Riebeckite
A variety of Minerals
What is Riebeckite?
Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na2(Fe3Fe2)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.
Uses & Applications
Riebeckite is primarily of scientific interest for understanding blueschist metamorphism and high-pressure geological processes. Fibrous varieties (crocidolite) were historically used as asbestos but are now avoided due to health risks.
Optical Properties
- Refractive Index
- 1.702-1.719
- Pleochroism
- Strong
- Optical Character
- Biaxial negative
- Dispersion
- 0.015
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The Meaning
Named after Emil Riebeck, a German explorer and mineralogist. The mineral is significant in understanding plate tectonics and high-pressure metamorphic environments.
Geochemistry
Forms under high-pressure, low-temperature metamorphic conditions characteristic of subduction zones. Its presence indicates specific pressure-temperature conditions and is important for understanding regional metamorphic history.
Key Characteristics
Formation of Riebeckite
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.
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Quick Facts
Physical Properties
- Color
- Usually black, also commonly light blue to blue-black, gray-blue, gray, brown
- Hardness (Mohs)
- 5 - 5.5
- Streak
- Pale gray to bluish-grey.
Chemical Properties
- Chemical Formula
- ◻[Na2][Fe<sup>2+</sup><sub>3</sub>Fe<sup>3+</sup><sub>2</sub>]Si8O22(OH)2 Riebeckite is defined as a member of the riebeckite group with Fe2+>Mg in the C2+ position and (OH) dominant in the W position.
- Elements
- Fe, H, Na, O, Si
Also Known As
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