Chapter III · THE MINERALS (continued 3)

Witherite BaCO₃ Pl. 48

Occurs in crystals, or in granular or columnar masses; hardness, 3.5; specific gravity, 4.3; color white to gray; luster vitreous; translucent on thin edges.

Witherite is not an abundant mineral. Its crystals are really orthorhombic, but they are usually twinned, three crystals growing through each other in such a manner that the resulting crystal appears like a six-sided double pyramid, similar to the one figured on Plate 48. The commonest 156 mode of occurrence is in compact masses. Witherite effervesces when cold acid is dropped upon it, which, with its heavy weight, and the green color it gives to the flame, serves to distinguish the mineral. It is used for medicines, in chemical industries, and a considerable amount is made into rat poisons. The chief locality for witherite is in northern England, but in this country it is found along with barite, especially at Lexington, Ky., and in Michigan.

Carbon

Carbon is an element widely distributed in nature, occasionally appearing in its elementary form, as graphite or the diamond, but much more important in its compounds. Small quantities are present in the air as carbon dioxide, CO₂, immense quantities occurring in the carbonate minerals, which have been considered under their respective metallic salts, as calcite, malachite, siderite, cerrusite, smithsonite, witherite, etc., and still other large quantities being represented in organic compounds, which occur as rocks under the heads of petroleum, coal, etc. The occurrence of limestones, graphite, coal or petroleum is always indicative of the activity of living organisms, and in some cases is the only indication of life in the earlier rocks.

Graphite C Plumbago

Occurs in hexagonal scales or flakes, in layered masses, or earthy lumps; hardness, 1; specific gravity, 2.1; color black or steel-gray; streak gray; luster metallic; opaque on thin edges.

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Like the diamond graphite is pure carbon, but in this case it is in non-crystalline form. It occurs in both igneous and metamorphic rocks. In the former case it is either in flakes in the rock, or in veins, and has been derived directly from the molten magmas, having either precipitated in the hardening granite or lava, or having been carried into the fissures and there precipitated to make the veins of graphite. In either case the graphite probably represents organic deposits which have been melted into the igneous magma at the time of its formation. Graphite may also occur in metamorphic rocks, beds of coal or other organic deposits being altered by the heat. Such beds are often of considerable extent and economic importance.

The extreme softness, greasy feel, and the dark-gray streak readily distinguish graphite.

It is widely used in making crucibles and furnace linings for foundries, lead pencils, paint, lubricating powders, etc.

Graphite is found at Brandon, Vt., Sturbridge, Mass., Ashford, Conn., in Essex, Warren and Washington Cos., N. Y., Clay, Chilton and Coosa Cos., Ala., Raton, N. M., Dillon, Mont., etc.

Diamond C

Occurs in octahedral crystals; hardness, 10; specific gravity, 3.5; colorless to yellow, brown, blue, etc., luster adamantine; transparent on thin edges.

Like graphite the diamond is pure carbon, but in this case in crystal form. It is the hardest of all minerals, and as brilliant as any; so that in 158 spite of being by no means the rarest, it may easily be considered the most popular of all gems. Tiny diamonds have been made artificially under great heat and pressure; so that this mineral is thought of as forming in Nature in dark igneous lavas at great depths. The diamond has good cleavage parallel to the octahedron faces, and, in spite of some traditions to the contrary, is brittle.

There are not many diamond localities, the most famous being the Kimberley district of South Africa, which produces many times as many diamonds as all the others put together, though all the time some are being found in Borneo and Brazil. A very few have been found in the United States, only one locality however yielding them in the original matrix. That is at Murfreesboro, Ark., where they are mined in a disintegrating peridotite (a dark lava, mostly peridot), which has been extruded through the sedimentary rocks of that region. This matrix is similar to the “blue earth,” the matrix of the diamonds of South Africa, which occurs in “pipes,” representing the necks of ancient volcanoes. The American diamonds are of small size, averaging considerably less than a third of a carat in weight, which does not allow great value to the individual diamonds.

From time to time, especially large diamonds have been found in different parts of the world, the largest being the Cullinan diamond, found at the Premier Diamond Mine of South Africa. It weighed 3025 carats or about a pound and a quarter, and was valued at over $3,000,000. It 159 was presented to King Edward VII, who had it cut into 11 brilliants, four of which are larger than any other diamond yet found. Other famous diamonds, like the Kohinoor, 106 carats, found in India in 1304; the Regent, 136 carats, also found in India; the Orloff, 193 carats, set in the eye of an Indian idol; the South Star, 125 carats, the largest ever found in Brazil; the blue Hope, etc., have in many cases romantic and interesting stories woven about them.

