314 A TREATISE ON METAMORPHISM. 



and liberation of heat. The change of meionite to epidote is a reaction of 

 hydration and possibly of oxidation. 



Corresponding with these facts the alterations to kaolin and talc are 

 known to take place in the zone of katamorphism, and the same is probably 

 true of the alterations to muscovite and epidote, although the latter reac- 

 tions may be more characteristic of the belt of cementation than of the belt 

 of weathering. 



MELILITE. 

 Melilite: 



(CaMgNa 2 ) 6 (AlFe) 2 Si 5 19 . (Groth. ) 



Tetragonal. 



Sp. gr. 2.9-3.10. 



occurrence. — Melilite has a widespread distribution in the leucite and 

 nephelite rocks. Aside from leucite and nephelite the most characteristic 

 associates are augite and perovskite. Some of the rocks in which melilite 

 occurs are leucitophyre, nepheline-syenite, and basalt. 



Alterations. — The alterations of this mineral are not recorded, although 

 from its composition there can be no doubt that in the upper physical- 

 chemical zone it decomposes into less complicated silicates. 



GEHLENITE. 



Gehlenite: 



Ca 3 Al,Si. 2 O 10 . 

 Tetragonal. 

 Sp. gr. 2.9-3.07. 



occurrence. — The only occuiTence of gehlenite recorded in rocks is as a 

 contact product in limestone. 



Alterations. — According to Dana it alters to talc (orthorhombic or mono- 

 clinic; sp. gr. 2.75), to fassaite (monoclinic; sp. gr. 2.965-3.291), and to 

 grossularite (isometric; sp. gr. 3.605). 



The change to grossularite involves the addition of Si0 2 , thus: 



Ca 3 Al 2 Si 2 O 10 +SiO 2 =Ca 3 Al 2 Si3O 1 ,-)-k. 



The decrease in volume of the grossularite as compared with the gehlenite 

 is 4.42 per cent. If the Si0 2 be added as a solid, the decrease in volume 

 is 18.56. 



As gehlenite is so rare, and the manner of the alteration into talc and 

 fassaite is not clear, no attempt is made to write equations for the changes. 



