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Lead-antimony sulfosalts from Tuscany (Italy); XIX, Crystal chemistry of chovanite from two new occurrences in the Apuan Alps and its 8 A crystal structure

Cristian Biagioni and Yves Moelo
Lead-antimony sulfosalts from Tuscany (Italy); XIX, Crystal chemistry of chovanite from two new occurrences in the Apuan Alps and its 8 A crystal structure
Mineralogical Magazine (August 2017) 81 (4): 811-831

Abstract

Two new occurrences of the lead oxy-sulfosalt chovanite have been identified at the Monte Arsiccio and Pollone mines, Apuan Alps, Tuscany, Italy. Chovanite from Monte Arsiccio occurs as black acicular crystals, up to 5 mm long, associated with rouxelite, robinsonite, sphalerite, valentinite, baryte, dolomite, quartz and Ba-rich K-feldspar ('hyalophane') in metadolostone vugs. Its unit-cell parameters are a = 48.38(5), b = 4.11(4), c = 34.18(4) A, beta = 106.26(2) degrees , V = 6521(64) A (super 3) , space group C2/m Very weak reflections indicate the doubling of the b parameter. Electron-microprobe data gave (wt.%): Ag 0.28, Tl 1.51, Pb 45.57, Sb 31.00, As 1.09, S 19.73, Se 0.05, Cl 0.02, sum 99.25. On the basis of Sigma Me = 58 apfu, its formula is Ag (sub 0.30) Tl (sub 0.86) Pb (sub 25.56) Sb (sub 29.59) As (sub 1.69) S (sub 71.52) Se (sub 0.07) Cl (sub 0.05) . Adding one oxygen atom, it is close to the formula TlPb (sub 26) (Sb,As) (sub 31) S (sub 72) O. Chovanite from Pollone occurs as thick black acicular crystals, up to 1 cm long, associated with baryte and quartz. The high-diffraction quality of the available material allowed the solution and refinement of the 8 A crystal structure in the space group P2 (sub 1) /c, with unit-cell parameters a = 34.052(3), b = 8.2027(7), c = 48.078(4) A, beta = 106.258(4) degrees , V = 12891.9(19) A (super 3) . The refinement converged to R (sub 1) = 9.14% on the basis of 19,853 observed reflections with F (sub o) > 4sigma (F (sub o) ). Electron-microprobe data gave (wt.%): Ag 0.12, Tl 0.54, Pb 48.34, Bi 0.20, Sb 26.71, As 3.37, S 20.23, Cl 0.07, sum 99.57. It corresponds to the formula Ag (sub 0.13) Tl (sub 0.30) Pb (sub 26.94) Bi (sub 0.10) Sb (sub 25.33) As (sub 5.20) S (sub 72.85) Cl (sub 0.20) , close to the idealized formula Pb (sub 28) (Sb,As) (sub 30) S (sub 72) O, with a single oxygen atom bound to two (Sb/As) atoms alternating with a vacancy along b as in scainiite and in other Pb oxy-sulfosalts. The crystal chemistry of this 8 A crystal structure is detailed, taking into account its modular description, the (Sb,As) (sub m) S (sub n) polymerization, its topological derivation from pellouxite, and the oxygen non-stoichiometry.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 81
Serial Issue: 4
Title: Lead-antimony sulfosalts from Tuscany (Italy); XIX, Crystal chemistry of chovanite from two new occurrences in the Apuan Alps and its 8 A crystal structure
Affiliation: Universita di Pisa, Dipartimento di Scienze della Terra, Pisa, Italy
Pages: 811-831
Published: 201708
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 34
Accession Number: 2017-058813
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N42°19'60" - N44°19'60", E09°40'00" - E12°25'00"
Secondary Affiliation: Universite de Nantes, FRA, France
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: 201731
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