THE MINERALS—CHARACTERISTICS—CRYSTALS AND THEIR FORMS—DESCRIPTIONS OF MINERALS.

Minerals are constituent parts of the earth. All parts of minerals are alike. There are simple minerals and mixed. The former are the true minerals, and are generally considered under the heading Mineralogy. The others constitute a branch of Geology, as they form aggregate masses, and as such compose a large portion of the earth. We must learn to distinguish minerals and crystals as inorganic forms of nature. In the animal and vegetable kingdoms we have forms which are possessed of organs of sight, smell, taste, and certain structures indispensable to their existence and development. But in minerals we have no such attributes. They are INORGANIC, and have a similar structure; a fragment will tell us the story as well as a block of the same mineral. These inorganic substances are possessed of certain attributes or characteristics. We find they have FORM. They have chemical properties, and they behave differently when exposed to light and electricity. They are generally solid. All the elements are found in the mineral kingdom, and a mineral may be an element itself, or a chemical combination of elements. These compounds are classed according as the combination is more or less simple. An alliance of two elements is termed a binary compound, of three a ternary compound, forming a base and an acid.

We have learnt from our chemistry paper that there are between sixty and seventy elementary bodies in nature. When we speak of “elements,” we do not mean to apply the popular and erroneous definition of the word. Earth, air, fire, and water are not elements; they are compounds, as we have seen. The list of elements has been given; we will now give the names of the more important minerals. We have no space for a detailed description, but in the British Museum the cases contain some hundreds, and the student will find them classified and described with the greatest care, and according to the arrangement of Berzelius.

Principal Minerals as arranged by Professor Ansted.

The above is the arrangement best suited for beginners.

Professor Nichol prefers the following arrangement:—

These are only a portion of the minerals, but it would be scarcely interesting to give the list at greater length. In the foregoing we recognize the metals and various combustible and non-combustible substances familiar to us, existing, as people say sometimes, in “lumps.” But if any one will take the trouble to examine a “lump,” he will find the shape is definite and even. These regular forms of the minerals are called CRYSTALS, from the Greek word krustallos, ice. The term was originally applied to quartz, for in olden times it was thought that quartz was really congealed water. We can define a crystal as “an inorganic solid bounded by plane surfaces arranged round imaginary lines known as axes.” It must not be imagined that crystals are small bodies; they may be of any size. There are crystals of many hundredweight; and although the usual crystal is comparatively small, it may be any size.

Crystallization has occurred by cooling, or by other natural means; and we can form crystals by evaporation from certain salts deposited in water. So we may conclude also that the evaporation of water in the early periods deposited many forms of crystals. We have crystals in the air, such as snowflakes, which are vapours crystallized. Carbon, when crystallized, is the diamond. Boron is very like it. Oxygen cannot be crystallized. Alumina makes sapphires and ruby with silica. Alumina and earth give us spars, tourmaline, and garnets. Limestone also has beautiful forms, as in Iceland spar. Crystals, therefore, are certain forms of nature, corresponding in the inorganic kingdom to the animals and plants of the organic.

Let us look a little more at these. Here we have a group of crystals of different forms. Earths are metals combined with oxygen, and the principal earths are alumina, lime, and silica. To these three we are chiefly indebted for the ground we live on, and from which we dig so many useful metals and other minerals. Earths are coloured by the substances mixed with them. We can thus find copper, silver, gold, lead, etc., by noting the appearance of the soil. True earths are white. Strontia and baryta are also earths, and the latter is used in firework manufactories. Our chief assistants are Alumina, which furnishes us with bricks and slate; Lime, which gives us marble or stones for building in a carbonate form. Quicklime, by which is meant lime freed from the carbonic acid, is well known; and plaster of Paris is only lime and sulphuric acid in combination. The Silicates, such as sand and flint, are in daily demand. Agate, cornelian, Scotch pebbles, rock-crystal, etc., belong to the same family. Even our gems are crystallized earths, and, as already stated, diamonds are merely carbon.

Stone, as we know, is quarried; that is, it is dug out of the earth. But perhaps many readers do not know why a stone-mine is called a “quarry.” Most kinds of stone (granite and marble are the exceptions) are found in layers, or strata, rendering them easy of removal. The blocks of stone are cut with reference to these layers in a more or less square manner, and “squared up” before they are carried away. Thus the term “quarry,” from an old French word, quarrÉ, or carrÉ, as now written, signifying a square. In granite quarries the stone being very hard is bored, and loosened by means of gunpowder or dynamite blasting. Slate, on the contrary, is easily divided into slabs. We will now resume the subject of Crystals.

We have said that crystals vary in size, and this variety may be traced, in the cases of crystallization from fluids, to the slowness or the rapidity of the cooling process. If the work be done slowly, then the crystals obtain a size commensurate with the time of cooling, as they are deposited one upon the other. The form of minerals is the first important point, and to ascertain their forms and structure we must study Crystallography. We shall find faces, or planes,—the lines of contact of any two planes,—called edges, and the angles formed where these planes meet. We may add that crystals have, at least, four planes, making six edges and four angles. Nearly all crystals have more than this, for the forms are, if not infinite, very numerous, and are divided into six (by some writers into seven) different systems or fundamental forms from which the varieties are derived. The axis of a crystal is an imaginary line drawn from an angle to the opposite one.

