A low temperature hydrothermal mineral of the zeolite group, this mineral is found in amygdalules and vanities in basalt. It is characterized chiefly by its cleavage, pearly luster on the cleavage face, and common sheaf-like groups of crystals. The name is derived from a Greek word meaning “luster,” in allusion to the pearly luster.

Orthoclase is a rock-forming feldspar mineral that is common in granites, syenites, and high grade metamorphic rocks. It is usually recognized by its color, hardness, and cleavage. It is distinguished from the other feldspars by its right-angle cleavage and the lack of twin striations on the best cleavage surface. The name comes from the Greek orthos meaning “right” and kalo meaning “I cleave,” in allusion to its right angle of good cleavage.

Named after the English mineral collector, John Henry Heuland (1778-1856), this mineral is a member of the zeolite group of minerals. It is characterized by its crystal form and one direction of perfect cleavage with pearly luster. It is usually found in cavities in basalt and as a devitrification product from volcanic glasses.

Serpentine is a common and widely distributed mineral which forms as an alteration of magnesium silicates. The variety chrysotile is the chief source of asbestos. It was named from the Greek ophites, meaning “of a serpent.”

A comparatively rare mineral, pyrophyllite is found in metamorphic rocks and was named from the Greek word for “fire” and “leaf” for the way it exfoliates when heated. It is mined and used as low-grade talc.

A widespread member of the mica group, this mineral is characterized by its brownish-red color. It was named from the Greek phlogopos for “resembling fire,” in allusion to is red tint. It is found in contact and regional metamorphic rocks and as a common mineral in kimberlite.

The perfect cleavage of this mineral allows it to be split into very thin flexible sheets. It is the most common of the mica group minerals and is typically found occurring as “books.” The name was used as early as 1794 by Johann Gottfried Schmeisser in his System of Mineralogy and is derived from the term “Muscovy glass,” which was a substitute for glass.

Described by Mohs in 1820, this mica group mineral occurs with corundum. Its name is from the Greek word meaning “pearl.”

This mineral occurs within a great range of compositions and is considered a group. It is characterized by its green color, micaceous habit, and cleavage, but its sheets are not elastic. It is a common mineral formed from alteration during low grade metamorphism. The name is derived from a Greek word meaning “green,” in allusion to the common color of the mineral.

Tremolite is found most frequently in metamorphosed dolomitic limestone. It was named by J.G.A. Hopfner for the Tremola Valley, Switzerland. A felted aggregate of tremolite fibers goes under the name of “mountain leather” or “mountain cork.” The mineral frequently is fibrous and is the material to which the name “asbestos” was originally given.