Dacostaite, K(Mg<sub>2</sub>Al)[Mg(H<sub>2</sub>O)<sub>6</sub>]<sub>2</sub>(AsO<sub>4</sub>)<sub>2</sub>F<sub>6</sub>  ⋅  2H<sub>2</sub>O, a new fluoride–arsenate mineral from the Cetine di Cotorniano Mine (Tuscany, Italy)
<p>The new mineral dacostaite, K(Mg<span class="inline-formula"><sub>2</sub></span>Al)[Mg(H<span class="inline-formula"><sub>2</sub></span>O)<span class="inline-formula"><sub>6</sub></span>]&l...
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| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-01-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/37/39/2025/ejm-37-39-2025.pdf |
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| Summary: | <p>The new mineral dacostaite, K(Mg<span class="inline-formula"><sub>2</sub></span>Al)[Mg(H<span class="inline-formula"><sub>2</sub></span>O)<span class="inline-formula"><sub>6</sub></span>]<span class="inline-formula"><sub>2</sub></span>(AsO<span class="inline-formula"><sub>4</sub></span>)<span class="inline-formula"><sub>2</sub></span>F<span class="inline-formula"><sub>6</sub></span> <span class="inline-formula">⋅</span> 2H<span class="inline-formula"><sub>2</sub></span>O, has been discovered in the Cetine di Cotorniano Mine, Chiusdino, Siena, Tuscany, Italy. It occurs as thin, colourless-to-white pseudo-hexagonal micaceous crystals up to 0.5 mm in size. The streak is white, and the lustre is silky. The cleavage is perfect on <span class="inline-formula"><i>{</i>001<i>}</i></span>. The empirical formula of dacostaite, based on (As <span class="inline-formula">+</span> P) <span class="inline-formula">=</span> 2 atoms per formula unit, is (K<span class="inline-formula"><sub>0.56</sub></span>Ca<span class="inline-formula"><sub>0.04</sub></span>Na<span class="inline-formula"><sub>0.03</sub>□<sub>0.37</sub></span>)<span class="inline-formula"><sub>Σ1.00</sub></span> (Al<span class="inline-formula"><sub>1.54</sub></span>Mg<span class="inline-formula"><sub>1.38</sub></span>Cu<span class="inline-formula"><sub>0.03</sub></span>Zn<span class="inline-formula"><sub>0.03</sub></span>)<span class="inline-formula"><sub>Σ2.98</sub></span> [Mg(H<span class="inline-formula"><sub>2</sub></span>O)<span class="inline-formula"><sub>6</sub></span>]<span class="inline-formula"><sub>2</sub></span> [(As<span class="inline-formula"><sub>0.99</sub></span>P<span class="inline-formula"><sub>0.01</sub>)</span>O<span class="inline-formula"><sub>4</sub></span>]<span class="inline-formula"><sub>2</sub></span> [F<span class="inline-formula"><sub>4.46</sub></span>(OH)<span class="inline-formula"><sub>1.46</sub></span>O<span class="inline-formula"><sub>0.08</sub></span>]<span class="inline-formula"><sub>Σ6.00</sub></span> <span class="inline-formula">⋅</span> 2H<span class="inline-formula"><sub>2</sub></span>O (<span class="inline-formula"><i>Z</i>=2</span>). Dacostaite is monoclinic, with a space group of <span class="inline-formula"><i>C</i></span>2/<span class="inline-formula"><i>m</i></span> and <span class="inline-formula"><i>a</i>=12.474(5)</span>, <span class="inline-formula"><i>b</i>=7.198(3)</span>, <span class="inline-formula"><i>c</i>=13.724(6)</span> Å, <span class="inline-formula"><i>β</i>=99.518(13)</span>°, and <span class="inline-formula"><i>V</i>=1215.3(8)</span> Å<span class="inline-formula"><sup>3</sup></span>. The crystal structure was solved using single-crystal X-ray diffraction data and refined to <span class="inline-formula"><i>R</i><sub>1</sub>=0.0927</span> for 1022 unique reflections with <span class="inline-formula"><i>F</i><sub><i>o</i></sub></span> <span class="inline-formula">></span> 4<span class="inline-formula"><i>σ</i></span> (<span class="inline-formula"><i>F</i><sub><i>o</i></sub></span>). The crystal structure of dacostaite can be described as formed by heteropolyhedral <span class="inline-formula"><i>{</i>001<i>}</i></span> layers and isolated Mg(H<span class="inline-formula"><sub>2</sub></span>O)<span class="inline-formula"><sub>6</sub></span> groups connected by H bonds. In the type material, dacostaite is associated with quartz, sulfur, gypsum, and a pharmacosiderite-like mineral in a small cavity of silicified limestone. Its genesis is related to the activity of oxidized (Al,F)-rich fluids during the late-stage evolution of the Sb ore deposit formerly exploited at the Cetine di Cotorniano Mine.</p> |
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| ISSN: | 0935-1221 1617-4011 |