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Eckerite, Ag (sub 2) CuAsS (sub 3) , a new Cu-bearing sulfosalt from Lengenbach Quarry, Binn Valley, Switzerland; description and crystal structure

Luca Bindi, Fabrizio Nestola, S. Graeser, P. Tropper and T. Raber
Eckerite, Ag (sub 2) CuAsS (sub 3) , a new Cu-bearing sulfosalt from Lengenbach Quarry, Binn Valley, Switzerland; description and crystal structure
Mineralogical Magazine (June 2015) 79 (3): 687-694

Abstract

Eckerite, ideally Ag (sub 2) CuAsS (sub 3) , is a new mineral from the Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs as very rare euhedral crystals up to 300 mu m across associated with realgar, sinnerite, hatchite, trechmannite and yellow, fibrous smithite. In thick section eckerite is opaque with a metallic lustre and shows a dark orange-red streak. It is brittle; the Vickers hardness (VHN (sub 25) ) is 70 kg/mm (super 2) (range: 64-78) (Mohs hardness of approximately 21/2-3). In reflected light, eckerite is moderately bireflectant and weakly pleochroic from light grey to a slightly bluish grey. Internal reflections are absent. Under crossed nicols, it is weakly anisotropic with greyish to light blue rotation tints. Reflectance percentages for R (sub min) and R (sub max) are 27.6, 31.7 (471.1 nm), 22.8, 26.1 (548.3 nm), 21.5, 24.5 (586.6 nm) and 19.4, 22.3 (652.3 nm), respectively. Eckerite is monoclinic, space group C2/c, with a=11.8643(3), b=6.2338(1), c=16.6785(4) Aa, beta =110.842(3) degrees , V=1152.81(5) Aa (super 3) , Z=8. The crystal structure [R (sub 1) =0.0769 for 1606 reflections with F (sub o) >4sigma (F (sub o) )] is topologically identical to that of xanthoconite and pyrostilpnite. In the structure, AsS (sub 3) pyramids are joined by AgS (sub 3) triangles to form double sheets parallel to (001); the sheets are linked by Cu(Ag) atoms in a quasi-tetrahedral coordination. Among the three metals sites, Ag2 is dominated by Cu. The mean metal-S distances reflect well the Ag<-->Cu substitution occurring at this site.The eight strongest powder X-ray diffraction lines [d in Aa (I/I (sub o) )(hkl)] are: 3.336 (70) (312); 2.941 (100) (314,114); 2.776 (80) (400,206); 2.677 (40) (312); 2.134 (50) (421); 2.084 (40) (208,206); 2.076 (40) (420); 1.738 (40) (228,226). A mean of five electron microprobe analyses gave Ag 52.08(16), Cu 11.18(9), Pb 0.04(1), Sb 0.29(3), As 15.28(11), S 20.73(13), total 99.60 wt.%, corresponding, on the basis of a total of 7 atoms per formula unit, to Ag (sub 2.24) Cu (sub 0.82) As (sub 0.94) Sb (sub 0.01) S (sub 2.99) . The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (2014-063) and named for Markus Ecker, a well known mineral expert on the Lengenbach minerals for more than 25 years.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 79
Serial Issue: 3
Title: Eckerite, Ag (sub 2) CuAsS (sub 3) , a new Cu-bearing sulfosalt from Lengenbach Quarry, Binn Valley, Switzerland; description and crystal structure
Affiliation: Universita di Firenze, Dipartimento di Scienze della Terra, Florence, Italy
Pages: 687-694
Published: 201506
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 24
Accession Number: 2016-007019
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 8 tables
N46°22'00" - N46°22'00", E08°11'30" - E08°11'30"
Secondary Affiliation: Universita di Padova, ITA, ItalyUniversitaet Basel, CHE, SwitzerlandUniversity of Innsbruck, AUT, AustriaForschungsgemeinschaft Lengenbach, DEU, Germany
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201604
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