87 



by ROSENBUSCH (1) as arising in connection with the alteration of olivine. 

 WILLIAMS, however, remarks that they occur at the junction of divine and 

 felspar, when both minerals are perfectly fresh ; and he is inclined to regard 

 them as due to processes connected with the consolidation of the rock. 

 As these needles are unquestionably of secondary origin in the case of 

 BECKE'S j9///fc it seems natural to regard them as of like origin in the other 

 cases. 



Rhombic pyroxenes. Under this head we include the minerals usually known 

 as enstatite, bronzite and hypersthene. Professor ROSEN BUSCH uses enstatite as 

 the general name for the group ; Professor TSCHERMAK, on the other hand, uses 

 the term bronzite. The different varieties depend on the relative proportions 

 of magnesia and ferrous oxide ; and what is important is that there 

 appears to be a perfectly continuous series from typical enstatite, a pure 

 magnesian bisilicate (Mg 2 Si0 4 ), to the mineral with 33 per cent, of ferrous 

 oxide for which TOM RATH proposed the term amblystegite. Professor JUDD uses 

 the term enstatite for the group, and designates the different varieties as 

 enstatite, ferriferous enstatite, highly ferriferous enstatite and excessively 

 ferriferous enstatite. Professor TSCHERMAK (2) proposes that varieties with less 

 than 5 per cent, of ferrous oxide should be termed enstatite ; those containing 

 from 5 to 15 per cent, bronzite ; and those containing more than 15 per cent. 

 kypersthene. Professor JUDD suggests that the latter group should be further 

 subdivided; the term hypersthene or proto-hypersthene being applied to 

 varieties containing between 15 and 25 per cent, of ferrous oxide and 

 TOM RATH'S term amWystegfa to varieties containing more than the latter 

 amount. 



The optical and other physical properties of the rhombic pyroxenes have been 

 shown by DES CLOISEAUX to stand in the closest relation to their chemical 

 properties and to change continuously with the latter. Thus, as the percentage 

 of ferrous oxide increases the negative optic axial angle diminishes from 

 133 8' (in oil) in enstatite (proper) with 2 '76 per cent, of ferrous oxide 

 to 59 20' in the amblystegite from Mt. Dore with 33'6 per cent. The 

 pleochroism, hardness and specific gravity increase as the amount of iron 

 increases. In the examination of thin sections colour and pleochroism are 

 the only tests available for discriminating between the diffei'ent varieties. 

 Enstatite is practically colourless, and as a natural consequence devoid of 

 pleochroism. The pleochroism of the other varieties is indicated in the 

 following scheme. (3) 



(1) Mik. Physiog., Band I., ISSo, p. 413. 



(2) Lehrbuch der Mineralogie, 1884, p. 43G. 



(3) It will be remembered that cross sections of the prism contain the a and ft axes of 

 elasticity; vertical sections contain the 7 axis. ", ft and 7 represent the greatest, mean and 

 least axes respectively. 



