Riebeckite
| Riebeckite | |
|---|---|
.jpg) Black riebeckite in an alkaline pegmatite from Lindinosa near Évisa, Corsica, France | |
| General | |
| Category | Silicate mineral |
| Formula (repeating unit) | Na2Fe2+3Fe3+2Si8O22(OH)2 |
| 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 Na2Fe2+3Fe3+2Si8O22(OH)2. 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.
Name and discovery
It was first described in 1888 for an occurrence on Socotra Island, Aden 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 (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
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.
Crocidolite (fibrous riebeckite)
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The fibrous forms of riebeckite are known as crocidolite and are one of the six recognised types of asbestos. Often referred to as blue asbestos, it is considered the most hazardous. In 1964 Dr Christopher Wagner discovered an association between blue asbestos and mesothelioma.[4]
Crocidolite asbestos was mined in South Africa, Bolivia and also at Wittenoom, Western Australia. Bolivian crocidolite was used in Kent Micronite cigarette filters in the 1950s. Blue asbestos was also formerly used in early gas masks. In the mid 20th Century, asbestos was found to be harmful so production stopped.
See also
References
- ↑ 1.0 1.1 "Riebeckite", Handbook of Mineralogy, Mineral Data Publishing, 2001
- ↑ 2.0 2.1 "Riebeckite", Mindat.org
- ↑ "Riebeckite", The Mineralogy Database (webmineral.com)
- ↑ McDonald, Corbett. Obituary: Dr Christopher Wagner, The Independent 4 July 2000
External links
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Wikimedia Commons has media related to Riebeckite. |
Chisholm, Hugh, ed. (1911). "Crocidolite". Encyclopædia Britannica 7 (11th ed.). Cambridge University Press. p. 477