38 



NEW YORK STATE MUSEUM 



the forms are identical in nomenclature with those of the ortho- 

 rhombic system. 



Two examples of triclinic crystals are shown in fig. 150 and 

 151 which represent respectively axinite and albite. 



Variations In form 



Reference has been made (p. 13) to the variations between 

 the mathematical development of a crystal form or combination 

 of forms and the actual mineral, crystallizing in those forms, as 

 it is found in nature. This distortion is often misleading to a 

 beginner, cubes and other forms of the isometric system being 

 frequently elongated in the direction of one axis to such an 

 extent as to resemble crystals of the tetragonal, the ortho- 

 rhombic or even the hexagonal system. The reader is advised 

 to observe carefully crystals of known minerals and to bear 

 constantly in mind the symmetry of the group to which they 

 belong. 



Crystals of mineral species from the same locality show a 

 predominance of one or two forms, which gives to such crystals 

 a distinguishing character known as crystal habit. Minerals 

 which occur widely distributed often show great variety in 

 crystal habit, producing forms which are of great interest and 

 beauty; quartz and calcite are notable examples. 



Grouping of crystals 



Though the crystals of many minerals occur isolated and 

 developed alike on all sides, having somewhat Ihe regularity of 

 the ideal representations, it is far more common to find them 

 grouped together in clusters, lining the interior of cavities, 

 springing from the accompanying rock or lying embedded in the 

 matrix. In some species the crystals show a tendency to ar- 

 range themselves in pairs, the faces of one individual being 

 symmetrically disposed with respect to the other but in reverse 

 position. This intergrowth of like crystals produces what is 

 known as a twin crystal, the resulting solid being frequently 

 of considerable complexity. A twin crystal may be recognized 

 by reentrant angles which distinguish it from a simple crys- 



