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
Category	Borate mineral
Formula (repeating unit)	Mg2Fe3+BO5
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
Identification
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 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, Caraș-Severin County, Romania and named for Ernst Ludwig (1842–1915), an Austrian chemist at the University of Vienna.[1]