Allactite
Picture width 3 mm
General
CategoryArsenate mineral
Formula
(repeating unit)
Mn7(AsO4)2(OH)8
IMA symbolAla[1]
Strunz classification8.BE.30
Dana classification41.2.1.1
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/a
Unit cella = 11.03, b = 12.12
c = 5.51 [Å], β = 114°, Z = 2
Identification
ColorBrown, dark to light purplish red, brownish red, colorless to white
Crystal habitElongated prisms, bladed, tabular, rosette-like aggregates
CleavageDistinct, {001}
FractureUneven
TenacityBrittle
Mohs scale hardness4.5
LusterVitreous, slightly greasy on fracture surfaces
StreakGray to faint brown
Specific gravity3.83 (meas.), 3.94 (calc.)
Optical propertiesBiaxial (-)
Refractive indexnα = 1.755–1.761
nβ = 1.772–1.786
nγ = 1.774–1.787
PleochroismX = blood-red; Y = pale yellow; Z = sea-green
2V angle~0°
References[2][3][4]

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.[3]

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Allactite. Mindat
  3. 1 2 Allactite. Handbook of Mineralogy
  4. Moore P. (1968). "Crystal chemistry of the basic manganese arsenate minerals: II. The crystal structure of allactite" (PDF). American Mineralogist. 53: 733–741.

Bibliography

  • Palache, P.; Berman H.; Frondel, C. (1960). "Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)" John Wiley and Sons, Inc., New York, pp. 785-787.
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