Heyrovskýite, Lillianite Solid Solution and Galena from the Yakuki Mine, Fukushima Prefecture, Japan
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概要
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Heyrovskyite from the Yakuki mine, Fukushima Prefecture, occurs as aggregates of minute blades in clinopyroxene skarns with two different mineral assemblages. One of them accompanies galena, lillianite solid solution, pyrrhotite, chalcopyrite and magnetite, whereas the other does magnetite, chalcopyrite, sphalerite and galena. The averages of five analyses of them are, respectively: Ag 1.13, 2.17; Cu 0.05, 0.07; Pb 59.33, 56.00; Bi 24.27, 26.72; S 14.83, 15.00; totals 99.61, 99.96%. Their empirical formulae calculated on the basis of S atoms=9 are, respectively: Pb<SUB>5.53</SUB>Ag<SUB>0.20</SUB>Cu<SUB>0.02</SUB>Bi<SUB>2</SUB>.24S<SUB>9</SUB>and Pb<SUB>5.15</SUB>Ag<SUB>0.38</SUB>Bi<SUB>2.44</SUB>S<SUB>9</SUB>. They are Ag-poorer heyrovskyites closer to the ideal formula, Pb<SUB>6</SUB>Bi<SUB>2</SUB>S<SUB>9</SUB>, from which they are derived by the coupled substitution of AgBi for PbPb, though the minor effect of the substitution of 2Bi for 3Pb exists. The average of four analyses of lillianite solid solution is: Ag 1.99, Cu 0.14, Pb 42.30, Bi 39.59, S 15.75, total 99.77%. The range of chemical analyses indicates that this phase demonstrates the substitution of 2Bi for 3Pb. Those of two galena analyses in them are, respectively: Ag 0.37, 0.97; Pb 84.01, 82.16; Bi 1.95, 3.15; S 13.35, 13.29; totals 99.68, 99.56%. In galena the coupled substitution of AgBi for PbPb is less developed than in heyrovskýite, but that of 2Bi for 3Pb is more developed. The cell parameters of two heyrovskýites are, respectively: a=13.719, 13.704; b=31.260, 31.247; c=4.127Å, 4.124Å. Also, the role of Ag is discussed after the reference to the variable Ag contents in this mineral. Probably it serves to prevent the phase from the inversion to its monoclinic derivative aschamalmite, which has real compositions closer to Pb<SUB>6</SUB>Bi<SUB>2</SUB>S<SUB>9</SUB> and would be stable under lower temperature condition. Ag-free heyrovskyites hitherto described by only chemical analyses needs verification.
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