SILK 'ATI'S, TITANATl.S, I ,T< '. 



457 



pale yellow, hrown, red, ureen. white, and at times nearly 

 l>l:ick. Streak, white; Transparent to opaque; o> = 1.923; 

 c = 1.9GS; o)-6 = .()i:.; Optically ( + ). 



B.B. - \Yhitens lint infu>il>le. Only very slightly affected by 

 acids. Yields a /ircoiiium reaction with turmeric paper. 



General description. Crystals are short prismatic combinations 

 of t he unit prism and pyramid of the first order. When x is present 

 the crystals arc acutely pointed. Microcrystals are more apt to 

 be pyramidal in habit. Twins 

 are not common, but genicu- 

 late twins like those found 

 in rutile occur in Renfrew 

 County, Quebec. Zircon is 

 related in its angles and axial 

 ratio to cassiterite and rutile, 

 and these with thorite, ThSiO 4 , 

 constitute an isomorphous 

 group. In rock sections zircon 

 appears either in crystalline 

 outlines or rounded, with a 

 very high relief, and white 

 or pale yellow or brown in 

 color. Cleavage cracks are 

 not marked. 



Interference colors are high 



fourth order, and the interference figure shows several colored 

 circles in addition to the dark cross. Optically positive. 



Zircon is very widely distributed, occurring as one of the most 

 common accessory minerals of the igneous rocks, as the granites, 

 syenites, and diorites, but never in very large quantities; in such 

 mamnas it is the first silicate to separate. It is also of common 

 occurrence in pegmatites, as at Green River, North Carolina, 

 where it is separated in commercial quantities; near Cash, Okla- 

 homa, in a pegmatite. At Greenville, Canada, and Amity, New 

 York, it occurs in a crystalline limestone. At several points in 

 Essex and Orange Counties, New York, deep brown to almost 

 black crystals occur. 



It is decomposed by weathering with difficulty and is found in 

 alluvial depo-iN and gold-bearing sands, with garnets, cassiterite, 

 magnetite, and other heavy minerals, still in a fresh unaltered 



FIG. 490. Zircon, Buncombe County, 

 North Carolina. The Small Crystal is 

 from Essex County, New York. 



