The crystal structure of launayite from Taylor Pit, Madoc, Ontario, Canada: crystal chemistry, modulated superstructures, and parent modular structure compared with rouxelite
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| Pubblicato in: | European Journal of Mineralogy vol. 37, no. 6 (2025), p. 971-995 |
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| Autore principale: | |
| Altri autori: | , , |
| Pubblicazione: |
Copernicus GmbH
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| Accesso online: | Citation/Abstract Full Text Full Text - PDF |
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| Abstract: | The crystal structure of launayite, ideally <inline-formula><mml:math display="inline" id="M1"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">Cu</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Pb</mml:mi><mml:mn mathvariant="normal">20</mml:mn></mml:msub><mml:mo>(</mml:mo><mml:mi mathvariant="normal">Sb</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">As</mml:mi><mml:msub><mml:mo>)</mml:mo><mml:mn mathvariant="normal">26</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn mathvariant="normal">60</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> (<inline-formula><mml:math display="inline" id="M2"><mml:mrow><mml:mi>Z</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">4</mml:mn></mml:mrow></mml:math></inline-formula>) from Taylor Pit, Madoc, Ontario, Canada, has been solved for the first time using the single-crystal X-ray diffraction (SCXRD) method. The mineral is composed of distinct superstructures that can be derived from the same parent structure. The structure of the main component is monoclinic and has been solved in the space group <inline-formula><mml:math display="inline" id="M3"><mml:mrow><mml:mi>P</mml:mi><mml:mn mathvariant="normal">2</mml:mn><mml:mo>/</mml:mo><mml:mi>a</mml:mi></mml:mrow></mml:math></inline-formula>, with cell parameters <inline-formula><mml:math display="inline" id="M4"><mml:mrow><mml:mi>a</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">42.6466</mml:mn><mml:mo>(</mml:mo><mml:mn mathvariant="normal">14</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math></inline-formula>, <inline-formula><mml:math display="inline" id="M5"><mml:mrow><mml:mi>b</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">8.0381</mml:mn><mml:mo>(</mml:mo><mml:mn mathvariant="normal">2</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math></inline-formula>, <inline-formula><mml:math display="inline" id="M6"><mml:mrow><mml:mi>c</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">34.3957</mml:mn><mml:mo>(</mml:mo><mml:mn mathvariant="normal">10</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math></inline-formula> Å, <inline-formula><mml:math display="inline" id="M7"><mml:mrow><mml:mi mathvariant="italic">β</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">64.684</mml:mn><mml:mo>(</mml:mo><mml:mn mathvariant="normal">2</mml:mn><mml:mo>)</mml:mo><mml:mspace linebreak="nobreak" width="0.125em"></mml:mspace><mml:mi mathvariant="italic">°</mml:mi></mml:mrow></mml:math></inline-formula>, and <inline-formula><mml:math display="inline" id="M8"><mml:mrow><mml:mi>V</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">10</mml:mn><mml:mspace linebreak="nobreak" width="0.125em"></mml:mspace><mml:mn mathvariant="normal">658.4</mml:mn><mml:mo>(</mml:mo><mml:mn mathvariant="normal">6</mml:mn><mml:mo>)</mml:mo><mml:mspace linebreak="nobreak" width="0.125em"></mml:mspace><mml:msup><mml:mi mathvariant="italic">Å</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula> from an untwined crystal. The asymmetric unit of launayite contains 48 cation sites and 60 sulfur sites. Final refinement resulted in an <inline-formula><mml:math display="inline" id="M9"><mml:mrow><mml:msub><mml:mi>R</mml:mi><mml:mn mathvariant="normal">1</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> value of <inline-formula><mml:math display="inline" id="M10"><mml:mn mathvariant="normal">0.0955</mml:mn></mml:math></inline-formula> for 11 741 unique reflections. The structural formula obtained from SCXRD study is <inline-formula><mml:math display="inline" id="M11"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">Cu</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Pb</mml:mi><mml:mn mathvariant="normal">20.330</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Sb</mml:mi><mml:mn mathvariant="normal">23.024</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">As</mml:mi><mml:mn mathvariant="normal">2.689</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn mathvariant="normal">60</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>, <inline-formula><mml:math display="inline" id="M12"><mml:mrow><mml:mi>Z</mml:mi><mml:mo>=</mml:mo><mml:mn mathvariant="normal">4</mml:mn></mml:mrow></mml:math></inline-formula>, in agreement with the formula <inline-formula><mml:math display="inline" id="M13"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">Cu</mml:mi><mml:mn mathvariant="normal">2.078</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Ag</mml:mi><mml:mn mathvariant="normal">0.059</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Tl</mml:mi><mml:mn mathvariant="normal">0.057</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Pb</mml:mi><mml:mn mathvariant="normal">20.404</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Sb</mml:mi><mml:mn mathvariant="normal">22.830</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">As</mml:mi><mml:mn mathvariant="normal">2.772</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn mathvariant="normal">59.80</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> from microprobe analysis. The structure of launayite can be viewed both as a boxwork structure and as a rod-based structure. The modular description of the launayite structure reveals a very close relationship with the structure of rouxelite: the parent structures of both can be regarded as merotypes. A full comparison of the crystal chemistry and modular description of both structures is presented. |
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| ISSN: | 0935-1221 1617-4011 |
| DOI: | 10.5194/ejm-37-971-2025 |
| Fonte: | Publicly Available Content Database |