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technology, compositional mapping with resolution ˜ 1 nm is possible. Unlike EDS mapping, energy-filtered imaging is only semi-quantitative, but it offers the huge advantage of especially high (collection) efficiency for light element analysis and mapping. Thus, biogenic nanostructures containing organic and inorganic matter could be mapped using energy-filtered imaging. Because thin specimens (ideally <100 nm thick) are required for TEM, specialized sample preparation procedures are required. Ultramicrotomy, ion milling, chemical etching, and precision polishing are the most commonly used methods for producing thin TEM specimens.
Electron Microscopy of Biominerals
Three approaches are potentially useful for detecting evidence of biomineralization in rocks using electron microscopy. They are morphological studies using high-resolution SEM imaging, mineralogical studies using TEM, and compositional studies using TEM.
The morphological approach usually relies on using high-resolution SEM imaging to identify shapes or forms on surfaces (e.g., worms) that are consistent with past biological activity. This approach has been used to search for nanofossils in meteorites and terrestrial rocks [1,5]. However, image interpretation is subject to uncertainties, and it is usually difficult to obtain corroborating compositional and structural data from the same specimen . Conductive coatings produce nanometer-sized morphological forms that have been confused with biological forms . A variety of exotic morphological forms similar to biogenic structures can be produced by strictly non-biological processes [4,6]. Even if the morphology of a particular form is consistent with biogenic activity, it may not be unique to biogenic activity.
The TEM has proven ideal for probing the mineralogy of biominerals [2,3]. Common biominerals include iron oxides (e.g., magnetite) (see Figure 2), iron sulfides (e.g., greigite and pyrrhotite), carbonates, and other minerals. Some biominerals are arranged in distinctive configurations. For example, magnetotactic bacteria are a group of organisms that orient and navigate along geomagnetic field lines, and they do so by precipitating chains of magnetite (or iron sulfide) nanocrystals. Unfortunately, the chains may not survive geological processing, and the individual bacterial magnetosomes that make up the chains can be difficult to distinguish from some inorganically produced magnetites (see Figure 2).
Compositional analyses at the nanometer scale can be useful for investigating biogenic structures. The distribution of heavy elements can be mapped with resolution on the order of ˜5 nm using EDS. Electron energy-loss energy-filtered imaging can be used to investigate the distribution and speciation of biogenically important light elements C, N, and O at the nanometer scale. Organic compounds (e.g., PAHs) associated with potential biominerals may be indicators of past biogenic activity . Although molecular species cannot be directly detected using electron microscopy, it is possible to probe the local (atomic and molecular) bonding environment of C, N, and O (and heavier elements), using electron energy-loss spectroscopy.
1. D.S. McKay et al. (1996). Science273, 924-930.
2. M. Pósfai et al. (1998). Science280, 880-883.
3. A. Iida and J. Akai (1996). Sci. Rep. Niigata Univ., Ser E (Geology)11, 43-66.
4. J.P. Bradley et al. (1997). Nature390, 5145-5146.
5. V.A. Pedone and R.L. Folk (1996). Geology24, 763-765.