THE IDENTIFICATION OF MINERALS. 427 



minerals, however, as nearly as we need to get it in order to identify them, can 

 generally be determined without recourse to the scale. The hardness of common 

 window-glass is about 6.5, and any mineral that will scratch it must beat least as 

 hard as quartz, and any mineral that can be scratched by a knife cannot be much 

 harder than 5.5. By the judicious use of the point of a knife and a piece of glass 

 one can soon learn to estimate hardness well enough for practical purposes. In 

 general, different specimens of the same mineral vary but little in hardness. There 

 are exceptions to this rule, however, and some mineral species, as serpentine and 

 calcite, vary greatly in this respect; the former ranging from 2.5 to 5.5, and the 

 latter from i to 3.5. 



4. The specific gravity or weight of minerals is one of their most constant 

 characteristics. It is more difficult to discover, however, than hardness, and is 

 therefore of less practical value as an aid in determining species. If the speci- 

 men is not too small, its weight can generally be estimated with sufficient accu- 

 racy for practical purposes by lifting it in the hand. Barytes or heavy spar can 

 be readily distinguished from all minerals which it 'otherwise resembles by its 

 much greater weight. 



5. Most minerals occur more or less commonly in crystals, that is, in figures 

 bounded by plane surfaces arranged regularly about a center. Minerals of the 

 same species always crystallize in similar or allied shapes, and therefore the deter- 

 mination of the crystalline form is an important aid in identification. For in- 

 stance, iron pyrites commonly crystallizes in cubes, thus rendering it easy to dis- 

 tinguish it from copper pyrites, which it somewhat resembles. Tourmaline and 

 hornblende, when occurring in small fragments in rocks, are very similar in appear- 

 ance, but the tourmaline can usually be distinguished by its long, slender, trian- 

 gular crystals. In order to recognize any but the simpler forms of crystals a 

 knowledge of crystallography, the science "which treats of the forms resulting 

 from crystallization," is necessary, but as most minerals commonly occur uncrys- 

 tallized, we are often- obliged to depend upon other characteristics, and the de- 

 termination of the crystalline form is seldom absolutely necessary. 



6. Cleavage, or the tendency of a mineral to break along certain planes, is 

 a property closely allied to the crystalline form, and is frequently useful in the 

 identification of minerals. Common feldspar (orthoclase) can be distinguished 

 from similar minerals by its peculiarity of breaking or cleaving in certain direc- 

 tions with a bright, even surface. 



7. When a mineral does not occur, as is commonly the case, in distinct 

 crystals, its general structure should be noted, whether it consists of an aggregate 

 of fine grains like granular quartz, or forms a compact mass like flint or chalce- 

 dony. Notice if it is made up of a number of slender columns like some tourma- 

 line, or of fine fibers like asbestos or satin spar. Sometimes a mineral has a 

 lamellar structure, consisting of a succession of plates or leaves, like common 

 mica. Again, it may be found in globular forms like marcasite, (white iron 

 pyrites,) or in a shape resembling a bunch of grapes, termed botryoidal, like 

 limonite or chalcedony. Minerals also occur coralloidaL (coral-like) forms, as 



