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

Abstract. Marioantofilliite (IMA 2025-012), ideally [Cu4Al2(OH)12](CO3)⋅ 3H2O, 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 µm-sized prismatic crystals. The streak is light blue, and lustre is greasy. Calculated density is 2.825 g cm−3. Marioantofilliite is optically biaxial (–), with α=1.613(4), β=1.626(3), and γ=1.633(5) (in 589 nm light). 2Vcalc is 72°. It is distinctly pleochroic, ranging from colourless to pale blue. The empirical chemical formula of marioantofilliite (with rounding errors) is [Cu4.232+Mg0.02Al1.76(OH)12](CO3)0.82(SO4)0.01[Si(OH)6]0.05⋅3H2O. Unit-cell parameters of marioantofilliite are a=5.590(3), b=2.9358(11), c=7.675(3) Å, β=100.958(17)°, and V=123.66(9) Å3, with space group C2/m and Z=1/3. The crystal structure was refined to R1=0.0372 for 181 unique reflections with F>4σ(F) and 23 refined parameters. It is topologically similar to that of other hydrotalcite-supergroup minerals and shows a distorted {001} brucite-like layer with Cu and Al statistically occupying an octahedrally coordinated M(1) site. The interlayer hosts disordered CO3 and H2O 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.