The crystal chemistry of rathite based on new electron-microprobe data and single-crystal structure refinements

Autor(en)

Dan Topa, Uwe Kolitsch

Abstrakt

Crystal-structure refinements in space group P2

₁/c were performed on five grains of rathite with different types and degrees of thallium, silver, and antimony substitutions, as well as quantitative electron-microprobe analyses of more than 800 different rathite samples. The results of these studies both enlarged and clarified the complex spectrum of cation substitutions and the crystal chemistry of rathite. The [Tl

⁺ + As3

⁺] ↔ 2Pb

²⁺ scheme of substitution acts at the structural sites Pb1, Pb2, and Me6, the [Ag

⁺ + As

³⁺] ↔ 2Pb

²⁺ substitution at Me5, and the Sb-for-As substitution at the Me3 site only. The homogeneity range of rathite was determined to be unusually large, ranging from very Tl-poor compositions (0.16 wt%; refined single-crystal unit-cell parameters: a = 8.471(2), b = 7.926(2), c = 25.186(5) Å, β = 100.58(3)°, V = 1662.4(6) Å

³) to very Tl-rich compositions (11.78 wt%; a = 8.521(2), b = 8.005(2), c = 25.031(5) Å, β = 100.56(3)°, V = 1678.4(6) Å

³). The Ag content is only slightly variable (3.1 wt%-4.1 wt%) with a mean value of 3.6 wt%. The Sb content is strongly variable (0.20 wt%-7.71 wt%) and not correlated with the Tl content. With increasing Tl content (0.16 wt%-11.78 wt%), a clear increase of the unit-cell parameters a, b, and V, and a slight decrease of c is observed, although this is somewhat masked by the randomly variable Sb content. The revised general formula of rathite may be written as Ag

_xTl

_yPb

_16−2(x+y)As

_16+x+y−zSb

_zS

₄₀ (with 1.6 < x < 2, 0 < y < 3, 0 < z < 3.5). Based on Pb-S bond lengths, polyhedral characteristics and Pb-site bond-valence sums, we conclude that the Pb1 site is more affected by Tl substitution than the Pb2 site. When Tl substitution reaches values above 13 wt% (or 3 apfu), a new phase (“SR”), belonging to the rahite group, appears as lamellar exsolution intergrowths with Tl-rich rathite (11.78 wt%). Rathite is found only in the Lengenbach and Reckibach deposits, Binntal, Canton Wallis, Switzerland.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Naturhistorisches Museum Wien (NHM)

Journal

Minerals

Band

8

Seiten

1-20

Anzahl der Seiten

20

ISSN

2075-163X

DOI

https://doi.org/10.3390/min8100466

Publikationsdatum

10-2018

Peer-reviewed

Ja

ÖFOS 2012

105116 Mineralogie, 105113 Kristallographie

Schlagwörter

ASJC Scopus Sachgebiete

Geology, Geotechnical Engineering and Engineering Geology

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/db47d401-f01d-4991-8106-fb837c2f0547