Calcium
Calcium is one of the most abundant of metals, but never occurs as such in nature, nor is it used as a metal by man. In its metallic form it is yellowish-white, and intermediate between lead and gold in hardness. Exposed to air it soon tarnishes by oxidation, and in water rapidly decomposes the water, forming the oxide. However, it has a great affinity for other elements, and makes a large number of minerals, the most important of which are calcite, aragonite, gypsum and fluorite, while it is an essential component of some garnets, anorthite, epidote, amphibole and pyroxene. It is very widely distributed as limestone, and is found in solution in most all natural waters, and in the shells and bones of many animals and some plants.
[Calcite]
CaCO₃
[Pl. 45]
Occurs in well defined crystals in incrustations, and in stalactitic, oolitic, and granular masses; hardness, 3; specific gravity 2.7; colorless to white, or when impure, yellow, brown, green, red or blue; luster vitreous to dull; transparent on thin edges.
Next to quartz, calcite is the most abundant of all minerals, and occurs in an almost endless variety of forms, over 300 having been described. It belongs to the hemihedral section of the hexagonal system, the form of the crystals being all sorts of variations of the rhombohedron, and combinations of left and right handed rhombohedrons. The cleavage is entirely uniform, in three directions, parallel to the faces of the rhombohedron, and at an angle of 74° 55′ with each other. Crystals may occur in the form characteristic of the cleavage, but not often. The commonest forms are a more or less elongated scalenohedron, made by combining right and left handed rhombohedrons, so that the resulting pyramid is six-sided, as in figure C, [Plate 45]. Such a scalenohedron may be combined with other forms in a great variety of ways. The six-sided prism with the ends terminated by one or more sets of rhombohedral faces is also fairly common. Twinning occurs occasionally.
The quickest way to determine calcite is by the hardness (3), combined with the fact that it effervesces, when hydrochloric acid is dropped upon it.
An interesting feature of this mineral is its marked property of deflecting light rays, so that a line or object placed behind a piece of clear calcite appears double. It was with pieces of calcite from Iceland that this was first seen; so that large transparent crystals of calcite are still called Iceland spar; and such calcite is used to make the Nichol’s prisms for microscopes, which are so useful in the study of minerals. This power of refracting light is present in all minerals, but not to such a marked degree as in calcite. The elongated scalenohedrons of calcite are often called “dog-toothed spar” from a fancied resemblance between them and the dog’s tooth.
Calcite is present in solution in the water of the sea and most streams, from which it is withdrawn by many animals and some plants, to make their shells, and bones. The foraminifera, some sponges, the echinoderms, corals and molluscs all draw large quantities from the water in which they live, and build more or less permanent structures from it. These shells when they fall to the bottom, or after being broken to bits, accumulate on the bottom and make limestone, which is widely distributed over the country. This same limestone, when metamorphosed and crystalline, is marble.
Calcite then is readily soluble in water, and streams flowing along crevices and fissures in limestone dissolve out great cavities or caves, like the Mammoth Cave of Kentucky. Other water, percolating through the limestone, comes to these cavities saturated with lime in solution and drips from the roofs and walls; then as part of the water evaporates, it deposits part of its lime in icicle-like masses, hanging from the roof. Such masses of non-crystalline calcite are called stalactites. Below on the floor of the cave, conical masses are built up in the same manner where the dripping water falls on the floor. These are stalagmites. In these limestone caves and in smaller cavities many of the most beautiful crystals grow. Somewhat similarly, when hot water from deep springs comes to the surface, it cools and can not carry as much lime, and so around the spring is laid down layer after layer of non-crystalline calcite making a mass known as travertine. Sometimes this is colored by iron or other impurities and a banded effect results. Such travertine as the “Suisun marble” from California, “California onyx,” “Mexican onyx,” and “satin spar” all belong to this class.
