Haloisitas em saprólitos de rochas vulcânicas do Rio Grande do Sul

The dominance ofdisordered kaolinite, and the low frequency of 1.0 nm halloysites reported in saprolites of volcanic rocks in southern Brazil may be attributed to the natural dehydrated state of these halloysites, or to the mineralogical pretreatments that induce their dehydration. To test the rehydration capacity of halloysites, selected samples from saprolites were dehydrated by heating to 110°C or by extraction of iron oxides with Na-dithionite-citrate-bicarbonate (DCB). The saprolite samples came from basalt under Oxisols and Vertisols, and from ryolite under Inceptisols. According to XRD the different samples contained 1.0 nm halloysite developed from zeolite and from the plurimineral assemblage of basalt, an association of 1.0 nm and 0. 7 nm halloysite, and an interstratified halloysite-smectite. After the induced collapse to 0.7 nm the main part of the halloysite recovered the original 1.0 nm space with a 48h formamide treatment. Although the original hydrated condition of the halloysite was partially recovered, the unexpanded portion would be erroneously identified as disordered kaolinite.