Abstract

Zincalstibite-9R, 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-9R 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 (Zn1.092+Cu0.872+Al0.04)Σ2.00Al1.01 (Sb0.975+Si0.02)Σ0.99(OH)12, and the ideal formula is (Zn,Cu)2Al(OH)6[Sb(OH)6]. Zincalstibite-9R crystallizes in space group R3İ, with a = 5.340(2), c = 88.01(2) Å, V = 2173.70(15) Å3 and Z = 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.

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