Allactite

Allactite

Picture width 3 mm
General
Category Arsenate mineral
Formula
(repeating unit)		 Mn7(AsO4)2(OH)8
Strunz classification 8.BE.30
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group P21/a
Unit cell a = 11.03, b = 12.12
c = 5.51 [Å], β = 114°, Z = 2
Identification
Color Brown, dark to light purplish red, brownish red, colorless to white
Cleavage Distinct, {001}
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 4.5
Luster Vitreous, slightly greasy on fracture surfaces
Streak Gray to faint brown
Specific gravity 3.83 (meas.), 3.94 (calc.)
Optical properties Biaxial (-)
Refractive index nα = 1.755–1.761
nβ = 1.772–1.786
nγ = 1.774–1.787
Pleochroism X = blood-red; Y = pale yellow; Z = sea-green
2V angle ~0°
References [1][2][3]

Allactite is a rare arsenate mineral of metamorphosed manganese zinc ore deposits. It is found in Sweden and New Jersey, US. Its name originated from Greek αλλάκτειν (allaktein) meaning "to change", referring to the strong pleochroism of the mineral.[2]

References

  1. ↑ Allactite. Mindat
  2. 1 2 Allactite. Handbook of Mineralogy
  3. ↑ Moore P. (1968). "Crystal chemistry of the basic manganese arsenate minerals: II. The crystal structure of allactite" (PDF). American Mineralogist. 53: 733–741.
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