Min′eral′′ogy, the science which treats of minerals, does not embrace all that relates to the mineral kingdom. Simple minerals alone are regarded as the subjects of mineralogy; rocks, formed by the aggregation of simple minerals, and their relations to each other are the subjects of petrology (the science of rocks) and geology. Mineralogy considers the composition, structure, formation and classification of minerals. Physical Mineralogy embraces the outside form (generally shown by crystallization) of minerals and the other physical characteristics of each of the different species, as specific gravity (relative weight), luster, hardness, fusibility, optical properties and color. The latter usually is variable, and hence not characteristic of a mineral. Chemical Mineralogy considers the character of minerals as chemical compounds, embracing also methods of using chemical tests as an aid in this determination. Descriptive Mineralogy shows the classification of minerals and a description of the various species and their varieties as found in nature.
Although the ancients recognized many of the gems and the minerals containing the useful metals, their knowledge of mineralogy was crude. Not until the development of modern chemistry, about the beginning of the 19th century, did minerals begin to be properly studied and classified. In 1820 Mohr of Vienna presented a system of grouping minerals upon their similarity of form, taste, luster, gravity, streak and hardness. In the methods now in use in this country the system of Mohr is largely followed, with additional aid from blowpipe examination and simple chemical tests. When new species are being described, the chemical analysis and the determination of the crystalline form must be absolutely complete. A mineral species always has a definite chemical composition which varies only within certain limits, and, if it crystallize at all as most minerals do, it always has the same general form. When a substance crystallizes in two distinct forms in nature, these are looked upon as different minerals and are given separate names.
Minerals vary greatly in hardness, from soft substances that may be scratched with the finger-nails, like gypsum, to the hardest stones, as the sapphire and diamond. The same wide difference exists as to their color, even in the same species, as for example, in tourmaline, which exhibits different shades of brown, blue, green, red and sometimes is colorless, and frequently black and opaque.
The study of minerals has led to the development of the science of crystallography, but most chemical compounds (see Chemistry) that are prepared artificially are capable also of forming crystals, and crystallography is applied to them as well as to the natural substances.
All the minerals that crystallize, as well as all artificial crystals, may be arranged in six groups or systems, according to the relation of the faces to certain imaginary lines passing through them, termed axes.
The Isometric system has three axes, all at right angles to each other, and of equal lengths. A familiar example is fluorspar, which usually occurs in cubes.
The Tetragonal system has three axes at right angles, but one of them may be of varying lengths as compared with the other two. Zircon crystallizes after this form.
The Orthorhombic system has three axes at right angles, but all unequal. Stibnite or antimony sulphide belongs to this system.
The Monoclinic system has three unequal axes, two at right angles and one oblique. The common hornblende illustrates this form.
In the Triclinic system all the axes are of unequal lengths and at varying angles with each other. A number of the feldspars are classed here.
The Hexagonal system has three axes at angles of 60° with each other, and a fourth at right angles to the plane of the other three. Beryl is a familiar example of this system.
While each mineral species that is crystallized follows its own form of crystallization, there are numerous modifications of the planes and angles of all the systems, giving rise to many complex forms. The molecular arrangement of minerals, which results in their crystalline form, also influences their capacity for transmitting light and heat. The form of the crystals in various minerals is often complicated also by what is termed twinning, when one or more parts in a crystal are in a reversed position to the other parts. This gives rise to many beautiful and complex forms. The dendritic form of magnetite between two flakes of mica is an instance of such twinning.
Minerals have been formed in these principal ways: (1) by the solidification by cooling of molten masses, as, for example, the quartz, feldspar and mica of granite; (2) by the action of heat upon rocks below the point of fusion, as in the formation of garnets in mica-schists, (3) by the action of water dissolving substances in one place and depositing the same or other compounds in another place, as in the deposition of calcite in veins; and (4) by the action of volcanic gases, which, upon cooling or coming in contact with substances with which they act chemically, deposit certain minerals. It is not always easy to decide in what way a given mineral has been formed, and it is evident that certain minerals may be deposited in more than one way.
An interesting and peculiar condition of some mineral species is what is known as pseudomorphism, where one mineral is replaced by another which usually retains the form of the original crystal. This arises from the substitution of one mineral for another, as for instance, smithsonite after calcite. Here the calcite crystals seem to have been gradually dissolved, while the smithsonite replaced them. Petrified wood, which is common in many places, occurring in Arizona in the shape of whole forests of silicified tree-trunks, is an example of pseudomorphism. Here the action has evidently been from the infiltration of water charged with silicic acid through the beds in which the forests were buried. As the original wood decayed or was dissolved, the silica, in the form of rough opal, took its place.
Dana's Treatise on Mineralogy, as revised to the present time, may be considered to be the standard for descriptive mineralogy. Brush and Penfield's Blowpipe Analysis is the most elaborate work on determinative mineralogy. Mineralogy in the United States, as bearing upon ornamental and precious stones, has been lately dwelt upon in detail in Gems and Precious Stones of North America by George F. Kunz.