Geology

1. Hrematite, or Specular Iron, is ferric oxide, Fe₂0₃. Sp. gr. = 4.5-5.3; H = 6.5. Crystallizes in rhombohedrons, or more commonly, in nodular masses, which are composed internally of very flat crystals. The color is black or steel-grey, which becomes red when the mineral is finely powdered. Hsernatite frequently contains earthy and other impurities and is one of the most important ores of iron.
2. Limonite, or Brown Hsernatite, is hydrated ferric oxide (2 Fe₂0₃, 3 H₂0) containing more than 14% of water. It is softer than hsematite and of a yellow or brown color. Sp. gr. = 3. 6-4; H= 5-5.5.
3. Magnetite is the black oxide of iron, Fe₃0₄ (or FeO, Fe₂0₃).
   Sp. gr. = 4.9-5.2; H = 5.5-6.5. Crystallizes in the isometric system, usually in octahedrons, sometimes in dodecahedrons. This mineral is strongly magnetic and is black in color, with a bluish-black metallic luster, when viewed in reflected light. Magnetite is widely diffused in certain classes of rocks, and also occurs in veins and beds, which form an important source of supply of the metal.
4. Ilmenite is a titaniferous ferric oxide, (Ti, Fe)₂0₃ Sp. gr. = 4.5-5.2; H = 5-6. When crystallized, this mineral is rhombohedral, but is generally massive.
5. Siderite is ferrous carbonate, FeC0₃ Sp. gr. = 3.7-3.9; H = 3.5-4.5. Crystallizes in rhombohedrons, the faces of the crystals frequently much curved, and often the crystals are very much flattened. When fresh, the mineral is grey or brown. It is but slightly acted on by cold acids; hot acids dissolve it with effervescence. Mixed with clay, siderite forms clay iron-stone, a valuable ore.
6. Iron Pyrites, bisulphide of iron, FeS₂ Sp. gr. = 4.9-5.2; H = 6-6.5. Crystallizes in the isometric system, usually in cubes, sometimes in dodecahedrons, and has a very characteristic brassy luster and color, to which it owes the popular name of “fools’ gold.” It is very hard, cannot be scratched with a knife, and strikes fire, like flint, when struck with steel. The mineral is soluble in nitric acid: it is widely disseminated in the rocks.
7. Marcasite, or White Iron Pyrites, has the same composition as pyrites, but crystallizes in the orthorhombic system, in modified prisms, but more commonly occurs in nodular masses, with a radial structure. It has the same hardness as pyrites, but is not quite so heavy. Sp. gr. = 4.68-4.85. In color it is paler than pyrites, with a tendency to grey, green, or even black. It decomposes very readily and after a few months’ exposure, even to dry air, often crumbles to a whitish powder.

The iron minerals are seldom largely represented in any given rock, with the exception of the ore beds, but iron is one of the most widely diffused of substances, few rocks being altogether free from it, and its various compounds playa very important role as coloring-matter in the rocks. Ferrous carbonate gives no color to the rock in which it occurs, and such rocks are apt to have a blue or grey tint, due to other substances, both organic and inorganic. When such rocks are exposed to the action of air and moisture, ferric oxide and ferric hydrates are formed, the former giving a red color and the latter various shades of yellow and brown.

A blue clay containing ferrous carbonate will, when fired in a kiln, give rise to red bricks or pottery, by the conversion of FeC0₃ into Fe₂0₃. Exposure to moist air produces a similar effect in nature, and the contrast in color between the superficial and deep-seated layers of the same rock is often as great as between blue clay and red brick.

Weathered blocks stained red on the outside are often blue, grey, or nearly black on the inside, the change not having penetrated through the whole mass. Such changes are most conspicuous in the sandstones, because their porous character allows a comparatively free circulation of air and water through them, but similar effects are frequently to be observed in other rocks also.

