Spaltiite, Tl₂Cu₂As₂S₅, one more new thallium sulfosalt mineral from Lengenbach quarry, Binn, Switzerland

<p>Spaltiite is a new thallium sulfosalt with the ideal formula of Tl<span class="inline-formula">₂</span>Cu<span class="inline-formula">₂</span>As<span class="inline-formula">₂</span>S<span class="inline-formula">₅</span>. It was found on a dump of the famous mineral locality Lengenbach (Binntal, Canton Valais, Switzerland). A small piece of pure white Triassic dolomite belonging to the Penninic Monte Leone Nappe hosts three euhedral long prismatic to lath-like spaltiite crystals, each approximately 2 mm in length but only <span class="inline-formula">∼0.2</span> mm thin. The hand specimen contains small quantities of pyrite, drechslerite and hatchite. The spaltiite crystals are greyish to black in colour and extremely soft. The Mohs' hardness is 1.5–2 (VHN<span class="inline-formula">₁₅</span> ranges from 30 to 65, mean 47 kg mm<span class="inline-formula">⁻²</span>). The mono-clinic crystals have a perfect cleavage parallel to <span class="inline-formula"><i>{</i>100<i>}</i></span>, which produces minute and plastic slabs. Reflectance measurements in air yield the following <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>R</mi><mi mathvariant="normal">min</mi></msub><mo>/</mo><msub><mi>R</mi><mi mathvariant="normal">max</mi></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="d02c9cdc34358548d654e324c6929bce"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00001.svg" width="52pt" height="14pt" src="ejm-38-27-2026-ie00001.png"/></svg:svg></span></span> values based on the standard wavelengths (Commission on Ore Mineralogy, COM): 27.0 % <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="539a58614ea8688159b8effbc6d3da8d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00002.svg" width="8pt" height="14pt" src="ejm-38-27-2026-ie00002.png"/></svg:svg></span></span> 32.6 % (470 nm); 26.8 % <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7572a9d7afeaa92ba0e8bb6f686362bd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00003.svg" width="8pt" height="14pt" src="ejm-38-27-2026-ie00003.png"/></svg:svg></span></span> 32.1 % (546 nm); 26.0 % <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c6f00d13d95b9183e3e2526db4298e27"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00004.svg" width="8pt" height="14pt" src="ejm-38-27-2026-ie00004.png"/></svg:svg></span></span> 31.1 % (589 nm); and 24.8 % <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a3a809672b156f3719eee3cbaf593ee5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00005.svg" width="8pt" height="14pt" src="ejm-38-27-2026-ie00005.png"/></svg:svg></span></span> 29.3 % (650 nm). Averaged electron-microprobe analyses (<span class="inline-formula"><i>n</i>=10</span>) gave (in wt %) Tl 47.41(19), Cu 15.46(12), Ag 0.15(6), As 17.36(14), Sb 0.41(5) and S 19.20(8), total 99.99(32). The empirical formula is Tl<span class="inline-formula">_1.94</span>Cu<span class="inline-formula">_2.04</span>Ag<span class="inline-formula">_0.01</span>As<span class="inline-formula">_1.95</span>Sb<span class="inline-formula">_0.03</span>S<span class="inline-formula">_5.03</span>, calculated based on 11 apfu. The large crystals exhibit a remarkably homogeneous composition. Spaltiite crystallises in space group <span class="inline-formula"><i>P</i></span>2<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mn mathvariant="normal">1</mn></msub><mo>/</mo><mi>c</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8612d9a728947694db06929c712e1bc6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00006.svg" width="20pt" height="14pt" src="ejm-38-27-2026-ie00006.png"/></svg:svg></span></span> (<span class="inline-formula"><i>a</i>=15.791(8)</span>, <span class="inline-formula"><i>b</i>=10.000(5)</span>, <span class="inline-formula"><i>c</i>=6.323(3)</span> Å, <span class="inline-formula"><i>β</i>=99.25(8)</span>°, <span class="inline-formula"><i>V</i>=985.5(8)</span> Å<span class="inline-formula">³</span>). The crystal structure was determined from single-crystal X-ray diffraction data (<span class="inline-formula"><i>R</i>₁=12.18</span> % for 4753 data, with F<span class="inline-formula">_<i>o</i>&gt;4<i>σ</i></span> (F<span class="inline-formula">_<i>o</i></span>) and 101 variable parameters). Spaltiite exhibits a pronounced layered atomic arrangement: two polar Cu–As layers in (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1</mn><mo>/</mo><mn mathvariant="normal">4</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c9e563c04ef0bd34b79af4cb2e191aff"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00007.svg" width="20pt" height="14pt" src="ejm-38-27-2026-ie00007.png"/></svg:svg></span></span> <span class="inline-formula"><i>y</i> <i>z</i></span>) and (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M38" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">3</mn><mo>/</mo><mn mathvariant="normal">4</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="30b07de17d2d38641add3bc8b83fb127"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00008.svg" width="20pt" height="14pt" src="ejm-38-27-2026-ie00008.png"/></svg:svg></span></span> <span class="inline-formula"><i>y</i> <i>z</i></span>), respectively, are related by inversion symmetry. Sandwiched between them are the Tl atoms. These two layers are centred in (0 <span class="inline-formula"><i>y</i> <i>z</i></span>) and (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M41" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">1</mn><mo>/</mo><mn mathvariant="normal">2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="383b6ebff07a68cc6459808254fe9b9c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00009.svg" width="20pt" height="14pt" src="ejm-38-27-2026-ie00009.png"/></svg:svg></span></span> <span class="inline-formula"><i>y</i> <i>z</i></span>), centrosymmetric but topologically and crystallographically distinct. The eight strongest intensities in the X-ray powder diagram are [<span class="inline-formula"><i>d</i></span> in Å (intensity) <i>hkl</i>]: 3.914 (40) 021; 2.988 (63) 510; 3.496 (45) 311; 2.869 (45) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M44" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mn mathvariant="normal">5</mn><mo mathvariant="normal">‾</mo></mover><mn mathvariant="normal">11</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="19a87194d2a60b38e097aed50cc1c11c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00010.svg" width="20pt" height="13pt" src="ejm-38-27-2026-ie00010.png"/></svg:svg></span></span>; 2.652 (36) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M45" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mn mathvariant="normal">3</mn><mo mathvariant="normal">‾</mo></mover><mn mathvariant="normal">31</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="d70d08c65379357dd1f9af3d89df6479"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00011.svg" width="20pt" height="13pt" src="ejm-38-27-2026-ie00011.png"/></svg:svg></span></span>; 3.646 (34) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M46" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mn mathvariant="normal">2</mn><mo mathvariant="normal">‾</mo></mover><mn mathvariant="normal">21</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="ef82265a0ec277239d16659eb9c33232"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-27-2026-ie00012.svg" width="20pt" height="13pt" src="ejm-38-27-2026-ie00012.png"/></svg:svg></span></span>; 2.506 (29) 040; 2.762 (26) 202. The name of the new mineral originates from the nickname “spalti”, which was used during laboratory studies, illustrating the extremely pronounced cleavage (in German, “<i>spalten</i>” means cleave).</p>