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American Mineralogist; January; v. 94; no. 1; p. 151-155; DOI: 10.2138/am.2009.3034
© 2009 Mineralogical Society of America
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Adding further complexity to the polybasite structure: The role of Ag in the B layer of the -M2a2b2c polytype

Luca Bindi1,* and Silvio Menchetti2,3

1 Museo di Storia Naturale, Sezione di Mineralogia, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy
2 Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy
3 C.N.R., Istituto di Geoscienze e Georisorse sezione di Firenze, Via La Pira 4, I-50121 Firenze, Italy

Correspondence: * E-mail: luca.bindi{at}unifi.it

We report data on the composition and crystal structure of the most Ag-rich (15.63 apfu) natural polybasite yet discovered. It shows the -M2a2b2c polytype. The crystal studied was found in a sample (mineralogical collection of the Royal Ontario Museum) from Gowganda, Timiskaming District, Ontario, Canada. Electron microprobe analysis yields the formula [Ag6(Sb1.78As0.18){sum}=1.96S7] [Ag9(Ag0.63Cu0.43){sum}=1.06S4]. Lattice parameters are a = 26.2625(4), b = 15.1623(5), c = 24.1061(6) Å, β = 90.045(5)°, V = 9599.0(4) Å3. The structure was refined in the space group C2/c to R = 0.0581 using 7725 observed reflections [I > 2{sigma}(I)]. The refinement shows that one of the three structural positions of the B module layer usually occupied by Cu is dominated by Ag. Crystal-chemical characteristics are compared with published data on the other members of the pearceite-polybasite group. Some remarks concerning nomenclature are also given.

Key Words: Crystal structure • chemical composition • polytype • silver minerals






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