Vol. 4] Murgoci. — Classification of the Amphiboles. 



375 



glaucophane from Zermatt, etc., and the uniaxial glaucophane 

 from San Pablo, have almost equal Na 2 0, but are very different 

 in their y — /?. Nevertheless, the ideas of Lane served as a start- 

 ing point for the following discussion. 



The statement of Tsehermak (Min. Mith., 1871, 38, 40), that 

 with the increase of Fe in the aluminous amphiboles the optic 

 angle increases around C and decreases around ft (usually the 

 first bisectrix), is well known. If we consider the Fe'" (better 

 Fe 2 Si 3 O ), this statement is quite true, not only in the glauco- 

 phane series but in general in the FeAl amphiboles with meta- 

 silicate formula. Of course, when 2 V becomes smaller around 

 a negative bisectrix, y — /3 also becomes smaller, and finally at- 

 tains the value zero when the amphibole becomes uniaxial. But 

 the rule is still more general; the variation of y and /3 can pro- 

 ceed in such a manner that y — (3 passing through zero then be- 

 comes negative, i.e., (3 takes the place of the former y, and vice 

 versa. In this case we have an amphibole with transverse axial 

 plane, and have established a continuous variation from the com- 

 mon amphibole with parallel axial plane, through the uniaxial 

 amphibole to the amphibole with transverse axial plane. 



Fig. 4. — Ellipsoid of elasticity, section parallel to c and b , of glauco- 

 phane, a, uniaxial glaucophane, to, and crossite, c. 



We have seen this whole series with all possible intermediate 

 members in the glaucophane group, and it is easy to see that it 

 is a function of Fe 2 Si 3 9 . The more Fe„Si0 3 replaces Al„Si0 3 , 

 the more the axial angle and y — /3 decrease : for Fe'" : Al = 1 :3 

 corresponds to the uniaxial glaucophane (y = /?), while for Fe : 

 Al = 6 :5 we meet with a crossite with transverse axial plane and 

 2 V almost 90°. Perhaps rhodusite, which is the Fe'" glauco- 

 phane, is a crossite of positive optic character, or even a uniaxial 

 positive glaucophane, with the optic axis perpendicular to the 



