We report the characterisation of two new tetrahedrite-group minerals, rozhdestvenskayaite, Ag10Zn2Sb4S13, and argentotetrahedrite, Ag6Cu4(Fe2+,Zn)2Sb4S13, and discuss the structural chemistry of the Ag-rich tetrahedrite group in order to identify correlations between structure and composition. Argentotetrahedrite is the Sb-analogue of argentotennantite. Rozhdestvenskayaite is named for Irina Rodzhdestvenskaya in recognition of her important contributions to mineralogy, and to the crystal chemistry of the tetrahedrite group in particular. Rozhdestvenskayaite is cubic with unit-cell parameter a = 10.9845(7) Å and V = 1325.37(15) Å3, and has a Mohs’ hardness of 3.0. Argentotetrahedrite is cubic with unit-cell parameter a = 10.6116(1) Å and V = 1194.92(2) Å3, and has a Mohs’ hardness of 3.9. The five strongest peaks in the X-ray powder diffraction patterns ([(hkl), dobs (Å), I/Imax (%)]) are: rozhdestvenskayaite [(222), 3.161, 100], [(004), 2.738, 35], [(044), 1.936, 24], [(226), 1.651, 19], [(134,015), 2.147, 18]; argentotetrahedrite [(222), 3.063, 100], [(044), 1.876, 35], [(004), 2.652, 28], [(226), 1.599, 25], [(134,015), 2.081, 19]. The large unit-cell volume of rozhdestvenskayaite is due to the high Ag content (70%) of the BS4 tetrahedron. A major issue for understanding the crystal chemistry of the tetrahedrite group relates to the occupancy of the Z site in which sulfur is coordinated octahedrally to monovalent metals (Cu or Ag). It is shown that this site can be completely empty, as in freibergite, and that this vacancy leads to a marked reduction in the volume of the Z(A6) octahedron from 16 Å3 in tetrahedrite with a fully occupied site to 11 Å3 in freibergite with a vacant Z site. In freibergite, the Ag–Ag distance of the Ag6 octahedron is 2.84 Å and is almost the same as that of silver metal (2.85 Å), strongly suggesting the presence of metallic bonding in this octahedral group. In contrast, the corresponding Ag–Ag distances of argentotetrahedrite and rozhdestvenskayaite are 3.23 Å and 3.24 Å, respectively, indicative of no metallic bonding. An important consequence of the metallically bonded Ag6 group of freibergite is that it has an aggregate formal charge of +4 (not +6), leading to a charge-balanced ideal formula (Ag6)4+(Cu+)4(Zn,Fe2+)2Sb4S12.
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Research Article|
November 01, 2018
Rozhdestvenskayaite Ag10Zn2Sb4S13 and argentotetrahedrite Ag6Cu4(Fe2+,Zn)2Sb4S13: two Ag-dominant members of the tetrahedrite group
Mark D. Welch
;
1
Department of Earth Sciences, Natural History Museum
, LondonSW7 5BD, United Kingdom
Corresponding author, e-mail: mdw@nhm.ac.uk
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Chris J. Stanley
;
Chris J. Stanley
1
Department of Earth Sciences, Natural History Museum
, LondonSW7 5BD, United Kingdom
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John Spratt
;
John Spratt
2
Core Research Laboratories, Natural History Museum
, LondonSW7 5BD, United Kingdom
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Stuart J. Mills
Stuart J. Mills
3
Geosciences, Museums Victoria
, GPO Box 666, Melbourne3001, Australia
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European Journal of Mineralogy (2018) 30 (6): 1163-1172.
Article history
received:
11 Jan 2018
rev-recd:
09 Apr 2018
accepted:
11 Apr 2018
first online:
04 Sep 2018
Citation
Mark D. Welch, Chris J. Stanley, John Spratt, Stuart J. Mills; Rozhdestvenskayaite Ag10Zn2Sb4S13 and argentotetrahedrite Ag6Cu4(Fe2+,Zn)2Sb4S13: two Ag-dominant members of the tetrahedrite group. European Journal of Mineralogy ; 30 (6): 1163–1172. doi: https://doi.org/10.1127/ejm/2018/0030-2773
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Index Terms/Descriptors
- bonding
- Canada
- Central Europe
- coordination
- crystal chemistry
- crystal structure
- electron probe data
- Europe
- formula
- Freiberg Germany
- freibergite
- Germany
- Harz Mountains
- Keno Hill Yukon Territory
- metals
- Mexico
- Moctezuma Mexico
- new minerals
- physical properties
- refinement
- Saxony Germany
- silver
- Sonora Mexico
- sulfantimonites
- sulfosalts
- tetrahedrite
- unit cell
- Western Canada
- X-ray diffraction data
- Yukon Territory
- Moctezuma Mine
- Andreasberg Germany
- argentotetrahedrite
- Saint Andreaskreuz Mine
- Beschert Gluck Mine
- rozhdestvenskayaite
Latitude & Longitude