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Yellow sprays of ferrimolybdite on quartz from the Liège Province, Belgium
Category Molybdate mineral
(repeating unit)
Fe3+2(MoO4)3·8(H2O) or Fe3+2(MoO4)3·n(H2O)
Strunz classification 7.GB.30
Crystal system Orthorombic
Dipyramidal class
Space group Orthorhombic
H-M symbol: (2/m 2/m 2/m)
Space group: Pmmn
Unit cell a = 6.665, b = 15.423
c = 29.901 [Å]; Z = 8
Color Canary-yellow, straw-yellow, greenish yellow
Crystal habit Acicular tufted to radial aggregates; powdery, earthy
Cleavage Distinct on {001}
Fracture Uneven
Mohs scale hardness 1-2
Luster Adamantine, silky, earthy
Streak Light yellow
Diaphaneity Transparent to translucent
Specific gravity 2.99
Optical properties Biaxial (+)
Refractive index nα = 1.720 - 1.810 nβ = 1.730 - 1.830 nγ = 1.850 - 2.040
Birefringence δ = 0.130 - 0.230
Pleochroism X = Y = clear to nearly colorless; Z = dirty gray to canary-yellow
2V angle 26° to 32° (calculated)
References [1][2][3]

Ferrimolybdite is a hydrous iron molybdate mineral with formula: Fe3+2(MoO4)3·8(H2O)[2] or Fe3+2(MoO4)3·n(H2O).[1] It forms coatings and radial aggregates of soft yellow needles which crystallize in the orthorhombic system.

Ferrimolybdite crystals from the Kingman District, Mohave County, Arizona

Discovery and occurrence[edit]

It was first described in 1914 for an occurrence in the Alekseevskii Mine in the Karysh River Basin, Khakassia Republic, Siberia, Russia.[1] It was named for its composition (ferric iron and molybdenum).

It occurs as an oxidation product of molybdenum bearing ore deposits. Associated minerals include: molybdenite, pyrite and chalcopyrite.[2]