AUGITIC AND HORNBLENDIC MINERALS. 



The Family of Augites and Hornblendes. 



Augite and hornblende are usually dark-green or black minerals 

 which belong to the monoclinic crystalline system, and are commonly 

 a little more easily scratched than the felspars with which they always 

 occur. There are distinct crystalline forms and cleavages for horn- 

 blende and augite ; but when some varieties of hornblende are 

 melted, they assume, on cooling, the crystalline form of augite. Hence 

 the conclusion has not unnaturally been drawn that they are essentially 

 one mineral which assumes the form of hornblende on cooling slowly 

 under great pressure, and that of augite when cooling in lava streams ; 

 and this view is supported by the fact that the hornblende is chiefly 

 found in syenite and completely crystalline rocks, while augite abounds 

 in basalts and those rocks which have cooled at the surface. But 

 though conditions of cooling may have some influence on the develop- 

 ment of these minerals, their formation is probably more dependent 

 upon the chemical composition of the rock matter in which they occur, 

 since hornblende is associated with the felspars which are rich in 

 silica, and rocks which contain quartz, while augite is met with in 

 association with felspars which contain less silica, and rocks from 

 which quartz is absent. Occasionally they occur together as in 

 trachytes of Etna, and they have been found artificially formed in 

 slags. Chemically there is no great difference between these minerals 

 beyond the fact that hornblende contains more alumina and magnesia, 

 and more iron than augite, while augite contains more lime ; in the 

 pale-coloured varieties of both minerals alumina is almost absent, and 

 the quantity of iron is greatly reduced. The chief varieties of the 

 augite or pyroxene group are augite, hypersthene, diallage, bronzite; 

 the pale-coloured varieties are named diopside, sahlite, and mala- 

 colite ; the chief varieties of amphibole or hornblende are tremolite, 

 actinolite, asbestus, and hornblende. 