The first form, the monometric, or cubic system, with three equal axes at right angles, is represented by fig. 436. This crystal is limited by eight equilateral triangles. It has twelve edges and six angles. If we describe a line from any one angle to an opposite one, that line is called an axis, and in the case before us there are three such axes, which intersect each other at right angles.23 Such crystals are regular octohedra. There are irregular forms also, whose axes do not come at right angles, or they may be of unequal length. The substances which we find crystallized in this form or system are the diamond, nearly all metals, chloride of sodium (salt), fluor-spar, alum, etc.

When we say in this form we do not mean that all the minerals are shaped like the illustration (fig. 436). We shall at once see that the system admits of other shapes. For instance, a regular crystal may have been cut or rubbed (and the experiment can be made with a raw turnip). Suppose we cut off the angles in fig. 436; we then shall have a totally different appearance, and yet the crystal is the same, and by cutting that down we can obtain a cube (fig. 437). Take off its angles again we obtain a regular octohedron once more, as shown in the diagram opposite.

We will exhibit the gradations. Suppose we cut fig. 437; we will obtain (fig. 438) the cube. The next is merely the cube with angles and edges cut off; and if we proceed regularly we shall arrive at fig. 442, the rhombic dodecahedron, or twelve-sided figure, whose equal planes are rhombs.

We can, by taking away alternate angles or edges situated opposite, arrive at other secondary crystals. From the original octohedron we can thus obtain figs. 443 and 444. These are known as tetrahedron. The pentagonal dodecahedron is another secondary form (fig. 445).

The cube, or hexahedron, the octohedron, and the rhombohedron are all simple forms, being each bounded by equal and similar faces, or surfaces. We can thus understand how certain primary or original natural forms of crystals can be changed in appearance by connection. Of the various substances crystallizing in this system we find salt, iron pyrites, gold, silver, copper, and platinum, and the sulphide of lead called galena, in the cube or hexahedron form. The diamond and fluor-spar, alum, etc., appear in the first form (I), fig. 436 (octohedron). The cube, we see, has six equal faces, eight equal angles, and twelve equal edges. Galena, as will be observed from the illustration herewith, shows this peculiarity in a very marked manner (fig. 446).

The second crystalline form is the Hexagonal, and in this system three of the four axes are equal and in the same plane, inclined at an angle of 60°, with a principal axis at right angles to the others. In crystals of this system are found quartz and calc-spar.

The third system is termed the Quadratic or the diametric. This form has three axes, all at right angles, two being equal and the other longer or shorter than the former two. In this system crystallize sulphate of nickel, zircon, oxide of tin, etc.

The fourth, or Rhombic system, or the trimetric. Here we have three rectangular axes, all unequal and intersecting at right angles. The sulphate and nitrate of potassium crystallize in this system.

The fifth is the oblique, or Monoclinic system, which displays three unequal axes, two of which are at right angles; the third, or principal axis, is at right angles to one and oblique to the other of the preceding. Ferrous sulphate, tartaric acid, and gypsum crystallize in this system.

The sixth, or Triclinic system, or the doubly oblique. In this system we have three axes differing in length, and all forms which can be arranged about these unequal and oblique axes. Sulphate of copper will be found in this group. The system has been called anorthic, or triclinic, because the axes are unequal and inclined, as in the oblique prism based upon an obliqued angled parallelogram. Axinite crystal, as annexed, will show one form in this system.

As may be gathered from the foregoing, it is not easy to determine a crystalline form with certainty,—a great part of the crystal may be invisible. A crystalline mass is a mineral, which consists of an arrangement of crystals heaped together. If it does not possess these the mineral is amorphous, or shapeless. We will now endeavour to describe some of the physical characteristics of minerals.

The Goniometer (see fig. 455) is the instrument used for measuring the angles of crystals. Wollaston’s reflecting instrument is most generally used. It consists of a divided circle, graduated to degrees, and subdivided with the vernier. The manner of working is easy, though apparently complicated. The vernier is brought to zero, when an object is reflected in one face of the crystal. The crystal is turned till the same object is viewed from another face. The angle of reflection is then measured, and can be read off on the circle.

We have already referred to the physical characteristics of the minerals, and one of these attributes is cohesion. When we find a substance is difficult to break, we say it is “hard.” This means that the cohesion of the different particles is very great. Minerals vary in hardness; some are extremely difficult to act upon by force, and a file appears useless. At the other side we find some which can be pricked or scratched with a pin; and these degrees of hardness being put as extremes, we can in a manner relatively estimate the hardness of all other minerals. We can test this by scratching one against another; whichever scratches the other is the harder of the two, and thus by taking up and discarding alternately, we can at length arrive at a comparative estimate of the hardness of all. Such a scale was arrived at by Mohs, and arranged in the following order. The softest mineral comes first:—

1. Talc.
2. Gypsum (rock-salt).
3. Calcareous spar.
4. Fluor-spar.
5. Apatite-spar.
6. Felspar.
7. Quartz.
8. Topaz.
9. Corundum.
10. Diamond.

