Lansfordite is a mineral of magnesium carbonate (). It represents the pentahydrate of magnesium carbonate, and has the total formula . Landsfordite was discovered in 1888 in a coal mine in Lansford, Pennsylvania. It crystallizes in the monoclinic system (space group P2<sub>1</sub>/c) and typically occurs as colorless to white prismatic crystals and stalactitic masses. It is a soft mineral, Mohs hardness of 2.5, with a low specific gravity of 1.7. It is transparent to translucent with refractive indices of 1.46 to 1.51. The mineral will effloresce at room temperature, producing nesquehonite.

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