Determination of the Feldspars. — Winchell. 
21 
broad and regular, while those of pericline are very frequently 
distributed only in certain ones of the albite bands, which they 
cross at varying angles according to the direction of the plate. 
Figure 1 8 represents a triclinic feldspar crystal with the 
Fig. 17. — Made of Roc Totirne (Descloizeanx). 
albite and pericline striations much amplified, to show their 
positions and direction for the species albite. The front face, 
lOO, is represented in part, but it is very rarely seen in nature. 
The prism planes, no, no, obliterate it. 
Chemical Composition . 
According to the law of Tschermak, which is generally 
adopted as a working- hypothesis, at least, the plagioclase feld- 
spars contain such proportions of soda and lime that each can 
be considered as a mixture of a definite number of the com- 
pound molecules Ab and An, in which — 
One albite molecule^Ab = Na./3. Al.^O.^.e SiO.^. or Na 
AlSigO^. 
One anorthite molecule =^ An =^^ CaO. Al./),,. 2SiO.^. or 
CaAl^SigOg. 
Thus the basicity grades from albite to anorthite in a some- 
what regular series. 
Tschermak groups them as follows:* 
Specific 
Gravity 
Comi)()iind 
Molecules. 
Percentage 
of Ab 
Percentage 
of An 
Albite 
Oligoclase . . 
Andesine .. . 
Labradorite 
B\ townite . . 
Anorthite. . . 
2.62 
2.64 
2.65 
2.6q 
2.71 
2.75 
AbiAno to.AbgAni 
AbeAn, to AbzAn, 
Ab3All2 to Ab4An3 
Ab,Ati, to AbiAns 
AbiAn, to Ab,An, 
Ab,A„„ to AboAu, 
100.00 to 88.88 
85.71 to 66.66 
60.40 to 57-14 
50.00 to 33.33 
0.00 to 
14.28 to 
40.00 to 
^0.00 to 
25.00 to 14.2875.10 to 
II. II 
33-33 
42.86 
66.66 
85.71 
1 1. II to 0.0088.88 to 100.00 
*Mineraux des Roches, p. 196. 
