FAYALITE IN CERTAIN IGNEOUS ROCKS 



555 



in many thin sections of this variety from widely different parts of 

 the district. In the rocks bearing quartz it is associated with the 

 feldspars, orthoclase, albite, and microperthite ; with the calcium- 

 iron- amphibole, arfvedsonite ; with the calcium-iron-pyroxene, heden- 

 bergite; and with iron mica containing large amounts of potassium. 

 In phases of the non-quartzose 

 and nepheline-bearing rocks 

 of the series the fayalite also 

 contributes a fraction of i per 

 cent, to as much as 5 per cent, 

 of the rock. In some of the 

 nepheline-rich rocks the faya- 

 lite is the only dark- colored 

 mineral noted in the thin sec- 

 tions. In these rocks the faya- 

 lite occurs with orthoclase and 

 microperthite, nepheline, soda- 

 lite, the soda amphiboles of 

 the riebeckite, and crocidolite 

 type, and the calcium-iron 

 pyroxene, hedenbergite, and 

 the potash-iron mica lepido- 

 melane. 



The yarious rocks in which 

 the fayalite occurs is thus seen 

 to have a considerable range 

 in certain chemical constitu- 

 ents, such as silica, alumina, 

 and the alkalies, and also in 

 mineral composition. The 

 area over which these rocks 

 are distributed is quite exten- 

 sive. The amphibole granite and quartz syenite cover several hun- 

 dred square miles, and the nepheline-bearing and related basic rocks, 

 from fifteen to twenty square miles. 



The fayalite, in the various phases of rock in which it occurs, has 

 the associations and relations of a normal, original constituent of 



Fig. I. — Photomicrograph of fayalite in 

 nepheline syenite; X6o. The fayalite, light- 

 colored, is surrounded on nearly all sides by 

 a black border of magnetic iron oxide. Por- 

 tions of it are in contact with feldspar and 

 hedenbergite. The greatest length of the 

 fayalite in this section is along the c axis, 

 parallel to which is indistinct parting or 

 cleavage. The areas within the fayalite are 

 colored dark by photography, and in reality 

 are yellowish -brown and bluish mineral 

 inclusions. The yellowish-brown inclusions 

 are in irregular ai^eas, and are probably yel- 

 lowish hydroxide of iron, gothite, formed by 

 alteration of the fayalite. The bluish inclu- 

 sions are finely striated crystals of a bluish 

 amphibole. 



