454 PROCEEDINGS OF SECTION C. 



The mineral is of a brownish-black color, and on a fresh fracture has a 

 bright metallic lustre. Some fragments are wholly or partially coated 

 with a smooth, hard, and strongly adherent coating of ferruginous 

 bauxite, as much as 3 or 4 millimetres thick in places. This peculiarity 

 extends to the tin ore also. Other fragments have no decided coating, 

 but a slightly rusty tinge, which enables them to be readily picked out 

 from the associated tin ore. Quartz, weathered felspar, and stibio- 

 tantalite are found adherent to it, whilst associated with it in the 

 gravels are cassiterite, tourmaline, quartz, mica, cyanite, and other 

 minerals. 



The most interesting mineral found at Greenbushes is the pre- 

 viously mentioned stibiotantalite. This has not yet been located in 

 lode material ; but in some parts of the southern part of the field was 

 at one time somewhat plentiful in the stream deposits. It was never 

 shown to exist in amounts which would justify its being put on the' 

 market as a tantalum ore, and as time goes on it appears to become 

 more and more rare. Greenbushes was, up till recently, the only known 

 locality for this mineral, but quite recently well-defined orthorhombic 

 crystals have been found at Mesa Grande, California, in a pegmatite 

 vein, with tourmaline, beryl, &c. 



The chemical composition of the Greenbushes mineral is shown 

 by the following two analyses by Mr. G. A. Goyder : — 



A B 



Tantalum pentoxide 51-13 .. 51-95 



Niobium pentoxide 7-56 . . 4-49 



Silica — . . 3-14 



Combined water — . . '61 



Antimony trioxide 40-23 . . 38-04 



Bismuth trioxide -82 . . -79 



Iron peroxide Trace . . -39 



Manganese protoxide — . . Trace 



Nickel protoxide -08 . . Trace 



Copper oxide — . . -20 



99-82 . . 99-Gl 



Specific gravity 7-37 . . G-47 



Its formula is Sb (Ta Nb)04, or more probably, according to Penfield 

 and Ford, (SbO)2 (Ta Nb)206, which would explain the resemblance 

 of its crystals to those of columbite. 



Two well-marked features of the Greenbushes ore are of great 

 interest, as indicating the probable origin of the mineral. In the first 

 place stibiotantalite is often seen filling minute veins and occupying 

 small vughs in masses of tantalite. Secondly, a very large number of 

 the water-worn pebbles contain a central core of tantalite, the boun- 

 daries of which are very irregular, and which penetrates often in thin 

 tongues out into the surrounding stibiotantalite. It would appear 

 that the original mineral has been tantalite, and that this has been 

 more or less replaced by pseudomorphs of stibiotantalite on the in- 

 filtration at a later stage of antimonial solutions into the lodes. 



