Riebeckite

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na₂(Fe,Mg)₅Si₈O₂₂(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, Adan 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 ((crow-SEE-doe-lite) 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.

References

1. ^ a b http://rruff.geo.arizona.edu/doclib/hom/riebeckite.pdf Handbook of Mineralogy
2. ^ a b http://www.mindat.org/min-3418.html Mindat.org
3. ^ http://webmineral.com/data/Riebeckite.shtml Webmineral data

List of minerals