tarr] some common MINERALS 317 



It also cleaves in cubic form as will be described later. Hexagonal 

 system or in six-sided crystals. Quartz is an example of this sys- 

 tem (see fig. IT). Tetragonal system or in four-sided crystals, et 

 cetera. 



Minerals have several physical properties which it is neces- 

 sary, to know about in order to determine them even by simple 

 methods. The most important of these are structure, hardness, 

 cleavage, fracture, specific gravity, color, streak, luster, degree of 

 transparency, taste, odor and feel. Xo mineral possesses all 

 these properties but some one of them may be the distinguish- 

 ing feature of some mineral. We will describe these properties 

 as briefly and clearly as possible. 



Structure. — This is the arrangement of the molecules in 

 the crystal. In the single crystal this arrangement may produce 



firisms, pyramids, cubes, octahedrons, etc.. and combinations of 

 hese. (See figures.) 



Quartz illustrates such a combination as it invariably oc- 

 curs in a form which shows a hexagonal prism and hexagonal 

 pyramid (fig. 17). But when we deal with a mass of crystals, 

 the individual crjstals are generallly imperfect and the resulting 

 forms may have various shapes depending on the shape of the 

 imperfect crystals. Thus, we have fibrous forms when the min- 

 eral is made up of fibers as in the case of asbestus. or columnar, 

 when it is composed of slender rods or columns. The fibers 

 may radiate from a center and produce star like forms or they 

 may be reticulated, the fibers crossing at various angles as in 

 a net. When the mineral consists of leaves or plates, like in 

 mica, the structure is lamellar. Sometimes the structure is 

 granular, the mineral consisting of an aggregate of grains. \'ery 

 commonly the mineral takes on an external form imitative of 

 other familiar objects and this shape is named after it. When 

 the shape is spherical or nearly so the form is known as globu- 

 lar. (The globules generally consist of radiating fibers.) Min- 

 erals containing long hairlike crystals are known as capillary 

 minerals. Xeedle shaped forms are designated as acicular min- 

 erals and pendent columns, cylinders, or elongated cones are 

 called stalactites. 



Hardness. — This is the resistance a mineral ofifers to be- 

 ing scratched. We have found that all minerals can be scratched 

 by the diamond so we say it is the hardest. As there are all 

 grades of hardness a scale of hardness has been adopted rang- 

 ing from one to ten. the diamond of course being number ten. 



