

DEFINITIONS. 



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THE object of the science of Crystallography, 

 regarded as a branch of mineralogy, is to trace and to 

 demonstrate those relations and differences between 

 the various crystalline forms of minerals, by means of 

 which we are enabled generally to discriminate the 

 different species of crystallized minerals from each 

 other. 



A crystal^ in mineralogy, is any symmetrical mineral 

 solid, whether transparent or opaque, contained with- 

 in plane, or sometimes within curved surfaces. 



These surfaces, as , 6, c, fig. 1., are called planes 

 or faces. 



Fig- I- 



The exterior planes of a crystal as they occur in 

 nature, are called its natural planes. 



Crystals may sometimes be split in directions 

 parallel to their natural planes, and frequently in 

 other directions. 



The splitting a crystal in any direction, so as to 

 obtain a new plane, is termed cleaving it, and the 

 crystal is said to have a cleavage in the direction in 

 which it may be so split. 



The planes produced by cleaving a crystal are called 

 its cleavage planes. 



An edge, as d fig. 1, is the line produced by the 



meeting of two planes. 



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