1. Calcite, carbonate of lime, CaC0₃, Sp. gr. = 2.72; H = 3. This mineral crystallizes in the hexagonal system, in a great variety of forms; rhombohedrons and scalenohedrons are common; hexagonal prisms and pyramids less so. Cleavage is very perfect, parallel to the faces of a rhombohedron, and the mineral breaks up into rhombohedrons when struck a sharp blow. Calcite is rapidly attacked, even by cold and weak acids, CO₂ escaping with effervescence. When pure, as in Iceland spar, the mineral is colorless, very transparent and lustrous, and displays the phenomenon of double refraction strongly; but more commonly it is cloudy or white, or stained red or yellow by iron. The decomposition of silicated minerals containing lime gives rise to calcite, and as this is soluble in water holding CO₂, nearly all natural waters have more or less of it in solution. It is widely diffused among the rocks, and in a state of varying purity forms great masses of limestone.
2. Aragonite (CaC03) is a somewhat harder and heavier form of calcite, with a specific gravity of 2.93 and a hardness of 3.5-4, and crystallizes in compound prismatic forms which belong to the orthorhombic system. It has not the marked cleavage of calcite and is very unstable; when heated it is converted into calcite and falls into tiny rhombohedrons of that mineral.
3. Dolomite is a carbonate of lime and magnesia, (Ca, Mg)CO₃, but with variable proportions of the two bases; it resembles calcite in appearance, and crystallizes in rhombohedrons which often have curved faces. Sp. gr. = 2.8-2.9; H=3.5-4. Dolomite may be readily distinguished from calcite by the fact that cold acids affect it but little.
4. Selenite, hydrated sulphate of lime, CaS0₄, 2 H₂0. Sp. gr. = 2.31-2.33; H = 1.5-2. It crystallizes in right rhomboidal prisms, belonging to the monoclinic system, and cleaves into thin non-elastic leaves. When pure, selenite is transparent and colorless, but is often stained by iron. This mineral occurs largely in granular masses, called gypsum, from which plaster of Paris is made by calcining the gypsum and so driving off the water of crystallization. Gypsum is slightly soluble and is present in most natural waters. Alabaster is a gypsum of especially fine grain, mottled in pale colours, or white.
5. Anhydrite, CaS0₄, is sulphate of lime without water; it is harder and heavier than gypsum (Sp. gr. = 2.9-2.98; H = 3-3.5), and crystallizes in a different system, the orthorhombic. The crystals have three sets of cleavage planes, which intersect each other at right angles.
6. Apatite is a phosphate of lime, with chloride and fluoride of calcium, which vary in relative amounts, 3 (Ca₃P₂0₈), 2 (Ca, Cl, F). Sp. gr. = 2.92-3.25; H = 5. It crystallizes in hexagonal prisms, terminated by hexagonal pyramids, and also occurs in masses The original and unchanged mineral is transparent and colorless, changing on alteration to opaque brown or green. Apatite is soluable in acids, and in water containing carbon dioxide, or ammonia i it is thus dissolved out of the rocks and widely diffused in the soils, where it forms a valuable plant food.
7. Fluor-spar, fluoride of calcium, CaF₂. Sp. gr. = 3.01-3.25; H = 4. Crystallizes in the isometric system, usually in cubes, and has a perfect octahedral cleavage. When pure, fluorspar is clear and colorless, but it is more commonly stained blue, green, yellow, or brown.

Chlorite

Under this name are grouped a number of closely allied minerals, which are hydrated silicates of alumina, magnesia, and iron. They are soft minerals, with a hardness of 1-1.5 and a specific gravity of 2.6-2.96, and are of a green color. The crystalline form is somewhat uncertain, but is now generally regarded as monoclinic, with a pseudo-hexagonal symmetry. These minerals are laminated and split readily into thin leaves, as do the micas, from which they may be distinguished by the fact that the leaves are not elastic.

The chlorites result from the decomposition of hornblende, augite, or the magnesian micas.

