Riebeckite: Properties and Occurrence

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 commonly forms prismatic crystals, but it also occurs as a fibrous asbestiform mineral.

It is a moderately hard mineral with a glassy luster. It forms prismatic crystals that are dark blue or black in color. A fibrous variety, crocidolite, is of metamorphic origin and is commonly called blue asbestos. It was first described in 1888 for an occurrence on Socotra Island, Aden Governorate, Yemen and named for German explorer Emil Riebeck (1853–1885).

General Information

  • Category: Silicate mineral
  • Formula: Na2(Fe2+3Fe3+2)Si8O22(OH)2
  • Crystal system: Monoclinic.

Properties

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.

  • 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 parallels 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

Occurrence

It typically forms dark-blue elongated to fibrous crystals in highly alkali granites, syenites, rarely in felsic volcanic, 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.

 

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