Luxembourgite, AgCuPbBi₄Se₈, a new mineral species from Bivels, Grand Duchy of Luxembourg

<p>Luxembourgite, ideally <span class="inline-formula">AgCuPbBi₄Se₈</span>, is a new selenide discovered at Bivels, Grand Duchy of Luxembourg. The mineral forms tiny fibres reaching 200&thinsp;<span class="inline-formula">µ</span>m in length and 5&thinsp;<span class="inline-formula">µ</span>m in diameter, which are deposited on dolomite crystals. Luxembourgite is grey, with a metallic lustre and without cleavage planes; its Mohs hardness is 3 and its calculated density is 8.00&thinsp;g&thinsp;cm<span class="inline-formula">⁻³</span>. Electron-microprobe analyses indicate an empirical formula <span class="inline-formula">Ag_1.00(Cu_0.82Ag_0.20Fe_0.01)_Σ1.03Pb_1.13Bi_4.11(Se_7.72S_0.01)_Σ7.73</span>, calculated on the basis of 15 atoms per formula unit. A single-crystal structure refinement was performed to <span class="inline-formula"><i>R</i>₁=0.0476</span>, in the <span class="inline-formula"><i>P</i>2₁∕<i>m</i></span> space group, with <span class="inline-formula"><i>a</i>=13.002(1)</span>, <span class="inline-formula"><i>b</i>=4.1543(3)</span>, <span class="inline-formula"><i>c</i>=15.312(2) <i>Å</i></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">β</mi><mo>=</mo><mn mathvariant="normal">108.92</mn><mo>(</mo><mn mathvariant="normal">1</mn><mo>)</mo><msup><mi/><mo>∘</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="4d920107eb4a98473050fd7bdaf16dee"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-449-2020-ie00001.svg" width="73pt" height="13pt" src="ejm-32-449-2020-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><i>V</i>=782.4(2) <i>Å</i>³</span>, <span class="inline-formula"><i>Z</i>=2</span>. The crystal structure is similar to that of litochlebite and watkinsonite and can be described as an alternation of two types of anionic layers: a pseudotetragonal layer four atoms thick and a pseudohexagonal layer that is one atom thick. In the pseudotetragonal layers the Bi1, Bi2 ,Bi3, Pb, and Ag1 atoms are localised, while the Cu2 and Bi4 atoms occur between the pseudotetragonal and the pseudohexagonal layers. Bi1, Bi2, and Bi3 atoms occur in weakly distorted octahedral sites, whereas Bi4 occurs in a distorted 7-coordinated site. Ag1 occupies a fairly regular octahedral site, Cu2 a tetrahedral position, and Pb occurs on a very distorted 8-coordinated site.</p>