Allactite Locality: Sterling Mine, Sterling Hill, Ogdensburg, Franklin Mining District, Sussex County, New Jersey, USA FOV 3.7 x 2.5 mm. Via Nick Zipco (1993) MOB coll. Note: In my (limited) experience, the color of allactite at Sterling Hill is NOT purplish - or red, scarlet, crimson etc. — Photo: Modris Baum · Public domain
Allactite
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 the Greek αλλάκτειν (allaktein) meaning "to change", referring to the strong pleochroism of the mineral.
Intro adapted from Wikipedia (CC BY-SA).
Allactite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Mn²⁺₇(AsO₄)₂(OH)₈.
It crystallizes in the monoclinic crystal system (space group P2_1/a). It is a member of the Allactite structural group.
Key properties: Mohs hardness 4.5; specific gravity 3.83; Gray to faint brown streak; Vitreous, slightly greasy on fracture surfaces luster; refractive index 1.773; optically biaxial.
IMA symbol Ala, IMA status: approved, named in 1884.
Found as a rare secondary mineral in veinlets through metamorphosed manganese deposits (Sweden); in a metamorphosed stratiform zinc orebody (Franklin, New Jersey, USA).
It is found associated with Synadelphite, hematolite, hausmannite, pyrochroite, fluorite (Moss mine, Sweden); pyroaurite, leucophoenicite, hodgkinsonite, adelite, franklinite, willemite.
The name comes from the Greek, to change, a reference to the strong pleochroism.