FIG. 7.

SEM images showing modes of early diagenetic mineralization of cyanobacterial mats forming Niuafo‘ou stromatolites. (A) Vertical section of a stromatolite composed of alternating layers of aragonite and silica (white arrows). (B) Section of a filamentous cyanobacterial mat with silicified cyanobacteria and aragonite matrix. (C) Section of a cyanobacterial mat mineralized with aragonite but with cell walls of filamentous taxa permineralized by silicates. (D) Two cyanobacterial filaments mineralized with aragonite in an aragonite matrix; note aragonite fan on the left side. (E) Comparison of two mineralized filaments, in which the larger is mineralized with silica and the smaller, partially degraded, with aragonite; both filaments are entombed in an aragonite matrix. (F) Silicified filament in aragonite matrix; note the perfect silica infilling. All samples etched with formic and hydrofluoric acids.