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Nanometer-scale complexity, growth, and diagenesis in desert varnish

¹ School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA
² School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA and Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA

View larger version (95K): [in this window] [in a new window]   Figure 1. Optical micrographs (A and B) from ultrathin section and powder X-ray diffraction profile (C) of typical varnish from sample 2. A: Low-magnification image showing the varnish layers parallel to the air-varnish surface (LV—layered varnish) and rubbly bottom between varnish and underlying rock. RV—rubbly varnish; Q—quartz grains from the rock. B: Higher-magnification optical micrograph of flaring-upward structure common in some varnish. Bands are 1–2 µm thick. C: Diffraction pattern shows broad peaks from phyllosilicates and iron oxides and sharp reflections from quartz. I—illite, S—smectite, K—kaolinite, Q—quartz, H—hematite, G—goethite.

View larger version (92K): [in this window] [in a new window]   Figure 2. TEM and EFTEM images of the varnish FIB section. A: Low-magnification, bright-field TEM image of the FIB section. The section is resting on an amorphous-C film that is supported on a Cu TEM grid. The dark bars at the sides of the FIB section are thicker areas of the section used for support. The blue arrows show the same point on the low-resolution (A) and higher-resolution (B) TEM images. B: Bright-field TEM image of a portion of the FIB section showing the location (white outline) of the composite EFTEM image in (C). The white areas at the bottom are holes. C: Color Mn-Fe map of a portion of the desert varnish FIB section outlined in (B). The image illustrates the distribution of iron (red) and manganese (green). Two pores, indicated by arrows, are rimmed by Mn-rich material. The majority of the black areas in the EFTEM image are rich in Si. This image is a composite of 20 separate Fe and Mn EFTEM images.

View larger version (165K): [in this window] [in a new window]   Figure 3. Comparison of transmission electron microscopy (TEM) and energy-filtered (EF) TEM images from sample 2. A, C: Bright-field TEM image. B, D: Corresponding EFTEM images. EFTEM images show distribution of Fe (red), Mn (green), Si (blue), and Ti (yellow). Panel C shows mottled Mn-rich material (white arrow), clay flake (black arrow), and Ti-oxide particle (blue arrow).

View larger version (116K): [in this window] [in a new window]   Figure 4. Transmission electron microscopy (TEM) and spectroscopic data of particles of disaggregated varnish from sample 3 adhering to a lacy-C film. A: Bright-field TEM image of desert varnish fragments. Black arrow shows single Mn-oxide particle. B: Mn map (green). C: High-resolution TEM image of particle indicated by arrow in A. Blue bar shows area analyzed in D. D: Histogram of intensities from barred region in C showing the 1 nm periodicity. E: Portion of electron energy-loss spectroscopy (EELS) spectrum from the Mn-oxide particle showing the presence of Ba and Ce.