Pervasive poorly crystalline Mg-rich hydrous silicate material in C-type asteroid matrices

C-type asteroid-derived samples, including Ryugu and carbonaceous chondrites, consist primarily of fine-grained matrices of magnesium-rich hydrous silicates formed by aqueous alteration. These matrices are commonly interpreted as mixtures of serpentine- and saponite-like phyllosilicates, although their mineralogical characteristics do not fully correspond to well-crystalline terrestrial layered silicates. Here we compare magnesium K-edge X-ray absorption near-edge structure spectra from Ryugu and carbonaceous chondrite matrices with those of magnesium-bearing hydrous silicates. The results indicate that the recorded magnesium coordination environments include a substantial contribution from poorly crystalline magnesium silicate hydrate-like material, in addition to serpentine- or saponite-like components. Integrated analysis of solubility data for magnesium silicate hydrate further suggests that this material exhibits coherent physicochemical behavior across a wide compositional range. Assuming its initial precipitation and subsequent mineralogical transformations, a wide range of compositional variation can be produced without requiring separate aqueous conditions for each local composition. This framework provides a physicochemical basis for explaining the compositional variability and associated phyllosilicate diversity of C-type chondritic matrices.