Ordonezite from the Theodoso Soto Mine, Sapioris, Durango, Mexico: New data and structure refinement

A new occurrence of the rare mineral species ordonezite has been found at the Theodoso Soto tin mine, near Sapioris, Durango, Mexico. The average composition of the samples is (Zn_0.98Fe_0.02)_Σ1.00Sb_2.00O₆; the core of the crystals is slightly richer in Fe than the rim. The mineral is uniaxial (+), with all n greater than 2.30 (n_calc is 2.38 at 589 nm); D_obs 6.66 g/cm³, D_calc 6.67 g/cm³. It is isostructural with ferrotapiolite, with a 4.668(2), c 9.269(6) A (from powder-diffraction data), space group P4₂/mnm, and is twinned on {103}. Its crystal structure, refined to R=4.1, wR=4.3% using 163 observed reflections, is fully cation-ordered.