Aragonite is found widely, as at Haddam, Conn., Edenville, N. Y., Hoboken, N. J., New Garden, Penn., Warsaw, Ill., etc.
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.
Occurs in crystals, in cleavable masses, or in fibrous masses; hardness, 2; specific gravity, 2.3; colorless, white, amber, gray, or pink; luster vitreous, silky or pearly; transparent on thin edges.
Gypsum crystals are monoclinic as seen on Plate 47, the perfect ones usually occurring in clay, as at Oxford, O., or in cavities; while crystals of less perfect outline, but with fine cleavages, are found in Utah, Kansas, and Colorado. The cleavage is very perfect in one direction, making it possible to strip off thin sheets almost like mica, and less perfect in two other directions, which appear on the smooth surface of the first cleavage as lines intersecting at 66° 14′. Twinning is also common in such a way, that the two united crystals make forms similar to arrowheads. These cleavages and the twinning show nicely in the photograph of gypsum on Plate 47.