The coral animals, especially in tropical waters precipitate an enormous amount of lime, until whole reefs are built of lime in this non-crystalline form. In places it is hundreds of feet thick and hundreds of miles in extent. Some of this coral has become popular for personal adornment. This is particularly a small, fine-grained variety, Corallum rubrum, which lives almost exclusively in the Mediterranean Sea. This coral is red in color, varying all the way from a deep red to white. It grows in small masses, three pounds being a good sized mass, in water 60 to 100 feet deep, requires some ten years to develop a full-sized mass. The making of this into beads and ornaments is an Italian industry. The demand is growing, while at the same time the supply is diminishing, and search is being widely made for more such coral, but up to the present time with little success. This precious coral is much worn as a protection against the “evil eye” and is widely imitated, apparently with as much protection to the wearer. When coral beads are offered cheap, they are probably something else, red gypsum being much used. This and all imitations can be readily detected by trying a drop of acid in the bead. Coral will effervesce, but gypsum and other substitutes will not.
The bulk of the shells of most molluscs is made of lime, but the mother-of-pearl layer inside is usually aragonite. The chalk of the cliffs on either side of the English channel is lime, and composed of the shells of single celled animals. See [p. 213]. When lime is deposited in loose porous masses, as around grass, etc., and below hot springs, this mass is termed calcareous tufa.
Calcite will be found almost everywhere, some of the localities for the finest crystals being Antwerp and Lockport, N. Y., Middletown, Conn., the caves of Kentucky, Warsaw, Ill., Joplin, Mo., Hazel Green, Wis., etc.
[Aragonite]
CaCO₃
[Pl. 46]
Occurs in crystals, in columnar or fibrous masses, or incrustations; hardness, 3.5; specific gravity, 2.9; colorless, white or amber; luster vitreous; transparent on thin edges.
Aragonite has the same chemical composition as calcite, but it crystallizes in the orthorhombic system, either in simple forms like A on [Plate 46], or twinned, so as to make forms which seem hexagonal. When in simple crystals its form easily distinguishes it from calcite and dolomite, but when twinned it appears much like either of these two minerals. From calcite it can then be distinguished by its greater hardness and the fact that it has cleavage in one direction only, and that imperfect. The cleavage is the only easy method of distinguishing it from dolomite. However, aragonite is most always easily distinguished by its habits, for it generally forms long slender crystals, which appear more like fibers than crystals. Neither calcite nor dolomite is at all fibrous.
Aragonite is much less abundant than calcite, and has resulted, either from deposition from hot waters, or from waters having sulphates in solution as well as lime. Much of the travertine, and many stalagmites and stalactites are composed of aragonites, forming as outlined under calcite. The mother-of-pearl layer in the shells of bivalves is generally aragonite. The pearly luster of this layer is due to its being formed by the successive deposition of one thin layer upon another; so that light falling on the mother-of-pearl, penetrates, part of it to one layer and part to another, and is then reflected. Certain molluscs have this layer composed of especially thin layers, one, the Unios or freshwater clams, the other, the “pearl oysters” or Aviculidæ, these latter, however, being only distantly related to the edible oysters. In the cases, where molluscs of either of these two families are of large size, large pieces of mother-of-pearl can be recovered, and are used for buttons, handles, and various ornamental objects. A further peculiarity of these same molluscs is the formation of pearls in the sheet of flesh, lining the shells. The pearls are round or rounded concretions of aragonite. At the center there is a grain of sand, or more often a tiny dead parasite. Either was an irritant to the mollusc, and to be rid of it, a layer of aragonite was secreted around it. Then as the mollusc continued to grow and secrete layers for its shell, it also added each time another layer around the sand-grain or parasite, until in time a pearl of noticeable, and then of considerable size resulted. These have all the pearly luster of the mother-of-pearl in a sphere which tends to make the luster even more marked.
Pearls were in use as ornaments in China some twenty-three centuries before Christ, and in India over 500 B.C. They were very highly prized by the Romans and since their times the rulers of India have shown a remarkable fondness for them. Today the finest come from the Gulf of Persia and the Red Sea, while still others are found about Australia and in the Caribbean Sea. In the United States not a few are collected every year from the fresh water clams, some of them beautifully tinted with pink or yellow.