Talc, we see, is the softest, and diamond the hardest. Thus “diamond cut diamond” has passed into a proverb expressive of the difficulties one “sharp” person has to circumvent or “cut out” another. Diamond is used by glass-cutters. When geologists wish to express the degree of hardness of any substance, they mention it with reference to the foregoing list; and if the substance be harder than fluor-spar, but not so hard as felspar, they determine its hardness five, or perhaps between five and six, or between four and five, according as it is harder or less hard than apatite. Thus hardness, or power of cohesion, resistance to exterior force and pressure, is a prime characteristic of the mineral kingdom. The file is the best test.

We now come to another phase of the physical character of our minerals—cleavage. This is the term employed to express the facility of cutting in a certain direction which in the mineral is its direction of cleavage. Take mica, for instance. There is no difficulty in separating mica into thin layers; we can do so with our fingers. The layers, or flakes, or laminÆ are so arranged that they exhibit less cohesion in one direction than when tried in other ways. We cut with the grain, as it were in the direction of the fibre when wood is concerned. Here we have another popular saying expressive of this,—“against the grain,”—which signifies an act performed unwillingly and unpleasantly. Cleavability, therefore, means cutting with the grain, as it were, and various minerals are possessed of different degrees of cleavage. It sometimes happens that electric excitement is observed when cleavage takes place. One place will become positive, and the other negative. Mica, arragonite, and calcareous spar will exhibit this action after cleavage or pressure. When a crystal of tourmaline is heated, it will develop positive electricity at one end of its principal axis, and negative at the other. Even if it be broken, the extremities of the fragments will exhibit similar phenomena, and so far like a magnet, which, as we have seen, possesses this attribute of “polarity.” But a curious fact in connection with this is that, if the heating cease the polarity ceases for a second or two, and yet as cooling goes on the polarity is restored, with the difference that the positive end has become negative, and the end previously negative has come over to the opposite pole. Electricity, therefore, must be hidden away in every portion of our globe, and will some day be proved to be the mainspring of all life.

Fracture in minerals is also to be noticed. Those substances which we cannot laminate we are obliged to break, and we may require to break a mineral in a direction different from or opposed to its direction of cleavage. Under such circumstances we must break it, disintegrate it, and observe the fracture. Sometimes we shall find the surfaces very even, or uneven, or what is termed conchoidal. This is observable in the breaking of flint. There are various ways in which minerals display fracture, and the particular manner and appearance denotes the class to which the mineral belongs.

We may pass over the question of the specific gravity of minerals, as we have in a former part explained this. It is important, however, to ascertain the specific gravity. As a general rule, minerals containing heavy metals are of high specific gravity.

But the relation of minerals (crystals) with regard to light is of great interest and importance. When we were writing of polarization, we mentioned the faculty a crystal has for double refraction, by which it divides a ray of light into two prolonged rays taking different directions, the plane of vibration of one being at right angles to that of the other. This property is not possessed by all crystals. Some act as ordinary transparent media. Some crystals transmit only one polarized ray, and tourmaline is called a polarizer; and if light be passed through it to another polarizer, it will be transmitted if the latter be similarly held; but if the second be held at right angles to it the ray will be stopped. We can easily understand this if we suppose a grating through which a strip of tin is passed; but the strip will be stopped by bars at right angles to it. The coloured rings in crystals can be observed when a slice of a double refracting crystal is examined. The rings are seen surrounding a black cross in some instances, and a white cross in another. The effect when examined in the polariscope is very beautiful. Selenite is probably the best crystal for exhibiting colours.

Minerals sometimes reflect, sometimes refract light; they are said to possess lustre and phosphorescence. All these properties may be considered as belonging to the crystals which are transparent, semi-transparent, translucent, or opaque, according to the degrees in which they permit light to pass through them. All minerals are electric or non-electric, and the variety can be ascertained by rubbing and placing the mineral near the electrometer. But all do not exhibit magnetic properties. Taste and smell are strongly marked in some minerals—salts, for instance, and sulphur; some are soapy to the touch, some appear cold to the fingers. Chemistry is very useful to us in determining the nature of the mineral, and the amount of it enclosed in the substance under examination. These delicate operations are termed qualitative and quantitative analysis. The application of heat is increased by means of the blowpipe, which is in effect a small bellows. We can thus, and particularly by means of the oxy-hydrogen blowpipe, obtain a very intense heat with little trouble. When the fragments of a mineral are held in the flame by platinum “tweezers,” or tongs, then the fusibility of the substance, and the colour of the blow-pipe flame will be of great assistance in determining the nature of the mineral. It is also curious to observe the different forms into which the various substances expand or contract under the influence of the blowpipe. We may have a rugged slag, an enamel, or a glass, or a bead, or “drop” of metal. The varied substances produce various colours—yellow, green, orange, or red, according to circumstances. Strontia is a vivid red, copper is green, lime orange, and so on.

