Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

CALCITE

Online Encyclopedia
Originally appearing in Volume V04, Page 970 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

CALCITE , a See also:

mineral consisting of naturally occurring See also:calcium carbonate, CaCO3, crystallizing in the See also:rhombohedral See also:system. With the exception of See also:quartz, it is the most widely distributed of minerals, whilst in the beautiful development and extraordinary variety of See also:form of its crystals it is surpassed by none. In the massive See also:condition it occurs as large See also:rock-masses (See also:marble, See also:lime-See also:stone, See also:chalk) which are often of organic origin, being formed of the remains of molluscs, See also:corals, crinoids, &c., the hard parts of which consist largely of calcite. The name calcite (See also:Lat. calx, calcis, meaning burnt lime) is of comparatively See also:recent origin, and was first applied, in 1836, to the " barleycorn " pseudomorphs of calcium carbonate after celestite from See also:Sangerhausen in Thuringia; it was not until about 1$43 that the name was used in its See also:present sense. The mineral had, however, See also:long been known under the names calcareous spar and calc-spar, and the beautifully transparent variety called See also:Iceland-spar had been much studied. The strong See also:double See also:refraction and perfect cleavages of Iceland-spar were described in detail by See also:Erasmus See also:Bartholinus in 1669 in his See also:book Experimenta Crystalli Islandici disdiaclastici; the study of the same mineral led Christiaan See also:Huygens to discover in 1690 the See also:laws of double refraction, and E. L. See also:Malus in 1808 the polarization of See also:light. An important See also:property of calcite is the See also:great ease with which it may be cleaved in three directions; the three perfect cleavages 969 are parallel to the faces of the See also:primitive rhombohedron, and the See also:angle between them was determined by W. H. See also:Wollaston in r8i2, with the aid of his newly invented reflective See also:goniometer, to be 74 55'. The cleavage is of great help in distinguishing calcite from other minerals of similar See also:appearance.

The hardness of 3 (it is readily scratched with a See also:

knife), the specific gravity of 2.72, and the fact that it effervesces briskly in contact with See also:cold dilute acids are also characters of determinative value. Crystals of calcite are extremely varied in form, but, as a See also:rule, they may be referred to four distinct habits, namely: rhombohedral, prismatic, scalenohedral and See also:tabular. The primitive rhombohedron, r {Too} (fig. 1), is comparatively rare except in See also:combination with other forms. A flatter rhombohedron, e { I to}, is shown in fig. 2, and a more acute one, f {rif}, in fig. 3. These three rhombohedra are related in such a manner that, when in combination, the faces of r truncate the polar edges of f, and the faces of e truncate the edges of r. The crystal of prismatic See also:habit shown in fig. 4 is a combination of the See also:prism m {211} and the rhombohedron e { T lc)} ; fig. 5 is a combination of the scalenohedron v {201} and the rhombohedron r {See also:roc) ; and the crystal of tabular habit represented in fig. 6 is a combination of the basal pinacoid c 1 1, prism m i f ) , and rhombohedron e { T To}.

In these figures only six distinct forms (r, e, f, m, v, c) are Floc. 1-6.-Crystals of Calcite. represented, but more than 400 have been recorded for calcite, whilst the combinations of them are almost endless. Depending on the habits of the crystals, certain trivial names have been used, such, for example, as See also:

dog-tooth-spar for the crystals of scalenohedral habit, so See also:common in the See also:Derbyshire See also:lead mines and See also:limestone caverns; See also:nail-See also:head-spar for crystals terminated by the obtuse rhombohedron e, which are common in the lead mines of See also:Alston See also:Moor in See also:Cumberland; See also:slate-spar (See also:German Schieferspath) for crystals of tabular habit, and some-times as thin as See also:paper: See also:cannon-spar for crystals of prismatic habit terminated by the basal pinacoid c. Calcite is also remarkable for the variety and perfection of its twinned crystals. Twinned crystals, though not of infrequent occurrence, are, however, far less common than See also:simple (untwinned) crystals. No less than four well-defined twin-laws are to be distinguished: i. Twin-See also:plane c (III).—Here there is rotation of one portion with respect to the other through 18o° about the See also:principal (trigonal) See also:axis, which is perpendicular to the plane c (III); or the same result may be obtained by reflection across this plane. Fig. 7 shows a prismatic crystal (like fig. 4) twinned in this manner, and fig. 8 represents a twinned scalenohedron v {2oi}.

ii. Twin-plane e (11o).—The principal axes of the two portions are inclined at an angle of 52° 3o1'. Repeated twinning on this plane is very common, and the twin-lamellae (fig. 9) to which it gives rise are often to be observed in the grains of calcite of crystalline limestones which have been subjected to pressure. This lamellar twinning is of secondary origin; it may be readily produced artificially by pressure, for example, by pressing a knife into the edge of a cleavage rhombohedron. t 970 iii. Twin-plane r (roo).—Here the principal axes of the two portions are nearly at right angles (8q° 14'), and one of the directions of cleavage in both portions is parallel to the twin-plane. See also:

