J. B. Scrivenor — A Tin-bearing Mineral from Siam. 123 



V. — Pleochrotsm in a Tin-bearing Mineral from Siam. 

 By J. B. Scrivenor, M.A., F.G.S. 



IN 1915 a heavy concentrate of sand from an unknown locality in 

 Siam was submitted to me for identification. The sand consisted 

 of rather coarse grains of a dark mineral, and finer grains of ilmenite, 

 monazite, tourmaline, zircon, and topaz, with some of the same dark 

 mineral as that occurring in coarse grains. 



The dark mineral was isolated and examined by partial chemical 

 analysis and by optical methods, the results proving that it was 

 a tin-bearing mineral with a remarkable pleochroism. The specific 

 gravity was found to be 6 - 913. 



The chemical examination, carried out by Mr. C. Salter, gave the 

 following constituents: Sn, 74-50 per cent; Ti 2 , 0-17 per cent; 

 and some iron and alumina. Metallic tin was readily obtained on 

 fusion with KCN. 



Optically the mineral was found to be uniaxial and positive, and 

 it exhibited a very marked pleochroism of deep red to green, 

 suggesting the colours seen in hypersthene. 



Unfortunately no crystals were available^ therefore it was 

 impossible to prove the mineral to be cassiterite, but the optical 

 properties, apart from the pleochroism and the chemical composition, 

 point to that identification. 



I have little doubt myself that this dark mineral is cassiterite, 

 showing abnormal pleochroism, because in undoubted specimens of 

 that mineral I have frequently noted a less marked pleochroism 

 of (E) carmine to (0) pale green or colourless. This pleochroism is 

 not always equally distributed over a section, but may appear in 

 irregular patches that suggest local variation in chemical composition. 

 I have been informed that a similar pleochroism has been noted in 

 a tin-bearing mineral from Nigeria, and have little doubt that it has 

 been observed by mineralogists who have examined numerous tin-ore 

 specimens from elsewhere. I published a note on this pleochroism 

 in my Annual Report (Federated Malay States Government) for 

 1904, and have mentioned it in subsequent publications, but this 

 specimen from Siam shows the phenomenon in a much more marked 

 degree than any other specimen I have seen, and now that one has 

 time to return to such matters I would suggest that the connexion 

 between coloration, pleochroism, and chemical composition of 

 cassiterite is a subject that might be pursued with some hope 

 of arriving at interesting results. In crystals and in grains, viewed 

 by reflected light, there is a wide range of colour; but in thin 

 sections examined by transmitted polarized light the differences of 

 coloration are more pronounced still. Pleochroism is sometimes 

 present, sometimes not. The most distinct pleochroic effects I have 

 seen are this deep red to green, deep brown to a lighter brown, and 

 violet to almost colourless. 



It would, of course, be necessary to examine large quantities of 

 material before one could ascribe any particular coloration to a 

 particular chemical constituent ; but it is perhaps worth noting one 

 point here in connexion with this Siamese mineral. One might 



