48 OUR PHYSICAL WORLD 



and to understand some descriptive terms. It would be well 

 for the reader- to obtain from some dealer in minerals and rocks 

 a collection of those described in the following pages in order 

 that he may have in hand a specimen to observe as he reads 

 the description. 



In solid form, minerals may be crystalline or non-crystalline. 

 In the latter case they are described as amorphous, the terms 

 amorphous and non-crystalline being synonymous. The forms 

 of the crystals of any specific mineral are always constant. Thus 



FIG. 26. Crystals 



quartz crystals are always six-sided prisms with a six-sided 

 pyramid on each end if the crystal is perfect. Hematite crystal- 

 lizes in cubes, pyrite in cubes, octohedra, or duodecahedra. The 

 very definite form of the crystals, if the mineral is crystalline, 

 is one means of distinguishing it (Fig. 26). 



Many minerals break along definite planes so that the frag- 

 ments are bounded by smooth surfaces that meet always at the 

 same angle. This property is called cleavage. Thus galena 

 always cleaves into cubes, calcite and feldspar into rhombs, 

 though the angle between the faces of the rhombs are different 

 in the two cases. Mica cleaves into thin plates and asbestos 



