Riebeckite

Riebeckite
Black riebeckite in an alkaline pegmatite from Lindinosa near Évisa, Corsica, France
General
CategorySilicate mineral
Formula
(repeating unit)
Na2(Fe2+3Fe3+2)Si8O22(OH)2
IMA symbolRbk[1]
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 9.76 Å, b = 18.04 Å
c = 5.33 Å; β = 103.59°; Z = 2
Identification
ColorBlack, dark blue; dark blue to yellow-green in thin section
Crystal habitAs prismatic crystals, commonly fibrous, asbestiform; earthy, massive
TwinningSimple or multiple twinning parallel to {100}
CleavagePerfect on {110}, intersecting at 56° and 124°; partings on {100} and {010}
FractureConchoidal to uneven
TenacityBrittle
Mohs scale hardness6
LusterVitreous to silky
StreakPale to bluish gray
DiaphaneitySemitransparent
Specific gravity3.28–3.44
Optical propertiesBiaxial (−)
Refractive indexnα = 1.680–1.698 nβ = 1.683–1.700 nγ = 1.685–1.706
Birefringenceδ = 0.005–0.008
PleochroismX = blue, indigo; Y = yellowish green, yellow brown; Z = dark blue
2V angleMeasured: 68° to 85°, Calculated: 62° to 78°
DispersionStrong
References[2][3][4][5]

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.

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ "Riebeckite", Handbook of Mineralogy, Mineral Data Publishing, 2001
  3. ^ "Riebeckite", Mindat.org
  4. ^ "Riebeckite", The Mineralogy Database (webmineral.com)
  5. ^ "IMA Master List". Archived from the original on 2015-01-05. Retrieved 2014-05-12.