116 Dr Brewster on Oxahverite, ^ 



and sometimes in insulated crystals implanted in the calcareous 

 spar. 



The crystals are acute octohedrons, with a square base, and 

 therefore belong to the pyramidal system of Mohs. The 

 angles at the base are truncated by planes parallel to the axis 

 of the octohedron, and equally inclined to the adjacent sides 

 of the base, so as to form, when enlarged, the faces of a square 

 prism. 



In submitting the most perfect crystals to measurement, a 

 very remarkable peculiarity, which I have never before ob- 

 served, is distinctly seen. Every face of the octohedron is a 

 surface of double curvature, in consequence of which the 

 maximum angle of the two opposite faces of the pyramid is 

 58°, while the minimum angle is 42°, giving a change of in- 

 clination of no less than 16°. The maximum inclination oc- 

 curs at the base, and at the vertex of the pyramid, and the 

 minimum inclination at an intermediate point, so that the cry- 

 stal has a form very unusual among minerals. See Plate II. 

 Fig. 5. 



The general size of the crystals is about one-tenth of an 

 inch in length, their surfaces are even, but not brilliant, and 

 the small truncations of the angles at the base of the pyramid 

 are more imperfect than those of the octohedron, the imper- 

 fections having the direction of the axis. 



The crystals cleave with some facility perpendicular to the 

 axis, and the plane of cleavage is considerably rounded, the 

 convex surface being turned towards the apex of the pyramid. 

 In no other direction have I been able to discover the least 

 appearance of cleavage. 



The hardness of this mineral is between 4.5 and 5.0, — 

 bet ween ^?f or spar and apatite^ but nearer to the latter. Its 

 specific gravity, as determined by a mass of forty-three grains, 

 is 2.218. 



The colour of the crystals is light-grey, leek-green, olive- 

 green, and reddish-brown. They have one axis of double re- 

 fraction coincident with the axis of the octohedron, like all 

 other crystals belonging to the pyramidal class. This axis is 

 negative like all those of the pyramidal class, exceptingapophyl- 

 lite, that cleave perpendicular to the axis, and its intensity is 



