Cuatrocapaite-(NH (sub 4) ) and cuatrocapaite-(K), two new minerals from the Torrecillas Mine, Iquique Province, Chile, related to lucabindiite and gajardoite

The new minerals cuatrocapaite-(NH (sub 4) ) (IMA2018-083) and cuatrocapaite-(K) (IMA2018-084) are the NH (sub 4) - and K-dominant members of a series with the general formula (NH (sub 4) ,K) (sub 3) (NaMg (sub []) )(As (sub 2) O (sub 3) ) (sub 6) Cl (sub 6) .16H (sub 2) O. Both minerals were found at the Torrecillas mine, Iquique Province, Chile, where they occur as secondary alteration phases. Both minerals occur as hexagonal tablets up to approximately 0.3 mm in diameter. They are transparent, with a vitreous lustre and white streak. For both, the Mohs hardness is ca. 21/2, the crystals are somewhat flexible, but not elastic, the fracture is irregular and the cleavage is perfect on {001}. The measured densities are 2.65(2) and 2.76(2) g/cm (super 3) for the NH (sub 4) - and K-dominant species, respectively. Optically, cuatrocapaite-(NH (sub 4) ) is uniaxial (-) with omega =1.779(3) and epsilon =1.541(3) and cuatrocapaite-(K) is uniaxial (-) with omega =1.777(3) and epsilon =1.539(3) (white light). The minerals are insoluble in acids, but decompose in NaOH(aq). The empirical formulas, determined from electron-microprobe analyses, are (NH (sub 4) ) (sub 2.48) Na (sub 1.66) Mg (sub 0.87) K (sub 0.09) (As (sub 12) O (sub 18.05) )C (sub l5.88) .16.02H (sub 2) O and K (sub 2.68) Na (sub 1.33) Mg (sub 0.93) (NH (sub 4) ) (sub 0.31) (As (sub 12) O (sub 18.01) )Cl (sub 6.16) .16.04H (sub 2) O. The minerals are trigonal, space group R3 m; the cuatrocapaite-(NH (sub 4) ) cell parameters are a=5.25321(19), c=46.6882(19) Aa, V=1115.80(9) Aa (super 3) and Z=1; the cuatrocapaite-(K) cell parameters are a=5.2637(15), c=46.228(8) Aa, V=1109.2(7) Aa (super 3) and Z=1. The structures, refined for cuatrocapaite-(NH (sub 4) ) to R (sub 1) =1.78% for 544 Io >2sigma I reflections, contain four types of layers: (1) a planar neutral As (sub 2) O (sub 3) (arsenite) sheet; (2) an (NH (super +) (sub 4) , K (super +) ) layer that links adjacent arsenite sheets; (3) a Cl (super -) layer placed on the As side of each arsenite; and (4) a layer containing partially occupied Na, Mg and H (sub 2) O sites that is flanked on either side by Cl layers. The layer sequence for the type 1, 2 and 3 layers is identical to the Cl-As (sub 2) O (sub 3) -K-As (sub 2) O (sub 3) -Cl layer sequence in the structures of lucabindiite and gajardoite.