The control of calcium silicate weathering and climate evolution by dissolution-precipitation processes at the mineral interface

The kinetics of mineral weathering on the Earth’s surface
and the feedback through atmospheric and ocean chemistry
controls climate evolution. The formation of surface coatings
during early low-temperature silicate weathering may
significantly affect the rate of silicate dissolution over
geological timescales, and thus influence predictions made by
climate models. The mechanism of formation of these layers
is critical for understanding and evaluating dissolution
kinetics of major rock-forming minerals, and is a matter of
vigorous debate and controversy. Here we present in situ,
high spatial resolution data on the evolution of mineral
surface topography and interfacial fluid composition during
dissolution of wollastonite (CaSiO3). Our study provides
clear, direct experimental evidence that leached layers are
formed in a two-step process: stoichiometric dissolution of
the pristine mineral surfaces and subsequent precipitation of
amorphous silica from a supersaturated layer of fluid in
contact with the mineral surface. This occurs despite the fact
that the bulk solution is undersaturated with respect to the
secondary phase. The thickness of the supersaturated zone
depends on fluid flow rate and suggests an origin for the wellknown
discrepancy, of up to four orders of magnitude,
between dissolution rates measured in the field and in the
laboratory. Our model suggests that transport-controlled
dissolution kinetics due to the formation of surface altered
layers leads to higher pCO2 and T (up to 2 °C) estimates over
the last 90 Myr if compared with scenarios where these layers
do not develop.