Primitive rhombohedron (r), with acute form (r½).
Obtuse rhomtrahedron (r½), ending in prism (g).
Equal six-sided prism (a), ending in regular (r).
Obtuse rhombohedron.
Fig. 461.—Crystals of Carbonate of Lime.
Carbonate of lime, not content with one system of crystals, makes its appearance in two. It is therefore divided into two minerals—namely, calcareous spar and arragonite. In the former it possesses various forms, as will be observed in the accompanying diagrams. It is a very important mineral, as will be readily acknowledged; it enters largely into the composition of all shells and bones. The minute shells, deposited by millions at the bottom of the sea, have combined to raise our chalk cliffs. Carbonate of lime is a constituent of water, as the deposits at the bottoms of kettles, upon the sides and bottoms of water-bottles, and the stalactites all testify. A little good vinegar will quickly dissolve this deposit. Calc-spar is crystallized, and the Iceland spar is celebrated. Marble, which is another form of carbonate of lime, is white, hard, and granular. It is sometimes varied, but the pure white is the most valuable. Chalk, we know well, is soft, and is useful for writing. We have also aphrite, schiefer spar,—compact limestone in various forms,—and finally, arragonite, called from the place of its nativity, Arragon,—a colourless and somewhat transparent vitreous crystal.
Barytes. The sulphate of baryta is known as heavy spar; the crystals are of tabular forms, but numerous modifications exist. One of the forms is represented in the margin.