Talc is a hydrated silicate of magnesia, 3 Mg O, 4 SiO₂, H₂O; the water varies in amount to as much as 7%. Sp. gr. = 2.56-2.8, H = 1. It is of a white or pale green color, with a pearly luster and a greasy, soapy feeling to the touch. Talc is rarely found crystallized j the crystals have a false hexagonal symmetry, and it is doubtful whether they should be referred to the orthorhombic or monoclinic systems. Usually it occurs in flakes or foliated masses, which split into thin, non-elastic leaves. Talc results from the alteration of magnesian minerals.

Steatite, or Soapstone, has the same composition as talc, but is not foliated, and may be much harder, as much as 2.5.

Serpentine is a hydrated silicate of magnesia and iron: 3 (MgO, FeO) 2 SiO₂, 2 H₂O. It does not crystallize, except sometimes in pseudomorphs. Sp. gr.= 2.5-2.65, H = 2.5-4. Its proper color is green, but it is usually mottled with red or yellow by iron stains. Serpentine is generally formed from the decay of olivine, less commonly from augite, or hornblende.

Kaolinite is the hydrated silicate of alumina, Al₂0₂, 2 SiO₂, 2 H₂O. It is usually soft and plastic, but orthorhombic crystals of pseudo-hexagonal symmetry may be sometimes detected with the microscope. Kaolinite arises from the decomposition of the felspars and especially of orthoclase. Glauconite is a hydrated silicate of alumina and iron, with small quantities of lime, magnesia, potash, and soda. It is of a green color, soft and friable, and results largely from the decay of augite.

Olivine is the only mineral of this group of sufficient importance to require mention; it is a silicate of magnesia and iron 2 (MgO, FeO)Si02, though the percentage of iron varies greatly.
Sp. gr. = 3.2-3.5; H = 6.5-7. Olivine crystallizes in the orthorhombic system, and occurs in prisms, flat tables, or irregular grains. Hydrochloric acid decomposes the mineral, with separation of gelatinous silica. The colour varies from olive green to yellow, or it may be colorless, and usually the irregular grains look like fragments of bottle glass.

Other Silicates, Chiefly Decomposition Products

Many of the complex silicates, when long exposed to the action of the weather and of percolating waters, become more or less profoundly changed. One of the commonest of these changes is hydration, or the taking up of water into chemical union, and this may be accompanied by the loss of soluble ingredients, or the replacement of some constituents by others.

Zeolites

In this group are included a large number of minerals, which are hydrated silicates of alumina, potash, soda, lime, etc. They all contain large quantities of water and hence boil and effervesce when heated before the blowpipe. All these minerals are products of decomposition and d) not occur as original constituents of rocks.

These two groups contain parallel series of minerals of similar chemical composition, but differing in their crystalline form and physical properties. In composition they are silicates of various protoxide bases, and range from silicates of magnesia to those of lime and lime-alumina, while silicate of iron is present in most of them. In crystalline form they belong to the orthorhombic and monoclinic systems, and can be distinguished by their cleavage. The pyroxenes have a prismatic cleavage of nearly 90°, while in the amphiboles the angles are 124° 30′ and 55° 30′. The orthorhombic amphiboles are rare and unimportant as rockforming minerals, but the pyroxenes of this form are widely distributed, though less so than the monoclinic.

1. Orthorhombic Pyroxenes are silicates of magnesia and iron (Mg, Fe)O, Si02.
   1. Enstatite has less than 5% of FeO.
   2. HBronzite has 5-14% of FeO.
   3. HHypersthene has more than 14% of FeO.

   The colour becomes darker and the optical properties change with the increase in the percentage of iron.

2. Monoclinic Pyroxenes
1. Augite - this very abundant and important mineral is a silicate of lime, magnesia, iron, and alumina (Ca, Mg, Fe)O, (AI, Fe) 203, 4 Si02. Sp. gr. = 3.3-3.5 ; H = 5-6. It crystallizes in oblique rhombic prisms, and in colour is green to black and opaque.
2. Diallage - is a variety of augite, usually of a green color, which is distinguished by its laminated structure, with lustrous faces.
3. Monoclinic Amphiboles
1. Hornblende, like augite, which it closely resembles in chemical composition, is among the most important of rock-forming minerals. In color it is usually green, brown, or black, and it crystallizes in modified oblique rhombic prisms. Sp. gr. = 2.9-3.5; H = 5-6.
2. Tremolite is a silicate of magnesia and lime (CaO, MgO), Si02. This mineral is pale green or white and occurs in laminse or long, blade-like crystals.
3. HActinolite resembles tremolite in composition, with the addition of iron (CaO, MgO, FeO) Si02. Color, green; sp. gr. = 3-3.2; usually occurs in long and thin crystals. Asbestus is a fibrous variety of tremolite or actinolite, in which the fibers are often like flexible threads and may be woven into cloth.

