670 WATSON AND STEIGER : SPINELLITE FROM VIRGINIA 



less often as scattered single crystals. The corundum crystals 

 measure up to 2.5 cm. long and 1 cm. wide, ]but probably most of 

 them range from 1 to 3 mm. and under 1 mm. in size. They are 

 usually clear and nearly white or colorless to light grayish, but 

 many are light to deep blue. Megascopic corundum, however, 

 has been identified only in the ore in the schist, none having yet 

 been observed in the ore in the granite. Otherwise the ore in 

 its two rock associates is identical in appearance and cannot 

 be distinguished in either hand specimens or large masses. 



As a rule the ore is entirely massive, and is cut by one or more 

 sets of joints irregularly spaced, which break it into blocks of dif- 

 ferent sizes. Exceptions, however, are noted in some of the 

 schist occurrences where thin bands of ore alternate with similar 

 ones of schist, and in those places where tiny bands of light-col- 

 ored corundum are developed, or under conditions mentioned 

 above where difference in granularity is shown. In each case a 

 rather distinct banded structure is apparent. 



Microscopic study of more than fifty thin sections of the ore^ 

 shows it to consist of a fine-grained interlocking aggregate of 

 spinel (pleonaste), magnetite, and conmdum, 'with some ilmenite, 

 and in some sections a brown mineral whose identity has not 

 yet I een established. From the optical data it is possible that 

 the brown mineral may prove to be a new species. In addition 

 to the minerals mentioned some sections show sillimanite, anda- 

 lusite (chiastolite), and a pale-green chlorite, possibly corundo- 

 pbyllite. The dominant mineral is spinel (pleonaste), which 

 makes up 50 per cent or more of the rock. All the principal 

 minerals vary considerably in amount. Corundum, for ex- 

 ample, may be abundant in some sections and almost or entirely 

 wanting in others. 



The spinel occurs in rich green, sometimes brown, grains, which 

 may show partial crystal outline, and is, of course, isotropic. In 

 the coarser grained rock the spinel grains Co not exceed 2 mm. 

 in diameter, while in the finer grained rock they will average less 



1 The microscopic description that follows relates entirely to the schist occur- 

 rences of the ore; study of thin sections of the granite occurrences of ore has not 

 been completed. 



