320 A TREATISE ON METAMORPH1SM. 



EPIDOTE GROUP. 

 ZOISITE, EPIDOTE, PIEDMONTITE. AND ALLAMTE. 



The epidote group includes the following minerals: 



Zoisite : 



Ca,(A10H)Al 2 (Si0 4 ) s . 



Orthorhombic. 



Sp. gr. 3.25-3.37. 



Epidote: 



Ca. 2 (A10H)(AlFe) 2 (Si0 4 ) 3 where Al:Fe as 6:1 to 3:2. 



Monoclinic. 



Sp. gr. 3.25-3.50. 



Piedmontite : 



Ca 2 (A10H) (MnAl) 2 (SiOJ 3 . 



Monoclinic. 



Sp. gr. 3.404. 



Allanite (orthite): 



Ca, (AlOH) ( AlCeFe) 2 (Si0 4 ) 3 . 



Monoclinic. 



Sp. gr. 3.5-4.2. 



occurrence. — Zoisite is not known as an original pyrogenic constituent of 

 igneous rocks. It is found in the schists and gneisses, especially those 

 containing the amphiboles. Thus it is very common in the amphibolites, 

 glaucophane-schists, eclogites, etc. Zoisite frequently occurs with albite 

 as one of the constituents of the so-called saussurites, which develop as 

 an alteration of the basic feldspars, especially in gabbros. Zoisite also 

 occurs in the altered granites and other acid igneous rocks, although it is, 

 on the whole, less abundant than in the more basic rocks, but in some 

 localities it is plentiful even in the acid rocks. Zoisite is a very frequent 

 constituent in grits, graywackes, and other sediments of similar composition. 

 In such rocks the minerals were partly altered to zoisite during the forma- 

 tion of the sedimentary rocks, and this zoisite is to be classed with the 

 allogenic constituents of the mechanical sediments. Zoisite further develops 

 in the altered sedimentary rocks as a frequent and sometimes abundant 

 product of metamorphism. From the foregoing statement of occurrence it 

 is plain that zoisite develops in the zone of katamorphism, and especially 

 in the belt of cementation. As shown under the discussion of the other 

 minerals, it is seen that zoisite may be derived from the following minerals: 

 Corundum, diaspore, gibbsite, grossularite, and the plagioclases. 



