Ludwigite

(repeating unit)Mg₂Fe³⁺BO₅IMA symbolLdw^[1]Strunz classification6.AB.30Crystal systemOrthorhombicCrystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)Space groupPbamUnit cella = 9.26, b = 12.26
c = 3.05 [Å]; Z = 4IdentificationFormula mass195.26 g/molColorPitch-black, olive-blackCrystal habitMassive – fibrous commonly in fanlike to felted aggregatesCleavage[001] PerfectFractureBrittle – Conchoidal – Very brittle fracture producing small, conchoidal fragments.Mohs scale hardness5.5LusterSilky to submetallicStreakGreenish blackDiaphaneityOpaque, translucent in thin fragmentsSpecific gravity3.6 – 3.8Optical propertiesBiaxial (+)Refractive indexn_α = 1.830 – 1.850 n_β = 1.830 – 1.850 n_γ = 1.940 – 2.020Birefringenceδ = 0.110 – 0.170PleochroismX = Y = dark green; Z = dark reddish brown2V angleMeasured: 20° to 45°SolubilitySlowly soluble in acidAlters tolimoniteReferences^[2][3]

Ludwigite is a magnesium-iron borate mineral: Mg₂FeBO₅.

Ludwigite typically occurs in magnesian iron skarn and other high temperature contact metamorphic deposits. It occurs in association with magnetite, forsterite, clinohumite and the borates vonsenite and szaibelyite.^[3] It forms a solid solution series with the iron(II)-iron(III) borate mineral vonsenite.^[2]

It was first described in 1874 for an occurrence in Ocna de Fier, Banat Mountains, Caraș-Severin County, Romania and named for Ernst Ludwig (1842–1915), an Austrian chemist at the University of Vienna.^[2]

References