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Although clays are crucial mineral phases in Earth's weathering engine, it is unclear how they form in surface environments under (near-)ambient pressures and temperature. Most synthesis routes, attempting to give insights into the plausible mechanisms, rely on hydrothermal conditions, yet many geological studies showed that clays may actually form at moderate temperatures (< 100 deg. C) in most terrestrial settings. Here, we present the mechanism of the low-temperature (25-95 deg. C) crystallization of a synthetic Mg-clay, saponite. We describe the pathway at the various sub-stages of the reaction, as we derived from high-energy X-ray diffraction, infrared spectroscopy and transmission electron microscopy data. Our results reveal that saponite crystallizes via a two stage process: 1) a rapid (several minutes) co-precipitation where ~20% of the available magnesium becomes incorporated into an aluminosilicate network followed by 2) a much slower crystallization mechanism (many hours to days) where the remaining magnesium becomes gradually incorporated into the growing saponite sheet structure., manuscript + supporting information