Embryonic Halloysites in Ecuadorian Soils Derived from Volcanic Ash

Abstract

AbstractA unique mineral association that represents embryonic stages of halloysite formation in weathering of andesitic volcanic ash was found in three Ecuadorian soils which did not contain allophane. This mineral association consisted of more or less poorly ordered halloysites and a “1.0‐nm mineral”. The more poorly ordered halloysite was nearly x‐ray amorphous and showed infrared (IR) absorption bands which were characteristic of halloysite but poorly resolved. It was dissolved congruently (Si/Al ratio = 1.0–2.0) by hot M/3 sodium citrate solution and incongruently (Si/Al ratio = 0.37–0.40) by 0.2 M ammonium oxalate‐oxalic acid (pH 3.0). The 1.0‐nm mineral and the less poorly ordered halloysite were not dissolved by these reagents. The former was not distinguished from the latter on the IR spectra and both had a Si/Al ratio of, or close to, 1.0. The 1.0‐nm mineral showed, however, a broad 1.0‐nm reflection not only at 25 but at 300°C and even at 550°C. Embryonic halloysites varied in morphology from thinly spread flakes to elongated, rolled thick plates, and spherical particles. Soils containing the more poorly ordered halloysites with Si/Al ratios of ca 1.5 to 2.0 and 1.0 showed a low and a fairly high “Al reactivity”, respectively. It was inferred that embryonic halloysites have formed in base and silica rich environments. Structural schemes of the 1.0‐nm mineral and the more poorly ordered halloysite were discussed.