MINERALOGICAL NOTES: No. XI. 



BY 



C. Ani>kkso.\, M.A., D.Sc, Director. 

 (Plates xxxviii.-xli.) 



DIAMOND. 



Near Bo<>gy Camp, Inverell, N.S.Wales. 



(PI. xxxviii., fig. 1, PI. xxxix., fig. 1.) 



An interesting diamond crystal from tliis field was lent by Mr. D. A. 

 Porter for measurement and description. The diamonds at this localit}^ 

 ai'e found in deposits of sand and gravel, probably of Pliocene age, 

 underlying the basalt capping of a number of isolated hills ; they are 

 accompanied by sti^eam tin and a little gold. 



The crystal weighs -0443 grams. It is colourless, and consists of a 

 hexakis-octahedrou with indices near (111), twinned on an octahedral face 

 (spinel law), and flattened [)arallel to the twin plane to form a triaugnlar 

 plate; diamonds of this shape are known at Amsterdam a,H uaadsteenen 

 (suture stones). Only six faces of each half of the twin are developed, 

 forming a very low pyramid with curved edges and planes, each face 

 striated in lines running roughly parallel to its intersections with an 

 octahedral face, but towards the periphery the striations curve in con- 

 formity with the crystal edges and gi'adually disap{)ear. A few small 

 triangular depressions appear near the apex, the corners of the pits, as is 

 usual in natural etch pits of the diamond, being directed towards an 

 adjacent octahedral edge. 



In order to investigate the " light paths," the crystal was mounted 

 on a two-circle goniometer so that the plane of the triangular plate was 

 approximately parallel to the plane of the vertical circle. Six trails of 

 reflection were found, ladiating in pairs from the apex (the centre of 

 PI. xxxviii., fig. 1), but not reaching quite to the centre. The crystal was 

 adjusted so that the point of intersection of these six paths was approxi- 

 mately polar, and then a large number of readings was taken along each 

 path. The results are plotted in stereographic projection in PI. xxxix., 

 fig. 1. 



Two other diamonds from the same locality in Mr. Porter's collection 

 merit a short description. Both are slightly j'ellowish ; one is a distorted 

 octahedron weighing -1 giam and built up by a number of parallel and 

 sub-parallel plates ; the other, which weighs VS grams, is a symmetrical 

 triakis-octahedron with rounded edges. 



