MINERALOGY. II3 



73. Talc [H^ Mg' Si^ O'^]. 



Talc, as it occurs in New Hampshire, is chiefly of the variety called 

 steatite, or soapstone, of which we have large beds that have been exten- 

 sively worked. At Franccstown there is a large quarry where talc of ex- 

 ceptional purity has been mined since 1802. At Orford there are five 

 beds, but the mineral is of a slaty character, and not so easily worked. 

 At Richmond the beds are still more impure, and the mineral con- 

 tains anthophyllite and pyrites, which interfere with the sawing of it 

 into blocks. Keenc, Weare, Warner, Canterbury, and Lancaster are 

 other localities where steatite has been found. The impure varieties 

 are of economic value, though they may not be transported to markets 

 at a distance. At Haverhill a large bed of steatite was long ago found, 

 and very large boulders of soapstone are found at Pelham, which have 

 been transported from some unknown locality. At Norwich Vt., the 

 pretty green foliated variety of talc has been found. 



Talc is not found with well formed crystal faces, but it is considered 

 orthorhombic, and its microscopic characters correspond to those of such 

 crystals. The steatite in thin sections appears to be a fibrous mass, and 

 where the fibres do not overlie one another so as to interfere with one 

 another, the fibres are dark when parallel to the plane of vibration of the 

 light. If we take one of the foliae of the nice green talc from Norwich, 

 and put it under the microscope, take out the ocular and cross the Nicols, 

 we shall see a black cross which opens out into two hyperbolas as the 

 section is revolved, and which shows the biaxial character of the crystal, 

 and that the optic axial angle is not large {iy°-i(f in the air). These 

 characters agree with those belonging to talc, in which the cleav^age is 

 basal, the bisectrix normal to the base, and the optic axes in the plane of 

 the macrodiagonal. 



Talc is a constituent of some of our rocks. In thin sections of them 

 it is usually in radiated masses. It is distinguished from chlorite in that 

 it shows neither dichroism nor absorption of light, and is usually fresh 

 and undecomposed. 



Talc enters into some granitic rocks, forming protogene, and with 

 some accessories forms the stratified talc schist. Much of the so-called 

 talcoid schist of Vermont and New Hampshire was shown by Prof. G. F. 

 VOL, IV. 15 



