In:
Mineralogical Magazine, Mineralogical Society, Vol. 83, No. 5 ( 2019-10), p. 741-748
Abstract:
The new minerals cuatrocapaite-(NH 4 ) (IMA2018-083) and cuatrocapaite-(K) (IMA2018-084) are the NH 4 - and K-dominant members of a series with the general formula (NH 4 ,K) 3 (NaMg□)(As 2 O 3 ) 6 Cl 6 ·16H 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 ~0.3 mm in diameter. They are transparent, with a vitreous lustre and white streak. For both, the Mohs hardness is ca . 2½, 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 3 for the NH 4 - and K-dominant species, respectively. Optically, cuatrocapaite-(NH 4 ) is uniaxial (–) with ω = 1.779(3) and ε = 1.541(3) and cuatrocapaite-(K) is uniaxial (–) with ω = 1.777(3) and ε = 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 4 ) 2.48 Na 1.66 Mg 0.87 K 0.09 (As 12 O 18.05 )Cl 5.88 ·16.02H 2 O and K 2.68 Na 1.33 Mg 0.93 (NH 4 ) 0.31 (As 12 O 18.01 )Cl 6.16 ·16.04H 2 O. The minerals are trigonal, space group R ${\bar 3}$ m ; the cuatrocapaite-(NH 4 ) cell parameters are a = 5.25321(19), c = 46.6882(19) Å, V = 1115.80(9) Å 3 and Z = 1; the cuatrocapaite-(K) cell parameters are a = 5.2637(15), c = 46.228(8) Å, V = 1109.2(7) Å 3 and Z = 1. The structures, refined for cuatrocapaite-(NH 4 ) to R 1 = 1.78% for 544 I o 〉 2σ I reflections, contain four types of layers: (1) a planar neutral As 2 O 3 (arsenite) sheet; (2) an ( ${\rm NH}_{\rm 4}^{\vskip -2pt\rm \scale65% +} $ ,K + ) layer that links adjacent arsenite sheets; (3) a Cl – layer placed on the As side of each arsenite; and (4) a layer containing partially occupied Na, Mg and H 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 2 O 3 –K–As 2 O 3 –Cl layer sequence in the structures of lucabindiite and gajardoite.
Type of Medium:
Online Resource
ISSN:
0026-461X
,
1471-8022
Language:
English
Publisher:
Mineralogical Society
Publication Date:
2019
detail.hit.zdb_id:
2034522-7
SSG:
13
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