common with spinel and diamond and is occasionally seen in magnetite. Penetration twins result from the union of two complete crystals by the revolution of one on the twinning axis through 180°. One of the commonest illustrations is the Carlsbad twin of orthoclase, figure 6. Eleven distinct types of twinning are known in orthoclase. Among the triclinic feldspars polysynthetic twinning is very common. This is a repeated twinning, usually of lamellæ so thin as to manifest their presence only by fine striations.

While crystals usually occur isolated or in irregular clusters, they are frequently found grouped in parallel position, as shown in figure 9 of amethyst from Montana.

faces often results from this oscillatory com bination, as in many calcites from Joplin, Mo., but curvature is also produced by pressure, as in twisted stibnite and quartz crystals, or from certain intermolecular forces, such as those which produce the beautiful arborescent frost crystals often seen on windows or stone walks, as well as the nearly spherical diamonds of Brazil and

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South Africa and the peculiar forms of gypsum found in the Mammoth Cave, figure 14. Liquid and gaseous inclusions often manifest their presence by moving bubbles. Enclosures of other minerals, such as needles of rutile and tourmaline, are very varied.

Usually the conditions coincident with the formation or deposition of minerals have been such that only confused crystalline masses have been produced. Such aggregates are shown by the structure of the mineral and include (1) Columnar and fibrous, respectively denoting coarse or fine fibres, examples of which are furnished by some tremolite and by cyanite (bladed-columnar) and asbestos. Stellated (e. g., wavellite) and radiated (e. g., pectolite) are varieties of fibrous aggregates. (2) Lamellar, in plates, e. g., selenite. Under this are included concentric, the plates circling crudely around a common centre, e. g., malachite and some banded agate. Foliated, the plates thin and separable, e. g., foliated talc. Micaceous is the most perfectly developed type of a foliated structure. (3) Granular, including coarsegranular, e. g., Tuckahoe dolomitic marble; finegranular, e. g., Carrara statuary marble; impal-. pable, e. g., lithographic limestone.

Linking crystallized with amorphous minerals are the many interesting imitative forms, a few of which are: reniform, kidney-shaped, e. g., hematite variety kidney ore, figure 10; botryoidal resembling a cluster of grapes, common in limonite, chalcedony, psilomelane; globular or spherical, e. g., pectolite, hyalite; nodular, e. g., flint; oolitic, in masses of small concretions resembling a fish's roe, e. g., oolite; dendritic and arborescent, in tree-like forms, e. g., wad, figure 12; reticulated, net-like, usually due to twinning, e. g., cerussite, figure 16; coralloidal, resembling coral, e. g., aragonite variety flos-ferri.

When one mineral exhibits the characteristic form of another it is known as a pseudomorph after the original mineral; thus quartz (a rhombohedral mineral) appearing in cubes which were originally fluorite is called quartz pseudomorph after fluorite. Pseudomorphs are formed, 1. By a change in chemical composition, e. g, limonite (hydrated iron sesquioxide) pseudomorph after pyrite (iron disulphide). 2. By substitution, e. g., cassiterite after orthoclase, also silicified wood, in which the original wood has been replaced by silica. 3. By incrustation, as in the coating of various objects by the hot springs in the Yellowstone Park.

Physical Mineralogy.- The external form of crystals has been shown to be due to the existence of a definite, internal, molecular struc ture. Physical mineralogy aims to explain the phenomena observed in connection with this structure.

1. Characters Due to Cohesion and Elasticity. Cohesion is the force which holds the molecules of a homogeneous substance together. Elasticity is the force which tends to bring the molecules when separated back to their original position. These forces manifest themselves in the properties of cleavage, fracture, tenacity, hardness, etc.

(a) Cleavage is the tendency of certain crystallized minerals to break in definite directions, invariably parallel to one or more of the simpler faces of the crystal. In the isometric system cubic cleavage is well illustrated by halite, figure 13; octahedral cleavage by fluorite and diamond; dodecahedral, by sphalerite. In the tetragonal system basal cleavage is shown by apophyllite, prismatic by rutile and wernerite. In the orthorhombic system, topaz has marked basal cleavage, while barite cleaves readily parallel to the prism. In the monoclinic system clinopinacoidal cleavage is exhibited by orthoclase and selenite, basal by the micas and orthoclase, prismatic by amphibole. Pinacoidal cleavage in the triclinic system is prominent in anorthite, albite and rhodonite. In the hexagonal system basal cleavage is shown by beryl, prismatic by nephelite, rhombohedral by calcite and dolomite. Cleavage is one of the most important aids to the mineralogist in the determination of unknown minerals by physical tests and it often enables him to recognize at a glance the crystal system of the mineral. Parting is separation along secondary twinning planes and much resembes cleavage. Pyroxene is a good illustration, figure 8.

