Field book of common rocks and minerals

A set for measuring hardness may be purchased from any dealer in mineral supplies. For rough determination, as in the field, the following objects have the hardness indicated; the finger nail 2¼, a penny 3, a knife blade about 5.5, and glass not over 6. In testing, a mineral is harder than the one it will scratch, and softer than the one by which it is scratched. For instance, if a mineral will scratch calcite and is scratched by fluorite, it is between 3 and 4 in hardness, say 3.5. When two samples mutually scratch each other they are of equal hardness. Care must be used in determining hardness, especially with the harder minerals; for often, when testing a mineral, the softer one will leave a streak of powder on the harder one, which is not a scratch. One should always rub the mark to make sure it is really a groove made by scratching.

Cleavage

Cleavage is the tendency, characteristic of most minerals, and due to the arrangement of their molecules, to cleave or break along definite planes. The cleavage of any mineral is not irregular or indefinite, but characteristic for each mineral, and always parallel to possible or actual faces on the crystal, and always so described. For instance galena has three cleavages, all equally good, and parallel to the cube faces; so it is said to have cubic cleavage. In the same way fluorite has octahedral cleavage, and calcite rhombic cleavage. In some minerals cleavage is well developed in one plane, and less developed in other planes, or it may be lacking altogether. The varying degrees of perfection by which a mineral cleaves are expressed as, perfect or imperfect, distinct or indistinct, good or poor, etc.

Specific gravity

The specific gravity of a mineral is its weight compared with the weight of an equal volume of water, and is therefore the expression of how many times as heavy as water the mineral is. For instance the specific gravity of pyrite is 5.1, which is saying it is 5.1 times as heavy as water. In a pure mineral the specific gravity is constant, and an important factor in making final determinations. As ordinarily obtained, a piece of pure mineral is weighed in air, which value may be called x. It is then immersed in water and again weighed, and this value is called y. The difference between the weight in air and that in water is the weight of an equal volume of water. Then we have the following formula:

Various balances have been devised for making these measurements, but any balance which will weigh small objects accurately, may be adapted to specific gravity work, by hanging a small pan under the regular weighing pan. When using this balance, care is taken to see that the lower pan is always submerged in water, even while the mineral is being weighed in air, so that when weighed in water in the lower pan, the weight of this lower pan has already been considered.

Streak

By streak is meant the color of the mineral when powdered. For some minerals, especially metallic ores, it is of great importance, for it remains constant, though the color of the surface of the mineral changes materially. It is most readily determined by rubbing a corner of the mineral on a piece of unglazed porcelain. Small plates, known as “streak plates” are made for this purpose.

Luster

The luster of a mineral is the appearance of its surface by reflected light, and it is an important aid in determining many minerals. Two types of luster are recognized; metallic, the luster of metals, most sulphides and some oxides, all of which are opaque on their thin edges; and non-metallic, the luster of minerals which are more or less transparent on their thin edges, and most of which are light colored. The common non-metallic lusters are; vitreous, the luster of glass; resinous, the appearance of resin; greasy, oily appearance; pearly, the appearance of mother-of-pearl; silky, like silk due to the fibrous structure; adamantine, brilliant like a diamond; and dull, as is chalk.

Color

When used with caution color is of the utmost importance in determining minerals, especially in making rapid determinations. In metallic minerals it is constant and dependable; but in the non-metallic minerals it may vary, due to the presence of small amounts of impurities which act as pigments. Color depends on chemical composition, and when not influenced by impurities is termed natural; but when the color is due to some inclosed impurity it is termed exotic. In this latter case caution must be used in making determinations. Many minerals are primarily colorless, but take on exotic colors as a result of the presence of small quantities of impurities; for instance, pure corundum is colorless, but with a trace of iron oxide present becomes red, and is called the ruby, or with a trace of cobalt becomes blue and is called sapphire.

CHAPTER III
THE MINERALS

KEY TO THE MINERALS, BASED ON HARDNESS, COLOR, ETC.