Tee Stability ~ Foxyamphiboles : Existence of Ferric - Bearing Minerals under the Reducing Conditions on the Surface of Venus

Introduction: An enigma of Venusian mineralogy is the suggestion that Fe3+bearing minerals exist under the reducing conditions of the Venusian atmosphere. Analysis of the spectrophotometric data from the Venera 13 and 14 missions, combined with the laboratory reflectance spectral measurements of oxidized basalts at elevatedtemperatures, led to the suggestion that metastable hematite might exist on Venus [I]. Heating experiments at 475OC when f,-lo-", demonstrated that the hematite to magnetite conversion is rapid, indicating metastable hematite is not present on Venus [2]. In addition to hematite, several other ferric oxide and silicate minerals have been proposed to occur on Venus, including laihunite or ferrifayalite [3], l?e3+-bearing tephroite, ox amphiboles [4], and oxybiotites (41. Heating experiments performed on these Fez-bearing minerals under temperature-f, conditions existing on Venus suggest that only oxyamphiboles and oxybiotites may be stable on the surface of Venus. Bcperimental Procedure: To simulate the appropriate redox conditions of the Venusian atmosphere, a Ni/NiO solid buffer was used. The Ni/NiO buffer, when heated to 475OC in an evacuated quartz cell, provides an f , of approximately lo-" [5]. The Ni/NiO buffer and mineral samples were wrapped in separate silver foil sheets. After heating periods of 1-30 days, the samples were quenched and analyzed by Mossbauer spectroscopy. Results: The laihunite (25% Fe3+) and the tephroite (16% Fe3+) samples were each shown to have all of their l?e3+ reduced to l?e2+ when heated with the Ni/NiO buffer at 475OC for 7 days and 14 days, respectively. Conversion of the ~ e ~ + in these olivine8 to ~e'+ is demonstrated by the Mossbauer spectra of laihunite, illustrated in figure 1.