548 



Wright — Three Contact Minerals. 



angle could only be obtained approximately ; the best average 

 of the results is 79° with a probable error of at least ± j-°. 

 Fracture uneven to splintery. Brittle. Hardness, about 5. 

 Luster, vitreous to resinous. Color, pale gray with tints of 

 blue or yellow to colorless. Transparent to translucent. 

 Streak, white. 



On the hand specimens a weathering or alteration crust, 

 consisting chiefly of finely divided carbonate, occurs not infre- 

 quently. In the thin section the spurrite is well defined 

 optically and is excellent material for optical work. 



From the relations of the optic properties to the crystallo- 

 • graphic it is highly probable that spurrite is monoclinic and 



Fig. 16. 



that the cleavage faces are parallel with the orthodiagonal (b 

 axis). If the good cleavage plane be called the basal pinacoid, 

 the optical orientation is apparently the following : b — a; a : c, 

 very small and possibly zero, the cleavage cracks not being 

 sufficiently perfect for decisive measurements. 



Twinning after both 001 and polysynthetic twinning after 

 orthodomes at angles of 56° to 58° with the twinning lines of 

 001 occur, and occasionally divide the field into sextants of 

 the same birefringence and all cut approximately normal to 

 the acute bisectrix, the plane of the optic axes in the different 

 sextants occupying different positions, as shown in the accom- 

 panying sketches. (Figs, la, lb.) 



The polysynthetic lamellae are often extremely line and 

 resemble albite lamellae very closely. On a section almost 

 precisely normal to the acute bisectrix the angle between the 



