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Radial aggregates of lustrous, black, metallic, acicular ludwigite crystals to 0.5 cm, from Alta Stock, Salt Lake County, Utah, USA.
Category Borate mineral
(repeating unit)
Strunz classification 6.AB.30
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pbam
Unit cell a = 9.26, b = 12.26
c = 3.05 [Å]; Z = 4
Formula mass 195.26 g/mol
Color Pitch-black, olive-black
Crystal habit Massive - fibrous commonly in fanlike to felted aggregates
Cleavage [001] Perfect
Fracture Brittle - Conchoidal - Very brittle fracture producing small, conchoidal fragments.
Mohs scale hardness 5.5
Luster Silky to submetallic
Streak greenish black
Diaphaneity Opaque, translucent in thin fragments
Specific gravity 3.6 - 3.8
Optical properties Biaxial (+)
Refractive index nα = 1.830 - 1.850 nβ = 1.830 - 1.850 nγ = 1.940 - 2.020
Birefringence δ = 0.110 - 0.170
Pleochroism X = Y = dark green; Z = dark reddish brown
2V angle Measured: 20° to 45°
Solubility Slowly soluble in acid
Alters to limonite
References [1][2]

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.[2] It forma a solid solution series with the iron(II)-iron(III) borate mineral vonsenite.[1]

It was first described in 1874 for an occurrence in Ocna de Fier, Banat Mountains, Caras-Severin, Romania and named for Ernst Ludwig (1842–1915), an Austrian chemist at the University of Vienna.[1]


Ludwigite needles and sprays as inclusions in a peridot crystal from Sapat Gali, Kohistan District, Pakistan. Size 2.8 x 2 x 1.1 cm.