200 Watson and Wathins — Association of Rutile and 



up to 2 inches long and ■£ inch wide, and from colorless to 

 green and blue. It. is in long bladed and coarsely columnar 

 crystals, flattened parallel to 100, and without terminal planes. 

 Fractures crossing the longer directions of the crystals (100) 

 are rather common. 



The cyanite is colorless in the thin sections but in some of 

 the strongly colored individuals weak pleochroism (colorless to 

 taint blue) is noticeable. Cleavage parallel to 100 is usually 

 well developed and is sometimes observed parallel to 010. It 

 contains inclusions of quartz, rutile, and some feldspar (?), suffi- 

 ciently large and conspicuous as to give a micropoikilitic tex- 

 ture to the cyanite. 



The most interesting feature of the cyanite is its alteration 

 to muscovite. Some specimens of a coarse aggregate of 

 columnar individuals of deep green color are easily scratched 

 with the knife and on close examination are seen to be composed 

 mostly of scales and shreds of muscovite. Other specimens 

 show the alteration to be accompanied by the formation of 

 much deep red to yellowish-brown iron oxide. Thin sections 

 under the microscope indicate an aggregate of muscovite 

 shreds, discolored from iron oxide, and sometimes inclosing 

 grains of rutile which show little or no alteration (fig. 3). 

 The early stages of alteration progress partly along cleavage 

 and fracture directions, indicated usually by a somewhat darker 

 color along these directions. 



The literature contains a few references to the alteration of 

 cyanite. Among the more recent ones are Yan Hise * and 

 Dana, f who remark that cyanite alters to muscovite (damourite) 

 and talc (steatite). Clarke:}: says the reported alteration of 

 cyanite to talc and steatite is most questionable, and remarks 

 that probably a compact muscovite (damourite) has been mis- 

 taken for talc. The reactions involved in the change of 

 cyanite to muscovite (damourite) are those of hydration and 

 are therefore characteristic of the zone of katamorphisrn.§ 



Genesis. 



The mineral association in the area described is one of 

 quartz and sericite with the heavy minerals cyanite, some 

 rutile, and less tourmaline and garnet. Yan Hise |J has pointed 

 out that the conditions favorable for the development of gar- 



* Van Hise, C. E. A Treatise on Metainorphism, Mono, xlvii, U. S. Geo- 

 logical Survey, pp. 318-319, 1904. 



f Dana, E. S. A System of Mineralogy, p. 501, 1900. 



X Clarke, F. W. Bull. 330, U. S. Geological Survey, p. 528 (footnote), 

 1908. 



§ Van Hise, C. E. Loc. cit., p. 318. 



|| Van Hise, C. E. Ibid., p. 300. 



