464 Hidden and Pratt — Associated Minerals of Rhodolite. 



axes of the quartz have a definite relation to the dodecahedral 

 axes. 



Corundum. — About one-seventh of the heavier concentrates 

 are corundum crystals, in the form of hexagonal prisms termi- 

 nated by the base, some of which are l cni in diameter. The 

 surfaces and edges are usually rough, from natural corrosion 

 while in place. The common color is gray, but pale blue, 

 pink and amethystine shades are often found. Ruby-colored 

 crystals also occur and some of them are nearly transparent. 

 A few of the amethystine crystals would furnish fairly good 

 gems. No corundum has, as yet, been found in situ in the 

 valley and no matrix is to be seen left upon the crystals. 



Spinel Group. — This group constitutes about five per cent 

 of the concentrates and seems to include the three species, 

 pleonaste, gahnite and chromite. 



The gahnite has a density of 4*24-4-40 and has been analyzed 

 by Dr. Charles Baskerville* with the following results: 



Ratio. 



A1 2 3 61-09 -599 = 1-13 



F^O 7-78 -108 



ZnO 27-44 -339 \ '53 



MgO__ ._ 3-30 -083 



99-61 



From the above analysis, the mineral is shown to be the 

 kreittonite or zinc-iron variety of gahnite. The ratio of ZnO : 

 FeO : MgO is close to 12:4:3, which would give for the 

 formula of this gahnite (12Zn . 4Fe. 3Mg)0 . A1 2 3 . 



The mineral has a clean glassy fracture like that of some 

 dark obsidians and is deep bottle-green by transmitted light. 

 The octahedron alone and in combination with the rhombic 

 dodecahedron were the only forms observed on the crystals. 

 The faces are never smooth but have an irregular and rounded 

 appearance very similar to the so-called "fused quartz." 

 Smooth fragmentary masses are most common, varying in size 

 from 3 mm to l-5 cm in diameter and are very free from inclusions. 



Of the chromite only a few small pieces were found, but 

 these would seem to indicate that there is now or has been a 

 rock of the peridotite character in this valley from which this 

 chromite originated. No basic magnesia rocks are known to 

 be exposed in this locality. 



The specimens are kidney-form and about l cm in diameter. 

 They have a very brilliant luster upon a fractured surface and 

 are of a pitchy-black color. A specific gravity determination 

 gave approximately 4*7. 



* University of North Carolina. 



