GUIDE TO THE MIXERALOGIC COLLECTIONS 



43 



5 apatite 8 topaz 



G orthoclase {) corundum 



7 quartz (rock crystal) 10 diamond 



A sharp corner of the mineral to be tested for hardness is 

 rubbed across the surface of each successive member of the 

 scale, beginning with the high members, till one is found which 

 is distinctly scratched; the hardness thus determined lies be- 

 tween that of the scale mineral scratched and the next higher 

 member; thus, a mineral which scratches calcite but does not 

 scratch fluorite has a hardness of 3-4. A good knife will scratch 

 C with difficulty. 



Tenacity 



A mineral may be: 



1 brittle, when it falls to powder before a knife or hammer 



and can not be shaved in thin slices; 



2 sectile, when it can be shaved in thin slices but falls to 



.powder under the hammer; 



3 ductile, when slices shaved from it mav be flattened under 



the hammer; 



4 flexible, when it will bend without breaking. 



Characters depending on light 

 Luster 



Differences in the luster of minerals are due to the light which 

 is reflected from the surface; luster is independent of the color of 

 the mineral. The luster of a mineral may be: 



1 metallic, the luster exhibited by opaque metals, example 



pyrite ; 



2 adamantine, the oily luster of the uncut diamond, example 



cerussite; 



3 vitreous, the luster of glass, example quartz; 



4 resinous, the luster of yellow resin, example sphalerite; 



5 greasy, the luster of oiled glass, example elaeolite; 



6 pearly, the luster of the mother of pearl, example brucite; 



7 silky, the luster of silk produced by a fibrous structure, 



example satin spar; 



8 dull, void of luster, example kaolin. 