Though for ages diamonds have been highly prized gems, it is only in comparatively recent times that cutting and polishing have been resorted to, for the purpose of enhancing their brilliancy. This is done by grinding reflecting faces on the original stone, by the aid of discs of iron or tin in which diamond dust has been embedded. Diamond chips and cloudy or imperfect diamonds are used for making tools for cutting glass, rock drills, etc.

Phosphorus

The element phosphorus at ordinary temperatures is an almost colorless, faintly yellow, solid substance of glistening appearance and waxy consistency. In Nature it does not occur pure, but always as one of its compounds. It is of great importance to man for it is one of the essentials for plant growth and also for the higher animals, being required for the bones and to some extent for nervous tissue. Originally it is found in all the igneous rocks. Some of the phosphorus is removed by solution and carried to other regions 160 and to the sea. From this distribution it comes into the sedimentary rocks, and, when they are altered by heat, into the metamorphic rocks. Thus it has a wide, though by no means even, distribution. The soils formed by disintegration of these rocks probably all have some phosphorus in them; but where there is vigorous plant growth, it soon tends to become exhausted, and must be renewed. For this reason the use of phosphates has become of prime importance in Agriculture. The possession of beds of rock carrying phosphorus has come to be of international importance. The United States is particularly fortunate in this respect, and produces over 25% of the world’s supply of phosphates. Most all the phosphorus is recovered either from phosphate minerals, the most important of which is apatite, or from the non-crystalline and impure mixtures of phosphate minerals and other substances, discussed under phosphate rock.

Apatite Ca₅F(PO₄)₃ Pl. 49

Occurs in crystals, concretionary nodules, or in bedded masses; hardness, 5; specific gravity, 3.2; color reddish-brown or green, rarely white or colorless; luster vitreous; translucent on thin edges.

Apatite occurs in hexagonal prisms, usually with the ends truncated by a basal plane, and with one or more sets of pyramidal faces between the prism and the basal plane. Crystals range in size from tiny to over a foot in diameter. There is but one cleavage and that is basal. The crystal 161 form, cleavage, and hardness will easily determine this mineral. Apatite is usually associated with igneous or highly metamorphic rocks, such as granites, gneisses, and crystalline limestones. While the phosphoric acid of apatite is highly desirable for use in fertilizers, the crystals do not occur in sufficient abundance to make them commercially available, and non-crystalline phosphate rocks are resorted to for this purpose.

Crystals of apatite are found at Norwich and Bolton, Mass., Rossie and Edenville, N. Y., Suckasunny and Hurdstown, N. J., Leiperville, Penn., Wilmington, Del., etc. Templeton, Canada, is perhaps the best known locality for fine apatite.

Turquois H₅[Al(OH)₂]Cu(OH)(PO₄)₄

Occurs in seams and incrustations; hardness, 6; specific gravity, 2.7; color bluish-green; streak blue; luster waxy; translucent to opaque on thin edges.

In this country this complex phosphate of aluminum and copper is found in streaks and patches in volcanic rocks, but in Persia comes from metamorphic rocks. To the Persians it was a magical stone, protecting the wearer from injuries, and among the Pueblo Indians it was regarded as of religious value in warding off evil. The best turquois comes from Persia, but it has been found at several points in the United States, as in Los Cerrillos and Burro Mts., N. M., in Mohave Co., Ariz., San Bernardino Co., Cal., in Nevada and Colorado.

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Fluorine

At ordinary temperatures the element fluorine is a colorless gas, which was not obtained pure until 1888, because it could not be contained in vessels of glass, gold, platinum, etc. At that time it was made and kept in a vessel composed of an alloy of platinum and iridium. Its most important compound is hydrofluoric acid, a fuming liquid, which is mostly used to etch or dissolve glass. It occurs in several minerals, like tourmaline, turquois, etc., but the only one used to obtain the hydrofluoric acid is fluorite.

Fluorite CaF₂ Pl. 50 Fluor spar

Occurs in crystals and cleavable masses; hardness, 4; specific gravity, 3.2; colorless or some shade of violet, green, yellow, or rose; luster vitreous; transparent on thin edges.

Fluorite usually occurs in beautiful cubic crystals, often with the edges and corners beveled by smaller faces, and occasionally in twins, which seem to have grown through each other. There is perfect cleavage parallel to each of the octahedral faces, which often, as in the illustration on Plate 50, show as cracks cutting off the corners.

