CALCAREOUS SPAR. Under this term it is usual to include only those varieties of Carbonate of Lime which occur in distinct individual crystals of the rhombohedral system, the name never being used to denote Arragonite, or any crystals of carbonate of lime belonging to the prismatic system ; nor is it usual to apply it to those more or less crystalline limestones of which marble is the purest variety, where each crystal is so embedded in the mass as to have lust all individuality. In a word, these rocks are of such importance and intermt that they do not admit of our treating them as a minera logical variety, but as masses formed by the aggregation of numerous crystals of it. These are noticed therefore under the heads Li IIPSTON E and ManntE, while we shall hero confine ourselves to the individual crystals of which the others are composed.
This substance presents, us with one of the most interesting objects which can engage the attention of the mineralogist, not only on account of the important part it plays in the geological structure of the earth, being frequently almost the solo ingredient of beds of rock of great thickness and extent, produced at every geological epoch, but al.:o from the beauty and diversity of its crystalline forms, and from the peculiarity of several of its physical properties. Tho study and a correct knowledge of this mineral species have also become of still greater importance since the discovery of the principles of isomorphism, by which it is shown that it is the most perfectly developed individual of a very largo class of the mineral salts of carbonic acid, of which it may consequently be considered the type.
If any crystal of calcspar, whatever its form, be carefully examined, an appearance indicating a tendency in its substance to break or split in the direction of three planes symmetrically related to the form may be perce:ved, and by a gentle blow the whole is readily reduced to fragments, each of which may with a little care be brought to the form of the rhombohodron represented in fig. 2, the faces of which are parallel to the three planes of cleavage above mentioned.
in the language of Hatiy, is the primitive form of calcspar, and represents, according to his theory, the shape of the ultimate mole cules or atoms of carbonate of lime, by the aggregation of which, according to certain laws, its various crystals rve produced. Although this rhombohedron occurs rarely or never as an unbroken crystal of pure carbonate of lime, it is nevertheless the most convenient ground form, to the axis of which the faces of ell other crystals of this substance may be referred, and it is therefore selected fur that purpose. These forms, although far exceeding in number those
observed in any other mineral species, are however (omitting the regular hexagonal prism, c, and its terminal faces, o, fig. 6) but of two kinds, being either rhombobedi-ons, of which varieties are repre sented in figs. 1, 2, 3, and 4, or scalinoliedrons, one of the most common of which is seen in fly 5. Their relations to each other and their combinations have been developed principally by Haiiy, I3ournou, and Monteiro, by whom no less than 30 different iiom bohedrons and 50 scalinohedrons have been distinguished. As might naturally be expected, the combinations resulting from so large a number of simple forms are exceedingly great, and Boumou, who has written a treatise of three thick volumes on this mineral and Arra gonite, has distinguished no less than 700 varieties of form. Of these 154 are described in the large work by Hedy, accompanied by very accurate drawings of each.
• A general knowledge of the crystalline form of this mineral may however be easily obtained by acquiring a knowledge of the relation of the faces of the five simple forms and the hexagonal prism referred to above ; as in almost all the more ordinary combinations the general feature of the crystal is produced by one of these. The fig. 2, which, as has been already stated, is considered as the ground-form, is a rhombohedron, the faces of which are inclined to each other in the terminal edges at 105° 5'. This form, though exceedingly rare in pure calcspar, is however the prevailing crystal in the nearly allied species produced by the combinations of the carbonates of lime and magnesia, as will be seen by referring to the article DOLOMITE. In deter mining the relations of any form, the position of the planes of this rhombohedron in reference to the other parts must first be fixed, and this is readily accomplished in every case, owing to cleavage-planes running parallel to its faces. This being determined, all other rhom bohedrons are at once divided into rhombohedrons of the first order, such as fig. 4, which have their faces situated as the faces of the ground-form, or into rhombohedrons of the second order, the faces of which are situated as the edges of the ground-form, as is the case with figs. 1 and 3.