Bacterial mineralization in weathered pyroclastic deposits of the Kaimondake volcanic ash (4040 ybp) and the Koya pyroclastic flow (6400 ybp) was investigated to evaluate the impacts of bacteria on mineral formation, and to characterize the microbiogenic minerals in the weathering environment. The mineralogy of abiogenic weathering products was also investigated for comparison with the microbiogenic products, and mineral saturation indices were calculated for porewaters using the PHREEQC computer code. The results indicated that these weathered pyroclastic deposits contain 108–109 cells/g, consisting of spherical to rod-shaped bacteria. Associated abiogenic allophane had an Al/Si ratio ranging from 1.01 to 2.13. The bacterial cell surfaces were completely or partially covered by poorly-ordered silicate minerals, which could be divided into two groups based on their chemical and morphological characteristics. Group I was characterized by well developed fibrous to smectite-like flaky habits with variable Al, Si and Fe, corresponding to compositions between proto-imogolite allophane and chamosite. These Al-Si-Fe minerals were the most abundant and major microbiogenic products in both lithologies. Group II exhibited poorly-developed aggregates of allophane-like granular materials composed mainly of Al and Si with minor Fe. Geochemical calculations revealed that the porewaters were saturated with respect to allophane and other crystalline clay minerals such as halloysite, kaolinite, montmorillonite and nontronite. These microbiogenic minerals may be formed as the earliest phase of these clay minerals after interaction of the bacterial cell surfaces with dissolved cations mainly Si, Al and Fe, in the porewaters.

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