Marioantofilliite, [Cu₄Al₂(OH)₁₂](CO₃) ⋅ 3H₂O, a new member of the hydrotalcite supergroup from Liguria (Italy)

<p>Marioantofilliite (IMA 2025-012), ideally [Cu<span class="inline-formula">₄</span>Al<span class="inline-formula">₂</span>(OH)<span class="inline-formula">₁₂</span>](CO<span class="inline-formula">₃</span>)<span class="inline-formula">⋅</span> 3H<span class="inline-formula">₂</span>O, is a new member of the hydrotalcite supergroup discovered in the Cu–Fe ore deposit of Monte Copello–Reppia, Graveglia Valley, Liguria, Italy. It occurs as globular aggregates up to 1 mm in diameter formed by <span class="inline-formula">µm</span>-sized prismatic crystals. The streak is light blue, and lustre is greasy. Calculated density is 2.825 g cm<span class="inline-formula">⁻³</span>. Marioantofilliite is optically biaxial (–), with <span class="inline-formula"><i>α</i>=1.613(4)</span>, <span class="inline-formula"><i>β</i>=1.626(3)</span>, and <span class="inline-formula"><i>γ</i>=1.633(5)</span> (in 589 nm light). 2<span class="inline-formula"><i>V</i>_calc</span> is 72°. It is distinctly pleochroic, ranging from colourless to pale blue. The empirical chemical formula of marioantofilliite (with rounding errors) is [Cu<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4.23</mn><mrow><mn mathvariant="normal">2</mn><mo>+</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="18pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="89080c3eca3fd94521ffbc657cd70b55"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-37-733-2025-ie00001.svg" width="18pt" height="17pt" src="ejm-37-733-2025-ie00001.png"/></svg:svg></span></span>Mg<span class="inline-formula">_0.02</span>Al<span class="inline-formula">_1.76</span>(OH)<span class="inline-formula">₁₂</span>](CO<span class="inline-formula">₃)_0.82</span>(SO<span class="inline-formula">₄)_0.01</span>[Si(OH)<span class="inline-formula">₆</span>]<span class="inline-formula">_0.05⋅3</span>H<span class="inline-formula">₂</span>O. Unit-cell parameters of marioantofilliite are <span class="inline-formula"><i>a</i>=5.590(3)</span>, <span class="inline-formula"><i>b</i>=2.9358(11)</span>, <span class="inline-formula"><i>c</i>=7.675(3)</span> Å, <span class="inline-formula"><i>β</i>=100.958(17)</span>°, and <span class="inline-formula"><i>V</i>=123.66(9)</span> Å<span class="inline-formula">³</span>, with space group <span class="inline-formula"><i>C</i></span>2/<span class="inline-formula"><i>m</i></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>Z</mi><mo>=</mo><mn mathvariant="normal">1</mn><mo>/</mo><mn mathvariant="normal">3</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="828edc207d6ef858da7929ec24df6e45"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-37-733-2025-ie00002.svg" width="41pt" height="14pt" src="ejm-37-733-2025-ie00002.png"/></svg:svg></span></span>. The crystal structure was refined to <span class="inline-formula"><i>R</i>₁=0.0372</span> for 181 unique reflections with <span class="inline-formula"><i>F</i>&gt;4<i>σ</i>(<i>F</i>)</span> and 23 refined parameters. It is topologically similar to that of other hydrotalcite-supergroup minerals and shows a distorted <span class="inline-formula"><i>{</i>001<i>}</i></span> brucite-like layer with Cu and Al statistically occupying an octahedrally coordinated <span class="inline-formula"><i>M</i></span>(1) site. The interlayer hosts disordered CO<span class="inline-formula">₃</span> and H<span class="inline-formula">₂</span>O groups. Marioantofilliite formed through the oxidative dissolution of primary Cu ores by mine drainage aqueous solutions and neutralization by gangue carbonates. Its name honours Mario Antofilli (1920–1983) for his contributions to the knowledge of the mineralogy of Liguria.</p>