68 



ANNALS NEW YORK ACADEMY OF SCIENCES 



it is evident that no broad deductions can safely be made from the study 

 of one member, aside from the fact that in only one place does the ore 

 occur in a norite area. F. W. Clarke 57 quotes Williams as regarding the 

 ore as a segregation, as does also J. F. Kemp. 58 Lagorio, 59 however, 

 refers to the deposits as in contacts, although his reference is to Wil- 

 liams's first paper. 



PETROGRAPHY OF THE EMERY AND ASSOCIATED ROCKS 



Perhaps the description of the several types of occurrence may be 

 facilitated if the ore and some of the rocks peculiar to the mines are 

 considered first. 



Spinel Emery 



In the hand specimen, spinel emery is a heavy black fine grained aggre- 

 gate, with dark gray crystals of corundum appearing in the best varieties ; 



sometimes the corundum is pink 

 (approaching ruby) and sometimes 

 blue (approaching sapphire). Un- 

 less these crystals are large, however, 

 the amount of the mineral present 

 cannot be distinguished. The emery 

 often has an excellent fracture and 

 closely resembles the Turkish emery. 

 In thin section, it is seen to consist 

 of an aggregate of pleonaste, corun- 

 dum and magnetite. The pleonaste, 

 except in the highest grade of ore, 

 constitutes the bulk of the rock. It 

 is in rich green grains of irregular 

 shape, closely crowded. They are, of 

 course, isotropic and have a high in- 

 dex of refraction, though the surface appears smooth. They often carry 

 magnetite inclusions in the form of very fine parallel rods. The corundum 

 is present in all amounts ; it may constitute half the specimen, or it may 

 be lacking entirely. The grains, which may occasionally reach an inch in 

 size, are in this variety colorless in thin section. They exhibit a prismatic 

 shape, which is usually quite sharp and apparently always crystallize 

 before the spinel. They are always strongly cracked and are often con- 



Fig. 4. Relations of Spinel, Corundum 

 and Magnetite in Emery. SI. 326 



57 Data of Geochemistry, Bull. 330, U. S. G. S., p. 278. 1908. 



68 Ore Deposits of the U. S. and Canada, pp. 61 and 173. 1906. 



69 Zeit. Kryst. Mineral, vol. 24. p. 288. 1894-95. 



