NATURE OF DIOCTAHEDRAL MICAS IN SPANISH RED SOILS

Structural formulae and other crystallochemical parameters were used t o study different species of dioctahedral micas in clay and coarse gra vel fractions of horizons from a red soil (Ultic Haploxeralf) in south ern Spain. Mineralogical analyses using X-ray powder diffraction, and measurements of the bo parameter revealed dioctahedral micas, illite a nd paragonite. Structural formulae established from electron microprob e analysis and energy dispersive X-ray analysis showed the illites to be K mica related in elemental composition and structure to muscovite and phengite. The paragonites were found to be closer to ideal mica. S tructural formulae for Na-K dioctahedral micas were obtained with crys tallochemical characteristics intermediate between those of Na micas a nd K micas. The possibility of these micas representing individual min eral phases or intergrowths of Na and K micas is discussed. In the soi l profile, micas from the Bt horizon showed the largest crystallochemi cal changes induced by pedogenesis.