Riebeckite
mineral
riebeckite, a sodium-iron silicate mineral [Na2Fe2+3Fe3+2Si8O22(OH)2] in the amphibole family. It forms part of a solid-solution series that includes both magnesioriebeckite (formed when iron is replaced by magnesium) and glaucophane (formed when iron is replaced by magnesium and aluminum).
Riebeckite is a moderately hard mineral with a glassy lustre. It forms prismatic crystals that are dark blue or black in colour. A fibrous variety, crocidolite, is of metamorphic origin and is commonly called blue asbestos.
The mineral is associated with acidic igneous rocks such as granites and syenites. Common deposits are found in Arizona, Colorado, and Massachusetts, U.S.; Greenland; Portugal; Nigeria; South Africa; and portions of western Australia. For detailed physical properties, see amphibole (table).
alpha = 1.600–1.672
beta = 1.614–1.686
gamma = 1.627–1.693
alpha = 1.587–1.642
beta = 1.602–1.655
gamma = 1.613–1.661
alpha = 1.612–1.700
beta = 1.625–1.709
gamma = 1.630–1.710
alpha = 1.622–1.690
beta = 1.672–1.730
gamma = 1.680–1.760
alpha = 1.615–1.705
beta = 1.618–1.714
gamma = 1.632–1.730
alpha = 1.643–1.688
beta = 1.658–1.711
gamma = 1.663–1.731
alpha = 1.606–1.661
beta = 1.622–1.667
gamma = 1.627–1.670
alpha = 1.605–1.685
beta = 1.618–1.700
gamma = 1.627–1.712
alpha = 1.645–1.701
beta = 1.662–1.711
gamma = 1.668–1.717
name
colour
lustre
Mohs hardness
specific gravity
actinolite
colourless to gray; darkens with increased Fe through green to black
silky; oily
2.9–3.2
2.9–3.2
anthophyllite
white, gray, green, or various shades of brown
vitreous
5½–6
2.9–3.2
arfvedsonite
dark bluish green to greenish black or black
vitreous
34460
3–3.5
basaltic hornblende
brown to black
glassy
34460
3.2–3.3
common hornblende
pale to dark green
glassy
34460
3–3.4
cummingtonite
dark green; brown
silky
34460
3.1–3.6
glaucophane
gray or lavender-blue
vitreous
6
3.1–3.3
richterite
brown, yellow, brownish red, pale to dark green
vitreous
34460
3–3.4
riebeckite
dark blue or black
vitreous
5
3–3.4
name
habit or form
fracture or cleavage
refractive indices
crystal system
actinolite
fibrous massive
one perfect cleavage of 56 degrees
monoclinic
anthophyllite
fibrous or lamellar masses; bladed and prismatic crystal aggregates
one perfect cleavage of 54 degrees
orthorhombic
arfvedsonite
long prisms; prismatic aggregates
one perfect cleavage of 56 degrees
monoclinic
basaltic hornblende
massive
one perfect cleavage of 56 degrees
monoclinic
common hornblende
massive
one good cleavage of 56 degrees
monoclinic
cummingtonite
fibrous or lamellar massive
one good cleavage of 55 degrees
monoclinic
glaucophane
fibrous or columnar massive
one good cleavage of 58 degrees
monoclinic
richterite
elongated crystals
one perfect cleavage of 56 degrees
monoclinic
riebeckite
longitudinally striated prismatic crystals; fibrous massive
one good cleavage of 56 degrees
monoclinic
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