The most important for the colour-maker, however, is the variety known as chalk. This is really a fossil product, i. e. it consists of the microscopic shells of marine animals united into solid masses. Despite the smallness of these animals, their epoch lasted long enough for their shells to form entire mountains which are encountered all over the world. A large part of the coast of England, the island of Rügen, and many other localities, consist entirely of chalk.
In many cases, chalk is found interspersed with nodular masses of flint, and in some places it also contains great quantities of the remains of other marine animals, such as sea urchins, the spines of which occur in such numbers in certain kinds of chalk as to unfit them entirely for use as a pigment.
The foregoing varieties of calc spar are the most important, and also occur in large quantities; but, to complete the tale, it is necessary to mention also a few others which, however, are only found in small amounts. To these belong, for example, anthracolite, a limestone stained quite black by coal; the oolithic limestones or roe stones, which are composed of granules resembling fish roe; muschelkalk, which is also of fossil character and is almost entirely composed of mussel shells cemented together with lime; the marls, which consist of calc spar mixed with varying quantities of clay and consequently often bear a great resemblance to loam in their properties. A few of these varieties find extensive employment for certain purposes, some marls for instance being used for making hydraulic lime, whilst all modifications of calc spar that are sufficiently pure can be burned for quick lime.
It has already been stated that the mineral arragonite is identical, chemically, with calc spar, since both consist of calcium carbonate, but differ in their crystalline habit. Thus, whereas the crystals of calc spar belong to the rhombohedral or hexagonal system, those of arragonite are always rhombic. This occurrence of one and the same substance in two different crystalline forms is known as dimorphism, and calcium carbonate is therefore dimorphous. Whether calcium carbonate assumes the form of calcite or arragonite depends entirely on physical causes. When the deposition of the carbonate takes place from a cold solution the shape of the crystals is always one belonging to the hexagonal or rhombohedral system; but when it is from hot solution, rhombic crystals are invariably formed, calc spar resulting in the former case and arragonite in the latter.
These different methods of formation which can be carried out in the laboratory by producing the requisite conditions, occur on the large scale in many parts of the world. Wherever a hot spring comes to the surface, containing considerable amounts of lime in solution, this separates out in the form of arragonite, which received its name from the circumstance that specially handsome crystals of this mineral are found in Arragon.
One of the best-known places where the formation of arragonite can be observed at the present time is Carlsbad in Bohemia. The hot springs there deposit a very large amount of lime, which is stained more or less yellow or red by the presence of varying quantities of iron oxide, and, under the name of “sprudelstein” is used for producing various works of art. 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. When a solution of calcium bicarbonate—which can be prepared by passing carbonic acid gas through water containing finely divided calcium carbonate in suspension—is exposed for some time to the air, it soon becomes cloudy, and a deposit of calcium carbonate settles down at the bottom of the vessel, because, in the air the dissolved calcium bicarbonate is decomposed into free carbonic acid gas and calcium carbonate, which latter, as has been mentioned, is quite insoluble in water. It has already been stated that this phenomenon goes on in Nature in the formation of stalactites, lime sinter and calcareous tuff.
Calcium carbonate is readily soluble in acids, the contained carbonic acid being liberated (as carbon dioxide) with effervescence. When such acids are employed for solution as form readily soluble salts with lime, such as hydrochloric, nitric, acetic, etc. acids, a perfectly clear solution is obtained; but if sulphuric acid is used, a white pulpy mass is formed, consisting of calcium sulphate, or gypsum, which, owing to its low solubility, separates out as small crystals. Any sandy residue left when calcium carbonate is dissolved, mostly consists of quartz sand. In dissolving dark-coloured limestones, grey, or even black, flakes are left, which consist of organic material very high in carbon. On limestone being subjected to fairly strong calcination, all the carbonic acid is expelled, leaving behind the so-called quick or burnt lime, which is, chemically, calcium oxide:—
| CaCO3 | = | CaO | + | CO2 |
| Calcium carbonate | = | Quick lime | + | Carbon dioxide |
If burnt lime be left exposed to the air for some time, it again gradually absorbs carbon dioxide and is reconverted into calcium carbonate. When burnt lime is sprinkled with water it takes up the latter avidly, becoming very hot and finally crumbling down to a very friable white powder, consisting of slaked or hydrated lime (calcium hydroxide, Ca(OH)2). The considerable rise of temperature in quenching the lime is due to the chemical combination of the calcium oxide and water.