CRYSTALLINE FORMS 



occur as compounds, formed by the union of two or more of 

 them. 



A mineral is a natural, inorganic substance, which has a homo- 

 geneous structure, definite chemical composition and physical 

 properties, and usually a definite crystalline form. 



Crystals are solids of more or less regular and symmetrical shape, 

 bounded, usually, by plane surfaces. The number of known crys- 

 talline forms is already very great, and yet they may be all reduced 

 to thirteen fundamental shapes, which are prisms, octahedrons 

 (eight-sided), or dodecahedrons (twelve-sided). 



The thirteen fundamental forms and their innumerable secondary 

 derivatives fall into six systems, which are characterized by the re- 

 lations of their axes. The axes of a crystal are imaginary lines, 

 which connect the centres of opposite faces, or opposite edges, or 

 opposite solid angles, and which intersect one another at a point in 

 the interior of the crystal. 



The Systems of Crystalline Forms have received many names, 

 the following being those which are most generally used in this 

 country : — 



I. Isometric System (monometric, cubical, regular). — In this 

 system the three axes are of equal 

 length and intersect one another 

 at right angles ; it includes the 

 cube, regular octahedron, and 

 rhombic dodecahedron, forms 

 which are symmetrical in all 

 positions. 



II. Tetragonal System (dimetric, pyramidal) . 



s\ 



S 



1 







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FIG. i. — Forms of the Isometric 

 System : Cube ; Regular Octahedron ; 

 Rhombic Dodecahedron. 



The axes inter- 

 sect at right angles, but are not 

 all of equal length ; the two 

 lateral axes are of equal length, 

 but the vertical axis is longer or 

 shorter than the laterals, In- 

 Fig. 2. — Forms of the Tetragonal eludes the right square prism and 



System: Right Square Prism; Square the re octahedron, the faces 



Octahedron. . 



of which are isosceles triangles. 



