The chemical index of alteration (CIA) is a tool to calculate the weathering intensity (Nesbitt and Young, 1982). It has been largely used to reconstitute the past climates on Earth at different epochs and to determine the sediment source rocks from shale and graywacke stratigraphical series. However, because it induces some uncertainties, we propose a new approach based on the M⁺-4Si-R²⁺ system (M⁺ = Na⁺ + K⁺ + 2Ca²⁺; 4Si = Si/4; R²⁺ = Fe²⁺ + Mg²⁺) that takes silica into account. In these coordinates, the chemical compositions of the weathered granitic, mafic and ultramafic rocks determine clearly separated trends which all converge toward the 4Si pole, namely the kaolinite composition (chlorites plot near the R²⁺ pole). Consequently, the alteration intensity for a given parent rock can be measured by the migration of its chemical composition toward the kaolinite pole: Δ4Si% = [(4Si_{altered sample} − 4Si_{unaltered parent rock}) × 100]/(100 − 4Si_{unaltered parent rock}). The ultimate stage of weathering leads to the progressive accumulation of the insoluble R³⁺ components (R³⁺ = Al³⁺ + Fe³⁺). It is attained in bauxite deposits where kaolinite is replaced by gibbsite. This ratio varies from 0 to 1 with increasing weathering degree, namely the leaching of soluble components (M⁺ and R²⁺), the oxidation of Fe²⁺ and the concentration of the residual ones (Al³⁺, Fe³⁺). Because hydroxides cannot be represented in the M⁺-4Si-R²⁺ system, the Weathering Intensity Scale (WIS) including the ultimate bauxite stage must be based on the co-variation of the Δ4Si% parameter and the R³⁺/(R³⁺ + R²⁺ + M⁺) ratio.

The formation of chondrite materials represents one of the earliest mineralogical processes in the solar system. Phyllosilicates are encountered at various stages of the chondrule formation, from the initial stages (IDP agglomerates) to the final steps (chondrule internal alteration). While typically linked to aqueous alteration, recent studies reveal that phyllosilicates could precipitate directly from residual fluids in post-magmatic or deuteric conditions and under a wide range of temperatures, pressures, water/rock ratios, and H₂/H₂O ratio conditions. This study re-examined the formation of hydrated phyllosilicates in chondrules and associated fine-grained rims (FGRs) using published petrographical, mineralogical, and chemical data on carbonaceous chondrites. Given that chondrules originate from the melting of interplanetary dust particles, the water liberated by the devolatilization of primary phyllosilicates, including clay minerals or ice melting, reduces the melting temperature and leads to water dissolution into the silicate melt. Anhydrous minerals (e.g., olivine and diopside) form first, while volatile and incompatible components are concentrated in the residual liquid, diffusing into the matrix and forming less porous FGRs. Serpentine and cronstedtite are the products of thermal metamorphic-like mineral reactions. The mesostasis in some lobated chondrules is composed of anhydrous and hydrous minerals, i.e., diopside and serpentine. The latter is probably not the alteration product of a glassy precursor but rather a symplectite component (concomitant crystallization of diopside and serpentine). If so, the symplectite has been formed at the end of the cooling process (eutectic-like petrographical features). Water trapped inside chondrule porosity can lead to the local replacement of olivine by serpentine without external water input (auto-alteration). In the absence of water, hydrated phyllosilicates do not crystallize, forming a different mineral assemblage.