143 



in colour. A green variety, however, occurs at Broken Hill 

 (6696, 5179) and a blue* one at the Londonderry Mica Leases, 

 Coolgardie (8794). The crystalline form of orthoclase is very 

 characteristic, but is not very well shown by the specimens 

 exhibited (see Fig. A). The crystals are very frequently found 

 twinned, the usual appearance being as if one half had been spun 

 round through an angle of 180° (fig. B) . This twinning is indicated 

 by a division line passing down the faces of orthoclase crystals 

 as seen on a fractured surface of an igneous rock. 168, from 

 LTralla, is such a twin crystal, with crystals of smoky quartz 

 imbedded in it. Under the influence of weathering it passes into 

 kaolin (seep. 131). Both orthoclase and kaolin are used in pottery 

 making. Coolgardie, Cooyal, Grafton, Braid wood, Timbarra, 

 Broken Hill, Wilson's Downfall, and other localities. 



A B 



A — Simple crystal of orthoclase. "B — Twin crj'stal of orthoclase. 



5178, Broken Hill, shows the cleavage of orthoclase well. 



8392, Grulgong, etc. ; red felspar in granite. f 



8877, Port Victor, S.A. ; crystals of orthoclase that have had 

 their angles eaten away {corroded) by the igneous magma in 

 which they are imbedded. 



* As this blue colour disappears when the mineral is heated, it is probably due to organic 

 matter. 



t Crystals of orthoclase sometimes attain a very large size, and impart a very ornamental 

 appearance to the rock. Granites depend for their characteristic colour upon the tint of 

 the orthoclase present. 



