180 Robinson — Octahedrite and Brooleite. 



Art. XIX. — On Octahedrite and Brookite, from Brindle- 

 town, North Carolina ; by H. H. Kobinson. 



The two minerals described in this article were kindly sent 

 to Prof. S. L. Penfield by Mr. W. E. Hidden, and are from the 

 original locality on the north slope of Pilot Mountain near 

 Brindletown, Burke County, North Carolina, discovered by him 

 in 1879.* They occur, according to Mr. Hidden, in detached 

 crystals scattered through the gold-bearing gravels of the dis- 

 trict, having been derived from the disintegration of the local 

 schists. As accompanying minerals, are mentioned zircon, 

 monazite, xenotime, samarskite, fergusonite, and many others. 



It was thought that brief descriptions, with figures, of these 

 two minerals would be of interest both because of their rare 

 occurrence in the United States and because no careful investi- 

 gation of the crystals from this locality seems to have been 

 made. 



1. Octahedrite. 



The crystals studied vary in size from 2 to 5 mm in diameter 

 on the horizontal axes and from 0*50 to l*25 mm in thickness. 

 The larger ones are of a deep bluish-black color and then they 

 possess a metallic-adamantine luster, while a smaller one is 

 quite colorless. Under the microscope, the color is seen to be 

 due to a dark pigment which entirely pervades some crystals, 

 while in others transparent spots are left, around the borders 

 of which the pigment gradually fades out. A small transpar- 

 ent crystal, representing a section perpendicular to the vertical 

 axis, was dark in all positions between crossed nicols. In con- 

 verging polarized light, however, it was seen that the mineral 

 was not perfectly uniaxial, as the axes of the interference figure 

 opened slightly on revolving the section. 



In habit the crystals are tabular owing to the prominent 

 development of the basal pinacoid, c (001). This Mr. Hidden 

 mentions as being the common habit. The simplest crystal 

 examined consists of the forms c (001), e (101), x (103), and 

 p (111), and is shown by figures la and lb j of these, la is a 

 horizontal projection (on c) and lb shows the crystal in the 

 usual perspective.f In figures 2a and 2b a slightly more com- 



* This Journal (3), xxi, 160, 1881. 



f In this article the method has been adopted of giving with each clinographic 

 drawing of the crystals a plan, which represents a true horizontal projection of 

 the lower figure. In most cases the drawing of such a plan greatly facilitates 

 the construction of the customary crystal figure, giving a complete control over it, 

 and the two figures taken together give a better idea of the crystal shape than 

 either one alone. Since there exists the closest relationship between the two 

 figures, it has been considered advisable to represent them in the position in 

 which they are drawn. Accordingly each plan is shown as revolved a certain 

 angle (18° 26') from a horizontal position. 



