gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1968.036.284.01 Onoratoite,1 a New Antimony Oxychloride from Cetine Di Cotorniano, Rosia (Siena, Italy) Belluomini G. Fornaseri M. Nicoletti M. Istituto di Geochimica, Università di Roma

This new mineral has been approved by the Commission on New Minerals and New Mineral Names of the International Mineralogical Association. Some members of the Commission suggested that the name should be onoratoite instead of onoratite and this suggestion has been accepted by the authors of this paper.

 12 1968 36 284 1037 1044 23 5 1968 Copyright © 1968, The Mineralogical Society 1968 Summary

Onoratoite, a new antimony oxychloride Sb8O11Cl2 from the antimony mines of Cetine di Cotorniano, Rosia (Siena, Italy), is triclinic with lattice constants a 18·92, b 4·03, c 10·31 Å, α ≃ γ ≃ 90°, β 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}.

The new mineral is optically biaxial negative, extinction angle small and variable from 0° to 14°. Orientation β:[100] ca. 8°; α:[001] ca. 12°; γ:[010] 0 to 14°. Refractive indices are 2·26 > γ > 2·23, 2·23 > β > 2·18. Birefringence γ-β 0·024 (meas.).

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 SbCl3-Sb2O3-HCl-H2O. The probable mechanism of formation of onoratoite is discussed.

Onoratoite is named in honour of the Italian mineralogist Ettore Onorato.