These minerals have a complex chemical composition, and are so variable that it is difficult to give formulae for them; they are silicates of alumina, together with potash, lithia, magnesia, iron, or manganese. There is a difference of opinion regarding the crystalline system to which the micas should be referred. When crystallized, they all form six-sided prisms, but there are reasons for believing that this is a false symmetry. Usually certain micas are referred to the hexagonal, and others to the orthorhombic.

These minerals are very closely allied to the felspars in chemical composition, but differ from them in crystalline form and systems, but some authorities regard them all as monoclinic. All varieties have a remarkably perfect cleavage, and split into thin, elastic, and flexible leaves, by which they may be readily recognized. They are quite soft, and most of them may be scratched with the finger-nail.

1. Muscovite may be selected as the most important and wide-spread of the numerous alkaline micas, it being a hydrated potash-mica, with the general formula, K₂0, 3 AI₂0″ 6 Si0₂, 2 H₂0. It is a lustrous, silvery-white mineral, usually transparent and colorless in thin leaves; it has a specific gravity of 2.76-3.1, and a hardness of 2.1-3.

Sericite is a silvery or pale green form of muscovite, which is an alteration product and often is derived from a felspar.

2. Lepidolite is a mica in which part of the potash has been replaced by lithia.
3. Biotite is the most important and widely disseminated of the numerous dark-coloured, ferro magnesian micas. This mineral is black or dark green in mass, and smoky even in thin leaves; chemically it is a hydrated silicate of potash, alumina, iron, and magnesia. In hardness and specific gravity it differs little from muscovite.

These minerals are very closely allied to the felspars in chemical composition, but differ from them in crystalline form and physical properties. They have a much more restricted distribution than the felspars, but have, nevertheless, an important bearing upon the classification of certain groups of rocks in which they occur.

Nepheline - is a silicate of potash, soda, and alumina ( (Na, K)₂0, Al₂0₃, 2 Si02). It crystallizes in transparent and colorless six-sided prisms, of the hexagonal system. H = 5.5-6; sp. gr. 2.6. The mineral is soluble in hydrochloric acid, gelatinous hydrated silica separating out. It is an important constituent of certain lavas.

Leucite - is composed as follows: K₂0, AI₂0₃, 4 Si0₂, with some of the potash replaced by soda. It crystallizes in twenty-four-sided figures (trapezohedrons), which belong to the tetragonal system, but can be distinguished from the isometric only by very careful measurement. H = 5.5-5.6; sp. gr. = 2.44-2.56. It is slowly attacked by hydrochloric acid.

Leucite cannot be called a common mineral, but its significance will be better seen when we come to take up the study of rocks.

Analcite - This mineral is usually regarded as a decomposition product, and placed among the zeolites but recent investigations make it very probable that in some cases, at least, analcite is a mineral of primary origin. Its composition is:

Na₂0, Al₂0₃, 4 Si0₂, 2 H₂0. Crystallizes in the isometric system, and is colorless in transmitted .light. It is soluble in mineral acids, with separation of gelatinous silica. Sp. gr. = 2.15-2.28.

Silica is an acid and forms a very extensive series of compounds with various metallic bases. As rock-forming minerals the silicates are second only to the silica minerals in importance.

