PHYSIOGRAPHY. 223 



Talc. 



TACHYLITE. (See Pitchstone.) 

 TALC. Prismatic Talc- Mica. MOHS. 



Primary form. Right rhombic prism. M on M = 120 

 nearly. 



Secondary form. Primary, having its acute lateral 

 edges replaced by tangent planes. 



Cleavage, parallel with P, commonly very perfect. 

 Fracture not observable. Surface, P smooth, the other 

 3lanes striated horizontally.. 



Lustre pearly upon P, both as faces of crystallization and 

 cleavage. The other faces possess vitreous lustre, inclin- 

 ing to 'adamantine, generally low degrees. 



Color various shades of green, as blackish green, leek 

 green, celandine-green, and apple-green, passing into 

 greenish grey, greenish white and greyish white. Streak 

 corresponding to the color, green . . . white. Semi-trans- 

 parent . . . translucent. Different colors in different direc- 

 tions. Some individuals are of a bright green color if 

 viewed in a direction perpendicular to the laminae, while 

 parallel to them they exhibit a fine brown tinge. In the 

 latter direction they are much more transparent than in the 

 former. 



Sectile, in a high degree. Thin laminae are easily flexi- 

 ble, Hardness = l-0...1-5. Sp. gr. =2-713, a dark 

 green variety. 



Compound Varieties. Imperfect globules and stellular 

 groupes : composition imperfectly columnar. Sometimes 

 several crystals are engaged with each other, so as to pro- 

 duce conical and cylindrical aggregations. Massive : com- 

 position granular, of various sizes of individuals, often im- 

 palpable ; sometimes imperfectly columnar. The individu- 



