- Riebeckite
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Riebeckite 
Crocidolite, variety of Riebeckite - Locality: Pomfret Mine, Vryburg, South AfricaGeneral Category Silicate mineral Chemical formula [][Na2][(Fe2+)3(Fe3+)2][(OH)2|Si8O22] Crystal symmetry Monoclinic 2/m Unit cell a = 9.76 Å, b = 18.04 Å, c = 5.33 Å; β = 103.59°; Z=2 Identification Color Black, dark blue; dark blue to yellow-green in thin section Crystal habit As prismatic crystals, commonly fibrous, asbestiform; earthy, massive Crystal system Monoclinic - Prismatic Twinning Simple or multiple twinning parallel to {100} Cleavage Perfect on {110}, intersecting at 56° and 124°; partings on {100} and {010} Fracture Conchoidal to uneven Tenacity Brittle Mohs scale hardness 6 Luster Vitreous to silky Streak Pale to bluish gray Diaphaneity Semitransparent Specific gravity 3.28 - 3.44 Optical properties Biaxial (-) Refractive index nα = 1.680 - 1.698 nβ = 1.683 - 1.700 nγ = 1.685 - 1.706 Birefringence δ = 0.005 - 0.008 Pleochroism X = blue, indigo; Y = yellowish green, yellow brown; Z = dark blue 2V angle Measured: 68° to 85°, Calculated: 62° to 78° Dispersion Strong References [1][2][3] Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula [][Na2][(Fe2+)3(Fe3+)2][(OH)2|Si8O22]. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.
Contents
Name and discovery
It was first described in 1888 for an occurrence on Socotra Island, Adan Governorate, Yemen and named for German explorer Emil Riebeck (1853-1885).[2]
Occurrence
It typically forms dark-blue elongated to fibrous crystals in highly alkali granites, syenites, rarely in felsic volcanics, granite pegmatites and schist. It occurs in banded iron formations as the asbestiform variety crocidolite ((crow-SEE-doe-lite) blue asbestos). It occurs in association with aegirine, nepheline, albite, arfvedsonite in igneous rocks; with tremolite, ferro-actinolite in metamorphic rocks; and with grunerite, magnetite, hematite, stilpnomelane, ankerite, siderite, calcite, chalcedonic quartz in iron formations.[1]
Riebeckite granite
Canton viaduct
The riebeckite granite known as ailsite, found on the island of Ailsa Craig in western Scotland, is prized for its use in the manufacture of curling stones.
Riebeckite granite was used for the facing stones of the Canton Viaduct from Moyles Quarry (a.k.a. Canton Viaduct Quarry) now part of Borderland State Park in Massachusetts, USA.
See also
References
- ^ a b http://rruff.geo.arizona.edu/doclib/hom/riebeckite.pdf Handbook of Mineralogy
- ^ a b http://www.mindat.org/min-3418.html Mindat.org
- ^ http://webmineral.com/data/Riebeckite.shtml Webmineral data
External links
Chisholm, Hugh, ed (1911). "Crocidolite". Encyclopædia Britannica (11th ed.). Cambridge University Press.
Categories:- Sodium minerals
- Iron minerals
- Magnesium minerals
- Asbestos
- Amphibole group
- Monoclinic minerals
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