The Felspar Group

The felspars are essentially silicates of alumina (Al2O3) together with potash, soda, or lime. Three primary felspars occur: orthoclase, a potash felspar (K₂0, Al₂0₃, 6 Si0₂); albite, a soda felspar (Na₂0, Al₂0₃, 6 Si0₂); and anorthite, a lime felspar (2 CaO, 2 AI₂0₃, 4 Si0₂). From the combination of these primary minerals two series are formed: the lime-soda series, oligoclase, andesine, and labradorite; and the potash-soda series, as yet imperfectly known. The felspars crystallize in either the monoclinic or triclinic systems, but the forms of the crystals are very much alike. With few exceptions, these minerals are of pale colours and, except when decomposing, are very hard.

Monoclinic Fe/spars

Orthoclase is a potash felspar (K₂O, AI₂0₃, 6 Si0₂ = K, AI, Si₃O₈), though soda may replace part of the potash, and small quantities of lime and iron are usually present. Hardness = 6, sp. gr. = 2.54 - 2.57. Orthoclase crystallizes in oblique rhombic prisms and is very generally twinned; there are two sets of cleavage planes, which intersect at a right angle and have thus given its name to the mineral. Orthoclase is usually dull and turbid, which is due to the presence of various alteration products, and even thin sections under the microscope are commonly hazy. Sanidine is a glassy, transparent variety of orthoclase, which is found in lavas of late geological date. Its clearness is due to the absence of the decomposition products, which render ordinary orthoclase turbid.

Triclinic Fe/spars

The minerals of this series are grouped together under the comprehensive term of Plagioclase, because of the difficulty of distinguishing them from each other under the microscope; they are very generally characterized by polysynthetic twinning, which makes fine parallel lines on the basal cleavage planes. Chemically, they are soda, lime, or lime-soda felspars, of which the latter are isomorphous mixtures of albite and anorthite. The following table (from Levy and Lacroix) gives the composition of the various members of this series, representing the soda-felspar constituent, or albite, by Ab, and the lime-felspar constituent, or anorthite, by An:

It will be observed that the specific gravity increases with the lime constituent, and the fusibility diminishes in the same proportion. Anorthite is decomposed by hydrochloric acid, labradorite is slightly attacked by it, while the other members of the series are not affected.

Anorthoclase is a triclinic potash-soda felspar (Ab₂0r₁), but is not a common constituent of rocks.

Next to oxygen, silicon is by far the most abundant constituent of the earth’s crust, though never occurring alone. It is united with oxygen to form silica (Si0₂) or enters into the formation of more complex compounds. The oxide, silica, is the commonest mode of occurrence and forms the most abundant of all the minerals.

Quartz (Si0₂)

Quartz is anhydrous silica in a crystalline state; it belongs in the hexagonal system, and crystallizes in hexagonal prisms capped by six-sided pyramids, or in double six-sided pyramids, or in modifications of these forms. It is insoluble in any acid except hydrofluoric, and only very slowly soluble in boiling caustic alkalis. When dissolved, as may be done by somewhat complicated processes, silica shows a distinct acid reaction.

Quartz has no cleavage and is very hard (H = 7), scratching glass readily, while it cannot be scratched with a knife; the specific gravity (sp. gr.) is 2.6.

When pure and symmetrically crystallized, quartz is transparent, colorless, and lustrous (rock crystal), but it more commonly is found in dull masses. Different varieties are colored by metallic oxides: thus, amethyst is quartz stained purple by’ the oxide of manganese; smoky quartz, or cairngorm, owes its brownish or yellowish color to the oxide of iron; and there are many other kinds.

In its various forms quartz is the most abundant of minerals, and plays the most important part in the formation of the different classes of rocks.

Chalcedony

Chalcedony occurs in spheroidal or stalactitic masses, composed of more or less concentric shells. The structure is crystalline, and displays radiating fibers, which are perpendicular to the shells. The chemical composition and behavior of this mineral are the same as in quartz, but the specific gravity is somewhat lower (2.59-2.64), and the optical properties different. Chalcedony has a waxy appearance, and is translucent or semi-opaque, and of various pale colors.

