Cuatrocapaite-(NH4)
Cuatrocapaite-(NH4) is an IMA-recognized mineral species in the arsenate class, with the chemical formula (NH₄)₃(NaMg )(As₂O₃)₆Cl₆·16H₂O.
It crystallizes in the hexagonal crystal system (space group P-3m1). It is a member of the Cuatrocapaite structural group.
Key properties: Mohs hardness 2.5; specific gravity 2.65; White streak; Vitreous to pearly; silky (vein fillings) luster; optically uniaxial.
IMA symbol Ctc-NH4, IMA status: approved, named in 2018.
A secondary alteration phase formed under hyperarid conditions from the oxidation of native arsenic, and possibly other As-bearing primary phases, coupled with reaction with underground brines rich in mobile cations such as K +, Na+, NH4+.
It is found associated with Lavendulan, magnesiokoritnigite, torrecillasite, native arsenic, arsenolite, pyrite.
An allusion to the structure, which consists of four (cuatro in Spanish) different types of layers (capa in Spanish): (1) [As2O3]; (2) [(NH4), K]; (3) [Cl6]; and (4) [(Na, Mg, )3(H2O)16].