308 A TREATISE ON METAMORPHISM. 



CHRYSOLITE GROUP. 

 FOKSTERITE, OLIVINE, AND FAYALITE. 



The chrysolite group includes — 



Forsterite: 

 Mg 2 Si0 4 . 

 Orthorhornbic. 

 Sp. gr. 3.21-3.33. 



Olivine: 



(MgFe) 2 Si0 4 where Mg : Fe : : 16 : 1, 12 : 1, to 2:1, in the last case the mineral being 



known as hyalosiderite. (Sp. gr. 3.566.) 

 Orthorhornbic. 

 Sp. gr. 3.2-3.6 according to 13intze, but ordinarily being, according to Dana, 3.27-3.37. 



Fayalite: 

 Fe 2 Si0 4 . 

 Orthorhornbic. 

 Sp. gr. 4.1. 



occurrence. — Tsehermak considers olivine as an isomorphous mixture of 

 fayalite and forsterite. The occurrence of the three minerals is the same, 

 except that fayalite and forsterite are not nearly so widely known as the 

 intermediate common mineral, olivine. Olivine is an abundant constituent 

 in intermediate and basic igneous rocks, both plutonic and volcanic, in lavas 

 and tuffs alike. In rare cases in the volcanic rocks fayalite occurs, as, for 

 instance, in nodules in volcanic rocks and in lithophysae of the rhyolites of 

 the Yellowstone Park. Forsterite also very rarely occurs in connection 

 with volcanic rocks. Olivine is also an accessory constituent in the very 

 basic schists and gneisses, such as the amphibolites, pyroxenites, eclogites, 

 etc. Finally, it not infrequently occurs in marbles. In rocks of this class 

 forsterite also rarely occurs. It therefore appears that the chrysolite group 

 of minerals occurs most abundantly as original constituents, but are also 

 rather widely found as secondary developments in the metamorphosed rocks, 

 including both the carbonates and the basic schists. 



Alterations. — The alterations of fayalite and forsterite are exceptional; 

 therefore the chief alterations which are considered are those which pertain 

 to olivine. 



The most common alteration of olivine is to serpentine (monoclinic; 

 sp. gr. 2.50-2.65). This is a change from an anhydrous orthosilicate to a 

 hydrous orthosilicate. Doubtless this explains why serpentine rather than 



