Zincalstibite-9m>Rm>, a new polytype in the hydrotalcite supergroup is reported from the Monte Avanza mine, Italy. It occurs as pale blue curved disc-like tablets flattened on {001} intergrown to form rosettes typically less than 50 μm in diameter, with cyanophyllite and linarite in cavities in baryte. Zincalstibite-9m>Rm> is uniaxial (−), with refractive indices ω = 1.647(2) and ε = 1.626(2) measured in white light. The empirical formula (based on 12 OH groups) is (rmula" id="inline-formula-1">mg class="math mml" alt="Formula" src="1337/embed/mml-math-1.gif" /> Al0.04)Σ2.00Al1.01 (rmula" id="inline-formula-2">mg class="math mml" alt="Formula" src="1337/embed/mml-math-2.gif" /> Si0.02)Σ0.99(OH)12, and the ideal formula is (Zn,Cu)2Al(OH)6[Sb(OH)6]. Zincalstibite-9m>Rm> crystallizes in space group m>Rm>3İ, with m>am> = 5.340(2), m>cm> = 88.01(2) Å, m>Vm> = 2173.70(15) Å3 and m>Zm> = 9. The crystal structure was refined to R1 = 0.0931 for 370 unique reflections [Fo > 4σ(F)] and R1 = 0.0944 for all 381 unique reflections. It has the longest periodic layer stacking sequence for a layered double hydroxide compound reported to date.
