Chemistry:Galaxite

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Galaxite
Galaxite-Jacobsite-Wiserite-697074.jpg
Galaxite from the Kaso mine, Japan
General
CategoryOxide minerals
Spinel group
Normal Spinel structural group
Formula
(repeating unit)
MnAl2O4
Strunz classification4.BB.05
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
Unit cella = 8.271 Å; Z = 8
Identification
ColorBlack, red-brown, red to yellow
Crystal habitOctahedra and rounded grains and exolution blebs
TwinningSpinel law with {111} as both twin and composition plane
CleavageIndistinct to none
FractureConchoidal to irregular
TenacityBrittle
Mohs scale hardness7.5
|re|er}}Vitreous
StreakRed-brown
DiaphaneityOpaque; may be translucent in thin section
Specific gravity4.234
Optical propertiesIsotropic
Refractive indexn = 1.923
Other characteristicsweakly to moderately magnetic
References[1][2][3]

Galaxite, also known as 'mangan-spinel' is an isometric mineral belonging to the spinel group of oxides with the ideal chemical formula Mn2+
Al
2
O
4
.[4]

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+
)
2
O
4
.[2]

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.[2]

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.[2][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.[2]

It is sometimes used as a gemstone.[5]

References

  1. Mindat.org
  2. 2.0 2.1 2.2 2.3 2.4 Handbook of Mineralogy
  3. 3.0 3.1 Webmineral data
  4. http://rruff.info/ima/
  5. Tables of Gemstone Identification By Roger Dedeyne, Ivo Quintens, p.282