Laboratory experiments are useful to constrain the environmental parameters that have allowed the formation of the ancient hydrous mineralogical assemblages observed at the surface of Mars, which are dominated by ferric smectites. Weathering under a dense CO2 atmosphere on early Mars is a process frequently invoked to explain their formation but has proven difficult to test in the laboratory due to low reaction rates. Here, we present a long-term weathering experiment (470 days, 45°C) of forsteritic olivine specially designed to increase as much as possible the amount of reaction products and thus allow their detailed SEM/TEM petrological and chemical characterization. Our results show the formation of crystalline smectites both under 1 bar of CO2 and under ambient air. However, important 2 differences are observed between the two types of conditions. The smectite formed under CO2 has an average chemical formula per half unit-cell of Si3.92Al0.16Fe 3+ 0.78Mg1.66 Cr0.01Ni0.06K0.04Ca0.04.O10(OH)2. It is thus intermediate between a trioctahedral Mg-rich saponite and a dioctahedral ferric smectite. It is also clearly enriched in Fe compared its counterpart formed under ambient air (Si3.68Al0.12Fe 3+ 0.37 Mg2.61Cr0.01Ni0.02K0.04Ca0.25.O10(OH)2). This result demonstrates that the enrichment in Fe observed for Martian smectites is to be expected if they were formed by low-temperature weathering under a dense CO2 atmosphere. Another difference is the nature of the accompanying phases, which includes amorphous silica (in the form of opal spheres 10 to 100 nm in diameter) and Mg-carbonates under CO2, but are limited to rare kaolinite under ambient air. The observation of kaolinite particles and the significant amount of Al measured in smectites despite the Al-poor nature of the initial material shows that this element is easily concentrated by low-temperature processes. The concentration of this element by weathering could lead to the formation of Al-rich upper horizons, as frequently observed on Mars.