ALTERATIONS OF ORTHOCLASE AND MICROCLINE. 255 



The alteration may be to hydro-muscovite or damourite (monoclinic ; 

 sp. gr. 2.76-3.00). This is believed by most mineralogists to differ from 

 muscovite only in containing more water, but Dana states that a greater 

 content of water in damourite than that contained by ordinary muscovite 

 is not necessary. 



From orthoclase and microcline, with the addition of magnesium and 

 iron compounds, biotite (monoclinic; sp. gr. 2.7-3.1) may be formed. If 

 the hydrogen and potassium be supposed to be present in equal proportions 

 and the same supposition be made with reference to magnesium and iron, 

 and the latter elements are supposed to be present as carbonates, the 

 reaction may be as follows : 



(5) 4KAlSi 3 8 +2MgC0 3 +2FeC0 3 +H,0 = 



2HKMgFeA] 2 Si 3 12 +5Si0 2 +K 2 Si0 3 +4C0 2 +k. 



The decrease in volume of the feldspar, magnesium carbonate, and iron 

 carbonate in passing into the biotite and quartz is 22.64 per cent. But, as 

 with muscovite, the more frequent reaction probably involves gibbsite, 

 thus : 



(6) KAlSi 3 O 8 fMgC0 3 +Fe0O 3 +Al(OH) 3 =HKMgFeAl 2 Si 3 12 +H 2 O+2CO 2 +k. 



This greatly simplifies the equation. The decrease in volume of the bio- 

 tite as compared with the compounds from which it is derived is 22.33 

 per cent. 



Orthoclase and microcline are said also to alter to epidote (monoclinic; 

 sp. gr. 3.25-3.50); but if this be so calcium and iron must be introduced. 

 The forms in which these compounds are present during the alteration are 

 doubtless variable. If they be assumed to be present as calcium carbonafe 

 and iron sesquioxide, the reaction might be as follows: 



(7) 4KAlSi 3 8 4-Fe 2 3 +4CaC0 3 +H 2 0=2HCa 2 Al 2 FeSi 3 13 +6Si0 2 +2K 2 C0 3 +2C0 2 +k. 



Supposing the Al is to the Fe as 2 is to 1, the decrease in volume of the 

 epidote and quartz as compared with the feldspar, calcite, and iron oxide 

 together is 33.73 per cent. However it is so uncertain as to the forms of 

 the accessory compounds, both before and after reaction, that it is impos- 

 sible to make a definite statement as to the volume relations. 



The alteration of orthoclase and microcline to minerals of the kaolin 

 group and to gibbsite occurs in the zone of katamorphism. The process 

 takes place on the most extensive scale in the belt of weathering-, especially 