Since fluorite loses weight and color on heating, it is concluded that the colors are due to the presence of hydrocarbon compounds. The red and the green fluorite when heated to above 212° F. become phosphorescent, as may be seen if they are thus heated and exposed to the light, then taken into the dark.

Fluorite is quite commonly the gangue mineral 163 associated with metallic ores, and is also likely to occur with topaz, apatite, etc. It is generally in such places that it seems to have been deposited from hot vapors, rising from igneous magmas.

It is the only mineral at all common from which fluorine can be obtained, and is used for making hydrofluoric acid, and other chemical compounds of this element. It is, however, of much greater importance as a flux in reducing iron, silver, lead and copper ores. In the industries it finds a place, being used to make apochromatic lenses, cheap jewelry, and for the electrodes in flaming arc lamps.

Fluorite is widely distributed, some of the better known localities being Trumbull and Plymouth, Conn., Rossie and Muscalonge Lake, N. Y., Gallatin Co., Ill., Thunder Bay, Lake Superior, Missouri, etc.

Halite NaCl Pl. 50 Salt

Occurs in crystals, and in cleavable and granular masses; hardness, 2.5; specific gravity, 2.1; colorless to white; luster vitreous; transparent on thin edges.

Halite is common salt, occurring in cubic crystals, with perfect cubic cleavage. Its form, hardness, taste, and solubility in water make it easy to determine.

Halite is the most abundant salt in sea water, making about 2.5% out of the total of 3.5% of solids in solution. It is also a prominent, when not the leading, salt in solution in the waters of inland lakes, like Great Salt Lake, or the Dead Sea, there being 20% of halite in the former and 164 8% in the latter, though the total of solid in solution in the water of the Dead Sea is greater than that in Great Salt Lake.

The great salt deposits are mostly the result of the evaporation of the water of arms or isolated portions of former oceans; the salt, gypsum, etc., left by the drying sea, having been buried beneath later sediments. Other bodies of salt represent the disappearance of ancient lakes. There are also the curious “salt domes” of Louisiana and Texas, which are immense, roughly circular, subterranean masses of salt extending to as yet unknown depths which are thought to have been formed by masses of salt from some deep source bed pushing their way upward through the overlying formations by plastic flowage. As the upthrust took place the sediments were arched into domes. Some of these domes are today important sources of rock salt.

There are extensive beds of salt under parts of New York, Michigan, Ohio, Oklahoma, Kansas, etc., which are mostly worked by drilling wells into the salt layer, then introducing hot water to dissolve the salt. The brine thus formed is pumped to the surface, and the salt recovered by evaporation in pans. During the process, skeleton crystals of salt with concave faces may form, but in Nature the crystals are uniformly solid cubes.

Boracite Mg₇Cl₂B₁₆O₃₀

Occurs in small crystals or granular masses; hardness of crystals, 7; of the masses, 4.5; specific gravity 3; colorless to white; luster vitreous; transparent to translucent on thin edges.

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Small crystals, associated with salt and gypsum, occur in the beds and incrustations, which result from the drying up of alkaline lakes, especially in Nevada and southern California. The crystals are orthorhombic, but appear like perfect cubes, with the edges beveled and part of the corners cut. They are not easily dissolved in water, but quickly go into solution in hydrochloric acid.

Colemanite Ca₂B₆O₁₁ + 5H₂O

Occurs in crystals or compact masses; hardness, 4.5; specific gravity, 2.4; colorless to white; luster vitreous; translucent on thin edges.

The crystals when they occur, are monoclinic; but usually colemanite is a bedded deposit, which has resulted from the drying up of a saline lake. It was first found in Death Valley, Cal., in 1882, then near Daggett, Cal., and since then in several similar locations in Nevada and Oregon. The deposits are of all grades of purity, the colemanite being mixed with varying quantities of mud. Today this mineral is the chief source of borax, which is used in medicines, cosmetics, colored glazes, enamel, and as a preservative.

Borax NaB₄O₇ + 10H₂O

Occurs in crystals or in powdery incrustations; hardness, 2; specific gravity, 1.7; colorless to white; luster vitreous; translucent on thin edges.

The crystals are tiny and monoclinic, this mineral being usually obtained by the evaporation of the saline waters of such lakes as Clear and Borax Lakes in southern California, or from the muds of salt marshes, like Searles Borax Marsh 166 in California. Originally most of our borax came from a large saline lake in Tibet, but now most of it is obtained from colemanite. Borax is soluble in water, giving it a sweetish taste.

