PHYSICAL PIMPUITII.S _'7! 



In addition to metallic, sulnnetallic, or non-inrtallic lusters, which 

 are in a larue mea>ure dependent upon opacity to light, tin- follow- 

 ing tiTins arc used : Adamantine luster is a high, shiny, and brilliant 

 luster. u>ually connected with mineral.- with :i high specific gravity 

 and index of refraction. It also gives the impression of being very 

 hard, (loot! examples are cuprite, rutile, cassiterite, sphalerite, 

 eerus.-ite. and dianioiul. 



Vitreous or glassy luster, as of broken glass, bright and shining, 

 like quartz, apatite, beryl, and most of the silicates. 



Greasy or resinous is a vitreous luster as if oiled or like resins, as 

 serpentine. 



Waxy, very much like resinous, like calcedony. 



Pearly luster is well shown in mother-of-pearl, due to a combina- 

 tion of surface reflection and a shelly structure, as in brucite, talc, 

 and the basal cleavage of apophyllite and the pinacoidal cleavage 

 of heulandite. 



Silky luster is the luster of satin, due to a fibrous structure, as in 

 satin spar, asbestos, and enstatite. 



Dull, as in chalk or kaolinite, is where the reflected light is 

 diffused. 



Phosphorescence. Some minerals and chemical compounds 

 possess the property of transforming energy of other forms into 

 light and continue to emit a characteristic glow long after the 

 exciting agent or cause has been removed or ceased to act. Calcium 

 sulphide mixed with small amounts of bismuth is used as a luminous 

 paint and will continue to glow for hours after exposure to sunlight. 

 The hexagonal zinc blende, wurtzite, will glow under the emissions 

 of radium. Diamonds, willemite, and kunzite phosphoresce when 

 exposed to the Rongten ray or ultra-violet light. In the case of 

 willemite the ultra-violet light is used to test the completeness 

 of the mechanical separation from the gangue and other zinc min- 

 erals, as every remaining particle of willemite will glow brilliantly 

 when exposed to ultra-violet light. 



Other minerals, as quartz, become luminous by friction, or when 

 fractured, as some micas. Specimens of the same species may vary 

 greatly in their power to phosphoresce and it would seem not to be 

 a property of pure chemical compounds or pure minerals but is 

 caused in most cases by impurities and is often restricted to local- 

 ities, as all the minerals from Borax Lake, California, phosphoresce 

 under ultra-violet light, which is probably due to some common 

 constituent. 



