MINERAL RESOURCES OF THE STATE OF NEW YORK 69 



from outside, just as the titaniferous iron ores (ilmenite-magnetite 

 mixtures) are believed to have originated in the gabbros and 

 anorthosites. A small percentage of ilmenite is present in the 

 emery. 



References 

 Magnus, Harry C. Abrasives of New York State. N. Y. State Geol. 23d Ann. 



Rep't, 1904 

 Nevius, J. N. Emery Mines of Westchester County. N. Y. State Mus. 53d 



Ann. Rep't, 1901 

 Rogers, G. Sherburne. Geology of the Cortlandt Series and its Emery Deposits. 



N. Y. Acad. Sci. Annals 21, 191 1, p. 11-86 



FELDSPAR 



Feldspar includes several mineral species, of which the useful 

 kinds are orthoclase, microcHne and albite. Orthoclase and micro- 

 cline have the same chemical composition, but differ in crystal 

 properties; they are silicates of alumina and potash containing in 

 pure state 64.7 per cent silica, 18.4 per cent alumina and 16.8 per 

 cent potash. Albite is the soda variety with 68.7 per cent silica, 

 19.5 per cent alumina and 11.8 per cent soda. Intergrowths of 

 soda and potash feldspar are fotmd which give practically all 

 gradation in composition from the one extreme to the other; they 

 are called perthite. 



The sources of feldspar for commercial purposes are so-called 

 pegmatites, those coarsely textured rocks that occur in dikes and 

 small bosses as an accompaniment of deep-seated intrusions. Granite 

 pegmatite suppHes most of the product and is a mixture of potash 

 feldspar, quartz and mica; it differs from ordinary granite in the 

 size and more irregular distribution of the minerals. If the inter- 

 growth is only moderately coarse and the crystals average not 

 more than four or five inches in diameter, too much labor will be 

 required in cobbing and such deposits have value only in connection 

 with crushing plants for the preparation of material for artificial 

 stone, roofing gravel etc. In the coarser phases the quartz and 

 mica may be elements of value as well as the feldspar. 



Pegmatites in New York State are limited in occurrence to the 

 two areas of old crystalline rocks represented by the Adirondacks 

 and southeastern Highlands. They occur in the vicinity of the 

 larger granite intrusions but workable bodies are more often found 

 on the border of such intrusions and within the old country gneisses 

 and schists than in the midst of the granites themselves. They 

 also occur in localities where ordinary granites do not outcrop, 



