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TEM in Geology. Basics and applications 37 small grain size of minerals in very low-grade metamorphic rocks, since most natural tobelites have been described in subgreenschist-facies rocks. Nieto (2002) was the first to describe, using TEM, the textural characteristics of NH 4 micas in very low-grade metapelites with associated coal seams (from the Bacia Carbonífera do Douro-Beira, N Portugal), and he established their chemical and genetic relationships with coexisting K micas. NH 4 and K micas and berthierine form small subparallel packets of a few layers separated by low-angle boundaries, exhibiting all the characteristics commonly described for subgreenschist-facies, which includes a lack of textural and chemical equilibrium. In the Douro-Beira samples, the simultaneous presence of two groups of compositions of micas, one K-rich and the other K-poor, was demonstrated by XRD, lattice fringe images, electron diffraction, and analytical electron microscopy data (Fig. 4). The textural relationship between the two micas was the usual one between paragenetic phyllosilicates in very low-grade shales, like the one described for the muscovite/paragonite system by Shau et al. (1991). NH 4 - and K-dominated micas were segregated into well-separated packets with scarce intergrowth and almost no mixed-layers. Hence, they showed a solvus relationship. The compositional gap is narrow (Fig. 4), possibly near closure. The presence of N in the micas was also confirmed by electron energy loss spectroscopy (EELS) of powdered portions dispersed on holey-carbon grids (Livi et al. 2006). The method involves a simultaneous acquisition of EELS spectra over the energyloss range 275-475 eV and the acquisition of energy dispersive X-ray spectra (EDS). The EELS spectra contain the core-loss edges for K L2,3 (296.3 eV) and N K (400.9 eV). Thus, the N/K ratio can be calculated. Analysis of ammonium in white micas from the Central Alps, Switzerland, indicated that concentrations of N down to 0.01 atoms per formula unit could be detected.
38 Fernando Nieto FIGURE 4. Lattice-fringe image showing textural relationships between packets of muscovite (K) and tobelite (NH 4 ) in an organic-rich shale. Insets: Chemical contents of three samples determined by EDX, showing the gap between the two kinds of micas; K/N ratio of a tobelite crystal determined by EELS; electron diffraction pattern (bottom right) in which the existence of two slightly different (00l) spacings is evident in the third (3) order. The intermediate spot between second (2) and first (1) order corresponds to a minor local muscovite/tobelite mixed-layer (not visible in the image). From Nieto (2002) and Abad et al. (2007). Genesis of Mn oxides from marine sediments Mn oxy-hydroxides form as a result of diagenetic, hydrogenous, or hydrothermal processes along mid-ocean ridges or on the sea floor, using reduced dissolved Mn 2+ provided by springs. These oxides, which have a +3 or +4 valence state, are largely controlled by redox and pH conditions. Precipitation of Mn oxy-hydroxide in natural environments could result from microbial processes, which accelerate the rate of Mn 2+ oxidation. Buatier et al. (2004) studied Mn deposits and partially altered sediments from the flank of the Juan de Fuca Ridge. At SEM scale, samples exhibited abundant
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