Forms And Combinations

A form is an assemblage of faces, all of which have similar relations to the axes. Two or more forms occurring as a single crystal constitute a combination. Only forms belonging to the same system can occur in combination, but, even with this limitation, the variety and complexity of crystals are very great. Certain forms occur which may be regarded as developed from other forms by the suppression of one-half or three-quarters of the faces of the latter.

Irregularities of growth (distortion) are very common, some faces of a form being larger than others, while certain faces may even be obliterated but however great the variation, the angle at which corresponding faces meet invariably remains constant for each mineral.

Massive and imperfectly crystallized minerals may consist of grains, fibres, or thin layers (lamince).

Hardness

The hardness of minerals is a useful means of identifying them. For this purpose they are referred to a scale of hardness, ranging from such soft substances as may be readily scratched with the finger-nail, to the hardest known substance, diamond. The degree of hardness is expressed by the numerical place of the mineral in the scale, and intermediate grades are indicated by fractions. Thus a mineral which is scratched by quartz, and scratches orthoclase with equal ease, has a hardness of 6.5. The scale is as follows: -

1. Talc.

2. Gypsum.

3. Calcite.

4. Fluorite.

5. Apatite.

6. Orthoclase.

7. Quartz.

8. Topaz.

9. Sapphire. 10. Diamond.

Cleavage

Many minerals split more or less readily in certain fixed directions, while in other directions they break irregularly. This property is called cleavage. Cleavage is uniform in different crystals of the same mineral, and is parallel to actual or possible crystal faces.

Pseudomorphs occur when one mineral assumes the crystal form proper to another. This may take place either by the addition or the removal of certain constituents, or some constituents may be removed and others substituted for them. The entire substance of a mineral may be removed and its place taken, molecule by molecule, by another, retaining the form, sometimes even the cleavage, of the first. The study of pseudomorphs is often of the greatest service, as throwing light upon the history of the rock in which they occur.

Compound crystals are formed by the joining of simple crystals. When two half-crystals are united along a plane in such a way that their faces and axes do not correspond, they are said to be twinned. When the twinning is repeated along numerous parallel planes, the crystal is a polysynthetic twin. Two crystals united at the ends to form a right angle are called geniculate, while two geniculate crystals may be so combined as to form a cross, and then are said to be cruciform.

Rock-Forming Minerals

The number of known minerals is large and constantly increasing, but only a few enter in any important way into the constitution of the earth's crust. We now proceed to a consideration of these constituent minerals, which are called rock-forming minerals, because the rocks are aggregations of them. It must be emphasized that the student can gain no real knowledge of minerals or rocks by merely reading about them; it is necessary that he should familiarize himseh with actual specimens.