Formation Mechanisms of Hematite in Gale Crater, Mars: Insight From Hematite Crystallite Sizes

Hematite is a widespread Fe(III) oxide on Mars and has been identified in Gale crater by the Mars Science Laboratory Curiosity rover. Hematite was detected by the Chemistry and Mineralogy (CheMin) X-ray diffraction (XRD) instrument in the samples drilled along Curiosity traverse exploring ~600 m of vertical stratigraphy. The variations in the width of hematite XRD peaks indicated that the crystallite size of hematite varies between the samples [1, 2]. We calculated crystallite size from whole powder pattern modelling of the collected XRD data. Our results revealed that the crystallite size ranged from 5 to 60 nm at lower elevations but remained unchanged and did not exceed 10 nm at the highest positions of the stratigraphic column. Crystallite size calculations for (110) and (104) XRD peaks also revealed that hematite crystallites were of equant shape. These findings suggested that hematite grew equally in all crystallographic directions, but the formation mechanism of equant shape hematite remained unknown.

To determine how equant shape hematite formed in Gale crater, we calculated crystallite sizes for synthetic hematite formed by Fe(III) hydrolysis, aqueous alteration of amorphous Fe(III) oxyhydroxide ferrihydrite, and thermal alteration of various Fe(III) oxyhydroxides. We also fitted lognormal distributions of crystallite sizes in Gale crater to constrain hematite growth mechanism.

A comparison of the data for synthetic hematite with the results from Gale crater showed that hematite of < 10 nm size and of equant shape could form by aqueous alteration of ferrihydrite at neutral to alkaline pH and by thermal alteration of ferrihydrite at low temperatures. Calculated lognormal distributions of crystallite sizes showed that samples with larger hematite crystallites did not have contributions of small crystallites, suggesting Ostwald ripening as the growth mechanism. Our analysis of XRD data suggests that hematite crystallites of small size could form in Gale crater by alteration of ferrihydrite in aquatic systems or by low temperature solid state alteration of ferrihydrite. The increase in crystallite sizes at lower elevations could be due to Ostwald ripening during late diagenesis.

References:
[1] Rampe et (2020) JGR Planets 125 (9)
[2] Szczerba et al (2023), LPSC abstract