FIG. 8.

(b) Fracture is the break of minerals other than cleavage and parting. It is most noticeable when it is conchoidal or shell-like. This fracture which is quite common among minerals is illustrated by figure II of uintahite. A hackly or jagged fracture is shown by copper and other native metals. Wollastonite often well illustrates a splintery fracture.

(c) The hardness of a mineral is its resistance to abrasion, or the resistance which its molecules offer to a body trying to push them into a new position. It is usually, though very crudely, determined by comparison with the following Scale of Hardness introduced by Mohs: (1) Tale; (2) Selenite; (3) Calcite; (4) Fluorite; (5) Apatite; (6) Orthoclase; (7) Quartz; (8) Topaz; (9) Corundum; (10) Diamond. While Mohs' scale is universally used

in all mineralogical works, various methods hav been adopted to determine the absolute hardness (see SCLEROMETER). Hardness is often one of the most conclusive tests in identifying minerals by their physical properties.

(d) The tenacity of a mineral is the resistance which its molecules offer to an effort to completely separate them. When this resistance is but slight the mineral is termed brittle; if it is stronger so that shavings may be cut off with a knife, but the shavings can be powdered by a hammer, it is called sectile, e. g., talc. Malleable minerals exhibit still greater tenacity, as shavings are not powdered by the blow of a hammer. The native metals, copper, gold and silver, are malleable and also ductile, that is, capable of being drawn out into wire. Flexible minerals are those which can be bent, but in which the force of cohesion is not strong enough to cause the molecules to return to their original position, e. g., foliated talc.

(e) Elasticity in minerals not only involves resistance of their molecules to complete separation, but indicates such a development of cohesion as to prevent a permanent bending of the specimen and lead to its return to its original position when the disturbing force is removed. The micas exhibit this property to a marked degree.

2. The Specific Gravity of a mineral is the ratio of its weight to the weight of an equal volume of water. It is dependent upon the weight of its molecules and the closeness of their aggregation. Corundum, a compound of the exceedingly light metal aluminum with the gas oxygen, nevertheless is comparatively heavy, having a specific gravity of 4. As a single molecule of Al2O3 must be relatively light, it follows that corundum is heavy because its molecules are closely aggregated. This will become clear by comparing a cubical box with a single wooden sphere at each corner, to the same box filled with such spheres and imagining that these spheres represent the closely crowded molecules of aluminum in a cubical piece of corundum. The filled box would manifestly be very much heavier, but it would not be nearly so heavy as it would be if the spheres were of lead instead of wood. The average specific gravity among common minerals being about 2.7, any mineral whose specific gravity is considerably greater than 2.7 seems heavy when placed in the palm of the hand. The metals and metallic minerals and most of those minerals which are compounds of the heavier metals have a high specific gravity. Iridium (22.75) is the heaviest; gold and platinum also have a very high specific gravity, ranging from 14 to 19. Very few minerals exceed 10 and all of them are metallic. The specific gravity of a mineral is a property of greatest importance because it is subject to such slight variations unless the material is impure or contains cavities.

3. Characters Depending Upon Light.-The optical characters which are common to both crystallized and amorphous minerals include chiefly diaphaneity, color, lustre, asterism, fluorescence and phosphorescence.

Diaphaneity or transparency is the quantity of light which a mineral transmits. If a mineral offers but little if any resistance to the passage of light, that is to say, when all the details of an object can be readily seen through

it, it is described as transparent, e. g., rock crystal. If, on the other hand, a mineral permits no light to traverse it, and it is, therefore, impossible to see any object through it, it is called opaque, e. g., jasper. Between transparent and opaque minerals are those which are translucent, e. g., chalcedony, in which light is transmitted, but objects cannot be seen. Very many substances ordinarily perfectly opaque appear translucent or even transparent in very thin sections, e. g., gold.

The color of a mineral is the kind of light which it either transmits or reflects. If transparent a mineral may transmit one kind of light and reflect another; hematite, for example, ordinarily appears black by reflected light, but on looking through thin crystals they appear red. Some minerals transmit different colors in different directions (see DICHROIC CRYSTALS). The external or superficial colors are those which first appeal to the eye, but they are not usually of great importance because not constant. The essential color or streak of a mineral, which is the color of the mineral when powdered, is a character of first importance because it varies but little regardless of what the external color may be. Play of colors, change of colors, iridescence, tarnish and opalescence are properties whose study is full of interest.

