BY W. F. PETTERD, C.M.Z.S. 163 



common in the mine, and may be an unrecorded variety 

 of halloysite." 



302. SoDALiTE (ChlorosiJicate of Sodium and Aluminium). 

 An accessory constituent in the comparatively rare 



alkaline series of rocks that contain nephelite (or elaeolite) 

 and minerals of that character. It belongs to the isometric 

 system, and usually occurs crystallised in rhombic dedeca- 

 hedra ; it is also known massive. In colour it is invariably 

 of a pale tint and translucent. It occurs as natrolitic 

 pseudomorphs in the elseolite-syenite of Port Cygnet. 



303. Sphalerite (Sulphide of Zinc). 



This common substance is more generally known on 

 mining fields as '' blende," or " zinc blende," and more 

 rarely as " black jack." It crystallises in the cubic system, 

 but is subject to many modifications. It has a brilliant, at 

 times almost sub-metallic lustre, and varies from almost 

 colourless to clear translucent yellow to pale-brown, and 

 thence to opaque darker shades, increasing to intensely 

 black. In common occurrence it is amorphous, with dis- 

 tinct cleavage. It affects an intimate association with 

 galena, and but few occurrences of that mineral, if any. 

 are known without more or less admixture of blende, either 

 in the massive or crystallised form. As blende is less 

 stable than galena in the zone of oxidation, it is the first 

 ingredient to be attacked ; and as zinc sulphate is readily 

 soluble, and is more mobile than the lead sulphate, it is 

 usually left behind to indicate the lead contents of a 

 gossan. At times cadmium and iron replace portions of 

 the zinc, and on rare occasions it may carry fair values in 

 silver, as is the case at the Magnet Mine; while the rare 

 elements, iridium, gallium, and thallium, are also known 

 to occur as replacements. In the mixed lead-zinc deposits 

 of the Mt. Read and Rosebery districts it is extremely 

 profuse, while at the Hercules Mine very beautiful patches 

 of remarkably well-developed but complicated crystals 

 occur in the cleavages; they are not rarely of a clear 

 pellucid yellow colour, and with them small crystals of 

 galena occur. At the same mine large quantities of 

 intensely black massive blende occur, but the major por- 

 tion of the mineral is intimately mixed with the lead-sul- 

 phide in the form of diminutive grains, thus forming a 

 solid, compact, m.etalliferous mass, almost devoid of gan- 

 gue. It is plentiful at Zeehan in the argentiferous galena 

 lodes, associated with quartz and the prevailing gangue 



