Vol. 4] Murgoci. — Classification of the Amphiboles. 373 



color of absorption, all glaucamphiboles having almost equal 

 FeO. Grunerite (FeSiO.,) and the amphiboles related to it are 

 very faintly colored, with almost imperceptible pleochroism, and 

 it is known that by heating' glaucophane Oebbeke* has obtained 

 a variation of the pleochroism on C = reddish brown, on b and & 

 = yellowish green to colorless, explained as due to the oxidation 

 of FeO to Fe 2 3 . Inspection of the analyses shows clearly the 

 influence of Fe 2 3 on the absorption of these minerals; gastal- 

 dite and glaucophane very faintly colored, glaucophane uniaxial, 

 in general strongly colored, crossite and rhodusite very intensely 

 colored and with strong absorption. I may further add that the 

 other series of blue amphiboles with extremely strong absorption, 

 osannite, arfvedsonite, riebeckite, crocidolite, etc., are the am- 

 phiboles richest in Fe 2 3 ; then philipstadite and others (with 

 only 1 or 2 per cent. Na 2 but rich in Fe 2 3 ) show a strong ab- 

 sorption and pleochroism in blue. 



(b). Lacroix has stated also that the angle of extinction is 

 smaller in the normal glaueophanes than in those passing over 

 into common hornblende or actinolite. Many of our slides which 

 show glaueophanes and karinthine or actinolite as forming one 

 single prism verify this suggestion. 



Further, I have remarked in the glaucophane series that with 

 the increase of intensity of color, an increase in the value of the 

 angle of extinction occurs, t:C (glaucophane) or 6:C (crossite). 

 A comparison of the whole group of Al Fe'" amphiboles has con- 

 vinced me that in general the size of the angle of extinction C-.c 

 (or 6:c) is related neither to the amount of AI2O3 (as Wiik has 

 emphasized), nor to the amount of Fe 2 3 , but to the proportion 

 of their molecular coefficients of combination in the amphibole 

 constitution.-;- In this way we can explain why the different 

 members of the glaueophanes, riebeckites, barkevikites, common 

 hornblendes (soretite, karinthine, etc.), and basaltic hornblendes 

 are minerals in general of constant formula, but are very differ- 

 ent in their optic orientation; e.g., in the glaucophane series: 



* Oebbeke, Zeits, fur Kryst, 18S7, 12, 286. 



fBrogger was the first to state that the angle f.c increases with the 

 amount of iron and alkali. Blasdale says, loc. ext., in respect to glauco- 

 phane "there is perhaps a connection between the position of the acute 

 bisectrix and the relative amount of ferric and aluminum oxide. ' ' 