It is very little use to attempt a study of mineralogy without some acquaintance with chemistry. In dealing with minerals, and in studying geology, we must try to keep our knowledge of chemical science in our minds, and thus fortified we can more easily understand the steps leading to the classification of minerals. It is impossible to teach mineralogy or geology from books. Nature must be studied, the specimens must be seen, the earth must be examined. The advance in mineralogy may be—probably will be—slow, but crystals will teach something; and when we can pass a viva voce examination in chemistry and crystallography, expressing, by the symbols, the various substances under discussion, we shall have made a considerable advance in the science. We shall have an idea of the component parts of various substances, and be able to class the various minerals according to their chemical constitution. Beginning with the metalloids, we shall pass to the metals and various compounds, salts, resinous substances, etc., such as amber.

It is impossible in the space at our command to describe all the minerals, and yet it is necessary to enumerate the most important. We may, therefore, take them in the following order. It should be added that most of the simple minerals occur in comparatively small quantities, but sometimes we find them in aggregate masses (rocks). We append a table.

SYNOPTICAL TABLE OF THE MINERALS.

• First Class.—Metalloids.
• Sulphur.
• Boron.
• Carbon.
• Silicium (Silicon).
• Second Class.—Light Metals.
• Potassium.
• Sodium.
• Ammonium.
• Calcium.
• Barium.
• Strontium.
• Magnesium.
• Aluminum.
• Heavy Metals.
• Iron.
• Manganese.
• Cobalt.
• Copper.
• Bismuth.
• Lead.
• Tin.
• Zinc.
• Chromium.
• Antimony.
• Arsenic.
• Mercury.
• Silver.
• Gold.
• Platinum.
• Third Class.
• Salts.
• Earthy resins.

Sulphur is found in Sicily and Italy and other parts of Europe, in a native state, but as such has to be purified. The crystals take the form as shown in the margin. Cleavage imperfect; it is brittle. Sulphuric acid is a very important combination, and a very dangerous one in inexperienced hands. Sulphur combines with a number of elements, which combinations are “Sulphides.” (See Chemistry section.)

Selenium is a metalloid resembling sulphur, but less common. It is inodorous.

Boron is usually found near volcanic springs, and in combination with oxygen. It is soluble. Taste, acid bitter, and white in colour; friable. It is known as Sassoline, or boracic acid. (See Biborate of Soda for one of the borates.)

Carbon is one of the most important of our minerals. In the form of coal we have it in daily use, and in the form of diamond it is our most valuable gem. In the latter form it is the hardest of all minerals, a powerful refractor of light, lustrous, and transparent. It is found in the East Indies and Brazil; more lately Cape diamonds have been brought to Europe, but they do not equal the Eastern gem. Almost fabulous prices have been given for diamonds, which, after all, are only carbon in a pure state. Another form of carbon is graphite (plumbago, or blacklead). It is much used for pencils and in households. It is found in Cumberland, and in many other localities in Europe and Canada.

Carbon appears in one or other of the above forms in regular octahedrons or their allied shapes. Anthracite, another form of carbon, is used as fuel for strong furnaces. It leaves little “ash,” and is smokeless when burned. Coal, in all its forms, is evidently derived from wood. Thousands of years ago vegetable matter must have been embedded in the ground and subjected to carbonization. There are different kinds of coal, all of which come under one or other of the following heads: cubical coal, slate coal, cannel coal, glance-lignite,—the last being, as its name implies, an imperfect development of wood; it is a brown coal. We are not here concerned with coal as a fuel. Charcoal is also a form of carbon prepared from wood and finds a counterpart in coke, which is prepared from coal. Carbon, as we have already seen, plays an important part in electric lighting and in the Voltaic Battery. Peat, or as it is called in Ireland, “turf,” is one of the most recent of the carboniferous formations. It is much used as fuel. It is cut from moors (“bogs,” as they are sometimes called), and the various deposits can be traced. Bog-oak is no doubt the first step towards peat, as peat is the step towards coal. The brown turf is newer than the black, and both kinds may be seen stacked in small square “bricks” along the Irish canals and in the yards of retailers of fuel.

Silicon. Silica occurs generally in combination with alumina, and never in a free state. In combination with oxygen it is called silicic acid. Silica, when crystallized, is usually called quartz.

Quartz has several varieties. We need only enumerate them, they will all be immediately recognized. We give illustrations of the crystals of quartz (fig. 459):—

1. Rock crystal appears in beautiful six-sided prisms.

2. Amethyst is coloured by protoxide of manganese, supposed by the ancients to be a charm against drunkenness.

3. Common quartz, or quartz rock, forms granite in combination, and is also known as “cat’s-eye,” “rose” quartz, etc.

4. Chalcedony, sometimes termed cornelian: used for seals, etc.

5. Flint: much used in potteries. “Flint and steel” have been superseded by phosphorus.

6. Hornstone: something like flint, resembling horn.

7. Jasper: of various colours; opaque and dull in appearance.

8. Silicious slate: a combination; used as a whetstone.

9. Agate: a mixture of quartz, amethyst, jasper, and cornelian; very ornamental.

10. Opal: a peculiar variety, containing water. It is not found in the form of crystal, but in vitreous masses. Its changeableness of hue is proverbial. The “noble” opal is much prized.

11. Smoky quartz, or cairngorm.

12. Onyx and Sardonyx.

We now arrive at some minerals which contain metals.

