ORIGINAL RESEARCH article
Front. Clim.
Sec. Carbon Dioxide Removal
Volume 7 - 2025 | doi: 10.3389/fclim.2025.1572341
Microscopic investigation of incipient basalt breakdown in soils: Implications for selecting products for enhanced rock weathering
Provisionally accepted
- Queensland University of Technology, Brisbane, Australia
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Digital optical and scanning electron microscopy (SEM) was used to study advance of incipient weathering of basaltic rock particles for two enhanced rock weathering (ERW) sites in Eastern Australia and three natural basalt sites (New Zealand and Eastern Australia). At the ERW sites, weathering of amended rock particles (up to 8 mm in diameter) induced a significant increase (1 -1.5 pH unit) in shallow soil pH. After 14 months of incubation at the more recent site, 6 -8 mm basalt particles showed dissolution of glass and olivine while pyroxene and plagioclase remained largely fresh. No secondary minerals were identified by SEM and high-quality X-ray diffraction analysis. Compared to the fresh, quarried basalt, the measured specific surface area (SSA) increased by 33%, suggesting microporosity formation via dissolution. At the >20 years ERW site, results were complex because of inconsistent application of basalt and greenschist facies 'metabasalt'. Metamorphic rock particles showed negligible weathering while basalt particles could only be identified in the coarse (>6 mm) fraction of the shallowest (0 -5 cm) soil. Within the finer particles in the deeper (5 -10 cm) zone of elevated pH, potential 'ghost' basalt particles were identified by distribution patterns of ilmenite, suggesting near-complete basalt breakdown of mm-sized particles on decadal timescale. In variably weathered natural samples, dissolution also dominated over precipitation of secondary phases. Weathering progress in basaltic tephra deposited 150 -600 years ago strongly depended on emplacement context. Tephra in free-draining >10 cm thick lapilli beds was only weakly altered, even where covered by soil, likely due to lacking connectivity of fracture networks for water access. In all studied incipiently weathered particles, we found sequential breakdown of glass and olivine before attack of pyroxene and plagioclase. Recognisable secondary mineral formation was minimal, but SSA increased over least weathered particles. The presence of interconnected glass and fracture networks apparently aids the rate of incipient weathering, increases microporosity and promotes particle disaggregation. This may permit application of relatively coarse (>5 mm) basalt for ERW but only for potential amendments where microscopic investigation and SSA have established suitable weathering fluid access networks.
Keywords: Enhanced rock weathering (ERW)1, surface area2, basalt3, carbon sequestration4, microscopy5, particle size6
Received: 07 Feb 2025; Accepted: 24 Apr 2025.
Copyright: © 2025 Burke, Kamber and Rowlings. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Balz Samuel Kamber, Queensland University of Technology, Brisbane, Australia
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