Mineralogy

Minerals possess certain physical properties that are considered in this chapter in the following order:

1. Certain characters depending upon light, such as color, luster, transparency, translucency, phosphorescence and fluorescence. Other optical properties especially valuable in the recognition of minerals in thin section under the microscope are dealt with in a later chapter.
2. Characters depending upon certain senses, such as those of taste, odor and feel.
3. Characters depending upon the state of aggregation, such as form, pseudomorphism, polymorphism, hardness, tenacity, fracture, cleavage, and surface tension effects. Crystallography, - the study of crystals, is considered in the next chapter.
4. The specific gravity of minerals.
5. Characters depending upon heat, such as fusibility.
6. Characters depending upon magnetism, electricity and radioactivity.
7. Color, Luster, Transparency, Etc.

Color

Color depends upon the absorption of some and the reflection of others of the colored rays or vibrations which compose ordinary white light. When a body reflects light to so small an extent as not to affect the eye, it appears black, but when it reflects all the vibrations of the different colors which compose white light, it appears white. Again, if it reflects the red vibrations of ordinary light and absorbs all the other vibrations, it appears red.

A blue mineral, such as sapphire, absorbs all the vibrations of white light with the exception of those that give the sensation of blueness to the eye.

The color of a mineral is often its most striking property. Unfortunately for purposes of identification, however, the colors of minerals vary very greatly. Even in the same species specimens are found having very different colors. The mineral quartz, composed of silicon dioxide, is commonly colorless or white, but it is also found with pinkish-yellow, green, brown, amethystine and even black colors. Corundum, composed of alumina, varies in color from pale brown to deep red and dark blue, the two latter varieties being the gemstones ruby and sapphire. The same crystal of a mineral may exhibit different colors, sometimes arranged in a regular fashion as in some crystals of tourmaline, at other times in patches as in certain specimens of fluor-spar, calcium fluoride.

The streak of a mineral is the color of its powder and may be quite different from that of the mineral in mass. For instance, black hematite gives a red powder. Streak is observed by producing a small quantity of the powdered mineral by scratching with a knife or file or by rubbing the mineral on a piece of unglazed porcelain or roughened glass called a streak-plate.

Some minerals, when turned about or looked at in different directions, display a changing series of prismatic colors, such as are seen in the rainbow or on looking through a glass prism. This is called a play of colors. It is shown by the diamond and is produced by the splitting-up of a ray of white light into its colored constituents as it enters and emerges from the mineral. Change ‘of color is a somewhat similar phenomenon extending over broader surfaces, the succession of colors being produced as the mineral is turned. This phenomenon is excellently displayed by certain varieties of the mineral feldspar, the colors shown including blues, greens, yellows and reds. Such a feldspar is an abundant constituent of a rock from southern Norway, and polished slabs of this rock in which the feldspar crystals lie in various directions are used for ornamental purposes. The change of color is caused by the interference of light reflected from thin plates of other minerals enclosed in parallel planes within the feldspar. Schiller, a nearly metallic luster shown by certain surfaces of the minerals hypersthene, schiller-spar, etc., is due to a somewhat similar cause. Reflection takes place either from minute plates arranged on parallel planes, or from cavities due to chemical action along certain parallel planes within the mineral.

Opalescence is a somewhat pearly or milky appearance shown by opal and moonstone. Iridescence is a display of prismatic colors due to the interference of rays of light in minute fissures which wall in thin films of air or liquid. These fissures are often the result of incipient fracture. Iridescence may sometimes be seen in quartz, calcite and mica. The brilliant display of colors given by the precious opal is due to the presence of very thin curved or distorted layers with slightly different optical properties.

Some minerals tarnish on the surface when exposed to the air and sometimes exhibit iridescent colors. This tarnish may result either from oxidation, or from the chemical action of sulphur and other elements which are generally present in the atmosphere in minute quantities. Tarnish may be distinguished from the true color by chipping or scratching the mineral, when the superficial nature of the tarnish is revealed. Copper pyrites often tarnishes to an iridescent mixture of colors. The mineral erubescite tarnishes readily on exposure to the air, and some varieties are called peacock ore.

Some crystals display different colors when viewed in different directions by transmitted light. This property, called pleochroism, ‘is considered with the special optical properties on a later page.

Luster

The luster of minerals differs both in intensity and kind, depending upon the amount and type of reflection of light that takes place at their surfaces.

