362 University of California Publications. [Geology 



to form, as E. Weinschenk* has stated, a series from lime-magne- 

 sian to soda-aluminous amphiboles; the others seem to come in 

 some side ferruginous series; this is quite certain for gastaldite 

 — glaucophane — erossite — rhodusite — abriachanite. 



Now it is generally admitted that blue amphiboles or amphib- 

 oles with blue pleoehroism contain more or less alkali (soda) in 

 their constitution ; this is always the case with the amphiboles 

 from glaucophane rocks. 



Further, many penologists have observed that some green 

 amphiboles become bluish at the periphery, and also undergo 

 some changes in their birefringence. Dr. A. C. Lane attributed 

 this phenomenon to the increase of the soda content, and has 

 given an empirical formula which connects the birefringence with 

 the soda content of the mineral. It seems to me that this for- 

 mula is not applicable in general, especially not in the glauco- 

 phane series. The chemical constituent with the most influence 

 on the physical properties seems to be Fe^O.j (viz., Fe 2 Si a O n ), 

 and that not only in the glaucophane series, but in general in the 

 amphibole family. 



We know that the optic orientation, the optic constants, and 

 the pleoehroism vary very much in the alkali amphibole group 

 from one member to another ; but we can state that the pleoehro- 

 ism always varies on c' to blue, on b (axis of symmetry) to green, 

 on a to yellow, whatever the orientation of the ellipsoid of elas- 

 ticity may be. 



Further, the size of the angle of the optic axes, the position of 

 the axial plane, and even the angle of extinction, up to a certain 

 point, are in relation with the amount of Fe 2 3 . 



A. GLAUCOPHANE CROSSITE RHODUSITE. 



Gastaldite — Glaucophane. — There is a very extensive litera- 

 ture about glaucophane, so that its identification is in general 

 not difficult. It is well known, however, what a variation glau- 

 cophane shows both in its optical properties (extinction angle, 

 pleoehroism, birefringence, optic angle, etc), and in its chemical 

 constitution. 



* E. Weinschenk. Die gesteinsbildende Mineralien 1901. Amphibol- 

 gruppe. 



