Arágonite

Arágonite, a mineral essentially consisting of carbonate of lime, and so agreeing in chemical composition with Calcareous Spar (q.v.), but differing from it in the form of its crystals, of which the primary form is a rhombic prism with angles of $116^\{\backslash circ\}\; 16\text{'}$ and $63^\{\backslash circ\}\; 44\text{'}$, the secondary forms being generally prismatic and pyramidal. The effect of heat on them shows another difference, aragonite being reduced to powder by a heat in which calcareous spar will remain unchanged. Aragonite appears to be the product of a crystallisation taking place at a higher temperature than that in which calcareous spar is produced; and accordingly it is frequent in volcanic districts and in the neighbourhood of hot springs, as at Carlsbad. It is frequently found filling cavities (see AMYGDALOID), and lining cracks, joints, and other natural fissures in such igneous rocks as basalt and its varieties, porphyrite, &c. as in Scotland. It derives its name from the province of Aragon in Spain. It sometimes occurs in stalactitic form. Its crystals are sometimes prisms shortened into tables, sometimes they are lengthened into needles. Twin crystals (maclés) are very common. Satin spar is a variety of it, in which the crystals are of a fine fibrous silky appearance, and combined together into a compact mass. Flos ferrí (i.e. flower of iron) is a name given to a coralloidal variety which sometimes occurs in iron mines.