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Tl-bearing sulfosalt from the Lengenbach quarry, Binn Valley, Switzerland: Philrothite, TlAs3S5

Published online by Cambridge University Press:  05 July 2018

L. Bindi ,
F. Nestola ,
E. Makovicky ,
A. Guastoni  and
L. De Battisti
L. Bindi*
Affiliation:
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, I-50121 Florence, Italy CNR – Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via G. La Pira 4, I-50121 Florence, Italy
F. Nestola
Affiliation:
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, I-35131, Padua, Italy
E. Makovicky
Affiliation:
Department of Geoscience and Resource Management, University of Copenhagen, Østervoldgade 10, 1350 Copenhagen, Denmark
A. Guastoni
Affiliation:
Museo di Mineralogia, Università di Padova, Via Giotto 1, I-35122, Padua, Italy
L. De Battisti
Affiliation:
FGL (Forschungsgemeinschaft Lengenbach), Via dello Storno 18, I-20147 Milan, Italy
*
*E-mail: luca.bindi@unifi.it
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Abstract

Philrothite, ideally TlAs3S5, is a new mineral from the Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs as very rare crystals up to 200 mm across on realgar associated with smithite, rutile and sartorite. Philrothite is opaque with a metallic lustre and shows a dark brown streak. It is brittle; the Vickers hardness (VHN25) is 128 kg/mm2 (range: 120–137) (Mohs hardness of 3–3½). In reflected light philrothite is moderately bireflectant and weakly pleochroic from dark grey to light grey. Under crossed polars it is anisotropic with grey to bluish rotation tints. Internal reflections are absent. Reflectance percentages for the four COM wavelengths (Rmin and Rmax) are: 26.5, 28.8 (471.1 nm), 25.4, 27.2 (548.3 nm), 24.6, 26.3 (586.6 nm) and 24.0, 25.1 (652.3 nm), respectively.

Philrothite is monoclinic, space group P21/c, with a = 8.013(2), b = 24.829(4), c = 11.762(3) Å, β = 132.84(2)°, V = 1715.9(7) Å3, Z = 8. It represents the N = 4 homologue of the sartorite homologous series. In the crystal structure [R1 = 0.098 for 1217 reflections with I > 2σ(I)], Tl assumes tricapped prismatic sites alternating to form columns perpendicular to the b axis. Between the zigzag walls of Tl coordination prisms, coordination pyramids of As(Sb) form diagonally-oriented double layers separated by broader interspaces which house the lone electron pairs of these elements.

The eight strongest calculated powder-diffraction lines [d in Å(I/I0) (hkl)] are: 12.4145 (52) (020); 3.6768 (100) (61); 3.4535 (45) (131); 3.0150 (46) (53); 2.8941 (52) (81); 2.7685 (76) (230); 2.7642 (77) (34); 2.3239 (52) (092). A mean of five electron microprobe analyses gave Tl 26.28(12), Pb 6.69(8), Ag 2.50(4), Cu 0.04(2), Hg 0.07(2), As 32.50(13), Sb 3.15(3), S 26.35(10), total 97.58 wt.%, corresponding, on the basis of a total of nine atoms, to (Tl0.789Pb0.198)∑=0.987 (As2.662Sb0.159Ag0.142Cu0.004Hg0.002)∑=2.969S5.044. The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (2013-066) and named for Philippe Roth (b. 1963), geophysicist and well known mineral expert on the Lengenbach minerals for more than 25 years.

Keywords

philrothitenew mineralTl-sulfosaltLengenbachSwitzerland
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 1 , February 2014 , pp. 1 - 9
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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