Opal, Hyalite

Hydrated silica (Si02, xH20). These minerals are amorphous and have no crystalline form. Opal is either translucent or opaque and of various colors. Precious opal (a gem) and common opal differ in color, and in the fact that the former is iridescent, the latter not. Hyalite is colorless and transparent. Hydrated silica is lighter than the anhydrous (sp. gr. = 2.2) and more readily soluble in hot alkaline waters. These minerals are of much less importance as constituents of rock than the forms of quartz.

Agate

Agate is a banded mineral, composed of layers of amorphous and crystalline silica, chalcedony, jasper, amethyst, rock crystal, etc.

Flint and Chert

Flint and Chert are also believed to be mixtures of hydrated and anhydrous silica. They occur in amorphous masses of neutral or dark colors, and are opaque, or somewhat translucent in thin pieces.

Of the simple, undecomposable substances, which chemists call elements, and of which somewhat more than seventy have been identified on the earth, only about twenty enter at all largely into the composition of the earth’s crust,’ so far as this is accessible to examination. It is estimated that 97 per cent of the crust is made up of ten elements.

The remaining ten are far less abundant, but yet of considerable importance.

Only two of these elements, carbon and sulphur, are found in a more or less impure state as minerals or rock masses; the others occur as compounds, formed by the union of two or more of them.

A mineral is a natural, inorganic substance, which has a homogeneous structure, definite chemical composition and physical properties, and usually a definite crystalline form.

Crystals are solids of more or less regular and symmetrical shape, bounded, usually, by plane surfaces. The number of known crystalline forms is already very great, and yet they may be all reduced to thirteen fundamental shapes, which are prisms, octahedrons (eight-sided), or dodecahedrons (twelve-sided).

The thirteen fundamental forms and their innumerable secondary derivatives fall into six systems, which are characterized by the relations of their axes. The axes of a crystal are imaginary lines, which connect the centres of opposite faces, or opposite edges, or opposite solid angles, and which intersect one another at a point in the interior of the crystal.

The Systems of Crystalline Forms have received many names, the following being those which are most generally used in this country: -

Isometric System (monometric, cubical, regular)

In this system the three axes are of equal length and intersect one another at right angles; it includes the cube, regular octahedron, and rhombic dodecahedron, forms which are symmetrical in all positions.

Tetragonal System (dimetric, pyramidal)

The axes intersect at right angles, but are not all of equal length; the two lateral axes are of equal length, but the vertical axis is longer or shorter than the laterals. Includes the right square prism and the square octahedron, the faces of which are isosceles triangles.

Hexagonal System

Here four axes are employed, three equal lateral axes intersecting at angles of 60 degrees, and a vertical axis, which is perpendicular to and longer or shorter than the laterals. Includes the rhombohedron, hexagonal prism, and scalenohedron.

Orthorhombic System (rhombic, trimetric)

The three axes intersect at right angles and are all of different lengths; rectangular and rhombic prisms, and rhombic octahedron.

Monoclinic System (mono-symmetric, oblique)

All three axes are of different lengths; two of the axes, usually the laterals, are at right angles to each other, while the third is oblique: right rhomboidal and oblique rhombic prisms.

Tric1inic System (anorthic, asymmetric)

Three axes of unequal lengths and oblique to one another: oblique rhomboidal prism, doubly oblique octahedron.

It is important to bear in mind the relations which the fundamental forms sustain toward one another. For example, a regular octahedron may be derived from a cube by evenly paring off the eight solid angles, until the planes thus produced intersect one another, the centres of the faces of the cube becoming the apices of the solid angles of the octahedron. Conversely, a cube may be formed from an octahedron by symmetrically truncating the angles, until the planes thus formed intersect. By slicing away the twelve edges of a cube or an octahedron a dodecahedron will result. These crystalline forms are, therefore, so related as to be all derivable one from another, and the relations of their axes remain unchanged; all three forms may be assumed by the same mineral, and they thus properly belong in the same system. Similar relations may be observed between the crystalline forms of the other systems.

