Part 7

When a mineral is entirely devoid of crystalline structure, both externally and internally, it is said to be amorphous.

Perfect and distinct crystals are the rare exception, most mineral specimens being simply aggregates of imperfect crystals. In such cases, and when the mineral is amorphous, the structure of the mass may be:—

Columnar or fibrous.

Lamellar, foliaceous, or micaceous.

Granular.—When the grains or crystalline particles are invisible to the naked eye the mineral is called impalpable, compact, or massive.

And the external form of the mass may be:—

Botryoidal, having grape-like surfaces.

Stalactitic, forming stalactites or pendant columns.

Amygdaloidal or Concretionary, forming separate globular masses in the enclosing rock.

Dendritic, branching or arborescent.

3. Hardness.—By the hardness of a mineral we mean the resistance which it offers to abrasion. But hardness is a purely relative term, calcite, for example, being hard compared with talc, but very soft compared with quartz. Hence mineralogists have found it necessary to select certain minerals to be used as a standard of comparison for all others, and known as the scale of hardness. These are arranged at nearly equal intervals all the way from the softest mineral to the hardest, as follows:—

Scale of Hardness.

(1) Talc. (2) Gypsum. (3) Calcite. (4) Fluorite. (5) Apatite. (6) Orthoclase. (7) Quartz. (8) Topaz or Beryl. (9) Corundum. (10) Diamond.

If a mineral scratches calcite and is scratched by fluorite, we say its hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is scratched by orthoclase, its hardness is 6; and so on. There are very few minerals harder than quartz, and hence the first seven members of the scale are sufficient for all ordinary purposes; and these are all included in the series of specimens accompanying this Guide.

Although it is desirable to be acquainted with the scale of hardness, and to understand how to use it, still the student will learn, after a little practice, that almost as good results may be obtained much more conveniently by the use of his thumb-nail and a good knife-blade or file. Talc and gypsum are easily scratched with the nail; calcite and fluorite yield easily to the knife or file, apatite with more difficulty; while orthoclase is near the limit of the hardness of ordinary steel, and quartz is entirely beyond it.

4. Specific Gravity.—The specific gravity of a mineral, by which we mean its weight as compared with the weight of an equal volume of water, is determined by weighing it first in air and then in water, and dividing the weight in air by the difference of the two weights. Minerals exhibit a wide range in specific gravity; from petroleum, which floats on water, to gold, which is nearly twenty times heavier than water. Although this is one of the most important properties of minerals, yet, being more difficult to measure than hardness, it is less valuable as an aid in distinguishing species. One can with practice, however, estimate the density of a mineral pretty closely by lifting it in the hand.

5. Lustre.—Of all the properties of minerals depending on their relations to light the most important is lustre, by which we mean the quality of the light reflected by a mineral as determined by the character or minute structure of its surface. Two kinds of lustre, the metallic and vitreous, are of especial importance; in fact all other kinds are merely varieties of these.

The metallic lustre is the lustre of all true metals, as copper and tin, and characterizes nearly all minerals in which metallic elements predominate. The vitreous lustre is best exemplified in glass, but belongs to most minerals composed chiefly of non-metallic elements. Metallic minerals are always opaque, but vitreous minerals are often transparent.

Other kinds of lustre are the adamantine (the lustre of diamond), resinous, pearly, and silky. When a mineral has no lustre, like chalk, it is said to be dull.

It should be made clear to children that lustre and color are entirely distinct and independent. Thus, iron, copper, gold, silver, and lead are all metallic; while white or colorless quartz, black tourmaline, green beryl, red garnet, etc., are all vitreous. Generally speaking, any color may occur with any lustre.

6. Color and Streak.—The colors of minerals are of two kinds,—essential and non-essential. By the essential color in any case we mean the color of the mineral itself in its purest state. The non-essential colors, on the other hand, are chiefly the colors of the impurities contained in the minerals.

Metallic minerals, which are always opaque, usually have essential colors; but vitreous minerals, which are always more or less transparent, often have non-essential colors. The explanation is this: In opaque minerals we can only see the impurities immediately on the surface, and these are, as a rule, not enough to affect its color; but in diaphanous minerals we look into the specimen and see impurities below the surface, and thus bring into view, in many cases, sufficient impurity so that its color drowns that of the mineral.

To prove this we have only to take any mineral (serpentine is a good example in our series) having a non-essential color, and make it opaque by pulverizing it or abrading its surface, when the non-essential color, the color of the impurity, immediately disappears; just as water, yellow with suspended clay, becomes white when whipped into foam, and thus made opaque.

What we understand by the streak of a mineral is its essential color, the color of its powder; and it is so called because the powder is most readily observed by scratching the surface of the mineral, and thereby pulverizing a minute portion of it. The streak and hardness are thus determined at the same time. The streak of soft minerals is easily determined by rubbing them on any white surface of suitable hardness, as paper, porcelain, or Arkansas stone.

ESSENTIAL AND ACCESSORY MINERALS.—Lithologists, regarding minerals as constituents of rocks, divide them into two great classes: the essential and the accessory. The essential constituents of a rock are those minerals which are essential to the definition of the rock. For example, we cannot properly define granite without naming quartz and orthoclase; hence these are essential constituents of granite; and if either of these minerals were removed from granite it would not be granite any longer, but some other rock. But other minerals, like tourmaline and garnet, may be indifferently present or absent; it is granite still; hence they are merely accidental or accessory constituents. They determine the different varieties of granite, while the essential minerals make the species.

This classification, of course, is not absolute, for in many cases the same mineral forms an essential constituent of one rock and an accessory constituent of another. Thus, quartz is essential in granite, but accessory in diorite.

PRINCIPAL MINERALS CONSTITUTING ROCKS.—Having studied in a general way the more important characteristics of minerals, brief descriptions of the chief rock-forming species are next in order. We will notice first and principally those minerals occurring chiefly as essential constituents of rocks.

1. Graphite.—Essentially pure carbon, though often mixed with a little iron oxide. Crystallizes in hexagonal system, but usually foliated, granular, or massive. Hardness, 1-2, being easily scratched with the nail. Sp. gr., 2.1-2.3. Lustre, metallic; an exception to the rule that acidic elements have non-metallic or vitreous lustres. Streak, black and shining (see pencil-mark on white paper). Color, iron-black. Slippery or greasy feel. Every black-lead pencil is a specimen of graphite. Specimen 9.