Halloysite as a kinetically controlled end product of arid-zone basalt weathering

Abstract

Mineralogical and isotope results paired with field observations suggest that halloysite is the favored, albeit metastable, aluminosilicate end product of arid-zone basalt weathering on Kohala, Hawai'i, and that the formation of smectite has been inhibited by kinetic factors. Soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance, thus allowing us to interpret chemical and mineralogical changes as the result of a time-dependent process. The halloysite content of the soil increases with increasing soil age; its growth is at the expense of allophane, which, in turn, forms at the expense of primary minerals. These mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil. Smectite, the thermodynamically stable phyllosilicate phase predicted by soil solution composition, is not found in these arid soils. We determined that our soil system is controlled by kinetic factors, and that, therefore, thermodynamic predictions do not reflect reality. The main factors favoring kinetic control of halloysite formation are intense, but short wet periods followed by prolonged extremely dry seasons, and microenvironmental conditions leading to immediate uptake of released Al by the halloysite-precursor mineral allophane. The δ 18O relationship between present-day soil water and halloysite, formed over the last 170 ka, was used as a tracer of past soil conditions. Results from a reconstruction of paleo-climate and -soil conditions, combined with δ 18O data, observed mineral relationships along the 350 ka chronosequence, and field evidence for long-term aridity rule out the possibility that the arid side of Kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and, therefore, altered mineralogical stabilities. We conclude that pedogenic halloysite has been continually forming from the early stage of soil formation, and that it has consistently formed under isotopic equilibrium conditions, indicating that halloysite δ 18O compositions imply paleoclimatic conditions over the time of its formation that are similar to today's.