258 A TREATISE ON METAMORPHISM. 



the latter, chiefly in the andesitic lavas. As an allogenic mineral it also 

 is found in the sedimentary rocks. Whether it occurs as an autogenic 

 mineral in the metamorphic rocks has not been determined. 



Alterations.— Both ortlioclase and mi crocline contain some sodium. When 

 the sodium becomes important the mineral is anorthoclase. It naturally 

 follows from this fact that the alterations of anorthoclase are in all respects 

 like those of ortlioclase and microcline, with the exception that the freed 

 alkalies are in good part sodium. The reactions are analogous to those 

 already given for ortlioclase, but with the muscovite or biotite the soda-mica 

 paragonite (monoclinic; sp. gr. 2 84) is formed. Supposing the sodium 

 silicate is to the potassium silicate as 2 to 1, the more important reactions 

 may be written as follows: 



(1) 2(2NaAlSi 3 8 .KAlSi 3 8 )+6H 2 0+3C0 2 =3H 4 Al 2 Si 2 9 +12Si0 2 +K 2 C0 3 +2Na 2 C0 3 +k. 



(2) 2(2NaAlSi 3 8 .KAlSi 3 8 )+9H 2 0+3C0 2 =6Al(OH) 3 +18Si0 2 +K.,C0 3 +2Na 2 C0 3 +k. 



(3) 2NaAlSi 3 8 -KAlSi 3 8 +6Al(OH) 3 =KH 2 Al 3 Si 3 12 +2NaH 2 Al 3 Si 3 12 +6H 2 0-k. 



(4 ) 2NaAlSi 3 8 . KAlSi 3 8 +MgC0 3 +FeC0 3 4-5Al (OH ) 3 = 



HKMgFeAl 2 Si 3 12 +2NaH 2 Al 3 Si 3 12 4-5H 2 0+2C0 2 -k. 



Supposing the sodium silicate is to the potassium silicate as 3 to 1, we have: 



(5) 2(3NaAlSi 3 8 .KAlSi 3 8 )+2Fe 2 3 *4-8CaC0 3 +2H 2 0= 



4HCa 2 Al 2 FeSi 3 13 4-12Si0 2 4-K 2 C0 3 +3Na 2 C0 3 +4C0 2 +k. 



The equations corresponding to (3) and (5) under ortlioclase and 

 microcline are not written, since their occurrence is very doubtful. The 

 decrease in volume of the kaolinite and quartz as compared with the 

 anorthoclase, equation (1), is 9.56 per cent, or of the kaolinite alone is 52.19 

 per cent. The decrease in volume of the gibbsite and quartz as compared 

 with the anorthoclase, equation (2), is 3.30 per cent, or of the gibbsite alone 

 is 68.02 per cent. The decrease in volume of the muscovite and paragonite, 

 as compared with the anorthoclase and gibbsite, equation (3), is 20.04 per 

 cent, The decrease in volume of the biotite and paragonite, as compared 

 with the anorthoclase and gibbsite, equation (4), is 10.91 per cent. The 

 decrease in volume of the epidote and quartz, as compared with the anortho- 

 clase, hematite, and calcite, equation (5), is 28.30 per cent. Equations cor- 

 responding with the above and the volume relations can be easily worked 

 out along analogous lines for other ratios of the sodium-bearing and potas- 

 sium-bearing silicates, but the general results would be the same, so this is 

 hardly worth the while. 



The geological positions and physical conditions under which the 



