Distinct hematite populations from simultaneous fitting of Mössbauer spectra from Meridiani Planum, Mars

Abstract

At Meridiani Planum, Mars, hematite occurs as a lag of ∼5 mm diameter spherules and their fragments and within the matrix of the sulfate‐rich outcrop as <30 μm particles. Well crystalline and chemically pure bulk hematite undergoes a magnetic transition at ∼264 K (Morin transition) that can be detected by Mössbauer spectroscopy and is within the Martian diurnal temperature range. We analyzed outcrop and lag hematite Mössbauer spectra obtained by the Mars Exploration Rover (MER) Opportunity as a function of temperature using a simultaneous fitting procedure to determine the Morin temperature and the temperature interval over which it occurs. Mössbauer spectra for terrestrial hematite‐bearing samples were acquired for comparison at Martian temperatures using a MER‐like spectrometer. Both outcrop and spherule hematites are characterized by two populations of hematite whose Mössbauer sextets have different Morin transition behavior. Compared to pure bulk hematite, the Morin temperature for Martian hematite is lower or not present within the temperature measurement range (190–290 K) and occurs over a wider temperature interval, consistent with reduced crystallinity, reduced particle or crystallite size, and/or chemical impurities (e.g., Al3+, Ti4+, H2O, and OH−). If the Morin transition behavior for outcrop hematite results solely from particle size effects and the nonjarosite ferric doublet is interpreted as superparamagnetic hematite as a working hypothesis, outcrop hematite is characterized by a size distribution of hematite particles ranging from superparamagnetic to possibly ∼30 μm.