Aragonite is found widely, as at Haddam, Conn., Edenville, N. Y., Hoboken, N. J., New Garden, Penn., Warsaw, Ill., etc.
[Anhydrite]
CaSO₄
[Pl. 46]
Occurs in cleavable or granular masses, rarely in crystals; hardness, 3-3.5; specific gravity, 2.9; color white, gray, bluish or reddish; luster pearly on cleavage faces; transparent on thin edges.
When anhydrite occurs in crystals, they are orthorhombic, like the diagram on [Plate 46]. Usually, however, it is found in beds or layers, which were deposited by the evaporation of sea water, and so it is associated with salt. Anhydrite has three cleavage planes which are at right angles to one another, which produce rectangular or cube-like forms. Mostly anhydrite is associated with gypsum, from which it differs by its greater hardness, pseudo-cubic cleavage, and the fact that anhydrite is not readily soluble in acid, while gypsum is. Chemically it differs from gypsum in not having water of crystallization, which gypsum does have. The anhydrite is likely to occur as veins and irregular masses in beds of gypsum. Calcium sulphate is precipitated from sea water when 37% of the water has been evaporated, and it may be deposited either as anhydrite or as gypsum, the factors, which decide as to which of these two minerals it will be, being as yet unknown. After deposition, if exposed to moisture, the anhydrite may change to gypsum, irregular masses often remaining unchanged.
It is found in salt mines in Elsworth Co., Kan., in limestone cavities at Lockport, N. Y., in veins in Shasta Co., Calif., etc.
[Gypsum]
CaSO₄ + 2H₂O
[Pl. 47]
Occurs in crystals, in cleavable masses, or in fibrous masses; hardness, 2; specific gravity, 2.3; colorless, white, amber, gray, or pink; luster vitreous, silky or pearly; transparent on thin edges.
Gypsum crystals are monoclinic as seen on [Plate 47], the perfect ones usually occurring in clay, as at Oxford, O., or in cavities; while crystals of less perfect outline, but with fine cleavages, are found in Utah, Kansas, and Colorado. The cleavage is very perfect in one direction, making it possible to strip off thin sheets almost like mica, and less perfect in two other directions, which appear on the smooth surface of the first cleavage as lines intersecting at 66° 14′. Twinning is also common in such a way, that the two united crystals make forms similar to arrowheads. These cleavages and the twinning show nicely in the photograph of gypsum on [Plate 47].
Gypsum is distinguished from anhydrite by its lesser hardness, its cleavage and by being soluble in acids.
Most gypsum occurs in beds or granular masses which were deposited from evaporating sea-water, coming down when 37% of the water was lost. Such beds are often very extensive and are quarried as a source of gypsum to make plaster of Paris, stucco, neat plaster, Keene’s cement, plaster and wall board, partition tiles, etc. The use of the gypsum for plaster of Paris and all these other uses is based on its affinity for water of crystallization. The gypsum is first heated to about 400° C., which drives off the water of crystallization, and causes it to crumble to a powder, which is the plaster of Paris. When water is added, it is taken up and the powder “sets,” or recrystallizes back to gypsum. This simple reaction has made it very useful, for making moulds, casts, hard finish on walls, as stucco, etc.
When the granular type of gypsum is fine grained, it is known as alabaster, which is used for carving vases, statuettes, ornaments, etc. The fibrous variety is called satin spar, and is sometimes used for cheap jewelry and ornaments, but it is very soft and quickly wears out. At Niagara Falls there is a considerable trade in objects carved from this satin spar, tourists buying them on the assumption that the mineral is native and comes from under the falls. Most of it, however, comes from Wales, the small amount of gypsum of that region being mostly granular.
Gypsum is found all across the United States, as in New York, Michigan, Virginia, Ohio, Alabama, South Dakota, Wyoming, Colorado, Utah, California, etc.