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stone) may be used instead of gypsum as its hardness is 2, or zinc blende 4, a common mineral in gold and silver districts, may be used instead of fluor spar, etc. If a mineral is to be tested for its hardness, try to scratch it with the one which you think hard enough, if the mineral is softer it will be scratched, and you try the next softer mineral of the scale, until the hardness of the mineral you are testing is easily ascertained to lie between certain numbers of the scale. Thus, if the substance is scratched by fluor spar (or blende) but not by calcite its hardness is between three and four; if the fluor spar scratches it easily it is nearer three, if but little it is nearer four. In testing for hardness it is far better to use minerals than to trust to the fingernail, knife blade, glass, or file, though after some practice with minerals, these may be used with success.
In selecting the minerals of the scale be sure to take the crystals of the substances given (except, of course, roll sulphur if that is used instead of gypsum); small pieces are as good as larger ones. A whole scale might be carried in a vest pocket. Gypsum, calcite and fluor spar will be found of greatest use. The pieces should not be carried loosely together as the harder ones would destroy the others; wrap them separately in paper.
TENACITY may be brittle as when the mineral is readily splintered or broken in any direction, sectile easily split in one or several directions, but not splin
tering, or malleable, not breaking, but flattening under a blow from a hammer.
CRYSTALLINE FORM will be given only in cases where it is plainly marked. A complete description of crystal systems would in most cases be unnecessary, and in a volume of this size would occupy too much space. It will be sufficient to characterize such crystals as approach in form the cubical, having the angles, right angles and the faces square or rectangular as pyrites and galena; hexagonal having six angles and faces on the sides as quartz, and generally terminated by a pyramid on one end; rhomboidal having the faces inclined at angles greater or less than right angles, but in other respects resembling the cubical system as calcite and feldspar; and tabular arranged in layers or plates like mica, often showing an indefinite degree of cleavage.
CLEAVAGE is the tendency of crystal to split in certain definite directions, but not in others. Sometimes a crystal has a perfect cleavage in one, two or three directions, generally parallel to the surfaces of the crystal, good examples of cleavage will be found in fluor spar, calcite, galena, feldspar, etc. If instead of spliting in planes only a splinter is struck off leaving a concave depression, such a fracture is called conchoidal, if the splinter is irregular the fracture is called splintery; flint or anthracite coal furnish examples of the former, quartz crystals of the latter.
SPECIFIC GRAVITY is the weight of a substance com
pared with water, and is an important point to notice. To ascertain this accurately it is necessary to have a pair of balances and a set of accurate weights. The rule to follow is as follows. Suspend the mineral by a fine thread or hair and weigh it in the air, and afterwards immerse it in a glass of water and weigh it again; it will weigh less when immersed in water than when in the air. The difference between these two weights will be its loss of weight in water and will be exactly equal to the water displaced. To find the specific gravity of the mineral divide its weight in air by its loss of weight in water. Thus, for example, a piece of quartz which weighs 4 ounces in air will weigh about 2 1/2 ounces in water; its loss of weight will be 1 1/2 ounces. Dividing 4 ounces by this gives 2 2/3 (or nearly 2.7); thus quartz is found to weigh 2 2/3 times as much as water. Since it is not generally possible to be provided with scales and weights, it will be found convenient to learn to judge of the specific gravity of minerals by comparing them with substances that are known: thus for example water is one and objects lighter than it will float, heavier ones will sink; quartz may be taken as 2.5; iron pyrites 5; cast iron 8; lead 11; gold about 18.
The CHEMICAL COMPOSITION of minerals can not be very fully treated here. Very few of the minerals are simple or elementary. Gold and platinum, however, are generally found native or not in chemical combination with other elements, but in small grains me
chanically diffused among other rocks. Silver, mercury, copper and sulphur are sometimes found native, but like other minerals they are generally in combination with some other element.
The chemical name of any compound is formed from the names of the elements which compose it; if there are only two of these elements the name of the compound terminates in ide, as silver sulphide a compound of silver and sulphur, lead chloride a compound of lead and chlorine. Since it is more common to hear the names of these compounds given in the reverse order they will be so presented in this book, and the above mentioned substances will be called sulphide of silver, chloride of lead, etc.
If in addition to two other elements, oxygen enters into the compound, the name given it will terminate in ate or ite, generally the former; thus in the above compound the chemical union of oxygen with silver and sulphur would produce a sulphate of silver, a very different substance from the sulphide. Some substances unite with the oxgen of the air, and spontaneously undergo great changes as for example some forms of iron pyrites (sulphide of iron) unite with oxygen on exposure, and are converted to sulphate of iron (copperas or green vitriol).
In the following table for convenience of reference the minerals given will be classified into rock constituents and metallic ores. Under the head of ores will be classed those minerals which either sometimes carry
the precious metals or are associated with them or may be mistaken for them.
In the examination of minerals and the comparison with the following table, notice first the lustre and size. of the crystals. The minerals of the rocks form either large or small crystals, but the crystals of the ores are usually small and often present the appearance of being fine grains disseminated through the gangue-rock. In examining a mineral chemically or otherwise separate it as much as possible from the associated rock, and if the particles arc fine place them on a piece of white paper. A good magnifying glass will be found of great assistance. In judging of the specific gravity or hardness of a mineral it is necessary that it should be separated completely from adhering particles of different rock.
While there are certain exceptions to the rule, it may be generally stated that large crystals which are hard, relatively light, and possessed of glassy (or vitreous) lustre, are minerals of the rocks, whereas if they are small, granular, or spray like, dark in color, soft and heavy, they should be tested for metal as there is a strong probability of their containing it. It frequently happens too that in many valuable ores, the metallic compounds are so finely disseminated that they appear only as a stain upon the gangue-rock, this is particularly true with the sulphide and chloride of silver. It must also be borne in mind that even the same kind of crystals are rarely of the same constitution, but contain varying per cents of other minerals.