Calcite

“ CALCITE, a mineral consisting of naturally occurring calcium carbonate, CaCO3, crystallizing in the rhombohedral system. With the exception of quartz, it is the most widely distributed of minerals, whilst in the beautiful development and extraordinary variety of form of its crystals it is surpassed by none. In the massive condition it occurs as large rock-masses (marble, limestone, chalk) which are often of organic origin, being formed of the remains of molluscs, corals, crinoids, &c., the hard parts of which consist largely of calcite. ”

“ Although the double refraction of some other minerals is greater than that of calcite (e.g. for cinnabar it is 0.347, and for calomel 0.683) , yet this phenomenon can be best demonstrated in calcite, since it is a mineral obtainable in large pieces of perfect transparency. Owing to the strong double refraction and the consequent wide separation of the two polarized rays of light traversing the crystal, an object viewed through a cleavage rhombohedron of Iceland-spar is seen double, hence the name doubly-refracting spar. ”

“ Calcium is the element that is the foundation of limestones. The skeletons and shells of animals are made of calcite, a common mineral formed by the uniting of carbon, oxygen, and calcium. Marbles are, perhaps, the most permanent form of the limestone rocks. "Hard" water has filtered through rocks containing calcite, and absorbed particles of this mineral. From water thus impregnated, all animal life on the earth obtains its bone-building and shell-building materials. ”

“ Calcium is one of the commonest and most important elements of the earth’s crust, of which it has been estimated to compose about one-sixteenth. It does not occur free in nature, but its most common form is in combination with carbon dioxid, forming the mineral calcite, marble, and the very abundant limestone rocks. ”

“ The calcspar is extremely abundant at Bergen Hill, where it might be mistaken for many of the other minerals which I describe as occurring there, and even in preference to them, to one's great chagrin upon arriving home and testing it, to find that it is nothing but calcite. In order to avoid this and distinguish this mineral on the field, it should be tested with a single drop of acid, which on coming in contact with it bubbles up or effervesces like soda water, seidlitz powder, etc., while it does not do so with any of the minerals occurring in the same locality. ”

“ The vast majority of the rocks which we see around us every day consist of quartz, felspar, mica, chlorite, kaolin, calcite, epidote, olivine, augite, hornblende, magnetite, haematite, limonite and a few other minerals. Each of these has a recognized position in the economy of nature. A main determining factor is the chemical composition of the mass, for a certain mineral can be formed only when the necessary elements are present in the rock. Calcite is commonest in limestones, as these consist essentially of carbonate of lime ”

“ When the hot springs bring up particles of sand, the lime substance incrusts these sand grains, forming globular masses resembling peas, and consequently named pisolite.
In chemical composition, calcite and arragonite consist of a combination of calcium oxide (lime) and carbonic acid, the formula being expressed by CaCO3. Calcium carbonate is insoluble in pure water, but dissolves somewhat freely in water charged with free carbonic acid. It is assumed that a compound is formed, which is known as calcium bi- (or acid) carbonate, is very unstable and can only exist in a state of solution. ”

“ In calcite there is a triad axis of symmetry parallel to the prism edges, three dyad axes each perpendicular to a pair of prism edges and three planes of symmetry perpendicular to the prism faces; the etched figures shown in fig. 93 will be seen to conform to all these elements of symmetry. There being in calcite also a centre of symmetry, the equilateral triangles on the basal plane at the lower end of the crystal will be the same in form as those at the top, but they will occupy a reversed position. ”

“ If a mineral scratches calcite and is scratched by fluorite, we say its hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is scratched by orthoclase, its hardness is 6; and so on. There are very few minerals harder than quartz, and hence the first seven members of the scale are sufficient for all ordinary purposes ”

“ Well-crystallized material, such as Iceland-spar, usually consists of perfectly pure calcium carbonate, but at other times the calcium may be isomorphously replaced by small amounts of magnesium, barium, strontium, manganese, zinc or lead. When the elements named are present in large amount we have the varieties dolomitic calcite, baricalcite, strontianocalcite, ferrocalcite, manganocalcite, zincocalcite and plumbocalcite, respectively.
Mechanically enclosed impurities are also frequently present, and it is to these that the colour is often due. ”

“ All the counterfeits analysed from Vietnam contain calcite. The C and O stable isotope ratios of the calcite are consistent with a geologically high temperature intrusive or volcanic origin and not with a marine or terrestrial limestone or tap water precipitation origin (except one, type 12) . The presence of a small amount of aragonite could indicate marine limestone origin but heavy industrial crushing will convert non-marine calcite to aragonite, which is also found in open fissures in volcanic rocks. ”

“ It is fairly light weight and effervesces freely in cold dilute hydrochloric acid.
Calcite has a variety of crystal forms but in Illinois flattened block-shaped crystals and elongate crystals with tapering points (“dogtooth spar”) are the most common. When broken, calcite cleaves into six-sided blocks called rhombs.
Crystals of calcite are found in Illinois as linings in geodes in certain limestones and shales, especially in the Nauvoo-Hamilton-Warsaw area, and as crystalline masses in limestone and dolomite. ”

“ Other kinds of satin-spar consist of calcium carbonate, in the form of either aragonite or calcite, these being distinguished from the fibrous gypsum by greater hardness, and from each other by specific gravity and optical characters. The satin-spar of Alston, Cumberland, is a finely-fibrous calcite occurring in veins in a black shale of the Carboniferous series. Fibrous calcite is known sometimes to German mineralogists as Atlasspath. ”

“ 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. ”

“ Sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure. Where the original mass was decomposed and contained calcite, zeolites, chlorite and other secondary minerals either in veins or in cavities, there are usually rounded areas or irregular streaks containing a suite of new minerals, which may resemble those of the calc silicate hornfelses above described. ”

“ Aragonite, like ordinary limestone, is calcium carbonate, but its atoms seem to be differently arranged, so as to make it a less stable compound, and it has a different crystalline form. Some calcareous organisms are composed of aragonite, others of ordinary calcite. ”

“ This family is common throughout SE Asia but, except for the north east coastal plains of China, dies out in southern China north of the borders with Myanmar (Burma) , Vietnam, and Laos [27] . Three of the 11 samples containing calcite have little or no spore/pollen contamination suggesting that the contamination does not come from the calcite or its source. In terms of the different types of fake hologram/sticker, type 2 appears different from the others with no smooth trilete fern spores; type 1 contained little material but appears similar to type 3 ”

“ In the first are found veins of calcite and quartz from half-an-inch to three inches thick, in which especially in the calcite the gold is visible mixed with iron and copper pyrites, galena and blende. It is a remarkable circumstance that the most mineralized veins run always in an east and west direction, whilst the poor and sterile veins always follow another direction. The workings are entirely on the surface, as the abundance of water which flows to them prevents sinking shafts, and nothing is known of the richness at depth. ”

“ René Just Haüy in his Essai d’une théorie sur la structure des crystaux (Paris, 1784; see also his Treatises on Mineralogy and Crystallography, 1801, 1822) supported and extended these views, but took for his primitive forms the figures obtained by splitting crystals in their directions of easy fracture of “cleavage,” which are aways the same in the same kind of substance. Thus he found that all crystals of calcite, whatever their external form (see, for example, figs. 1-6 in the article Calcite) , could be reduced by cleavage to a rhombohedron with interfacial angles of 75°. ”

“ In some isomorphous groups of minerals replacement takes place to only a limited extent, and the element which is partly replaced always predominates; while in other groups the replacement may be indefinite in extent, and between the ends of the series the different members may vary indefinitely in composition, with no sharp demarcation between species. Thus in the group of rhombohedral carbonates the different species are usually sharply defined. In well-formed crystals of calcite the calcium is replaced by only small amounts of magnesium, iron, lead, &c. ”

“ Apatite—essentially calcium phosphate with chlorine or fluorine; hexagonal; H. 5; luster vitreous or subresinous; colors usually greenish to bluish, characterized by a hexagonal form.
Aragonite—a calcium carbonate; differs from calcite in cleavage, and in being orthorhombic; H. 3.5–4; luster vitreous or resinous; white, also gray, yellow, green, and violet.
Asphaltum—asphalt; mineral pitch, bitumen; a natural mixture of different hydrocarbons; odor bituminous; melts at 90 to 100 degrees C.; burns with a bright flame; graduates into mineral tars and through these into petroleum ”

“ Calcite.--This mineral occurs in great abundance in and about the tunnel, and from all the shafts. There are two forms occurring there, the most abundant of which is the rhombohedral, after Fig. 3. It can generally be obtained, however, in excellent crystals, which, although perfect in form, are opaque, but often large and beautiful. It is always packed with a thousand or its multiple of other crystals into veins of a few inches thick; and crystals are obtained by carefully breaking with edge of the cold chisel these masses down to the fundamental form shown. ”

“ Calcite when pure, as in the well-known Iceland-spar, is perfectly transparent and colourless. The lustre is vitreous. Owing to the presence of various impurities, the transparency and colour may vary considerably. Crystals are often nearly white or colourless, usually with a slight yellowish tinge. The yellowish colour is in most cases due to the presence of iron, but in some cases it has been proved to be due to organic matter (such as apocrenic acid) derived from the humus overlying the rocks in which the crystals were formed. ”

“ The next three cases (43-45) are chiefly devoted to the various crystallisations of calcite, including that generally known as the Fontainbleau crystallised sandstone, and the stalactic and fibrous varieties from Africa, Sweden, and Cumberland; while the two cases marked 45 A and B are covered with polished samples, known to people generally as marbles, including the beautiful fire marble. The forty-sixth case is also covered with calcites, including the reastone, the limestone incrusted upon a human skull, found in the Tiber at Rome. ”

“ Glassy luster; water-white, milky, honey-colored, pink, gray; may occur in six-sided crystals, sometimes pyramid-shaped; always breaks with flat glistening faces; always reacts in the lump with cold dilute muriatic (hydrochloric) acid. Calcite 16 13. Like calcite above but may have a pink, pearly luster and curved crystal faces; reacts with cold dilute acid when powdered but not readily in lump form. Dolomite 18 14. Opaque white, glassy or bluish, very heavy, lustrous on freshly broken surface; does not react with acid. Barite 61 15. ”

“ As the masses are never secured by the miners, they can always be picked from the piles of débris around the shafts and the dumps, and afford some little instruction as to the manner in which a mineral is built up by crystallization, and may be subdivided by cleavage to a crystal of the same shape exactly, but infinitesimally small. A crystal to be worth preserving should be about an inch in diameter, and as transparent as is attainable.
Another form of calcite which is to be sparingly found is what is called dogtooth spar, having the form shown in Fig. ”

“ A great variety of minerals occur in the form of concretions, but this mode of occurrence is especially characteristic of certain constituents of rocks, such as calcite, siderite, limonite, hematite, and quartz. Concretions may be classified according to the nature of the segregating minerals; and in each class we may distinguish the pure from the impure concretions. A pure concretion is one entirely composed of the segregating mineral. Most nodules of flint and chert, quartz, geodes, concretions of pyrite, and many hollow iron-balls are good illustrations of this class. ”