MICA. TALC. 



35 



ingredients; but different specimens of this mineral vary according 

 to the analyses of the same chemist. 



Silex 63—74 



Alumine 17 — 14 



Potash- - - 13— 



Lime - -- -- -- -- -- 3 — 6 



Oxide of iron -------- - 1 — 



Loss - - 3 — 6 



Others give the proportion of silex 46, alumine 24, lime 6. 



The existence of potash, or the vegetable alkali, in felspar, is a 

 fact deserving of particular attention.* It may be owing to this cir- 

 cumstance, that felspar is so frequently observed in a soft or decom- 

 posing state, although when undecayed its hardness is litde inferior 

 lo that of quartz. Those felspars which are durable are probably 

 free from potash. Felspar in a compact form, occurs in many rocks ; 

 it constitutes the principal part of most porphyries, and of the lighter- 

 coloured lavas. Compact felspar differs from hornstone, the latter 

 being infusible without the addition of alkalies. 



Mica derives its name from the Latin micans, glittering. It has 

 a splendid lustre and is known as the substance called Muscovy glass. 

 It consists of very thin leaves or laminae, which may be easily sepa- 

 rated with a knife. The plates are elastic, by which it may be dis- 

 tinguished from the mineral called talc. The thin plates are transpa- 

 rent. The colours of the thick plates are yellow, grey, blackish 

 green, white, and brown. The surface may be scratched with a 

 knife : it melts into an enamel with the blowpipe ; it is sometimes 

 crystallized in six-sided prisms. 



Talc nearly resembles mica in appearance. The plates are flex- 

 ible, but not elastic : it is much softer than mica, and is infusible ; its 

 colours generally incline towards green, but it is sometimes of a silver 

 while : it has a soapy feel. Chlorite, which is nearly allied to talc, 

 derives its name from chloros, the Greek word signifying green. Talc 

 and chlorite pass, by insensible gradations, into each other, and in 

 this state, they supply the place of mica in most of the granitic rocks 

 that I have examined in the vicinity of Mont Blanc. Chlorite is of 

 a darkish dull green colour ; it has a glistening lustre ; its structure 

 is minutely foliated ; it is soft, and rather unctuous. The constit- 

 uents of these three minerals are, — 



* It has recently been discovered, that, in some of the felspathic rocks, soda oc- 

 cupies the place of potash, and gives a slight change to the crystalline form : some 

 mineralogists are desirous of making this variety a new species, and have proposed 

 to give it the name of Cleavelandite; but geology and mineralogy are already too 

 much burdened with unmeaning terms, and if a new name must be introduced, that 

 of felsparite would be more appropriate, and convey an idea of its approximation 

 to felspar. 



