A new approach to compositional limits for sepiolite and palygorskite

Most bulk chemical analyses of sepiolite and palygorskite available in the literature are erroneous because samples analyzed are admixtures of mineral s that are difficult to separate or identify by other techniques. Some chem ical analyses performed on selected individual particles by energy dispersi ve X-ray analysis (EDX) are also influenced by the same problem. Chemical a nalyses are summarized for sepiolite and palygorskite reported in the liter ature (bulk and EDX analyses). The analyses are evaluated by comparison to three sepiolite and three palygorskite pure samples analyzed by EDX techniq ues. Results indicate that sepiolite is a true trioctahedral mineral, very pure (near end-member) with negligible structural substitution and with eig ht octahedral positions filled with magnesium, close to the theoretical for mula Mg8Si12O30(OH)(4)(OH2)(4)(H2O)(8). Palygorskite is intermediate betwee n di- and trioctahedral phyllosilicates. The octahedral sheet contains main ly Mg, Al, and Fe with a R-2/R-3 ratio close to 1, and with four of the fiv e structural positions occupied. The theoretical formula is close to (Mg(2) R(2)(3+)square(1))(Si8-xAlx)O-20(OH)(2)(OH2)(4). R-x/2(2+)(H2O)(4), where x = 0-0.5. Sepiolite and palygorskite are thus more compositionally limited than previously reported.