Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na<sub>2</sub>(Fe<sup>2+</sup><sub>3</sub>Fe<sup>3+</sup><sub>2</sub>)Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>. 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 rhombus; fracture is uneven, splintery. It is often translucent to nearly opaque.
Name and discovery
Riebeckite was first described in 1888 for an occurrence on Socotra Island, Aden Governorate, Yemen, and named for German explorer Emil Riebeck (1853–1885).
Occurrence
Riebeckite 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.
Crocidolite asbestos was mined in South Africa, Bolivia, and Wittenoom, Western Australia. Bolivian crocidolite was used in approximately 13 billion Kent Micronite cigarette filters, manufactured from March 1952 until at least May 1956 by the Lorillard Tobacco Company (now part of the R. J. Reynolds Tobacco Company). Blue asbestos was also used to similar effect, and hazard, in early gas masks.