The lustre of a mineral is the manner in which it reflects light. According to its intensity it is said to be splendent, shining, glistening, glimmering, or, if devoid of lustre, dull. In describing its quality minerals are separated into metallic, sub-metallic and unmetallic. Metallic minerals, of which galena is a good illustration, are not considered to possess a metallic lustre unless they are opaque. The unmetallic minerals are further divided into adamantine, the lustre of the diamond; vitreous, glassy, e. g., rock crystal; resinous or waxy, e. g., sulphur, amber; greasy, e. g., elaeolite; pearly, e. g., brucite; metallic-pearly, e. g., bronzite; silky or satiny, e. g., chrysotile, satin spar. Lustre is a quality of considerable importance, the first step in the determination of minerals being to decide whether or not they possess a metallic lustre.

Asterism is the property of showing a sixrayed star either by reflected or transmitted light. Star-mica and star sapphire, figure 15, are the best illustrations.

Fluorescence is the property of emitting from within light of one color during exposure to light of other colors, or to the emanations from radium. Fluorite has long been cited as the most prominent illustration of the property, but recent investigations with ultra-violet rays have shown that willemite from Franklin Furnace, New Jersey, is more magnificently fluorescent when exposed to ultra-violet rays than any other known mineral. Some calcite from Franklin, Furnace and from Longban, Sweden, shows a charming rose pink fluorescence under the ultraviolet rays. This property seems to be due to a transformation of the rays within the mineral and their emission as light of greater wave length. See FLUORESCENCE.

Phosphorescence is the property of emitting. light for a time after gentle heating or after exposure to radium, an electrical discharge or to light. The ultra-violet rays are especially powerful in exciting phosphorescence. Fluorite exhibits both fluorescence and phosphorescence,

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but many fluorescent minerals do not phosphoresce. Some New Jersey willemite is magnificently phosphorescent after exposure to the ultra-violet rays. Kunzite glows with a rich red light after excitation by the ultra-violet or the Roentgen rays. Many other minerals, including diamond, wollastonite, pectolite and all the minerals from Borax Lake, Cal., exhibit phosphorescence. Phosphorescence is explained by the transformation into light of the energy communicated to the body by some exciting agent. (See PHOSPHORESCENCE.)

In the more advanced study of mineralogy very great importance is attached to the consideration of the optical characters of crystals. The crystal system and even the identity of the mineral species may often be determined by an examination of its optical characters. Isometric crystals have like optical properties in all directions and are, therefore, called isotropic. The optical properties of all other crystals are unlike in different directions and they are consequently called anisotropic. Anisotropic crystals are subdivided into two groups, isodiametric, including crystals of the tetragonal and hexagonal systems, and anisometric, including crystals of the orthorhombic, monoclinic and triclinic systems. In isodiametric crystals there is no double refraction in the direction of the vertical crystallographic axis, which is called the optic axis; these crystals are consequently said to be uniaxial. The optical structure of uniaxial crystals is represented by a spheroid, a section of which normal to the optic axis is always a circle. Anisometric crystals are more complex, but there are always two directions analogous in character to the single axis of the uniaxial group, so that anisometric crystals are said to be biaxial. Their optical structure is represented by an ellipsoid with three unequal rectangular axes. There are two directions in which such ellipsoids can be cut so as to yield cross sections which are circles. The optic axes are normal to these planes. In orthorhombic crystals the axes of the ellipsoid coincide in direction with the crystallographic axes. In monoclinic crystals one of the ellipsoidal axes coincides with the axis of crystallographic symmetry, while the other two lie in the plane of symmetry. In triclinic crystals there is no essential relation between the ellipsoidal and the crystallographic axes. When examined in polarized light isometric crystals exhibit no special phenomena. (See PHYSICAL CRYSTALLOGRAPHY.)

4. Characters Depending upon Heat.- Fusibility is the relative ease with which a mineral melts. Mercury, water and petroleum are the only minerals which are liquid at ordinary temperatures of the atmosphere. Von Kobell's scale of fusibility is 1. Stibnite, fusible in a candle flame, even in large pieces. 2. Natrolite, fusible in a candle flame, but only when in small splinters. 3. Almandite Garnet, easily fusible in the blowpipe flame, even in large pieces. 4. Actinolite, fusible with difficulty in the blowpipe flame in large pieces. 5. Orthoclase, fusible with difficulty in the blowpipe flame, even when in small splinters. 6. Bronzite, scarcely fusible at all. The determination of the fusibility of a mineral is frequently of aid in its identification.

Other minor characters depending upon heat are conductivity, expansion, specific heat and diathermancy.