Abstract
Extensive carbonate precipitation has occurred on Mars. To gain insight into the carbonation mechanisms and formation processes under ancient Martian aqueous conditions, we examine the precipitation of carbonates resulting from atmospheric carbon fixation, focusing on interactions between various brines and silicate and perchlorate solutions in alkaline environments. The micro-scale morphology and composition of the resulting precipitates are analysed using ESEM micrographs, EDX chemical compositional analysis, X-ray diffraction, and micro-Raman spectroscopy. Our findings indicate a significant atmospheric carbonation process involving chlorate and sulphate brines reacting with alkaline perchlorate solutions, leading to the precipitation of calcium carbonate polymorphs, including vaterite, aragonite, and calcite, as well as other carbonates like siderite (iron carbonate) and zaratite (nickel carbonate). Some precipitates exhibit biomorphic structures (such as globular spherical aggregates, fine branched tubes, and flower-like morphologies) that should not be mistaken for fossils. These experiments demonstrate that various precipitates can form simultaneously in a single reaction vessel while being exposed to different micro-scale pH conditions. We propose that systematic laboratory studies of such precipitate reactions should be conducted in preparation for the analysis of the Mars Sample Return collection on Earth, aiding in the interpretation of carbonate presence in natural brine-rock carbonation processes under Martian conditions while also helping to distinguish potential biosignatures from purely geochemical processes.