Onoratoite, a new antimony oxychloride, from Cetine di Cotorniano, Rosia (Siena, Italy)

<jats:title>Summary</jats:title><jats:p>Onoratoite, a new antimony oxychloride Sb<jats:sub>8</jats:sub>O<jats:sub>11</jats:sub>Cl<jats:sub>2</jats:sub> from the antimony mines of Cetine di Cotorniano, Rosia (Siena, Italy), is triclinic with lattice constants <jats:italic>a</jats:italic> 18·92, <jats:italic>b</jats:italic> 4·03, <jats:italic>c</jats:italic> 10·31 Å, <jats:italic>α</jats:italic> ≃ <jats:italic>γ</jats:italic> ≃ 90°, <jats:italic>β</jats:italic> 110°. Density 5·3 meas., 5·49 calc. The needle-like crystals are elongated on [010] and show as prominent form {001}. Other forms present are {100}, {201}, {401}, {412}.</jats:p><jats:p>The new mineral is optically biaxial negative, extinction angle small and variable from 0° to 14°. Orientation <jats:italic>β</jats:italic>:[100] <jats:italic>ca.</jats:italic> 8°; <jats:italic>α</jats:italic>:[001] <jats:italic>ca.</jats:italic> 12°; <jats:italic>γ</jats:italic>:[010] 0 to 14°. Refractive indices are 2·26 > <jats:italic>γ</jats:italic> > 2·23, 2·23 > <jats:italic>β</jats:italic> > 2·18. Birefringence <jats:italic>γ</jats:italic>-<jats:italic>β</jats:italic> 0·024 (meas.).</jats:p><jats:p>The stability field of onoratoite has been investigated by systematic thermo-gravimetric and differential thermal analysis of antimony oxychlorides and by a direct study of equilibrium relationships in the system SbCl<jats:sub>3</jats:sub>-Sb<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>-HCl-H<jats:sub>2</jats:sub>O. The probable mechanism of formation of onoratoite is discussed.</jats:p><jats:p>Onoratoite is named in honour of the Italian mineralogist Ettore Onorato.</jats:p>