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Crocidolite  (variety of Riebeckite)
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Crocidolite is named from the Greek kpokus, meaning nap of cloth, in allusion to its finely fibrous habit. Riebeckite is named for Emil Riebeck (1853-1885), German explorer.

Discovered in 1815;   IMA status: Riebeckite is Valid (pre-IMA; Grandfathered)

Information below is for Riebeckite

Chemistry

Chemical Formula:

Na₂Fe²⁺₃Fe³⁺₂(Si₈O₂₂)(OH)₂

Iron Sodium Silicate Hydroxide

Molecular Weight:

935.90 gm

Composition:

Sodium

4.91 %

Na

6.62 %

Na₂O

Iron

29.84 %

Fe

23.03 %

FeO / 17.06% Fe₂O₃

Silicon

24.01 %

Si

51.36 %

SiO₂

Hydrogen

0.22 %

H

1.92 %

H₂O

Oxygen

41.03 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/F.08-50

Nickel-Strunz 10th Ed. ID:

9.DE.25

9 : SILICATES (Germanates)
D : Inosilicates
E : Inosilicates with 2-periodic double chains, Si₄O₁₁; Clinoamphiboles

Related to:

Amphibole Group. Sodic Clino-Amphibole Subgroup. Magnesioriebeckite-Riebeckite Series.

Varieties:

Crocidolite is a variety of Riebeckite

Synonyms:

Abriachanite, Blue Asbestos, Blue Ironstone, Cape Asbestos, Cape Blue, Krokydolith, South African cat's eye

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

As prismatic crystals, to 20 cm. Commonly fibrous, asbestiform; earthy, massive.

Twinning:

Simple or multiple twinning || {100}.

Physical Properties

Cleavage:

Perfect on {110} intersecting at 56° and 124°; partings on {100}, {010}

Fracture:

Conchoidal to Uneven, Splintery

Tenacity:

Brittle

Moh's Hardness:

5.0 - 6.0

Density:

3.28 - 3.44 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Other:

An asbestos mineral. Fibres, if breathed in, can cause serious and irreversable lung damage. Avoid exposure to dust.

Optical Properties

Color:

Gray-Blue to leek-Green

Transparency:

Semi-transparent to Opaque

Luster:

Vitreous to Silky

Refractive Index:

1.656 - 1.740  Biaxial ( + ) or ( - )

Birefringence:

0.005 - 0.008

Dispersion:

Strong

Pleochroism:

X = blue, indigo; Y = yellowish green, yellowbrown; Z = dark blue

Other:

Exhibits chatoyance and cat's eye effect

Occurances

Geological Setting:

In alkalic granites and syenites; rarer in felsic volcanics and granite pegmatites; in some schists. In iron formations as asbestiform "Crocidolite."

Common Associations:

Aegirine, Nepheline, Albite, Arfvedsonite (igneous); Tremolite, Ferro-actinolite (metamorphic); Grunerite, Magnetite, Hematite, Stilpnomelane, Ankerite, Siderite, Calcite, Chalcedony, Quartz (iron formations)

Common Impurities:

Fe, Ti, Mg, Al, Mn

Type Locality:

Socotra Island, Adan Governorate, Yemen

Year Discovered:

1815 (Crocidolite); 1888 (Riebeckite)

View mineral photos:

Crocidolite Mineral Photos and Locations

More Information

Mindat.org (Crocidolite)
Mindat.org (Riebeckite)
Webmineral.com (Riebeckite)

Crocidolite is a fibrous, or asbestiform, variety of Riebeckite and belongs to the Amphibole Group of minerals that also includes Actinolite , Arfvedsonite, Riebeckite and Tremolite. Riebeckite forms in alkalic granites and syenites while Crocidolite forms in iron formations. Crocidolite was first described in 1815 by M. H. Klaproth under the name Blaueisenstein meaning blue ironstone, and in 1831 by J. F. Hausmann, who gave it its present name Crocidolite which is from the Greek word kpokus meaning nap of cloth in allusion to its fibrous habit. Crocidolite fibers are finely textured and hair-like, occurring in naturally formed bundles or mats. Its resemblance to asbestos has earned it the name Cape Asbestos, the chief occurrence being in Cape Colony, South Africa. Crocidolite is formed by geologic alteration by the removal of alkali and peroxidation of the ferrous iron and furher by deposition of silica between the fibers or by their replacement by silica. A hard siliceous mineral is formed which can be polished and exhibits a beautiful chatoyance with silky luster and a smooth "cat's eye" effect. Upon further alteration and deposition of silica this mineral becomes the popular golden brown mineral known as "tiger-eye" or "hawk's-eye" when colored blue. Another alteration product of Crocidolite, consisting of silica and ferric hydrate, is called Griqualandite for its occurance in the Asbestos Mountains, Griqualand West, Cape Colony, South Africa.
 

  
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