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Calcite is present in solution in the water of the sea and most streams, from which it is withdrawn by many animals and some plants, to make their shells, and bones.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.