ARRAGONITE, Prismatic Carbonate of Lime, called by Mohs tho Prismatic Lime-Haloide. It is a mineral substance consisting of carbonfc acid and lime, admitting of cleavage in planes parallel to the faces of a right rhombic prism of 116° 5' and 63° 55', which may therefore be considered as its fundamental form (fig. 1). The most general modifications which occur consist either in the removal of the four acute angles at A by planes a intersecting each other in the short diagonal B B, and inclined to each other at an angle of 108° 18', by which the face P being entirely removed, the form of fig. 2 is pro duced ; or the change may be effected by the truncation of the acute lateral edges of the prism by planes parallel to the axis of the crystal, and therefore inclined to the faces L, at 121° 57', giving rise to the form seen in fig. 3. These modified forms usually present themselves in twin crystals, in which the short diagonals of the prism B B are placed at right angles to one another, when only two crystals are present, thus producing a very simple cross. It is usual however that three of the crystals of fig. 3 cross each other, producing a crystal of the appearance of fig. 4, which at first sight may be mistaken for an hexagonal prism, but on a closer inspection it will be found that what appeared to be a single face is really composed of two planes, making a re-entrant angle.
The intersections of the individual crystals with each other are visible both in the lateral and terminal faces, and are indicated in fig. 4 by the dotted lines. These crystals have been found abundantly in a ferruginous clay in Aragon in Spain, where they occur accom panied by a sulphate of lime. From this circumstance the mineral has derived its name. It has also been found very beautifully crystal lised in a vein of a massive variety of the same mineral traversing basalt at Bilin in Bohemia. (Mohs.) Fine specimens have been found at the following places in England :—in the Dufton lead-mines ; in a cavern of Grauwacke, near Merridge, Somersetahire ; and also in several parts of Devonshire, &c. In an old coal-mine six miles south
and as it will only thrive in tho colder districts, it was onto expected to form an important agricultural plant in Europe. It has however been found upon trial unable to accommodate itself to our uncertain climate, and to perish as soon as the cold nights and damp weather of autumn approach, without having been able during the summer to west of Cockfield, Durham, it is remarkable as occurring depending from a roof of clay-slate, and accompanied by tubular calcareous stalactites. (Phillips.) Varieties of this mineral are also common in beds of iron-ore in the mines of Eisenerz in Styria, and in several other iron-mines of Hungary, of Transylvania, &c., consisting of numerous fibrous crystals, of a satin-like lustre, radiating from a centre, and to these the name of Floe Ferri has been applied.
In a chemical and crystallographical point of view Arragonite is peculiarly interesting, as presenting to us carbonate of lime differing in its system of crystallisation from that of the common Calc-Spar, and thus affording us an instance of the influence of any difference in the aggregation of matter in changing its physical properties, as will be seen by comparing this substance with the rhombohedral Calc-Spar, with which it agrees in chemical constitution. In the scale of Molls its hardness varies from to 4, while that of Cale-Spar is 3. The specific gravity of Arragonite is 2'931 Cale-Spar 2'721 They act also differently on light, the index of ordinary refraction of Arragonite being . . . . . Cale-Spar . . . . . . 1.519 Attempts have been made to account for these differences by con sidering them the effects of small quantities of carbonato of strontia. which Professor Stromeyer first discovered to be contained in many specimens of Arragonite ; but the conclusion is unfounded, as will be seen by the results of two analyses given by Strorncyer :— First. Second.