Tripuhyite, FeSbO4, revisited

The exact nature of tripuhyite remains controversial more than 100 years after the first description of the mineral. Different stoichiometries and crystal structures (rutile or tri-rutile types) have been suggested for this Fe-Sb-oxide. To address these uncertainties, we studied tripuhyite from Tripuhy, Minas Gerais, Brazil (type material) and Falotta, Grisons, Switzerland using single-crystal and powder X-ray diffraction (XRD), optical microscopy and electron microprobe analysis. Electron microprobe analyses showed the Fe/Sb ratios to be close to one in tripuhyite from both localities. Single crystal XRD studies revealed that tripuhyite from the type locality and from Falotta have the rutile structure (P4₂/mnm, a = 4.625(4) c = 3.059(5) and a = 4.6433(10) c = 3.0815(9) Å, respectively). Despite careful examination, no evidence for a tripled c parameter, characteristic of the tri-rutile structure, was found and hence the structure was refined with the rutile model and complete Fe-Sb disorder over the cationic sites in both cases (type material: R₁ = 3.61%; Falotta material: R₁ = 3.96%). The specular reflectance values of type material tripuhyite and lewisite were measured and the following refractive indices calculated (after Koeningsberger): tripuhyite n_min = 2.14, n_max = 2.27; lewisite (cubic) n = 2.04. These results, together with those of ⁵⁷ Fe and ¹²¹ Sb Mössbauer spectroscopy on natural and synthetic tripuhyites reported in the literature, indicate that the chemical formula of tripuhyite is Fe³⁺Sb⁵⁼O₄ (FeSbO₄ ). Thus, tripuhyite can no longer be attributed to the tapiolite group of minerals of general type AB₂O₆. A comparison of the results presented with the mineralogical data of squawereekite suggests that tripuhyite and squawereekite are identical. In consequences, tripuhyite was redefined as Fe³⁺ Sb⁵⁺ O4 with a rutile-type structure. Both the proposed new formula and unit cell (rutile-type) of tripuhyite as well as the discreditation of squawcreekite have been approved by the Commission on New Mineral and Mineral Names (CNMMN) of the International Mineralogical Association (IMA).