K-FELDSPARS AS VOLCANIC PALEOSOIL INDICATORS - FIRST INVESTIGATION INTHE ALBAN HILLS VOLCANIC AREA, LATIUM, ITALY

A detailed mineralogical study was carried out on potassium feldspars contained in some ''paleosoil'' samples from the Alban Hills volcanic region (Latium, Italy). Chemical and structural correlation of the dat a allowed a comparison with data on sanidinite feldspars from the same area and with data on other nearby volcanic complexes. The chemistry (N-Or = 72.6 divided by 88.3 % Or) and lattice parameters [a(o) = 8.49 2 divided by 8.574(Angstrom), b(o) = 12.996 divided by 13.034(Angstrom ), c(o) = 7.163 divided by 7.190(Angstrom)] indicate a prevalently mor e sanidinitic character for the paleosoil feldspars compared with thos e in Alban ''sanidinites''. The particular structural state (HT disord er) of some paleosoil K-feldspars would suggest different crystallizat ion temperatures and, for some samples, also a possible pressure influ ence during their formation. The abundance of K-feldspars in the sampl es examined and their less orthoclasic features compared with feldspar s found in sanidinitic Alban ejecta would highlight a rather different genetic process for these paleosoils compared with that of the more t ypical Alban volcanic products, mainly of leucititic composition. The studied K-feldspars may be considered as good volcanic paleosoil indic ators and a probable allochthonous origin of the Alban paleosoils is h ypothesized.