90 



Vol. XLIII, Art. 6. — S. Tsuboi : 



p = 2.r8' (y' = Sir-2'2' 



(4) luclinatiou of optical plane to the side pinacoid : 



O.P. A i!f (010) = 75°35' 

 (Ö) Optic axial angle : 2r=81°40' 

 (6) Optical character negative. 



From the values of refractive indices, the mineral was identified 

 as anorthite with its chemical composition Ab.Ai'a-, according to 

 Becke.'^ Optical constants other than refractive indices also show 

 a close agreement with those of anortliite. 



The mineral encloses small crystals of olivine, (Fe,Si04)i3-i» 

 (Mg,8i04)s7-si, wdth the refractive indices «<1.678<<5<1.691< r. 



Plagioclase, as microphenocrysts, is also anorthite wdth refrac- 

 tive indices similar to the nbove, euhedral to subhedral, commonly 

 twinned according to tlie Carlsbad and albite laws, but rarely to 

 the pericline and Manebach laws. Some of the plagioclase crystals 

 are almost free from inclusions, but most of them include rather 

 abundantly anhedral grains of light greenish augite and uncrystalHzed 

 or partly crystallized opaque masses (slag), which latter may have 

 been imprisoned prior to crystaUization. Olivine is only rarely found 



as an inclusion. Zonal arrangement of 

 inclusions is very well developed in some 

 cases. Zonal structure due to chemical 

 difference is almost lacking except that the 

 less calcic plagioclase forms the shell of the 

 In veiy rare cases a more complex zoning 



1. Most calcic. 



2. Medium calcic. 



3. Least calcic 



Au 



Fig. 38. 



anorthite phenocryst 



(Fig. 38) is extremely faintly exhibited. 



1) As to the meanings of (f, p, i^', and p' see Fig. 34, p. 65. The values in (3)— (6) were 

 calculated by supposing /3 = 1.578. 



2) "Zur PhysiograpWe der Gemengteile der krystallinen Schiefer," Denkschriften der 

 kaiserlichen Akademie der Wissemchaßen, 75 Bd. S. 103, 1913 (Wien). 



