GEOLOGY OF GRAVES MOUNTAIN 217 



Cyanitc. Gyanite is very generally but irregularly distributed through 

 both phases of the quartzite composing the mountain, and is intimately 

 associated with rutile, lazulite, pyrophyllite, coarse crystaUine quartz, and 

 hematite. It is a prominent constituent of the rutile-bearing rock, chiefly 

 a mixture of cyanite and hematite with quartz ; and it is frequently a con- 

 spicuous mineral in the lazuhte-bearing quartzite (itacolumite), especially 

 in the lazulite areas. It forms long bladed and coarsely columnar forms 

 up to several inches in length, is usually colorless though green is not 

 uncommon. In the weathered portions of the rock cyanite is partially 

 discolored a yellowish brown from iron stain. Muscovite is a frequent 

 associate and is derived in part at least from the alteration of cyanite. 



In thin sections under the microscope the cyanite is colorless and non- 

 pleochroic. It usually shows polysynthetic twinning in broad bands, and 

 good cleavage parallel to the direction of elongation (100) and less often 

 and less well developed a second cleavage parallel to 010. Fracture is 

 common. The cyanite contains inclusions of rutile and quartz, and in 

 some cases it forms inclosures in the lazulite. The rutile inclusions usually 

 show perfect crystal boundaries and the mineral is some shade of red in 

 color. Rutile as formless grains sometimes occurs along the boundaries of 

 the cyanite and other minerals. Concerning the coarse rutiles Professor 

 Shepard* remarks: "the most perfect rutiles are generally imbedded in 

 the massive kyanite; and when detached leave behind impressions having 

 a polish and lustre equal to that of their own planes." 



An interesting feature of the cyanite is its alteration to muscovitef and 

 iron oxide. The cyanite of the rutile matrix has been designated by Shep- 

 ard+ a ferruginous cyanite. Muscovite in scales is frequently observed on 

 the cyanite by the naked eye and in such relations as to leave no doubt of 

 its derivation from the cyanite by alteration. This is entirely confirmed 

 by the microscope. Thin sections often show the cyanite to have frayed- 

 out or ragged ends altered to muscovite shreds, and at times the cyanite 

 substance is clouded by muscovite scales, not infrequently discolored by 

 reddish brown oxide of iron. Iron oxide has formed along cleavage and 

 fracture directions and, in case of the rutile matrix, it masks much of the 

 cyanite substance. 



In the table below is given an analysis of the Graves Mountain cyanite 



* Op. cit., 1859. 



t Watson, Thomas L. and Watkins, J. H., Association of Rutile and Cyanite from 

 a New Locality. Amer. Journ. Sci., 1911, vol. 32, p. 200. 

 i Op. cil.. 1859. 