Potassium. This metal is so frequently combined in minerals with alumina that we may refer to it with the latter in sequel. There are two natural potassa salts—nitre, and sulphate of potassa. Nitre is known as saltpetre, and is of great use in medicine. It is the chief ingredient in the composition of gunpowder.

Sodium. We have a number of minerals in this group—viz., nitrate of soda (nitratine), which occurs in large quantities in Peru; rock salt, chloride of sodium, known as salt. It crystallizes in the cubic system. Colour usually white, but it occurs in secondary rocks in company with gypsum, etc. It is sometimes of a reddish colour, or even green and yellow. Biborate of soda is borax, and is found in and on the borders of a Thibetian lake. There are several other combinations with soda: the sulphates of soda—viz., thenardite and glauberite, anhydrous and hydrated respectively, carbonate of soda, and so on.

Ammonia combinations occur in lava fissures, and are not often met with in consequence of their volatile nature.

Calcium. This forms an important group of the minerals, which are very white in colour, and not very hard in substance. Calcium is the metallic basis of lime. Fluoride of calcium, known as fluor-spar, most frequently crystallizes in cubes in the first system. Anhydrite is the anhydrous sulphate of calcium. The hydrated sulphate is called gypsum. One variety of the hydrated sulphate is selenite, another is known as alabaster. Apatite, or asparagus stone, and pharmacolite are in this group.

Carbonate of lime, not content with one system of crystals, makes its appearance in two. It is therefore divided into two minerals—namely, calcareous spar and arragonite. In the former it possesses various forms, as will be observed in the accompanying diagrams. It is a very important mineral, as will be readily acknowledged; it enters largely into the composition of all shells and bones. The minute shells, deposited by millions at the bottom of the sea, have combined to raise our chalk cliffs. Carbonate of lime is a constituent of water, as the deposits at the bottoms of kettles, upon the sides and bottoms of water-bottles, and the stalactites all testify. A little good vinegar will quickly dissolve this deposit. Calc-spar is crystallized, and the Iceland spar is celebrated. Marble, which is another form of carbonate of lime, is white, hard, and granular. It is sometimes varied, but the pure white is the most valuable. Chalk, we know well, is soft, and is useful for writing. We have also aphrite, schiefer spar,—compact limestone in various forms,—and finally, arragonite, called from the place of its nativity, Arragon,—a colourless and somewhat transparent vitreous crystal.

Barytes. The sulphate of baryta is known as heavy spar; the crystals are of tabular forms, but numerous modifications exist. One of the forms is represented in the margin.

Strontium is the metallic basis of strontia. Sulphate of strontium is celestine, the mineral which colours the blow-pipe flame a fine crimson. There are certain varieties. Strontia salts are chemical preparations. A beautiful pyrotechnic “red fire” is produced by mixing nitrate of strontia with sulphur, antimony, charcoal, and chlorate of potassia.24 There is a carbonate of strontia in the same crystalline system.

Magnesium. With this metal we have a large group of minerals. Magnesite is carbonate of magnesia, and occurs as talc-spar. The magnesium limestone crystallizes as bitter spar. This dolomite is like marble or common limestone, according to colour. Talc is a combination of magnesia with silicic acid. The hydrated carbonate is termed “white magnesia.” The sulphate of magnesia is found in Siberia, and we have boracite, and native magnesia called periclase. The sulphate is generally present in mineral waters, such as the Seidlitz and Epsom Springs. Large masses have been found in the extensive caverns of Kentucky and Tennessee, etc.

Meerschaum is a hydrated silicate of magnesia. It is found in Anatolia and Negropont, also in France and Australasia. Serpentine is another similar composition. It is found in Cornwall, where it is carved into various ornaments. It is sometimes called snakestone. There are many other hydrated silicates of magnesia—viz., gymnite, picrosmine, pycrophyll, etc.

There is another family allied to magnesia, called Augites. These minerals are black or dark-green, and are contained in lava and basalt: Augite and Hornblende are the chief representatives of this family. The former crystallizes in the fourth system (see fig. 463), and there are several varieties—diallage, bronzite, diopside, etc. Hornblende belongs to the same system, and is a large factor in the composition of gneiss, syenite, and porphyry. Tremolite is a hornblende, and asbestos (amianthos), and mountain-cork are also varieties. The attribute of asbestos for sustaining heat is well known, and may be usefully employed for fire-proof purposes. The well-known jade-stone of China and calamite are other varieties.

Aluminum, or Aluminium, gives us a large class of minerals. It is the metallic basis of alumina, which, combined with silica, is the chief component of our clay. Silicic acid and this base combine to form many minerals, and contains nearly all the precious stones. Corundums consist of pure alumina, and crystallize in the hexagonal system. The following stones are varieties of this mineral:—Sapphire, a beautiful blue; ruby, a red oriental; topaz, yellow oriental; amethyst, violet; all being sapphires more or less. The finest crystals are found in the East Indies in the sands of rivers and diluvial soils. The common corundum is very hard, and is used for polishing. Emery is well known, and is found in mica-slate. It is of a bluish-grey colour, and is also a polisher.

