Occurs in crystals, in columnar or fibrous masses, or incrustations; hardness, 3.5; specific gravity, 2.9; colorless, white or amber; luster vitreous; transparent on thin edges.
Aragonite has the same chemical composition as calcite, but it crystallizes in the orthorhombic system, either in simple forms like A on [Plate 46], or twinned, so as to make forms which seem hexagonal. When in simple crystals its form easily distinguishes it from calcite and dolomite, but when twinned it appears much like either of these two minerals. From calcite it can then be distinguished by its greater hardness and the fact that it has cleavage in one direction only, and that imperfect. The cleavage is the only easy method of distinguishing it from dolomite. However, aragonite is most always easily distinguished by its habits, for it generally forms long slender crystals, which appear more like fibers than crystals. Neither calcite nor dolomite is at all fibrous.
Aragonite is much less abundant than calcite, and has resulted, either from deposition from hot waters, or from waters having sulphates in solution as well as lime. Much of the travertine, and many stalagmites and stalactites are composed of aragonites, forming as outlined under calcite. The mother-of-pearl layer in the shells of bivalves is generally aragonite. The pearly luster of this layer is due to its being formed by the successive deposition of one thin layer upon another; so that light falling on the mother-of-pearl, penetrates, part of it to one layer and part to another, and is then reflected. Certain molluscs have this layer composed of especially thin layers, one, the Unios or freshwater clams, the other, the “pearl oysters” or Aviculidæ, these latter, however, being only distantly related to the edible oysters. In the cases, where molluscs of either of these two families are of large size, large pieces of mother-of-pearl can be recovered, and are used for buttons, handles, and various ornamental objects. A further peculiarity of these same molluscs is the formation of pearls in the sheet of flesh, lining the shells. The pearls are round or rounded concretions of aragonite. At the center there is a grain of sand, or more often a tiny dead parasite. Either was an irritant to the mollusc, and to be rid of it, a layer of aragonite was secreted around it. Then as the mollusc continued to grow and secrete layers for its shell, it also added each time another layer around the sand-grain or parasite, until in time a pearl of noticeable, and then of considerable size resulted. These have all the pearly luster of the mother-of-pearl in a sphere which tends to make the luster even more marked.
Pearls were in use as ornaments in China some twenty-three centuries before Christ, and in India over 500 B.C. They were very highly prized by the Romans and since their times the rulers of India have shown a remarkable fondness for them. Today the finest come from the Gulf of Persia and the Red Sea, while still others are found about Australia and in the Caribbean Sea. In the United States not a few are collected every year from the fresh water clams, some of them beautifully tinted with pink or yellow.
Aragonite is found widely, as at Haddam, Conn., Edenville, N. Y., Hoboken, N. J., New Garden, Penn., Warsaw, Ill., etc.
[Anhydrite]
CaSO₄
[Pl. 46]
Occurs in cleavable or granular masses, rarely in crystals; hardness, 3-3.5; specific gravity, 2.9; color white, gray, bluish or reddish; luster pearly on cleavage faces; transparent on thin edges.
When anhydrite occurs in crystals, they are orthorhombic, like the diagram on [Plate 46]. Usually, however, it is found in beds or layers, which were deposited by the evaporation of sea water, and so it is associated with salt. Anhydrite has three cleavage planes which are at right angles to one another, which produce rectangular or cube-like forms. Mostly anhydrite is associated with gypsum, from which it differs by its greater hardness, pseudo-cubic cleavage, and the fact that anhydrite is not readily soluble in acid, while gypsum is. Chemically it differs from gypsum in not having water of crystallization, which gypsum does have. The anhydrite is likely to occur as veins and irregular masses in beds of gypsum. Calcium sulphate is precipitated from sea water when 37% of the water has been evaporated, and it may be deposited either as anhydrite or as gypsum, the factors, which decide as to which of these two minerals it will be, being as yet unknown. After deposition, if exposed to moisture, the anhydrite may change to gypsum, irregular masses often remaining unchanged.
It is found in salt mines in Elsworth Co., Kan., in limestone cavities at Lockport, N. Y., in veins in Shasta Co., Calif., etc.