Ranciéite from Mazzano Romano (Latium, Italy)

Eugenio Barrese*, Ciriaco Giampaolo, Odino Grubessi and Annibale Mottana
Dipartimento di Scienze della Terra, Università di Roma, Città Universitaria, 00185 Roma, Italy
*Present Address: Dipartimento di Scienze della Terra, Università della Calabria, 87030 Castiglione Cosentino Scalo, Cosenza, Italy.

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.

Keywords: ranciéite • thermal analysis • Mazzano Romano • Italy

Mineralogical Magazine; March 1986 v. 50; no. 355; p. 111-118; DOI: 10.1180/minmag.1986.050.355.14
© 1986, The Mineralogical Society
Mineralogical Society (www.minersoc.org)