Olivine rarely occurs in crystals, but when it does they belong to the orthorhombic system. The dark-green grains or masses are recognized by the color, considerable hardness and indistinct cleavage. Serpentine may have a similar color, but its hardness is only 4. In hydrochloric acid olivine decomposes to a gelatinous mass.
Olivine is typically one of the constituents of the dark lavas, like basalt, gabbro, or peridotite. It is also a common mineral in meteorites. Olivine, in the presence of water, alters to other minerals, especially serpentine, with great facility.
It occurs fairly widely wherever the dark lavas are present, as in the White Mountains of N. H., in Loudoun Co., Va., in Lancaster Co., Penn., and in many localities in the Rocky Mountains and Cascade Range.
[Epidote]
Ca₂(AlOH)(AlFe₂)(SiO₄)₃
[Pl. 42]
Occurs in grains or columnar masses; hardness, 6.5; specific gravity 3.4; color green, usually a pistachio or yellow-green; luster vitreous; translucent on thin edges.
Rarely epidote occurs in crystals, which belong to the monoclinic system, and may be either short like the diagrams on [plate 42] or long and needle-like. The color and hardness will suffice to determine this mineral, as almost no other has the peculiar yellowish-green color which is characteristic of this form.
Epidote occurs primarily in metamorphic rocks at or near the contact with igneous rocks; or it may be a secondary mineral resulting from the weathering of granites, especially along seams. It sometimes occurs with hornblende in highly folded schists, as in New York City. It is often a mineral which has resulted from the alteration of other minerals, as pyroxene, amphibole, biotite, or even feldspars.
It is found at Chester and Athol, Mass., Haddam, Conn., Amity, Munroe and Warwick, N.Y., East Branch, Penn., in the Lake Superior region, in the Rocky Mountains, etc.
[Tourmaline]
(FeCrNaKLi)₄Mg₁₂B₆Al₁₆H₈Si₁₂O₆₃
[Pl. 42] & [frontispiece]
Occurs in three-sided prismatic crystals; hardness, 7; specific gravity, 3.1; colorless, red, green, brown, or black; luster vitreous; transparent on thin edges.