Chemical index of alteration formula
with alteration index <75 indicate sub-alkaline rhyolite magma composition. In contrast, most bulk-ash samples without glass shards plot in the dacite field and average 10% lower SiO 2 content and less than 3% sodium+potassium content. This is interpreted to be the result of diagenetic alteration. The Chemical Index of Alteration (CIA) is the most accepted of available weathering indices. Past conditions of physical and chemical weathering can be reliably inferred if application of the CIA is combined with a comprehensive facies analysis.