Alum forms another family, of which we may first mention aluminite, a “basic sulphate” of alumina and found in small quantities. Alum-stone is found in Italy. Alum occurs in large crystallized masses. (See illustration, fig. 464.) There are different minerals with a composition very similar to alum, in which the potassa base of alum is supplied by others. Thus we have the potassa alum, soda alum, manganese alum, ammonia alum—all being very nearly of the same constituents, and having similar crystals in the regular system, and are thus termed isomorphous, or similarly-formed. The potassa, or potash alum, is the commonest form, and is found abundantly in England, on the Continent of Europe, and the United States. Soda alum is called salfatarite, and magnesia alum pickeringite; manganese alum is apjohnite; phosphate of alumina is wavellite.

There are compounds of alumina and magnesia called Spinels. They are hard minerals, and the same isomorphous changes take place with them as are observable with the bases of alum. There are therefore varieties such as the spinel ruby found in the East Indies, very red in colour; the balas ruby not so red, and the orange-red, termed rubicelle. Ceylon is remarkable for some fine specimens of spinels. Chromite is like the spinel, but is known as chrome iron.

Zeolites are principally compositions of silica and alumina. They contain water, and are white, vitreous, and transparent. There are several varieties of them—natrolite, stilbite, etc. We will now pass on to the Clays, which are a very important family of the aluminum group.

There are a number of hard minerals which, when disintegrated, form certain earthy masses. These we term clay, or clays, which possess various colours and receive certain names, according to the proportion of metallic oxides they contain. All clays have an affinity for water, and retain it to a very great extent. The earth has also a peculiar smell. Clay is used in various ways; pottery, for instance, we read in the Bible as having been an employment from very ancient times. One attribute of clays, the retention of water, is of the greatest use to the world in providing moisture for plants and seeds. We may mention other characteristics of clay. It absorbs oil very quickly, and therefore is useful for removing grease-spots. It cannot be burned, so we have fire-bricks and fire-clay in our stoves and furnaces. There are various clays—pipe-clay, for instance, which is white; potters’ clay is coarser. There is porcelain clay as well as porcelain earth, of which more below. Yellow ochre and sienna are clays used by artists. Bole is a reddish clay; and tripoli is employed for polishing. There are, besides, andalusite, or chiastolite and disthene, crystalline forms of clay.

Porcelain has been known to the Chinese for centuries. In 1701 it was discovered in Germany by BÖttcher, a chemist, who while endeavouring to make gold by Royal command, found the porcelain, and was thereby enriched. Porcelain earth is frequently found; is known in many places as kaolin, and usually comes from the decomposition of felspar. But in Cornwall we find it as decomposed granite, and the filtering process can be viewed from the railway, while both gneiss and granulite have been known to yield kaolin. It is also found in China, Saxony, and France. It is free from iron, and when ground and mixed with silicic acid, it is handed to the potter or moulder. After the vessels have been dried in the air they are put into the kiln, and then become white and hard. After that they are glazed in a mixture of porcelain earth and gypsum, or ground flints and oxide of lead, made fluid with water in the glazing of earthenware. The vessel is then put into the furnace again, or “fired,” as the process is called, and then comes out white, hard, and partly transparent.

Earthenware utensils are made of a coarser material,—clay and powdered flints,—from which all the gross matter has been eliminated. Flint is not difficult to break, if made hot and thrown into cold water. A stamper is then used to break the flints. They are first ground in a mill and purified like the clay, then they are mixed and beaten, while moist, into “putty,” and turned, or forced, into moulds. The handles are fixed on afterwards. The ware is baked for two days and glazed. The various colours are obtained by mixing different clays and oxides—iron or manganese. Biscuit porcelain is made by pouring a creamy mixture of porcelain earth into plaster-of-Paris moulds, and when a thin case has formed within, the liquid is poured out again. It is then dried in the mould and shrinks. The mould is taken to pieces, and the thin biscuit porcelain is left.

Felspars are very like the zeolites, except that the former contain no water. Felspar crystallizes in a number of different forms. We annex illustrations of specimens. This spar is found in rocks, granite, gneiss, etc. One variety is the moonstone, of a peculiar lustre. Felsite is amorphous felspar. Albite contains soda instead of potash. Labradorite is nearly a pure lime felspar, and is remarkable for its colours, like a pigeon’s breast. Spodumene is like albite, and leucite, soda-lite, etc., belong to this family.

Lapis-Lazuli is a felspar distinguished by its blue tint. It was used for ultramarine colouring at one time, which colour can also be made chemically. Lapis-lazuli is found in Siberia and China. It is a mixture of mineral species. Hauyne is something like it. Obsidian is a sort of black glass, and occurs in various colours in vitreous masses. It is derived from the fusion of rocks, and is employed in the manufacture of boxes, etc. Pumice stone bears a resemblance in composition to the foregoing, but is porous, and so called spongy. It contains both potash and soda in some quantities. Pearlstone and pitchstone also attach themselves to this family group.

The Garnets possess many curious forms of crystals, which are coloured and used as gems. Tourmaline is a very particularly useful crystal, and is used in the investigations concerning the polarization of light. It is found of nearly all colours. The garnet and staurolite crystals are shown (figs. 470, 471).

