Ludwigite

Ludwigite
240px Radial aggregates of lustrous, black, metallic, acicular ludwigite crystals to 0.5 cm, from Alta Stock, Salt Lake County, Utah, USA.
General
Category	Borate mineral
Formula (repeating unit)	Mg2Fe3+BO5
Strunz classification	06.AB.30
Crystal symmetry	Orthorhombic 2/m 2/m 2/m Space Group: Pbam
Unit cell	a = 9.26 Å, b = 12.26 Å, c = 3.05 Å; Z = 4
Identification
Formula mass	195.26
Color	Pitch-black, olive-black
Crystal habit	Massive - fibrous commonly in fanlike to felted aggregates
Crystal system	Orthorhombic
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

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

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.

References

File:Forsterite-Ludwigite-34581.jpg

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.