Fig.1.Optical image (plane polarized light) showing the texture of phosphate stromatolites. Lamination is defined by alternating dark and light bands, dominated by phosphate and clastic grains, respectively.

Fig.2.(A) Secondary electron image (etched sample) of a phosphate stromatolite cupola; carbonate particles (mainly coccolith debris) and a detrital clay particle (arrow) lie on a phosphate lamina. (B) TEM image of a clastic lamina, with a muscovite particle (MU) between carbonate grains (C).

Fig.3.Secondary electron image (etched sample) of the surface of a phosphate lamina forming a stromatolitic dome. Phosphate grains have oval to ellipsoidal morphologies, sometimes grouped in complex aggregates.

Fig.4. TEM image corresponding to a stromatolitic phosphate laminae. The observed texture may be considered as a parallel aggregate of hexagonal prisms of francolite, with poorly crystalline silicate material located at the grain boundaries.

Fig.5. TEM image showing textural and structural features of poorly crystalline smectite phases: spindle-like packets 20-50nm thick, with layer terminations and wavy layers. Note the existence of an area with marked dark contrast within a smectite packet, corresponding to an amorphous substance containing material of relatively high atomic number (arrow).

Fig. 6. Smectite packets (SM) curved in the area adjacent to a phosphate crystal (PH). In the lower part of the photograph non-crystalline material (AM) has a mottled appearance.

Fig. 7. Spindle-like smectite (SM lacking complete fringe contrast as the result of slight misorientation of the layers. Dots delineate the upper boundary of the spindle. Mottled areas with dark contrast in the lower part of this image (arrow) correspond to non-crystalline material.

Fig.8.Association between Fe–Si–Al-rich non-crystalline substances and smectite. Amorphous substances (AM) show a mottled appearance and marked dark contrast, and are located between smectite packets (SM).

Fig.9. (A) Non-crystalline Fe-rich material (AM) with minor amounts of smectite surrounding francolite crystals (PH). The amorphous substance appears as dark mottled areas that form some rounded aggregates (arrows). (B) An expanded portion of (A), where the three phases are clearly visible: francolite crystal with well defined lattice fringes of 0·82nm/spacing (PH); smectite with poorly defined lattice fringes (SM); Fe-rich amorphous substance (AM).

Fig.10. (A) Triangular Fe–Si–Al composition diagram obtained by quantitative analysis of the phases located between phosphate grains (Mo: montmorillonite; No: nontronite). Analyses corresponding to a mixture of smectites and amorphous phases (triangles) project on the same areas as smectite analyses (circles). Two trends toward the Si and Fe apexes are observed. Two quantitative analyses of non-crystalline substances (squares) are located near the Fe apex; that nearest to the Fe apex is chemically equivalent to ferrihydrite. (B) Triangular Al2-FeAl-AlMg diagram with smectite analyses showing that these smectites correspond mainly to a Fe-rich montmorillonite (Bl: beidellite; FeBl: Fe-rich beidellite; Mo: montmorillonite; FeMo: Fe-rich montmorillonite).

Fig.11.Qualitative nannoprobe analyses illustrating the chemical composition of the amorphous phases. Spot size of microanalysis was 19nm. (A) Si-rich+Fe-rich amorphous phase, with minor amounts of Ca, Al, P, S, K and Mg. (B) Si-rich amorphous phase. (C) and (D) Fe-rich amorphous phase. Note in (C) the greater intensity of the AlKα peak relative to the SiKα peak.

Fig.12. Selected diagrams showing ranges of compositions of smectite.

Fig.13.Model for formation of authigenic clays within pelagic phosphate stromatolite