Abstract
Crystal-structure refinements in space group P21/c were performed on five grains of rathite with different types and degrees of thallium, silver, and antimony substitutions, as well as quantitative electron-microprobe analyses of more than 800 different rathite samples. The results of these studies both enlarged and clarified the complex spectrum of cation substitutions and the crystal chemistry of rathite. The [Tl+ + As3+] ↔ 2Pb2+ scheme of substitution acts at the structural sites Pb1, Pb2, and Me6, the [Ag+ + As3+] ↔ 2Pb2+ substitution at Me5, and the Sb-for-As substitution at the Me3 site only. The homogeneity range of rathite was determined to be unusually large, ranging from very Tl-poor compositions (0.16 wt%; refined single-crystal unit-cell parameters: a = 8.471(2), b = 7.926(2), c = 25.186(5) Å, β = 100.58(3)°, V = 1662.4(6) Å3) to very Tl-rich compositions (11.78 wt%; a = 8.521(2), b = 8.005(2), c = 25.031(5) Å, β = 100.56(3)°, V = 1678.4(6) Å3). The Ag content is only slightly variable (3.1 wt%–4.1 wt%) with a mean value of 3.6 wt%. The Sb content is strongly variable (0.20 wt%–7.71 wt%) and not correlated with the Tl content. With increasing Tl content (0.16 wt%–11.78 wt%), a clear increase of the unit-cell parameters a, b, and V, and a slight decrease of c is observed, although this is somewhat masked by the randomly variable Sb content. The revised general formula of rathite may be written as AgxTlyPb16−2(x+y)As16+x+y−zSbzS40 (with 1.6 < x < 2, 0 < y < 3, 0 < z < 3.5). Based on Pb–S bond lengths, polyhedral characteristics and Pb-site bond-valence sums, we conclude that the Pb1 site is more affected by Tl substitution than the Pb2 site. When Tl substitution reaches values above 13 wt% (or 3 apfu), a new phase (“SR”), belonging to the rahite group, appears as lamellar exsolution intergrowths with Tl-rich rathite (11.78 wt%). Rathite is found only in the Lengenbach and Reckibach deposits, Binntal, Canton Wallis, Switzerland.
Highlights
The name rathite was given by Baumhauer [1], in honor of Gerhard von Rath, Professor of Mineralogy, Bonn, Germany, to a new mineral of the sartorite–dufrénoysite series from the Lengenbach deposit (Binntal, Canton Wallis, Switzerland) with the formula 12PbS·5As2 S3 ·Sb2 S3(=Pb12 As10 Sb2 S30 )
No subsequent studies of rathite were conducted prior to our present work, Makovicky and Topa [13] recently present a set of analytical formulae for the calculation of the homolog order Ncalc and the substitution degree for Tl and Ag in the sartorite homologous series (Tlsubst and Agsubst ), from quantitative electron microprobe analyses
Our studies show that rathite is commonly intimately intergrown with sartorite, baumhauerite, liveingite, and dufrénoysite, and occurs as larger, single-phase homogenous grains
Summary
The name rathite was given by Baumhauer [1], in honor of Gerhard von Rath, Professor of Mineralogy, Bonn, Germany, to a new mineral of the sartorite–dufrénoysite series from the Lengenbach deposit (Binntal, Canton Wallis, Switzerland) with the formula 12PbS·5As2 S3 ·Sb2 S3. Berlepsch et al [2], on the basis of new single-crystal structure determination of chemically well-characterized rathite and summarizing past achievements in unraveling name, chemistry, structure, and classification made by Solly [3], Giuşcă [4], Berry [5], Le Bihan [6], Nowacki et al [7], Marumo and Nowacki [8], Makovicky [9], Laroussi et al [10], Pring [11], and Berlepsch et al [12], presented the last chapter of the complex story of this rare mineral They concluded that rathite is a member of sartorite homologous series with space group symmetry.
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