Yellow crystals of the very rare phosphate jahnsite-(CaMnMn) associated to greenish black frondelite. From: Silbergrube, Waidhaus, Vohenstrauss, Bavaria, Germany. — Photo: David Hospital · CC BY-SA 4.0
Jahnsite-(CaMnMn)
Jahnsite-(CaMnMn) is an IMA-recognized mineral species in the phosphate class, with the chemical formula CaMn²⁺Mn²⁺₂Fe³⁺₂(PO₄)₄(OH)₂·8H₂O.
It crystallizes in the monoclinic crystal system. It is a member of the Jahnsite structural group.
Key properties: Mohs hardness 4; specific gravity 2.78; Vitreous luster; optically biaxial.
IMA symbol Jah-CaMnMn, IMA status: approved, named in 1987.
A late stage hydrothermal decomposition product of primary phosphate minerals in complex granite pegmatites.
It is found associated with Phosphosiderite, zodacite, varulite, microcline (Mangualde pegmatite, Portugal).
By analogy to jahnsite-(CaMnMg); the suffix indicates sequentially the dominant atom in the X, M(1), and M(2) structural positions.