There are six kinds of luster:

1. M Dallic - The ordinary luster of metals. When feebly displayed this luster is termed sub-metallic, Gold, iron pyrites and galena have a metallic luster; chromite and cuprite have a submetallic luster.
2. Vitreous - The luster of broken glass. When less well developed, it is called subvitreous luster. Quartz and rock-salt afford examples of vitreous luster, calcite of subvitreous.
3. Resinous - The luster of resin. Opal, amber and some kinds of zinc blended have a resinous luster.
4. Pearly - The luster of a pearl. It is shown by surfaces parallel to which the mineral is more or less separated into thin plates, reproducing to some extent the conditions of a pile of thin glass sheets, such as cover-glasses. Talc, brucite and selenite show pearly luster.
5. Silky - The luster of silk. This luster is peculiar to minerals having a fibrous structure. The fibrous form of gypsum known as satin-spar, and the variety of asbestos called amianthus are good examples of minerals having a silky luster.
6. Adamantine - The luster of a diamond.

The luster of minerals may be of different degrees of intensity, according to the amount of light reflected from their surfaces. Thus, when the surface of a mineral is sufficiently brilliant to reflect objects distinctly, as a mirror would do, it is said to be splendent. Certain varieties of hematite have a splendent luster. When the surface is less brilliant and objects are reflected indistinctly, it is described as shining. When the surface is still less brilliant and is incapable of giving any image, it is termed glistening, and glimmering denotes a still feebler luster. Minerals with no luster are described as dull.

As shown later, the various surfaces of a crystal may show different kinds and degrees of luster.

Transparency and Translucency

A mineral IS transparent when the outlines of objects seen through it appear sharp and distinct. Rock crystal-a variety of quartz-and selenite are good examples. Minerals are said to be sub transparent or semitransparent when objects seen through them appear indistinct. A mineral which, though capable of transmitting light, cannot be seen through is translucent. This condition is very common among minerals. When no light is transmitted the mineral is opaque, but it must be noted that this refers only to the appearance as usually seen. A large number of apparently opaque minerals become translucent when cut into very thin sections, and this property is of great importance, as shown in a later chapter, in the identification of minerals in rocks.

Many minerals which are opaque in the mass are translucent on the sharply broken edges and in splinters, as in the case of the common black flint from the Chalk of the south of England.

Phosphorescence and Fluorescence

Phosphorescence is the property possessed by some substances of emitting light after having been subjected to certain conditions such as heating, rubbing, or exposure to electric radiation or to ultra-violet light. Some varieties of fluorspar, when powdered and heated on an iron plate, display bright phosphorescence. Pieces of quartz when rubbed together in a dark room emit a phosphorescent light. Exposure to sunlight or even ordinary diffused light elicits phosphorescence from many minerals, as may be observed by transferring them rapidly to a dark room. Diamond, ruby and certain other minerals show brilliant phosphorescence after exposure to X-rays. Willemite, zinc orthosilicate, phosphoresces when exposed to X-rays, a fact employed to make certain that this mineral has been completely extracted from its ore.

Some minerals emit light whilst exposed to certain electrical radiations. This phenomenon is best exhibited by fluor-spar and for this reason is called fluorescence.

Taste, Odor and Feel

Taste

The characters of minerals dependent upon taste are only perceptible when the minerals are soluble in water. The following are terms used in this connexion: saline, the taste of common salt; alkaline, that of potash and soda; cooling, that of nitre or potassium chlorate; astringent, that of green vitriol; sweetish astringent, that of alum; bitter that of Epsom salts, and sour, that of sulphuric acid.

Odor

Some minerals have characteristic odors when struck, rubbed, breathed upon or heated. Terms used are:

• Alliaceous - the odor of garlic, given when arsenic compounds are heated.
• Horse-radish - the odor of decaying horse-radish, given when selenium compounds are heated.
• Sulphurous - the odor of burning sulphur, given off by pyrites when struck, or by many sulphides when heated.
• Fcetid - the odor of rotten eggs, given by heating or rubbing certain varieties of quartz or limestone.
• Argillaceous or Clayey - the odor of clay when breathed upon.

Feel

Smooth, greasy or unctuous, harsh, or meager or rough, are kinds of feel of minerals that may aid in their identification. Certain minerals adhere to the tongue.