Measures of the Degree of Chemical Weathering of Rocks

TLDR

Silica‑to‑alumina ratios are used to quantify rock weathering, but their reliability depends on bedrock uniformity, constant alumina content, strong correlation between silica loss and total‑element loss, and the magnitude of the ratio difference between fresh and weathered rocks. The authors analyzed total silicate compositions of 64 non‑quartz volcanic and 48 igneous/metamorphic rocks from humid regions, finding a highly significant correlation between silica loss, total‑element loss, and the silica‑to‑alumina mole‑ratio. The silica‑to‑alumina mole‑ratio also correlates with complex absolute and relative weathering indices, indicating that this simple index can guide assessments of weathered rock physical, chemical, and engineering properties.

Abstract

In humid regions, the silica-to-alumina ratios have been found to provide an easy way to quantify the degree of rock weathering. The reliability of this index depends on the uniformity of the bedrock, the constancy of the alumina content during weathering, the degree of correlation between silica loss and total-element loss, and on the magnitude of the difference between the silica-to-alumina mole-ratio values of fresh and thoroughly weathered rock. Examination of total silicate analyses of fresh and derivative weathered rock from sixty-four non-quartz-bearing volcanic rocks in north-east Papua and forty-eight igneous and metamorphic rocks (some quartz-bearing) from other humid areas revealed extremely good correlation, with a high degree of significance, between the silica loss and the total-element loss, and between the silica-to-alumina mole-ratio and total-element loss. The silica-to-alumina mole-ratio also correlates well with the more complex absolute and relative weathering indices calculated from total-silicate analyses. The simple index should provide a useful guide to the physical, chemical, and engineering properties of weathered rocks.