Influence of Climate on the Iron Oxide Mineralogy of Terra Rossa

-Terra rossa samples were taken from the B horizons of soil profiles and from cracks within limestone in Italy. The average annual temperature (AAT) of the sites ranged from 8.4 to 20.3~ and the average annual precipitation (AAP) from 511 to 3113 mm, with either a 5-6 month water deficit or a large water surplus. Goethite and hematite were identified in all the samples. Under a moist (> 1700 mm AAP) and cool (13~ AAT) climate, a xeric, hematitic pedoenvironment was preserved by the well-litified carbonate rock. Hematite occurred in trace amounts, even with an AAT of 8.4~ and an AAP of 3300 mm, confirming the specific role of the hard limestone on the pedoclimate of terra rossa. The lowest mean crystallite dimension of goethite and hematite was found in the samples from the wettest sites, and in these samples hematite was nearly free of A1 substitution. Rubification in terra rossa appeared to be due to the specific pedoenvironment. The hematite cannot be considered a relict phase formed under another climate. Illite and kaolinite were the main clay minerals in samples from xeric sites whereas more weathered clays, such as Al-interlayered vermiculite, occurred in cool, moist sites. We postulate that the processes of rubification and vermiculitization could have taken place at the same time. Key Words--Goethite, Hematite, Pedoclimate, Terra rossa, XRD. Terra rossa is a red soil very common in areas of mediterranean climate. It forms a discontinuous and shallow cover on hard carbonate rock. The characteristic bright red color is due to the high pigmenting power of hematite, which masks the coexisting yellow of goethite (Torrent et al., 1983; Barron and Torrent, 1986; Schwertmann, 1988; Boero and Schwertmann, 1989). Red hematitic soils are generally considered the result of intense mineral weathering under subtropical and tropical climates. Therefore, the terra rossa that occur under temperate climates have been considered relict soils, formed under warmer paleoclimates (see Bronger et aL, 1983 for references). However, hematite has been demonstrated to form in axeric temperate areas, provided there is a dry pedoclimate during part o f the year (Bresson, 1974, 1976; Schwertmann et al., 1982). Therefore, the relict hypothesis is not the only explanation for the occurrence of hematite in terra