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APPENDIX B. 



Descriptive Labels used in Students' Mineealogical Case 33. 

 Fracture. 



The nature of the surface produced by fracturing a mmeral is a useful 

 character. It is best observed in minerals that have little or no cleavage ; 

 indeed, when cleavage is well developed it is very difficult to produce a 

 true fracture at all. 



The " Fracture " of a mineral is described as follows : — 



1. Conchoidal — When the surface produced is curved : quartz. 



2. Even — When the surface produced is nearly flat : magnesite. 



3. Uneven — When the surface produced is rough : copper- pyrites. 



4. Splintery — When the siirface j)roduced is splintery : Jihro2i^ 



hematite. 



5. Hackly — A jagged surface produced : native coppier. 



6. Earthy : chalk. 



Pseudoinorphism. 



One mineral is frequently found apparently crystallising with the form 

 of another. This is always the result of some form of secondary action. 

 Thus one mineral may 



1. Be coated by another ; 



2. Be dissolved away, and the resulting cavity subsequently occupied by 

 another ; 



3. Be gradually replaced, molecule by molecule, bj- another. 



VariouH Optical Phenomena. 

 Lustre : — 



Metallic : galena, j^yntes. 

 Sub-metallic : chrome iron. 



Adamantine : blende, zircon. 



Resinous : apatite, chloro-hromicle of silver. 



Vitreous : quartz. 



Pearly : talc, selenite (one face only). 



Silky : asbestos. 

 Play of colours : precious opal. 

 Change of colours : labradorite. 

 Opalescence : moonstone. 

 Iridescence : limonite. 

 Tarnish : *' peacock copper." 

 Double refraction : calcite. 



