Mineralogical and chemical composition of three tuffs from northern Israel

The mineralogical and chemical composition of three types (black, yellow and red) of tuffs from Mt. Peres in northern Israel, that differ in terms of degree of weathering were determined in order to understand the weathering sequence under Mediterranean or semi-arid conditions. The black tuff is very slightly weathered and contains 62% volcanic glass, 15% primary minerals, 12% titano-magnetite, 5% halloysite and 4.5% hydroxyapatite. The yellow tuff, considered to be the weathering product of the black tuff, contains only 4% volcanic glass and 21% primary minerals, 8% Fe-rich minerals, 51% amorphous material identified as halloysite-like allophane, 15% halloysite, and 5% hydroxyapatite. A different mineralogical composition from that of red tuff indicates that this material was formed during a different eruption. The red tuff consisted of 29% volcanic glass, 26% primary minerals (different from those of the black tuff), 11% iron-bearing minerals, 10% halloysite, 5% hydroxyapatite and 19% amorphous material. The specific surface area and cation exchange capacity of the black, red and yellow tuff (7, 28, 174 m² g^-1 and 107, 285, 601 mmol_c, kg^-1 at pH 7, respectively) were in accord with their amorphous material contents.