The former is silicate of alumina with the silicate of some other oxide, which is not always the same. This change, of course, gives us a series, as in the case of alum above mentioned.

The red varieties, called almandine, or precious garnets, are distinguished from the duller, “common” species by their clear colour. Bohemia is the most productive soil for the garnets.

Mica includes, as we have already noticed, a group of minerals which have a peculiarly laminated structure. These layers are by no means all alike, but they present a smoothness to the fingers which is highly characteristic. The chief constituents are alumina and silica, occasionally with magnesia. Mica slate is very common, and is often used instead of glass in window-frames. Muscovite, lepidolite, and phlogopite are all micas of the “potash,” “lithia,” and “magnesia” varieties.

In the list of minerals associated with the lighter metals, we need only now mention the Gems, so well known. These stones are very hard in many instances, infusible, and exhibiting beautiful colours. Amongst them are diamonds, sapphires, and rubies, of which we have spoken; the topaz, noticed under corundum. The chrysoberyl (of a pale green, or occasionally reddish hue), of which the alexandrite of Siberia is a variety, is a compound of glucina with alumina; the beryl, a silicate of the same, and the emerald of beautiful green. Zircon is another gem, and “hyacinth” is its most valued form. The latter is found in basaltic rocks. The emerald crystallizes in the hexagonal system.

We may now consider the minerals formed by the heavier metals, such as Iron, Copper, Nickel, etc.

Iron. This well-known metal fills a very important place in our mineral arrangements, for the substances formed with iron ores occur in great variety of structure, and occasionally in very large masses. They are highly magnetic, and very hard. Were we here treating of iron as a metal, we could give some information respecting its extraction and manifold uses. All we need mention here is the fact that iron occurs in nature in various ores which are essentially composed of iron and oxygen. The iron is extracted in the blast furnace, in which the process is continued for years. The “slag,” or glassy scum, protects the molten iron from the air; its presence is necessary in all blast furnaces. The most important of the iron group of minerals are Magnetic Iron (magnetite), or loadstone. This mineral occurs in Sweden and North America, and is found in primary rocks, and in Scandinavia forms mountains. It crystallizes in the regular (octahedron) system, and often in the form in illustration in the margin. It is highly magnetic, as its name implies.

Native iron very rarely occurs, and then only in thin layers. The most extraordinary specimens are those termed meteoric iron, which fall from the atmosphere in great masses; and the meteoric stones, which contain ninety per cent of iron, together with other constituents in small quantities—viz., nickel, cobalt, copper, manganese, carbon, sulphur, arsenic, etc.

Red hematite crystallizes in the hexagonal system. It possesses much the same (chemical) constitution as corundum (q. v.). It is brightly metallic, and shows a red streak. It occurs in various forms, as iron glance or specular iron, which is found in Sweden and Russia; micaceous iron, bloodstone, clay, ironstone, etc.

Brown hematite has not been found in crystals, but brown ironstone (fibrous) is crystalline. The earthy brown, containing clay, gives us yellow ochre and umber. Pea-iron ore and “morass” or “bog” ore also belong to this class. Limonite is the name given to these more recent formations, of which yellow ochre is a pure specimen.

The combinations of iron with sulphur (pyrites) are also important. Iron pyrites and magnetic iron pyrites are two which may be mentioned. The latter first.

Magnetic iron pyrites (or pyrrhotin) crystallizes in six-sided prisms, and is attracted by the magnet. The composition of this mineral has not been exactly ascertained, and no chemical formula has been found for it.

Iron Pyrites (bisulphide of iron) is known as cubic pyrites, yellow pyrites, and mundic. It is generally found in the regular system of crystals, either as a cube or as a pentagonal dodecahedron. (See first system of crystals, ante.) Its colour is yellowish. It is known also as green vitriol when oxidised, and forms beautiful green crystals (copperas). This salt is used in the preparation of Prussian blue and violet dyes. With gallic acid it makes ink.

There are many other “ferruginous” minerals, such as vivianite, green ironstone, white iron pyrites, arsenical pyrites, or mispickel, etc.

A carbonate of iron, called chalybite, or spathic ironstone, is very abundant in nature, and forms obtuse rhombohedrons. It is very useful for the production of steel, as it forms the clay iron ore found in coal districts in combination. In a fibrous form it is known as sphÆrosiderite. It is a most useful mineral.

Chrome iron (chromite) is useful for the preparation of chromium compounds. It crystallizes in the cubic system. It is magnetic, especially when treated. Chromic acid forms scarlet “needle” crystals, and by its assistance chromate of lead, or chrome yellow, is prepared. (Chromate of lead is found in a native state as crocoisite). See Chromium.

Manganese is contained in several minerals. It usually occurs as an oxide. It colours minerals variously. In a pure state manganese is white and brittle. The chief minerals are—

Pyrolusite (the binoxide of manganese of commerce) occurs in crystals. It is black. It is used in the preparation of chlorine and oxygen. The other minerals are known as manganite, which is also found associated with pyrolusite, as are hausmannite and braunite, the other oxides.

