Magnesioferrite

Magnesioferrite is a magnesium iron oxide mineral, a member of the magnetite series of spinels. Magnesioferrite crystallizes as black metallic octahedral crystals. It is named after its chemical composition of magnesium and ferric iron. The density is 4.6 - 4.7 (average = 4.65), and the diaphaniety is opaque. Occurs as well-formed fine sized crystals or massive and granular. Its hardness is 6-6.5. It has a metallic luster and a dark red streak.

Magnesioferrite
Magnesioferrite from Ochtendung, Eifel, Germany
General
Category	Oxide minerals Spinel group Spinel structural group
Formula	Mg(Fe3+)2O4
IMA symbol	Mfr[1]
Strunz classification	4.BB.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Unit cell	a = 8.3866 Å; Z = 8
Identification
Color	Black to brownish black
Crystal habit	As octahedral crystals, massive granular
Twinning	Twin plane {111}, contact twins
Cleavage	On {111}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	6 - 6.5
Luster	Metallic, semimetallic, dull
Streak	Dark red
Diaphaneity	Opaque, transparent in thin fragments
Specific gravity	4.55 – 4.65 measured
Optical properties	Isotropic
Refractive index	n = 2.38
Other characteristics	Magnetic
References	[2][3][4]

Occurrence

It occurs in fumaroles, as a result of combustion metamorphism and coal seam fires, in glass spherules related to meteorite impacts, and as accessory phase in kimberlites and carbonatites.^[2]

It has been reported from Vesuvius and Stromboli, Italy.