Allophane (Size ~ 3 cm) Locality: Maid of Sunshine Mine, Turquoise District (Courtland-Gleeson District), Dragoon Mts, Cochise County , Arizona, USA — Photo: Modris Baum · Public domain
Allophane
A poorly crystalline hydrous aluminium silicate — technically a mineraloid rather than a true mineral — common in weathered volcanic soils.
Intro adapted from Wikipedia (CC BY-SA).
Allophane is an IMA-recognized mineral species in the silicate class, with the chemical formula Al₂O₃(SiO₂)₁.₃₋₂.₀·2.5-3.0H₂O.
It is a member of the Allophane structural group.
Key properties: Mohs hardness 3; specific gravity 2.75; Vitreous to earthy, waxy luster.
IMA symbol Alp, grandfathered status — known before the IMA’s founding, named in 1816.
A weathering product of volcanic ash. In hydrothermally altered igneous rocks, from the breakdown of feldspars, and in hydrothermal veins, typically related to copper deposits. In sedimentary rocks, including chalk and coal beds.
It is found associated with Quartz, cristobalite, imogolite, gibbsite, vermiculite, chrysocolla, \limonite".
Its name comes from the Greek for other and to appear, referring to a change of appearance under the blowpipe.