Nickel and Cobalt are generally found together, both being similar, and the minerals are compounds of arsenic or sulphur, and occur under similar circumstances. The principal are of Nickel and of Cobalt—

• Sulphide of nickel (ullmanite).
• Arsenical nickel (nickeline).
• Nickel glance (gersdorffite).
• Nickel pyrites (siegenite).
• Arsenical cobalt (smaltine).
• Cobalt glance (cobaltine).
• Cobalt bloom (erythrine).
• Cobalt pyrites (linnÆite).

Nickel ores are used for extraction of the metal, which is used as a substitute for silver. The name is derived from the German, kupfernickel, or false copper. It was discovered in 1751.

Copper, again, forms a number of minerals, and the chief is the red oxide of the metal, called cuprite. It crystallizes in the cubic system. Its colour is red, and tinges a flame green. Cuprite yields excellent copper, and is found in Cornwall, and in many places on the continent. The black oxide is rarely found. It is known as melaconite.

Malachite (carbonate of copper) is remarkable for its beautiful green colour. In Australia it is worked for copper. It is chiefly ornamental. Siberia yields the finest specimens, but the mineral is found in Cornwall and Cumberland, as well as on the continent. Chessylite (from Chessy, in France) is frequently found with malachite. It has been called blue malachite, or the azure copper ore. It is used as a paint.

Besides the above, copper unites with sulphur to form minerals, such as the needle ore (bismuthic sulphide of copper), antimonial sulphide, bournonite; purple copper, and copper pyrites, which is very abundant, and furnishes us with most of our copper. There is also the “grey” copper ore, which contains various metals; even silver is obtained from it at times.

Bismuth gives us only a few minerals, of secondary importance. Native bismuth resembles antimony, but is reddish in hue. Bismuth ochre, bismuth blende, and bismuthine are the chief combinations.

Lead is more important, and is obtained from galena, the sulphide of lead, which is very abundant, and the principal lead ore. It can be at once distinguished by its high specific gravity and metallic lustre; the “cubic cleavage” also is very easy. It frequently is found containing silver, and even gold, antimony, iron, etc. There are several suphantimonites of lead, such as zinkenite, geocronite, etc., and the salts, such as sulphate of lead and white lead ore, or carbonate of lead (cerasite). The chromate of lead is found in the Ural Mountains.

Tin is not found in a native state, but as tinstone, or binoxide of tin, named cassiterite. It is found largely in Cornwall, and the mines there have yielded great quantities for generations. Tin pyrites, a union of sulphides of tin, iron, and copper, is also found in Cornwall.

Zinc is produced from the ore called (zinc) blende, or sulphide of zinc (black Jack). Its colour is very variable, sometimes red, but when pure is greenish-yellow. It is also found black and brown. The red oxide of zinc (or spartalite) is also worked for zinc. The carbonate, or zinc spar, is common, and used to make brass, as is calamine, which is possessed of a remarkable lustre, and is even luminous when rubbed. It is a silicious oxide of zinc, and is found in the sedimentary rocks. When heated, it displays strong electric properties.

Chromium occurs in very few mineral combinations; chromate of lead, chrome iron, and chrome ochre, or sesqui-oxide of chromium are the only important ones.

Antimony minerals are very hard; the tersulphide is the most common, and from this the metallic antimony is produced. Red antimony, the oxide, is a rarer ore.

Arsenic resembles antimony, and occurs in combination with many metals. White arsenic, or arsenious acid, is found in Bohemia, Alsace, Transylvania, etc. Orpiment and realgar are sulphides of arsenic, and are employed as colouring matters in paint and fireworks. Arsenic is very poisonous.

Mercury is occasionally found native, but more generally as cinnabar. Chloride of mercury (or calomel) is found associated with the cinnabar, or hepatic ore. Cinnabar is easily volatilized, and possesses high specific gravity. The Californian mines are very rich. Spain also produces a large quantity. It is opaque, and carmine in colour.

Silver occurs native, or in ores. The latter are as follows:—The sulphide, or the vitreous silver (argentite); antimonial silver; and the combined sulphides, of antimony and silver. There are many silver minerals, such as the chloride (horn silver, or kerargyrite), bromide, and carbonate of silver, bismuthic silver, etc. The bromide and iodide are bromargyrite and iodargyrite. Silver occurs most frequently associated with gold; natural alloys of these two metals are found, containing from 0·16 to 38·7 per cent. of silver, which causes considerable variations both of colour and density. In addition to this alloy, we may mention sylvanite (graphic tellurium), which contains, besides gold and silver, one of the rarer metals—viz., tellurium.

Gold is our most precious mineral, and is generally found native. It exists in sand and in certain rocks. It crystallizes in various forms, and in Mexico it is found in companionship with silver and copper sulphides. Australia and California render the most valuable supplies of the metal.

Platinum is also found native, and rarely is crystals. It is often alloyed with other metals, chiefly with iron or gold; also with diamonds. We have already considered it as a metal. Little remains to be said about salts and resins, for with the exception of those we have referred to under Chemistry, they are of little value. The bitumens, rock oil, etc., which exude from the earth, are very useful, and as asphalt and petroleum play an important part in the civilized world, but scarcely come under the strict rule of minerals as we consider them, and with this reference we close our sketch of Mineralogy.