ALTERATION OF FELDSPAR TO SERICITE. 233 



m&y be assumed to have crystallized in the presence of a less potent amount 

 of fluorine, and indeed the analyses given by Dana" do not show any fluorine, while 

 the analyses given for ordinary rnuscovite sometimes do and sometimes do not show 

 it. To determine its presence in the altered Tonopah andesite, two tests for it were 

 made, in No. 2 and No. 8 (pp. 213, 216). No. 2 showed 0.12 per cent, No. 8 a trace. 

 No sericite was identified in No. 2, while No. 8 (the vein) contains it. The tests 

 therefore are not convincing as to the fluorine being contained in the mica, but 

 indicate its presence in the waters which altered the rock. No. 2, it may be noted, 

 now contains between three and four times as much water as No. 8.* 



CHEMISTRY OF THE ALTERATION OF SODA-LIME FELDSPAR TO SERICITE. 



The alteration of soda-lime feldspar by carbonated waters, according to 

 Rosenbusch, c may produce calcite, sericite, and quartz. If the former is carried 

 away by the permeating waters only quartz and sericite results, as in the case of 

 orthoclase. rf Where orthoclase is similarly altered, some potassium carbonate goes 

 into solution. Similarly Bischof ^ suggests, as an explanation for pseudomorphs 

 consisting largely of muscovite (sericite) after feldspar, such as he describes, 

 that part of the alkaline silicates of the feldspar was decomposed by carbonic 

 acid, their silica remaining and their alkalies being removed as carbonates; another 

 part of the silicate was removed as such; and the rest of the silicate went to 

 form the mica. In this way a mixture of mica and quartz originated. 



The analyses of sericite pseudomorphs after feldspar, given by Bischof in 

 connection with his above-cited explanation, show in many cases the presence of 

 fluorine; whence the suggestion arises that though carbonic acid decomposes the 

 feldspar, it may still require the help of a small quantity of fluorine for the 

 decomposition products to crystallize as muscovite. 



CHANGES IN RARER CONSTITUENTS DURING ALTERATION OF EARLIER 



ANDESITE. 



The evidence afforded by the rarer constituents of the rock is less trust- 

 worthy, on account of the small amounts present. The percentages of titanium, 

 barium, and phosphorus in the different rocks are represented in the diagram 

 forming fig. 74, the scale being ten times that employed for the commoner rock con- 

 stituents in fig. 73 (p. 218). It is here seen that the titanium behaves much like 

 the alumina, increasing with the increasing silica in the first three specimens, and 



"System of Mineralogy, p. 618.. 



* Fluorine is abundant among the exhalations of cooling igneous rocks, is also found in many ordinary waters, in 

 spring waters, and even in sea water. (Bischof, Gustav, Chemische Geologic, vol. 2, pp. 86-89.) 

 c Elemente der Gesteinslehre, Stuttgart, 1898, pp. 70-71. 

 dThis change involves the substitution of potash for soda. 

 t Bischof, Gustav, Chemische Geologic, vol. 2, p. 743. 



