Abstract: Ranciéite occurs as stalactites coated with 10 Å halloysite in an open fissure crossing the Mazzano Romano Mn-impregnated tuffs. Chemical analysis (by EMPA) gave: MnO₂ 71.9, CaO 6.11, BaO 2.85, MgO 1.53, K₂O 0.86, Na₂O 0.53 plus minor oxides, total 85.07% (average of sixteen spots). Thermal analysis gave a total loss of ∼ 20 wt% of which 13.9% can be attributed to bonded water. The resulting formula contains 4H₂O rather than 3H₂O, as in most published references. The loss of water leads to the formation of hausmannite plus unidentified phases, after a complex sequence of reactions. Dehydration is directly followed by an X-ray amorphous stage. The X-ray pattern and IR spectrum indicate a poor structural order, in agreement with the likely formation conditions of the Mazzano ranciéite at atmospheric pressure and very low temperatures (less than 50 °C), from cold water percolating through the main Mn deposit.