166 



F. P. Meiniell — Corundum in RJiodesia. 



VI. — J^OTE ON AN OcCUKKENCE OF CoRUNDUM IN EhODESIA. 

 By r. P. Mennell, F.G.S. 



COEUITDUM is one of those rather exceptional rock-formers whose 

 mode of origin is not fully explained, and it is, accordingly, of 

 interest to put on record any new localities for the mineral, together 

 with the features they present. Fragments of it have frequently 

 heen found among the old gravels of the Limpopo River, along the 

 Transvaal-Rhodesian and Transvaal-Bechuanaland border, but although 

 the Transvaal Geological Survey has described an occurrence in the 

 Pietersburg district, the source of these fragments for some time 

 remained unknown. However, last year I received from the Rhodesian 

 side of the border, close to Rhodes' Drift on the Limpopo, and north- 

 ward, from the well-known Messina copper-mine, some rocks which 

 included large pieces of a type rich in corundum. The area is known" 

 to comprise gneissose granulites, consisting of the contact zone and 

 marginal modifications of a granite mass, and the specimens received 

 included large cleavage fragments of felspar and pieces of coarse 

 granite. 



Felspar. 



Spinel. 



Coiundum. 



Microscopic section of corundum-bearing rock, x 25 diam. 

 The corundum occurs in the form of irregular masses of a dull 

 pinkish or greyish colour, up to several inches across. These show 

 the usual well-marked rhombohedral parting, and are sometimes single 

 crystals and sometimes aggregates. The rock in which they are 

 embedded is a felspathic one, to which sparsely distributed hornblende 

 crystals give a somewhat gneissic aspect. Completely encircling each 

 mass of corundum, as if protecting it from the invasion of the 

 felspathic matrix, is a coating of what to the unaided eye looks like 

 magnetite. This sends out straggly offshoots among the surrounding 

 felspar grains, but is only seen near the corundum. The general 

 aspect of the rock is very much that of an augen-gneiss, with the 

 corundum forming the ' eyes '. 



