Abstract: Euhedral crystals of the low-temperature mineral lansfordite, MgCO3 · 5H2O, have been prepared from saturated magnesium bicarbonate solutions at temperatures below 10°C. The crystals are monoclinic P21/a with a = 12.4758(7), b = 7.6258(4), c = 7.3463(6)Å, β = 101.762(6)°, V = 684.24Å3, Dcalc. = 1.693 g cm−3, Dobs. = 1.70(1) g m−3. At room temperature, the crystals slowly effloresce to produce pseudomorphs of nesquehonite, MgCO3 · 3H2O. Dehydration is complete at 300°C, with decarbonation taking place in the interval to 560°C. A new X-ray powder diffraction pattern is presented, and details of the infra-red absorption spectrum are discussed.