Chemical and textural controls on the formation of sepiolite, palygorskite and dolomite in volcanic soils

Abstract

A 3-m deep soil profile from Gran Canaria, Canary Islands, has been studied. The profile is interesting because soil development on mafic tephra produced a pattern of clay mineral and carbonate distribution. Carbonates precipitated abundantly, increasingly towards the bottom, making up 20-90 wt.% of the soil. Sepiolite and calcite are dominant at the top, whereas palygorskite and dolomite are the major components at the lower part of the profile. Quartz is present in low amounts in most of the profile, and smectite and non-crystalline partially altered tephra occur in low amounts towards the bottom. Mineralogical, chemical and textural investigations indicate that sepiolite and calcite precipitated in the space between the original tephra grains, from Ca, Mg and Si dissolved in situ and transported in the runoff. Palygorskite and smectite precipitated within tephra grains, from in situ tephra weathering, as well as dolomite. The distribution of the clay minerals is due to clay composition and ion mobility. Sepiolite, consisting only of Si and Mg, precipitated outside tephra grains, where the mobile Si and Mg ions were abundant in the interstitial waters. Palygorskite and smectite contain Si, Al, Fe, and Mg and only precipitated within the tephra grains, where the immobile Al and Fe were available. Calcite precipitation in solution is kinetically favored over that of dolomite and thus calcite precipitated between the grains. Dolomite precipitated within altering tephra grains and spherical clay structures because tephra and clay generated a viscous medium where dolomite supersaturation increased and the kinetic barrier for precipitation was overcome. These results are relevant to the "dolomite problem", as they illustrate how dolomite can precipitate in surface conditions, and to CO2 sequestration, because dolomite can immobilize higher CO2 amounts (both Ca and Mg are involved and not only Ca) and in a much more stable manner (dolomite is eight orders of magnitude less soluble than calcite).