Ludwigite

Ludwigite is a magnesium-iron borate mineral: Mg2FeBO5.

Ludwigite
Radial aggregates of lustrous, black, metallic, acicular ludwigite crystals to 0.5 cm, from Alta Stock, Salt Lake County, Utah, USA.
General
CategoryBorate mineral
Formula
(repeating unit)
Mg2Fe3+BO5
IMA symbolLdw[1]
Strunz classification6.AB.30
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPbam
Unit cella = 9.26, b = 12.26
c = 3.05 [Å]; Z = 4
Identification
Formula mass195.26 g/mol
ColorPitch-black, olive-black
Crystal habitMassive - fibrous commonly in fanlike to felted aggregates
Cleavage[001] Perfect
FractureBrittle - Conchoidal - Very brittle fracture producing small, conchoidal fragments.
Mohs scale hardness5.5
LusterSilky to submetallic
Streakgreenish black
DiaphaneityOpaque, translucent in thin fragments
Specific gravity3.6 - 3.8
Optical propertiesBiaxial (+)
Refractive indexnα = 1.830 - 1.850 nβ = 1.830 - 1.850 nγ = 1.940 - 2.020
Birefringenceδ = 0.110 - 0.170
PleochroismX = Y = dark green; Z = dark reddish brown
2V angleMeasured: 20° to 45°
SolubilitySlowly soluble in acid
Alters tolimonite
References[2][3]

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]

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.

References


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