Ferrimolybdite

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

Ferrimolybdite
Ferrimolybdite crystals from the Kingman District, Mohave County, Arizona
General
Category	Molybdate mineral
Formula	Fe3+2(MoO4)3·8(H2O) or Fe3+2(MoO4)3·n(H2O)
IMA symbol	Fmyb[1]
Strunz classification	7.GB.30
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) 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
Identification
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	[2][3][4]

Discovery and occurrence

It was first described in 1914 for an occurrence in the Alekseevskii Mine in the Karysh River Basin, Khakassia Republic, Siberia, Russia.^[2] 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.^[3]