¹M. M. Morris, ¹J. A. Hurowitz, ¹L. Ehm
Journal of Geophysical Research: Planets, 131, e2026JE010007
Link to Article [DOI: 10.1029/2026JE010007]
¹Department of Geosciences, Stony Brook University, Stony Brook, NY, USA

Published by arrangement with John Wiley & Sons

Fe-Mg carbonates on Mars preserve key evidence of past aqueous chemistry and climate, yet their formation conditions remain poorly constrained. We conducted low-temperature (∼25°C) anoxic precipitation experiments under Mars-like conditions, systematically varying pH, Fe:Mg ratio, Ca²⁺ content, and carbonate saturation state. Results show initial pH and saturation state as primary controls: near-neutral pH favored siderite (FeCO₃) and chukanovite (Fe₂(CO₃)(OH)₂) precipitation, while alkaline conditions promoted the precipitation of hydrated Mg-carbonate and Fe-substituted Mg-hydroxide and layered double hydroxides. Ca²⁺ suppressed amorphous Fe-carbonate precursor precipitation, enhancing the growth of rhombohedral carbonate and enabling partial Ca²⁺ substitution. A limited number of elevated temperature experiments (50°C) did not enhance Mg²⁺ substitution in siderite, but instead resulted in the precipitation of very high-magnesium calcite (MgCa(CO₃)₂) overgrowths. Mg²⁺ incorporation into siderite (at the level of ∼10 mol%) occurred only under low Fe:Mg ratios and high magnesite (MgCO₃) saturation, highlighting kinetic barriers to Mg²⁺ substitution.

¹Emmanuel Jacquet
Meteoritics & Planetary Science (in Press)
Open Access Link to Article [DOI: 10.1111/maps.70247]
¹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590, Muséum National d’Histoire Naturelle, CNRS, Paris, France

Published by arrangement with John Wiley & Sons

Chondrule sizes are an important discriminant of chondrite groups and chondrule formation theories. Yet most studies report 2-D sizes without attempting the somewhat tedious corrections to 3-D proposed in the past literature. I show here that 3-D size averages can be simply calculated directly from 2-D averages of appropriate powers of sizes (and vice-versa), without having to reconstruct 3-D distributions, although explicit formulas to that end are also provided. This illuminates the comparison between 3-D and 2-D arithmetic size averages, which are confirmed to be little different for the published data sets. Yet I suggest another type of average, the “sphere-weighted average” (i.e., weighted by the chondrule equal-volume sphere area), a measure of the surface/volume ratio of the whole chondrule population, which is more robust against omission of small chondrules. While larger than the arithmetic averages, it is best in line with previous literature estimates based on less exhaustive surveys of chondrules.