Abstract

The paucity of weathering rates for quartz in the natural environment stems both from the slow rate
at which quartz dissolves and the difficulty in differentiating solute Si contributed by quartz from that derived
from other silicate minerals. This study, a first effort in quantifying natural rates of quartz dissolution, takes
advantage of extremely rapid tropical weathering, simple regolith mineralogy, and detailed information on
hydrologic and chemical transport. Quartz abundances and grain sizes are relatively constant with depth in a
thick saprolite. Limited quartz dissolution is indicated by solution rounding of primary angularity and by the
formation of etch pits. A low correlation of surface area (0.14 and 0.42 m2 g21) with grain size indicates that
internal microfractures and pitting are the principal contributors to total surface area.
Pore water silica concentration increases linearly with depth. On a molar basis, between one and three
quarters of pore water silica is derived from quartz with the remainder contributed from biotite weathering.
Average solute Si remains thermodynamically undersaturated with respect to recently revised estimates of
quartz solubility (,180 mM) but exceeds estimated critical saturation concentrations controlling the initiation
of etch pit formation (.17–81 mM). Etch pitting is more abundant on grains in the upper saprolite and is
associated with pore waters lower in dissolved silica. Rate constants describing quartz dissolution increase
with decreasing depth (from 10214.5–10215.1 mol m22 s21), which correlate with both greater thermodynamic
undersaturation and increasing etch pit densities. Unlike for many aluminosilicates, the calculated natural
weathering rates of quartz fall slightly below the rate constants previously reported for experimental studies
(10212.4–10214.2 mol m22 s21). This agreement reflects the structural simplicity of quartz, dilute solutes, and
near-hydrologic saturation.

Citation

Schulz, M.S., White, A.F. (1999): Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico III: Quartz dissolution rates. Geochimica Et Cosmochimica Acta.