Galaxite

Galaxite, also known as 'mangan-spinel' is an isometric mineral belonging to the spinel group of oxides with the ideal chemical formula Mn²⁺Al₂O₄.^[5] It is sometimes used as a gemstone.^[6]

Galaxite
Galaxite from the Kaso mine, Japan
General
Category	Oxide minerals Spinel group Normal Spinel structural group
Formula	MnAl2O4
IMA symbol	Glx[1]
Strunz classification	4.BB.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m (no. 227)
Unit cell	a = 8.271 Å; Z = 8
Identification
Color	Black, red-brown, red to yellow
Crystal habit	Octahedra and rounded grains and exolution blebs
Twinning	Spinel law with {111} as both twin and composition plane
Cleavage	Indistinct to none
Fracture	Conchoidal to irregular
Tenacity	Brittle
Mohs scale hardness	7.5
Luster	Vitreous
Streak	Red-brown
Diaphaneity	Opaque; may be translucent in thin section
Specific gravity	4.234
Optical properties	Isotropic
Refractive index	n = 1.923
Other characteristics	weakly to moderately magnetic
References	[2][3][4]

Occurrence

It was first described in 1932 for an occurrence at Bald Knob, Alleghany County, North Carolina near its namesakes, the town of Galax, Virginia, named after the plant galax or wandflower which grows in the area.^[3][4]

Galaxite generally occurs as small granular aggregates with a red-brownish tone. It has a vitreous luster and leaves a brownish-red streak. It is rated 7.5 on the Mohs Scale.^[3]

It occurs in carbonate-rich metamorphosed manganese ore deposits. It occurs associated with alleghanyite, rhodonite, sonolite, spessartine, tephroite, kutnohorite, manganhumite, jacobsite, kellyite and alabandite in the Bald Knob area. Associated minerals include katoptrite, magnetite, manganostibite, magnussonite, tephroite, manganhumite and manganosite in the Brattfors mine area of Nordmark, Värmland, Sweden.^[3]

Composition

Galaxite is the manganese (Mn) rich endmember of the aluminium (Al) series of the spinel group. Divalent iron (Fe) and magnesium (Mg) readily substitute for the manganese in the crystal structure. Trivalent iron may also substitute for the aluminium. Thus, reflecting most natural samples, the formula may be better represented as (Mn,Fe²⁺,Mg)(Al,Fe³⁺)₂O₄.^[3]