327 



Under the microscope the minerals seen to be present 

 are quartz and muscovite, the latter being slightly greenish 

 in colour, and so is slightly pleochroic. 



The muscovite is present in elongated flakes showing 

 good cleavages, and is often present in bunches or tufts, but 

 in all degrees of orientation. In the true greisen replace- 

 ment of felspar has been complete. A gradational alteration 

 of the aplite occurs, however, and some sections show the 

 incipient greisenization of the felspar. 



The quartz of the original aplite is unchanged, but some 

 secondary quartz has been introduced. 



(c) The Kaolin. — At a still later stage in pneumatolysis 

 kaolinization of the aplite has occurred. The kaolin is deve- 

 loped in bands along minute fissures, which may contain 

 thin quartz veins, and may be ascribed essentially to the 

 action of superheated water at a lower temperature than 

 that of greisenization. 



The evidence of pneumatolysis of the aplite is clearly 

 shown, and the progressive fall in temperature of the 

 pneumatolytic agents is reflected in the change of pneu- 

 matolytic product. The order of development of pneumato- 

 lytic product in these aplites is in accord with that worked 

 out for other fields. ( 4 > 



The formation of muscovite from microcline is doubtless 

 represented by the well-known equation : — 



(i.) 3 K AlSi 3 8 + H,0 *->■ H 2 K Al 3 (Si0 4 ) 3 + K 2 

 Si0 3 + 5 Si0 2 , and the development of kaolin by 



(ii.) 2 K AlSi 3 6 8 + 2 H 2 — ^ A1 2 3 2 Si0 2 

 2 H 2 + K 2 Si0 3 + 3 Si0 2 , or if C0 2 is regarded 

 as an active agent by 



(iii.) 2 K AlSi 3 8 + 2 H o + C0 2 — ^ A1 2 3 2 Si0 2 

 2 H 2 + K 2 C0 3 + 4 Si0 2 . 



The volume changes represented by these equations are 

 for (i.) the development of muscovite and quartz, a volume 

 decrease of 22 per cent. ; for (iii.) the production of kaolin 

 and quartz, a volume decrease of 13 per cent. 



During the greisenization kaolin can accompany the 

 production of mica, although in general the former is dis- 

 tinctly formed at a lower temperature. If this is so, the 

 porous nature of the greisen can be explained as due to the 

 weathering out of kaolin from the rock. Even so, it is possible 

 that the porosity may in part represent the volume decrease 

 on greisenization, as shown by the preceding equations. It is 

 difficult to understand, however, if this be correct, why the 



(4)0/. Flett, J. S.: Memoir of Geol. Surv. Eng. and Wales, 

 1909, Geology of Bodmin and St. Austell, p. 118. 



