Trioctahedral clay minerals occurring as phyllosilicate cement in diagenetically altered serpentinitic graywackes have been studied using X-ray powder diffraction, scanning electron microscopy, and microprobe analyses. Two distinct cement assemblages were observed: (1) saponite ± calcite ± dolomite ± opal-CT ± pyrite, which is restricted to quartz-rich graywackes, and (2) ordered mixed-layered chlorite/smectite (C/S) + saponite ± calcite ± dolomite ± opal-CT ± pyrite characteristic of serpentinite-rich graywackes. Swelling properties as well as chemical analyses reveal a predominance of high-charge layers both in saponite and C/S. Saponite and C/S show a unique composition of 2:1 layers with respect to Si/(Si+Al) ratio. The decrease in Mg/(Mg+Fe) ratio from saponite to C/S with respect to the increasing Al^IV content implies that Fe-Mg substitution was controlled by distortion of tetrahedral sheets caused by Si-Al substitution. The lack of correlation between whole-rock and phyllosilicate Mg/(Mg+Fe) ratios supports this interpretation. Textural as well as compositional evidences suggest that both saponite and C/S originated by the interaction of the sediment with pore-fluids during burial as direct precipitates. It is inferred that a different bulk rock composition, and consequently, a different chemistry of pore-fluids played an important role during authigenesis. Particularly, the availability of Al may have been a primary factor controlling whether saponite or C/S formed, while the role of Mg (or Mg/(Mg+Fe) ratio) as well as the role of temperature were only of secondary importance.