376 



University of California Publications. 



[Geology 



plane of symmetry. The whole phenomenon can be clearly indi- 

 cated in a diagram : In a system of coordinates, let the ordinates 

 represent the indices of refraction, and the abscissae represent 

 the amount of Fe 2 3 causing the variation of the former. Ac- 

 cordingly we have Fig. 5, which shows that we can have the fol- 

 lowing glaucamphiboles : 



n 









1M 



i 



c 









a 







at 



Gl 



i c 



§! 



J r 2?/.' 







z 



i | M t 3 



Fig. 5. — Diagram showing the variation of 7 — /3 and 7 — a with the ratio 

 of Fe 2 3 : ALO ; , ; Gl = glaueophane ; Cr = crossite; Eh? = 

 rhodusite. 





Coefficient of 

 AI2O3 FeoO:j 



Position 

 of axial 

 plane 



t:c 



b 



(or 



a) 



2V 



Optic 

 oh ar- 

 aeter 



Formula* 



Gastaldite 



21—15 



0- 6 



11(010) 



0- 



- 6° 



40°-60° 





2 G+M'; (6/4G +2/4R + M) 



Glaueophane 



14— 9 



0- :i 



11(010) 



0- 



- 6° 



30°— 50° 





G + M; (3/4G+1/4R + M) 



Glaueophane 

 uniaxial 



8 



4 







10° 



0° 





2/3G+1/3R + M 



Crossite 



6 



e 



1(010) 





15° 



50° 





1/2G + 1/2R + M 



Crossite 



4 





1(010) 



20°- 



-30° 



90° 





1/3G+2/3R+M 



Rhodusite 



0— 2 



12-10 



1(010) 



O 



-40°) 



(50°-30°) 



(+) 



R + M,(l/5G+4/5R+M) 



Abriaehanite 



0— 3 



21—18 



(?) 





(?) 



(?) 



(?) 



2R+M 



We shall see further on that just the same optic variation 

 takes place in the karinthine series and in the riebeckite (crocid- 

 olite) series. As regards karinthine, we shall discuss it later on, 

 but we may dwell on croeidolite and riebeckite a little in this 

 place. 



I do not know the constitution of Cross 's croeidolite, and still 

 less that of the croeidolite with transversal axial plane, but I 



*In these formulae G = Al.Sl.O,,; B = Fe J Si,O ; M, (or M') ; the sum 

 of the other metasilicates = Na.,SiO., + FeSi0 3 + 2 (or l)MgSiO, 

 (+H.,Si0 3 ). 



