28 



MINERALOGY 



Models. In the study of crystals and the relation of crystal 

 forms, models cut from wood are indispensable. The student should 



cut the simpler forms and their 

 combinations from cork, as rela- 

 tions once established in this way 

 are never forgotten. Crystal 

 models are cut showing similar 

 faces of the same size, or equally 

 developed, thus representing the 

 ideal symmetry of crystals, Fig. 

 43. In nature crystals seldom if 

 ever present the ideal symmetry, 

 as some faces are always enlarged 

 at the expense of others ; in this 

 way some faces may be entirely 

 obliterated, when the appearance 

 of a crystal may be so changed by the unequal development, or 

 distortion, as to be difficult of recognition. Distortions take the 

 form of elongation along a set of parallel edges, as in Fig. 44, a 



FIG. 43. 



FIG. 44. A Distorted Rhombic Dodecahedron of Garnet. 



distorted rhombic dodecahedron of garnet with the edges parallel 

 to ab elongated ; again, in the distortion of crystals, points are re- 

 placed by edges, as in Fig. 45 a and 45 b, a regular crystal of quartz 



