374 



University of California Publications. 



[Geology 



Fe 2 3 :Al 2 0. 



C:c or b .c 



Gastaldite and Glaueophane 

 Gastaldite of Skikoku 

 Mn Gastaldite of Miask 

 Glaueophane uniaxial 

 Crossite 

 Bhodusite 



0—1/4 



— 6° (rarely larger) 



1/1—3/2 

 >10/2 



2/5 

 3/4 

 1/2 



—11° 



( + ?)36° 

 —10° to —13° 

 —16° to —20° 

 ?(4°) 



In other series : 



Crocidolite 



Eiebeckite, Osannite 



Amph. of Karbole 



Barkevikite 



Barkevikite 



Hudsonite 



Hastingsite 



Soretite 



Basaltic hornblende 

 Karsulite 



Kataphorite (Sao Miguel) 

 Etc., etc 



>10/1 

 >10/1 



2/1 

 <2/3 



2/3 

 <l/2 



2/3 



3/5 

 <l/2 

 <l/2 



3/2 



to 



—15° 

 —14° 

 —20° 

 — 9° 

 —25° 

 —17° 

 —12° 

 —10° 

 —23° 



>70° 

 >75= 



So far as I know, only some arfvedsonites and common horn- 

 blendes (karinthine, pargasite, etc.) seem not to follow this rule, 

 but there is an explanation for this, as the corresponding series 

 is not well known. Very probably a large cpiantity of FeO to- 

 gether with Fe 2 3 affects the increase of the angle of extinction 

 in greater measure than Fe 2 3 only : e.g., in the case of crocido- 

 lite, arfvedsonite, riebeckite, etc. 



(c). An interesting, perhaps the most obvious, influence of 

 the amount of Fe„0 3 on the optic properties is the variation of 

 the birefringence, y — /?. Dr. A. C. Lane, as well as many other 

 petrologists, has remarked that the green amphiboles in some 

 alkali rocks become bluish at the periphery, and the birefringence 

 decreases. He explains this as an influence of the feldspars on 

 the amphiboles, and has given an empirical formula which con- 

 nects the amount of Na with the birefringence ring : 



where Na — soda content of the amphibole, and b = the bire- 

 fringence of the orthopinacoid section which is to be taken posi- 

 tive or negative according as the vertical axis is C or S (resp. fa) 

 Table I shows us that this formula is not general, because the 



90 



Na = — (0.012— b) 

 17 