Fine crystals of prismatic habit twinned according to this See also:law were formerly found in considerable See also:numbers at Wheal Wrey in See also:Cornwall, and of scalenohedral habit at Eyam in Derbyshire and Cleator Moor in Cumberland; those from the last two localities are known as " butterfly twins " or " See also:heart-shaped twins " (fig. ro), according to their shape. iv. Twin-plane f (111).—The principal axes are here inclined at 53° 46'. This is the rarest twin-law of calcite. 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 See also:

colour may vary considerably. Crystals are often nearly See also:white or colourless, usually with a slight yellowish tinge. The yellowish colour is in most cases due to the presence of See also:iron, but in some cases it has been proved to be due to organic See also:matter (such as apocrenic See also:acid) derived from the humus overlying the rocks in which the crystals were formed. An opaque calcite of a grass-See also:green colour, occurring as large cleavage masses in central See also:India and known as hislopite, owes its colour to enclosed " green-See also:earth " (See also:glauconite and celadonite). A stalagmitic calcite of a beautiful See also:purple colour, from Reichelsdorf in See also:Hesse, is coloured by colbalt. Optically, calcite is uniaxial with negative bi-refringence, the See also:index of refraction for the See also:ordinary See also:ray being greater than for the extraordinary ray; for See also:sodium-light the former is 1.6585 and the latter 1.4862. The difference, 0.1723, between these two indices gives a measure of the bi-refringence or double refraction. Although the double refraction of some other minerals is greater than that of calcite (e.g. for See also:cinnabar it is 0.347, and for See also: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 See also:object viewed through a cleavage rhombohedron of Iceland-spar is seen double, hence the name doubly-refracting spar. Iceland-spar is extensively used in the construction of See also:Nicol's prisms for polariscopes, polarizing microscopes and saccharimeters, and of dichroscopes•for testing the pleochroism of See also:gem-stones. Chemically, calcite has the same See also:composition as the orthorhombic See also:aragonite (q.v.), these minerals being dimorphous forms of calcium carbonate.

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 See also:

magnesium, See also:barium, See also:strontium, See also:manganese, See also: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. A remarkablecase of enclosed impurities is presented by the so-called See also:Fontainebleau limestone, which consists of crystals of calcite of an acute rhombohedral form (fig. 3) enclosing 5o to 6o% of quartz-See also:sand. Similar crystals, but with the form of an acute hexagonal See also:pyramid, and enclosing 64% of sand, have recently been found in large quantity over a wide See also:area in See also:South Dakota, See also:Nebraska and See also:Wyoming. The See also:case of hislopite, which encloses up to 20% of "green earth," has been noted above. In addition to the varieties of calcite noted above, some others, depending on the See also:state of See also:aggregation of the material, are distinguished. A finely fibrous form is known as satin-spar (q.v.), a name also applied to fibrous See also:gypsum: the most typical example of this is the See also:snow-white material, often with a rosy tinge and a pronounced silky lustre, which occurs in See also:veins in the Carboniferous shales of Alston Moor in Cumberland. Finely scaly varieties with a pearly lustre are known as See also:argentine and aphrite (German Schaumspath); soft, earthy and dull white varieties as agaric mineral, rock-See also:milk, rock-See also:meal, &c.—these form a transition to marls, chalk, &c. Of the granular and compact forms numerous varieties are distinguished (see LIME-STONE and MARBLE). In the form of See also:stalactites calcite is of extremely common occurrence.

Each stalactite usually consists of an aggregate of radially arranged crystalline individuals, though sometimes it may consist of a single individual with crystal faces See also:

developed at the See also:free end. See also:Onyx-See also:marbles or See also:Oriental See also:alabaster (see ALABASTER) and other stalagmitic de-posits also consist of calcite, and so do the allied deposits of travertine, calc-See also:sinter or calc-tufa. The modes of occurrence of calcite are very varied. It is a common See also:gangue mineral in metalliferous deposits, and in the form of crystals is often associated with ores of lead, iron, See also:copper and See also:silver. It is a common product of alteration in igneous rocks, and frequently occurs as well-developed crystals in association with See also:zeolites lining the amygdaloidal cavities of basaltic and other rocks. Veins and cavities in limestones are usually lined with crystals of calcite. The wide See also:distribution, under various conditions, of crystallized calcite is readily explained by the solubility of calcium carbonate in See also:water containing See also:carbon dioxide, and the ease with which the material is again deposited in the crystallized state when the carbon dioxide is liberated by evaporation. On this also depends the formation of stalactites and calc-sinter. Localities at which beautifully crystallized specimens of calcite are found are extremely numerous. For beauty of crystals and variety of forms the haernatite mines of the Cleator Moor See also:district in See also:west Cumberland and the See also:Furness district in See also:north See also:Lancashire are unsurpassed. The lead mines of Alston in Cumberland and of Derbyshire, and the silver mines of Andreas-See also:berg in the Harz and See also:Guanajuato in See also:Mexico have yielded many fine specimens. From the zinc mines of See also:Joplin in See also:Missouri enormous crystals of See also:golden-yellow and amethystine See also:colours have been recently obtained.

At all the localities here mentioned the crystals occur with metalliferous ores. In Iceland the mode of occurrence is quite distinct, the mineral being here found in a cavity in See also:

basalt. The See also:quarry, which since the 17th See also:century has supplied the fatuous Iceland-spar, is in a cavity in basalt, the cavity itself measuring 12 by 5 yds. in area and about ro ft. in height. It is situated quite See also:close to the See also:farm Helgustadir, about an See also:hour's ride from the trading station of Eskifjordur on Reydar Fjorc#ur, on the See also:east See also:coast of Iceland. This cavity when first found was filled with pure crystallized masses and enormous crystals. The crystals measure up to a yard across, and are rhombohedral or scalenohedral in habit; their faces are usually dull and corroded or coated with stilbite. In recent years much of the material taken out has not been of sufficient transparency for See also:optical purposes, and this, together with the very limited See also:supply, has caused a considerable rise in See also:price. Only very occasionally has calcite from any locality other than Iceland been used for the construction of a Nicol's prism. (L. J.

End of Article: CALCITE

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
CALCHAS 		[next]
CALCIUM [symbol Ca, atomic weight 40.0 (o= x6)]

Site © 2006 - Net Industries