Pseudomeisserite-(NH4)
Pseudomeisserite-(NH4) is an IMA-recognized mineral species in the sulfate class, with the chemical formula (NH₄)₂Na₄[(UO₂)₂(SO₄)₅]·4H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c).
Key properties: Mohs hardness 2.5; specific gravity 3.22; Very pale yellow streak; Vitreous luster; optically biaxial.
IMA symbol Pmss-NH4, IMA status: approved, named in 2019.
Secondary, post-mining phase in efflorescent crusts on mine walls by oxidation of primary uraninite, montroseite, and coffinite in sandstones in a humid underground environment.
It is found associated with Belakovskiite, blödite, changoite, ferrinatrite, gypsum, ivsite, metavoltine, tamarugite.
The prefix recognizes the similarity in appearance to meisserite and the suffix as the NH4- dominant member of a series that probably exists with a hypothetical K-dominant member.