ABSTRACT. Ranci6ite occurs as stalactites coated with 10 A halloysite in an open fissure crossing the Mazzano Romano Mn-impregnated tufts. Chemical analysis (by EMPA) gave: MnO 2 71.9, CaO 6.11, BaO 2.85, MgO 1.53, K20 0.86, Na20 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~o can be attributed to bonded water. The resulting formula contains 4H20 rather than 3H20, 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 ranci6ite at atmospheric pressure and very low temperatures (less than 50 ~ from cold water percolating through the main Mn deposit.
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