WATSON AND STEIGERI SPINELLITE FROM VIRGINIA 673 



Corundum is subject to wide variation in amount and, with 

 the exceptions noted above, it is usuaFy not visible megascopi- 

 cally. It may fail entirely in some thin sections and may be 

 present in considerable quantity in others. In the bulk sample 

 of the rock yielding the results on analysis given in table 2, 

 the corundum was estimated at about 18 per cent. It is devel- 

 oped both in grains and in distinct crystals of prismatic habit, 

 which measure up to 0.12 mm. in length. The corundum gen- 

 erally cuts sharply against the spinel and magnetite, but in 

 places the boundaries are irregular and rather indefinite, suggest- 

 ing, in such cases, a form of intergrowth with the ore minerals 

 indicating approximately simultaneous crystallization. How- 

 ever, it seems reasonably clear that a part of the corundum crys- 

 ta lized ahead of the spinel and magnetite. 



Inclusions of both spinel and magnetite occur, especially the 

 latter, which may be grouped at times in areas of minute rounded 

 black particles in such abundance as almost entirely to obscure 

 the substance of the coiimdum. Regular rhombohedral parting 

 is frequently well developed and, as a rule, the individuals are 

 marked by irregular fractures or cracks. 



CHEMICAL COMPOSITION 



Composition of the Virginia rock emery is shown in the de- 

 tailed chemical analysis in column I of table 3. For purposes of 

 comparison two analyses (II and III) by Rogers of the Peeks- 

 kill emery, New York, and two of the Naxos emery (IV and V) 

 by Papavasiliou are tabulated. 



The differences in chemical composition of the emery from the 

 three localities, strikingly shown in a comparison of the analyses, 

 are differences that express variation in the proportion and com- 

 position of the minerals that enter into the make-up of the 

 emery from the different localities. Higher combined iron ox- 

 ides and lower alumina characterize the Virginia emery. Mag- 

 nesia in the emery from Virginia and New York, widely variable 

 in the latter, may be considered low for rocks in which spinel is 

 a principal mineral, but in each case it can be accounted for on 



