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The Mineral riebeckite

Fibrous Crocidolite

Riebeckite forms in two very different habits. The darker-colored forms which are individually crystallized are generally of igneous origin, such as volcanic rock and pegmatites. The finely fibrous variety, known as Crocidolite, usually originates from altered metamorphic rocks. The Crocidolite variety is a type of asbestos, and is sometimes also referred to as "blue asbestos". It is considered to be the most hazardous form of asbestos, and should never be brought near the mouth. If its fibers or particles enter the lungs, they can cause asbestosis. Asbestosis is a lung disease caused by inhalation of asbestos particles, which causes several cancers, particularly lung cancer and mesothelioma. Symptoms of asbestosis do not arise until about 20 years after the inhalation. Due to the hazards, washing hands after handling specimens is highly recommended. Many mineral collectors avoid collecting asbestos minerals out of safety concerns.

Riebeckite forms a series with Magnesio-riebeckite, which is the magnesium-dominated end member which has magnesium replacing part of the iron in its structure. Riebeckite crystals are frequently included within larger crystals of Quartz, and will sometimes turn the Quartz a blue color if the inclusions are very fine. Riebeckite was named after the German explorer and mineralogist Emil Riebeck (1853–1885).

Tiger's Eye, a well-known inexpensive gemstone, is a pseudomorph of compact Quartz after finely the fibrous Crocidolite variety of Riebeckite. It is formed when the Quartz takes over and dissolves the Crocodolite, leaving the Quartz compact yet composed of solid fibrous - and chatoyant forms. When polished, it's silky luster creates a beautiful chatoyant effect of moving layers of brown and yellow lines and waves.

Chemical Formula

Na2Fe2+3Fe3+2Si8O22(OH)2

Color

Dark blue, grayish-blue, dark green, brown, dark gray, black

Crystal System

Monoclinic

Properties

Streak
Light Greenish-brown, bluish gray, white
Hardness
5 - 5.5
Transparency
Opaque. May be slightly translucent on thin cross-sections under strong back-lighting.
Specific Gravity
3.0 - 3.4
Luster
Vitreous, silky
Cleavage
1,2 - prismatic
Fracture
Splintery, uneven
Tenacity
Brittle. Fibrous form is elastic.
Other ID Marks
Finely fibrous crystals may exhibit chatoyancy

Crystal Habits

Crystals are usually prismatic and striated lengthwise. They often show a diamond-shaped cross-section. Most often in finely fibrous form in parallel lines, and in slender, acicular crystals and groupings. Also columnar, bladed, radiating, massive, and as intersecting prismatic crystals in matrix.

Additional Information

Composition
Basic sodium iron silicate, often with some magnesium
In Group
Silicates; Inosilicates; Amphibole Group
Striking Features
Crystal habits and color
Environment
In low temperature metamorphic rock, in sodium-rich granite pegmatites, and in nepheline syentite deposits.
Rock Type
Igneous, Metamorphic

Varieties

 -   Fibrous, asbestos variety of Riebeckite.
 -   Magnesium-rich form of Riebeckite, where magnesium replaces some of the iron content. Magnesioriebeckite is considered by the IMA as a distinct mineral species. The chemical formula of Magnesio-riebeckite is Na2Mg3Fe3+2Si8O22(OH)2
 -   Variety of Riebeckite, which can be altered to Quartz, found together with Carnelian in the Stirling Brook in Warren Co., New Jersey. It is named after Leon Touchon, a local store owner in the area.

Noteworthy Localities

Although Riebeckite is found worldwide, few localities have produced collectible species. Perhaps the most famous form of this mineral is as a pseudomorph, being replaced by Quartz in the variety Tiger's Eye. Tiger's Eye is most abundant in the region between Pomfret and Prieska, Northern Cape Province, South Africa.

Large, striated Riebeckite crystals come from Mount Malosa, Zomba District, Malawi; and massive Riebeckite is abundant at Alter Pedroso, near Alter do Chão, Portugal. Sharp crystals have come from Punta di u Mazzone, Ocana, Corsica, France; and at Zagi Mountain, Warsak District, Pakistan.

In the U.S., large Riebeckite crystals in Albite, often intersecting, come from St. Peters Dome and Mount Rosa, El Paso Co., Colorado. Large and well-formed black crystals have been found at the Golden Horn Batholith near Washington Pass, Okanogan Co., Washington. Stirling Brook, Warren Co., New Jersey has produced a variety known locally as Touchonite in association with Carnelian. In Canada, fibrous, flowery Riebeckite growths have come from Mont St. Hilaire, Quebec.

Common Mineral Associations

Aegirine, Nepheline, Quartz, Albite, Tremolite, Hematite

Distingushing Similar Minerals

Schorl - Harder and lacks good cleavage.
Glaucophane - Difficult to distinguish, though usually lighter in color.
Arfvedsonite - Can be very difficult to distinguish, although usually in thinner crystals and more needle-like.
Aegirine - Crystals have steep pyramidal terminations.

riebeckite Photos



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