Mineralogical and chemical studies of silica minerals from shales of Neogene Tertiary age in the Tempoku district, Hokkaido, Japan lead to a proposed mechanism of diagenesis and transformation of silica minerals in sedimentary rocks. A zonal distribution of amorphous silica, opalcristobalite, low-cristobalite and low-quartz is recognized successively from the younger to older strata. In particular, the value of d(101) and the peak width of 50% intensity in cristobalite show a progressive decrease with increasing depth of burial. It is suggested that most of the silica minerals in the samples studied originated by alteration of diatoms rather than any other source. The studied sequences have fairly uniform muddy facies with rare intercalation of pyroclastic sediments and show no evidence of hydrothermal alteration or metamorphism. Under the microscope amorphous silica is usually associated with abundant diatom fossils whereas cristobalite and diatoms rarely occur together. We interpret the zonal distribution of the neoformed silica minerals as an induction of diagenetic alteration of biogenic silica,

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