Sulphur S Pl. 51

Occurs in crystals, incrustations or compact masses; hardness, 2; specific gravity, 2; color yellow; streak yellow; luster resinous; translucent on thin edges.

Aside from the numerous compounds, such as the sulphides of the metals like pyrite, galena, sphalerite, etc., and the sulphates, like gypsum, barite, anglesite, etc., sulphur occurs in its elemental form in Nature. In this case it may be in crystals, which are orthorhombic and usually occur as octahedrons, with the upper and lower ends truncated, either by a basal plane, or by a lower octahedron, or by both. Incrustations and compact masses are, however, much the commoner mode of occurrence. The incrustations are found mostly about volcanic regions, where the sulphur has risen from the molten lavas as a sublimate, and on cooling has been deposited in crevices or on the adjacent surfaces. Irregular masses of sulphur are often found where sulphide minerals, like pyrite or galena have been decomposed in such a way as to leave the sulphur behind. The extensive beds of sulphur are usually associated with gypsum, and are thought to be the result of water, containing bituminous matter, so acting on gypsum as to remove the calcium and oxygen as lime, and leave the sulphur. Finally many waters carry sulphates in solution, from which the sulphur may be 167 precipitated by certain sulphur bacteria, making thus incrustations on the bottom of ponds or lakes.

Sulphur is used for making matches, gunpowder, fireworks, insecticides, in medicine, vulcanizing rubber, etc. It is widely distributed, however, most of the present world’s production is from deposits associated with the “salt domes” of Texas and Louisiana. A “caprock” of gypsum and anhydrite overlies many of these which often contains elemental sulphur. Wells are drilled into this, and the sulphur is melted by the introduction of hot steam. This melted sulphur is then pumped to the surface and run into molds.

Some of the best known localities are Sulphurdale, Utah, Cody and Thermopolis, Wyo., Santa Barbara Co., Cal., Humboldt Co., Nev., and about the hot springs of the Yellowstone Park.

Ice H₂O Pl. 51 water

Occurs solid as ice, snow and frost, or liquid as water; hardness, 2; specific gravity, .92; colorless to white; luster adamantine; transparent on thin edges.

Though we seldom think of ice, and its liquid form, water, as a mineral, still it is one, and perhaps the most important of all minerals, as well as the most common. Ice melts at 32° F. and vaporizes at 212° F., being then termed steam. Because it is so common and liquid at ordinary temperatures it acts as a solvent for a host of other minerals, and is therefore the agent by which they are transported from place to place and redeposited in veins and beds.

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Not only does water act as a transportation agent for minerals in solution, but is also the agent of erosion and weathering. Water vaporizes slowly when exposed to the air at all temperatures above freezing, and so it is slowly rising from the surface of the sea or lakes or moist ground into the air, where it would accumulate until the air was saturated, if the air would only keep still and at a uniform temperature. The air will hold a given amount of water vapor, which is, for example, 17 grams per cubic meter when the temperature is 68° F., but at 59° F. it will hold only 12½ grams, or at 50° F. only 9 grams. Thus the air is more or less completely saturated at higher temperatures, and when the temperature is lowered the air can not hold all it has taken up, and it is precipitated in dew, rain or snow, most often as rain. When the rain falls it mechanically carries away, and more or less slowly transports to other places particles of rock, being thus the agent of erosion; and when it is slowed down, as on entering the quiet water of a lake or the sea, it drops the mechanically carried sediment and makes sedimentary deposits.

Another very important and unique feature of water is that on freezing it expands about ¹/₁₁th of its former bulk, so that, as a result, ice floats, and also wherever water in crevices is frozen, the crevices are enlarged. In locations where this freezing and melting take place repeatedly throughout a year, there the breaking up of rocks is rapid.

This is hardly the place to take up a complete discussion of water, but its action as a solvent, 169 mechanically, and in freezing, melting, and vaporizing is the basis of a large part of the study of geology.

When water crystallizes, as in forming ice, it is in the hexagonal system. It tends to twinning and a snow-flake is made up of a large number of twinned crystals, each diverging from the other at 60°. When ice is formed in the air or on the surface of water it forms these complex and beautiful multiple twins, of which but a couple are suggested here. Beneath the surface the hexagonal crystals grow downward into the water, parallel to each other, making a fibrous structure, which is very apparent when ice is “rotten,” which is the time at which the surfaces of the prisms are separating, because the molecules leave the crystal in the reverse order to which they united with it. Frost in marshy or spongy ground will often show this fibrous growth beautifully.

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