Tectosilicates

Tectosilicates

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.

Usually distinguished by its blue color, this mineral is found in nepheline syentites. It was discovered in 1811 in Greenland and was named to reflect its sodium content. Crystals are rare, it is commonly found massive, in embedded grains. This massive variety is often used as a decorative stone.

The most common and abundant of all the minerals, quartz is a significant component of many igneous, metamorphic, and sedimentary rocks. It is found in an impressive range of varieties and colors; the purple variety is amethyst, which gets its color from small amounts of ferric iron; the yellow variety is called citrine; while the dark variety, smoky quartz, is formed from exposure to radioactive material. The name is a German word of ancient derivation.

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.

Although opal is regarded as a valid mineral species for historical reasons, it is not a true mineral in the accepted sense of the word. It is composed of amorphous silica. A gemstone, opal has numerous varieties such as fire opal which has intense orange-to-red reflections, and wood opal which occurs in petrified wood. It may be deposited by hot springs at shallow depths, but the largest accumulations are as siliceous tests of silica-secreting organisms.

A rock-forming mineral, nepheline is common in silica-deficient igneous rocks. Discovered in 1801 in the lavas of Mount Vesuvius, Italy, this mineral is characterized by its greasy luster. The name comes from the Greek word meaning “a cloud” because when immersed in acid, the mineral becomes cloudy.

A member of the zeolite group of minerals, this mineral was discovered in 1803 in Germany. It occurs in cavities in basalts. It was named from the Greek natron meaning "soda," in allusion to its sodium content and lithos meaning "stone."

A rock-forming potassium feldspar mineral, microcline was discovered in 1830 in Norway. The name comes from the Greek words meaning "little" and "to incline," in allusion to the small departure from monoclinic symmetry. It is found in plutonic felsic rocks, and in pegmatites, where it may be intimately intergrown with quartz forming graphic granite. The green variety is called Amazon stone.

Discovered in 1791 in the lavas of Mount Vesuvius, leucite is a rare mineral that can be found in recent potassium rich mafic and ultramafic lavas. It is found only in silica-deficient rocks and never in rock containing quartz. It is named from the Greek leucos for “white,” in allusion to its common color.

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.