GUIDE TO THE MIXER A LOGIC COLLECTIONS 



67 



forms are of particular interest from a crystallographic point 

 of view. Twinning occurs quite frequently. Massive forms 

 occur in great variety with granular manmiillary stalactite and 

 concretionary structure, plane or banded. 



Quartz has a vitreous luster in the crystallized varieties which 

 passes, in the massive forms, to greasy, splendent or dull. 



Fig- 182 Fig. 1S3 



Quartz 



When pure, quartz is transparent and colorless with a white 

 streak; various shades of yellow, red, brown, green, amethyst 

 and black are due to slight impurities. 



A CRYSTALLINE VARIETIES 



Rock crystal. Pure, colorless 



Amethyst. Purple to violet. Color probably due to manganese 

 Rose quartz. Pink to rose. Colored by titanium 

 Smoky quartz. Smoky brown to black. Color probably due to 

 carbon 



Milky quartz. Translucent white. Usually massive 

 Ferruginous quartz. Opaque brown to red. Colored by iron 

 Aventurin. Spangled with scales of mica, hematite etc. 



