Blue asbestos

Riebeckite
Black riebeckite in an alkaline pegmatite from Lindinosa near Évisa, Corsica, France
General
Category	Silicate mineral
Formula (repeating unit)	Na2(Fe2+3Fe3+2)Si8O22(OH)2
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/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
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

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na₂(Fe²⁺₃Fe³⁺₂)Si₈O₂₂(OH)₂. It forms a solid solution 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.

It was first described in 1888 for an occurrence on Socotra Island, Aden Governorate, Yemen and named for German explorer Emil Riebeck (1853–1885).

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.