ALTERATIONS OF ZOISITE AND EPIDOTE. 323 



It is not easy to approach accuracy in writing equations for the altera- 

 tions of the epidotes in the belt of weathering. In the equations given 

 below it is supposed that the calcium passes into carbonate, that the 

 Al (OH) goes into gibbsite, that the remainder of the aluminum goes into 

 kaolin, and that the excess of silica separates as quartz. In the epidote the 

 Al is supposed to be to the Fe as 2:1, and the iron is supposed to pass into 

 limonite (amorphous; sp. gr. 3.8). Upon these suppositions the alterations 

 stand — 



For zoisite — 



(1) Ca 2 (A10H)Al 2 (Si0 4 ) 3 +2C0 2 +3H 2 0=2CaC0 3 +Al(OH) 8 +H. 1 Al 2 SiA+Si0 2 +k 



and for epidote — 



( 2 ) Ca 6 ( AlOH ) 3 Al 4 Fe 2 Si 9 3 c +6C0 2 +8 iH 2 0= 



6CaC0 3 +3Al(OH) 3 +2H 4 Al 2 Si 2 9 +Fe 2 3 .lJH 2 0+5Si0 2 +k. 



The increase in volume of all the compounds formed as compared with 

 the zoisite is 66.22 per cent, and as compared with the epidote is 69.08 per 

 cent. 



Of course there are many other ways in which the equations could be 

 written. All of the aluminum might pass into gibbsite or diaspore and 

 more quartz form. The iron may pass into hematite in whole or in part, 

 etc. While all this is true, it is believed that the above equations represent 

 correctly the fundamental fact that by hydration and carbonation zoisite 

 and epidote in the belt of weathering pass into simpler compounds. 



Similar reactions could be written for the alterations of piedmontite 

 and allanite, but considering the comparative rarity of these compounds 

 this will not be done. 



A finite: 



HCa 3 Al 2 BSi 4 16 . (In some cases part of the Ca is replaced by Fe and Mn. ) 



Triclinic. 



Sp. gr. 3.271-3.294. 



occurrence. — Axiiiite occurs as a secondary constituent in basic eruptive 

 rocks, such as the diabases and gabbros. It is found to some extent in the 

 schists and gneisses, and particularly in those bearing abundant pyroxene 

 and amphibole. It also occurs in altered sedimentary rocks as a product 



