisilicate type in one of the two following condi- 
tions, and still be not an impurity. 
A. It may be basie silica ; in which case the general for- 
mula for the group would be— 
Old (R2, 81,812) Sit New _SiJ0,|(R,R,),8Al,78i)2 
and the special formulas for the species are: 
Labradorite (JRL §RI4 JSi}*Si* —_Sif0,1(4(R, w.)-+98 1+ Hi 
Andesine (PRe+gRI+ gSit)*Si? —SiJO.1G(R, Ba) +9841+ 475) 
Oligoclase (2 R94, 514 3,Si7)25i?  SiJO.1(3,(R, Ba) +; 8Alt IS 
Albite yNa®+g81448i2)2Sis — $i]0,4(gvas+-30Al+-$75i)2 
Orthoclase (@K34951445i2)2Si2 iO, (ax.+-39-Al+- 475i» 
Roor R,z,)in Labradorite, Andesine and Oligoclase = Ca, Na, 
_ B. It may be accessory silica ; by which is meant, not i 
ingredient unessential to the species, but a true component, 
ee the fundamental type of the series.* The formu- 
+88 on this view would be :— 
* This idea of the feldspars is recognized by the author in the last edition of his 
Mineralogy (1854) in which he heads the sf as follows: ‘Type ratio ia 
ony n of bases and silica : 1, and varying from this by the addition of 5" 
to the type”; and gives the following as ihe formulas: uv 
i : Oligoclase (R + RDS ae 
Albite i+ Be. 
Gitodade (k + 2Si* 
