504 MINERALOGY 



B.B. Fuses at 3 with intumescence, in most cases. The 

 powdered mineral fused with soda and dissolved in strong HC1 

 and heated with tin yields a violet-colored solution (titanium). 

 Only slightly attacked by HC1, but decomposed by sulphuric 

 acid. 



General description. Crystals are tabular parallel to the base 

 in habit, or elongated parallel to the prism, when they are terminated 

 by the unit pyramid. Usually combinations of the base, unit 



FIG. 512. Titanite. Renfrew, Canada. 



prism, and pyramid ; other faces, as x, a, and s, are less common. 

 A large number of rare forms have been described. 



Chemically, some of the titanium may be replaced by ferric 

 iron and aluminium, and a rose-red manganese variety occurs at 

 St. Marcel, Piedmont, associated with piedmontite, the man- 

 ganese epidote. 



In rock sections titanite appears in wedge-shaped out- 

 line or rounded grains, either colorless, pale yellow, or brown. 

 The relief is very marked ; cleavage in two directions distinct, 

 but peculiar from the fact that they are never parallel to the 

 crystal outline. The extinction due to dispersion is ill-defined. 

 Interference colors vary greatly according to the direction of the 

 section in the crystal, from gray of a high order to gray of the 

 first order, when the two rays a and p, which are nearly 

 equal, are vibrating in the section. 



