Figure 5
Chemical composition obtained by electron probe microanalysis (EPMA). (A) X-ray (Kα) mappings showing abundances of K, Fe and Al with voids, polish defects, and all other mineral phases masked out, overlain onto a grey-scale base-layer calculated with the expression ∑ [(counts)i·Ai], (where A is atomic number, and i is Si, Ti, Al, Fe, Mn, Mg, Ca, Ba, Na, K, P, S and O), which contains the basic textural information of the scanned areas. Color scales represent counts (cts) with warmer colors representing more intense X-ray signals. Altered rims (red arrows) have lower K and Fe contents, and higher Al content relative to cores (black arrows). (B) Triangular diagrams Al-K-Fe showing the chemical domains of glaucony and nontronite. First diagram to the left corresponds to the accumulated number of pixels obtained from X-ray (qualitative) maps presented in A. The second diagram to the right corresponds to single-spot quantitative EPMA analyses (a.p.f.u.). (C) HRTEM photomicrograph showing reversal of the glauconitization process. False color image (phase map) with colors corresponding to AEM compositions of: interstratified glauconite-smectite, (Si3.53Al0.29Fe3+0.18)4(Al0Mg0.34Fe1.68)2.02(K0.77Ca0.03)0.8 (green color), and nontronite, (Si3.73Al0.27)4(Al0.30Mg0.41Ti0.03Fe1.30)2.02 (K0.43Ca0.03)0.46 (red color).
