218 



PP. T. Schaller—Dumortierite. 



Fig. 3 shows three of the crystals. 



A is from California, size l ram X i mm . 



B " New York, about l mm X i mm . 

 C a Arizona. about l mm X l mm . 



c 



The minerals andalusite, sillimanite, staurolite and dumor- 

 tierite possess certain properties that are very similar. They 

 are all orthorhombic, their axial ratios are similar, their princi- 

 pal constituents are silica and alumina, and in many ways these 

 minerals can well be grouped together. A comparison of 

 their axial ratios is given below : 



a c 



Adalusite -9861 '7025 



Sillimanite '970 ? 



Staurolite '9795 '6942 



Dumortierite -8897 *6871 



