Yellowish crystals of the rare manganese sulphate mineral ilesite in a matrix of white apjohnite from one of the few localities known worldwide for ilesite. From: Santa Ana Mine, Mazarron, Murcia, Spain. — Photo: David Hospital · CC BY-SA 4.0
Apjohnite
A manganese-aluminium sulfate hydrate named for James Apjohn, professor at Trinity College Dublin.
Intro adapted from Wikipedia (CC BY-SA).
Apjohnite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Mn²⁺Al₂(SO₄)₄·22H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c). It is a member of the Halotrichite structural group.
Key properties: Mohs hardness 1.5; specific gravity 1.78–1.81; Silky in aggregates luster; optically biaxial.
IMA symbol Apj, grandfathered status — known before the IMA’s founding, named in 1847.
Occurs as efflorescences.
It is found associated with Halotrichite, pickeringite, gunningite, copiapite, epsomite.
The mineral was named to honor Dr. James Apjohn (1796–1886), Professor of Chemistry and Mineralogy, Trinity College, Dublin, Ireland.