Allactite

Allactite
Picture width 3 mm
General
Category	Arsenate mineral
Formula (repeating unit)	Mn7(AsO4)2(OH)8
Strunz classification	08.BE.30
Unit cell	a = 11.03 Å, b = 12.12 Å, c = 5.51 Å, β = 114°, Z = 2
Identification
Color	Brown, dark to light purplish red, brownish red, colorless to white
Crystal symmetry	Monoclinic, point group: 2/m, space group P21/a
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. 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.