It might be supposed that the crystalline systems and the relations of their imaginary axes were merely mathematical devices to reach a convenient classification of forms. Such a conclusion would, however, be a very erroneous one. Crystalline form is the expression of molecular structure, and many of the physical properties of minerals are determined by their mathematical figure. It is clear that the physical properties which depend upon form are not inherent in the molecules of the mineral, but are conditioned by the way in which the molecules are built up into the crystal. Amorphous substances refract light equally in all directions, and are thus called isotropic, but when an amorphous substance crystallizes, it assumes the qualities proper to its crystalline form. Thus water is isotropic, while the hexagonal crystals of ice are singly refractive in only one direction, doubly refractive in two. The same substance may, under different circumstances, crystallize in different systems, and will then display the properties appropriate to each system.

Not only the refractive powers of a crystal, but also its mode of expansion when heated, and its conductivity of electricity and heat depend upon its form.

The crystals of the isometric system, which have their three axes of equal length, are singly refractive in all directions, expand equally when heated, and conduct heat and electricity equally in all directions. Those of the tetragonal and hexagonal systems, which have one axis longer or shorter than the others, are doubly refractive along the lateral axes, expand equally when heated, and show equal conductivity along these axes. Along the principal axis they are singly refractive, expand to a different degree when heated, and display a different conductivity along this axis than along the others. In the orthorhombic, monoclinic, and triclinic systems, which have all the axes of unequal lengths, the crystals are singly refractive in two directions; they expand unequally and conduct differently along all their axes.

The optical properties of minerals are of great value in the study of rocks, and by the aid of the polarizing microscope very minute crystals may be identified.

Most substances which are solid under any circumstances are capable of assuming a crystalline form, so that solidification and crystallization are usually identical. For the formation of large and regular crystals, it is necessary that the process be gradual and that space be given for the individual crystals to grow. Usually crystallization begins at many points simultaneously, and the crystals crowd upon one another, resulting in a mass of more or less irregular crystalline grains. The same substance which, when very rapidly solidified, forms an amorphous glass, will give rise to distinct crystals, if slowly solidified.

Crystallization requires that the molecules be free to move upon each other, and thus to arrange themselves in a definite fashion. It may take place either by the deposition of a solid from solution, by cooling from a state of fusion, or by solidification from the condition of vapour. In all cases the size and regularity of the crystals depend upon the time and space allowed for their growth. In a manner not yet understood, amorphous solids may be converted into crystalline aggregates. This has been observed in the case of certain glassy volcanic rocks, which, though amorphous when first solidified, have gradually become crystalline, without losing their solidity. This process is called devitrification.

The actual steps of crystallization may be observed by slowly evaporating a solution of some crystalline salt under the microscope. The first visible step in the process is the appearance of innumerable dark points in the fluid, which rapidly grow, until their spherical shape is made apparent. The globules then begin to move about rapidly and arrange themselves in straight lines, like strings of beads, and next suddenly coalesce into straight rods. The rods arrange themselves into layers, and thus build up the crystals so rapidly, that it is hardly possible to follow the steps of change. In certain glassy rocks, which solidified too quickly to allow crystallization to take place, the incipient stages of crystals, in the form of globules and hair-like rods, may be detected with the microscope.

Secondary Forms of Crystals

A great variety of crystalline forms is produced by the occurrence of secondary planes or faces on the angles or edges of the primary forms. All the similar parts of the crystal may be modified in the same way, or alternating similar parts may be so modified.

Certain faces may be obliterated by the enlargement of others; but however great the variation, the angle at which corresponding faces meet almost invariably remains constant for each mineral.

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

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:

Cleavage

Many minerals split readily along certain planes, still retaining a crystalline form, while in other directions they break irregularly. This property is called cleavage. Cleavage is uniform in the different varieties of the same mineral, and takes place either in planes parallel to one or more faces of the fundamental form of the crystal, or along the diagonals of that form.

Pseudomorphs occur when one mineral assumes the crystalline 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.

Crystals of the same form may vary in length and in the size of their corresponding faces, which gives rise to numerous irregularities of shape.

Rock-forming Minerals

The number of known minerals is exceedingly great, and is 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 himself with actual specimens.