Polloneite, a new complex Pb(-Ag)-As-Sb sulfosalt from the Pollone mine, Apuan Alps, Tuscany, Italy

Author(s)

Dan Topa, Frank N. Keutsch, Emil Makovicky, Uwe Kolitsch, Hermann Werner Paar

Abstract

Polloneite, ideally AgPb

₄₆As

₂₆Sb

₂₃S

₁₂₀, is a new N = 4 member of the sartorite homologous series. It occurs in a matrix of baryte from the Pizzone level of the Pollone baryte-pyrite-(Pb-Zn-Ag) deposit at Valdicastello Carducci, near Pietrasanta, in the Apuan Alps, Tuscany, Italy, as anhedral grains up to 0.5 mm across. The mineral is opaque, greyish black with a metallic lustre. In reflected light polloneite is white, bireflectance is moderate. Internal reflections are absent. Under crossed polars, anisotropism is moderate with rotation tints in brown-violet and deep grey. The reflectance data (%, air) are: 30.2, 42.4 at 470 nm, 28.8, 41.0 at 546 nm, 27.9, 39.8 at 589 nm and 26.0, 37.4 at 650 nm. Mohs hardness is 3-3 1/2, microhardness VHN

₅₀ exhibits a mean value of 200 kg mm

^-2. The average results of 15 electron-microprobe analyses of three grains are Ag 0.71(5), Pb 52.05(21), As 10.61(22), Sb 15.40(12), S 21.16(8), total 99.92(15) wt.%, corresponding to Ag

_1.20Pb

_45.76As

_25.79Sb

_23.04S

_120.21 (on the basis of Me + S = 216 apfu). The simplified formula AgPb

₄₆As

₂₆Sb

₂₃S

₁₂₀ is in accordance with the results of a crystal structure determination. The calculated density is 5.77 g cm

³. Polloneite is monoclinic, space group P2

₁, a = 8.413(2), b = 25.901(5), c = 23.818(5) å, β = 90.01(3)°, V = 5189.8(18) å

³ Z = 1. The strongest eight lines in the calculated powder-diffraction pattern [d in å(I)hkl] are 3.795(100)(026), 3.414(60)(233), 3.238(69)(080), 3.020(97)(253), 2.922(82)(066), 2.738(73)(236), 2.375(79)(290) and 2.103(64)(400). Polloneite is a new N = 4 member of the sartorite homologous series with substantial Sb and small, but important, Ag content. It is a three-fold superstructure with a tripled unit-cell parameter, 7.9 å, of sartorite homologues. In the As-Sb rich slabs, several types of crankshaft chains and isolated (As,Sb)-S polyhedra occur. A sequence of three different, tightly bonded double-layer fragments (broad ribbons) contains two asymmetric fragments with long crankshaft chains whereas the third fragment type, with Ag, contains small mirror-symmetrical metalloid groups and no crankshaft chains. This configuration can potentially cause order-disorder phenomena in the structure. The threefold superstructure and the mixed As-Sb character distinguish polloneite from veenite and from dufrénoysite, respectively.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Harvard University, Naturhistorisches Museum Wien (NHM), University of Copenhagen, Paris-Lodron Universität Salzburg

Journal

Mineralogical Magazine

Volume

81

Pages

1303-1322

No. of pages

20

ISSN

0026-461X

DOI

https://doi.org/10.1180/minmag.2017.081.003

Publication date

12-2017

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 105113 Crystallography

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology

Portal url

https://ucrisportal.univie.ac.at/en/publications/78c480de-a570-4ab1-874f-d79760f257bd