152 



University of California Publications. 



| Geology 



the standpoint of its chemical composition. Professor Blasdale, 

 who kindly undertook the chemical analysis, reports : 





A. 



B. 



Average. 



Mol. Ratios. 



Si0 2 



43.56 



43.79 



43.68 



.723 



Ti0 2 



20.18 



20.00 



20.0!) 



.250 



BaO 



36.34 



36.31 



36.33 



.237 



100.08 100.1 o 



The suggested formula is BaTiSi..< >,,. which yields the follow"' 

 ing calculated values : 



Si0 2 43.71 

 Ti0 2 19.32 

 BaO 36.97 



100.00 



Professor Blasdale also reports that the mineral is easily decom- 

 posed by hydrofluoric acid, hut only slowly attacked by molten 

 potassium pyrosulphate. 



Benitoite is then a very acid titano-silicate of barium, and 

 stands in a class by itself, both as regards acid silicates and 

 titano-silicates. The possibility of the titanium acting as a base 

 was considered, but the summation of the analyses and the fact 

 that tlie crystals arc often perfectly colorless seem to point defi- 

 nitely to the above interpretation. The blue color of much of 

 tlie material may be due to a small amount of titanium in the 

 si squioxide condition. 



Associated with benitoite is a black or brownish black pris- 

 matic mineral that also appeal's to be new. Its most striking 

 characteristic is a very perfect prismatic cleavage of 80° 10'. Its 

 hardness is between 5 and (i and it melts easily, at about 1.5, 

 giving a sodium flame and becoming a lustrous black enamel 

 bead. It appears to be monoclinic, is biaxial, and gives an ex- 

 tinction angle of about 10 degrees on the cleavage face. Its cross 

 sections are six-sided, the four cleavage traces being truncated 

 by a lateral pinacoid. In thick pieces it is opaque, but in moder- 

 ately thin ones it is a deep rich red, which changes to a brownish 

 or ocrcous yellow as the thickness decreases. Pleochroism is 

 prominent. < )n a cleavage plate the ray vibrating near the prism 



