2.2 Electron Backscatter Diraction Analyses
2.2.4 High Lateral Resolution EBSD
The spatial or lateral resolution of EBSD is dertermined by the interaction volume for incoherent, elastic scattering processes induced by the primary electron beam. This interaction volume depends on the incoherent quasi-elastic scattering of the primary beam electrons and is inuenced by mainly two factors: material properties of the sample and the energy of the primary electrons.
The material properties that inuence the interaction volume are the atomic num- ber and electronic properties of the sample material. A lower atomic number of the investigated material leads to a larger interaction volume. Calcite consists of the light elements Ca, C and O and can thus be only be investigated with low lateral resolutions compared to steels or most other metallic compounds. The lateral reso- lution on calcite was determined to be around 0.5µm using an accelerating voltage
of 20 kV (Schmahl et al. 2008).
The energy of primary electrons is controlled by the accelerating voltage of the electron gun. A decrease of accelerating voltage leads to a decrease of the primary electron energy and thus to a reduction of the interaction volume (Ren et al. 1998, Steinmetz & Zaeerer 2010). This reduction results directly in a higher lateral reso- lution. The validity of these considerations was recently demonstrated (Steinmetz & Zaeerer 2010). The decrease of the accelerating voltage however results in patterns with a low pattern quality (dened by Wright & Nowell 2006), a measure of the signal-to-noise ratio.
The improvement of the lateral resolution of EBSD on calcitic samples was per- formed at the Max-Planck Institute for Iron Research (MPIE) in Düsseldorf. The used EBSD system was a prototype of a new generation of fast EBSD cameras. This camera allowed a fast collection of low signal to noise ratio diraction patterns. This implies that the dwell time of the electron beam on one measured point was short compared to other devices. However, in the used SEM4problems arose on the sample
due to beam damage: An amorphous carbon layer formed on the sample surface due to chemical cracking of organic molecules present within the sample. As the EBSD signal is obtained only from the upper 20 nm of the sample (e. g. Field 1997) a thin amorphous layer strongly decreases the pattern quality. To minimize these problems the beam aperture was reduced severely (30µm) to decrease the beam current and
therewith the beam damage. However, this reduction of the aperture results in a comparably low collection speed: Maps were measured with 200 ms per frame with 15 kV accelerating voltage and 500 ms per frame with 10 kV accelerating voltage.
The relatively low collection speed resulted again in beam damage due to the high dwell time. Nevertheless EBSD measurements could be performed successfully. A further decrease in beam damage can be achieved by increasing the step-size of the measurement, a factor that naturally again decreased the lateral resolution. High lateral resolution EBSD measurements with 10 kV and 15 kV could be performed successfully on brachiopod shells (Section 3.3). However, the lateral resolution with 15 kV is better as a smaller step size (100 nm) can be chosen.
3 Results and Discussion
3.1 On the Structure and Formation of Biotic and
Abiotic Schwertmannite
Abstract
We studied the structural variability of Schwertmannite as synthesized in dierent biotic and abiotic pathways from acidic solutions in the presence of sulfate. The biotic pathways included the bacterial strains Leptospirillum ferrooxidans EHS 2. The abiotic pathways started from ferric or ferrous-solutions and used various meth- ods of oxidation and/or precipitation to produce the iron-oxy-hydroxy-sulfates. The hedgehog-like morphology of schwertmannite, which is common in biotic synthesis environments, was obtained in biotic but also in abiotic syntheses. Further, the hedgehog particles in both synthesis routes are solidly lled with precipitate. As they are not hollow they do not reect overgrown bacteria. We further found that the contribution of bacteria such as Leptospirillum ferrooxidans EHS-2 to schwert- mannite formation is limited to the oxidation of ferrous to ferric iron in acidic conditions. A substrate eect of EPS secreted by the bacteria cannot be ruled out. The schwertmannites obtained from various synthesis pathways gave slightly dif- ferent x-ray diraction proles. In Rietveld renements these dierences could be attributed to a variation in the size and shape of coherently diracting nanocrys- talline domains. These domains are anisotropic with 5-11 nm along the channels and 1.8 - 2.4 nm perpendicular to the channels depending on the synthesis procedure. Mössbauer spectroscopy showed that the schwertmannite precipitates were purely ferric and contain octahedrally coordinated Fe(III) only.
This section is a manuscript that was submitted to: European Journal of Mineralogy
TITLE: On the Structure and Formation of Biotic and Abiotic Schwertmannite. AUTHORS: Andreas J. Goetz, Andreas Ziegler, Judith Kipry, Friedrich E. Wag- ner, Zi-Lin Wang, Kuan-Ying Hsieh, So-Hyun Park, Rossitza Pentcheva, Claudia Wiacek, Michael Schlömann, Eberhard Janneck, Wolfgang W. Schmahl.
3.1.1 Introduction
The mineral schwertmannite, a non-stoichiometric compound with an idealized com- position of Fe8O8(OH)6SO4, occurs widely in environments with acidic iron- and sulfate bearing waters. In spite of its high abundance it has only been known since the 1990s (Bigham et al. 1990, Bigham et al. 1994). The late identication of this compound can be attributed to its lack of long range order and the correspond- ing absence of a distinctive and easily recognizable diraction pattern. Moreover, schwertmannite shows x-ray diraction (XRD) peaks overlapping with those of 6-line ferrihydrite. The physicochemical conditions for the formation of schwertmannite are met in acid mine drainage or acid rock drainage waters (e. g. Bigham et al. 1996, Kawano and Tomita 2001, Giagliano et al. 2004, Regenspurg et al. 2004, Paraniuk and Siuda 2006, Adams et al. 2007, Kupka et al. 2007). In such regions the disso- lution of pyrite liberates typically Fe and S. Bacteria oxidize the sulfur to sulfate and the iron to Fe-(III) whereupon the lower solubility of Fe-(III) compared to Fe- (II) leads to the precipitation of schwertmannite. Natural schwertmannite is known for building aggregates of whiskers which form hedgehog-like particles (Ferris et al 2004). It has been suggested that these hedgehog-like structures result from a - prob- ably lethal - over-growth of the iron oxidizing bacteria by schwertmannite needles (Ferris et al. 2004).
Due to the amorphous to nanocrystalline nature of schwertmannite there is no nal agreement on its structure. Bigham et al. (1990), who rst described the mineral in detail, put forward the theory that schwertmannite has a β-FeOOH structure
resembling that of akaganeite. This structure is similar to that of hollandite: the Fe-OOH octahedra form a structure with open channels that are lled with anions, in the case of schwertmannite with sulfate. In a recently published paper a structural model based on a disruptedβ-FeOOH structure is described and evidence for this is
given by pair distribution function (PDF) tting (Fernandez-Martinez et al. 2010). However, the structural position of the sulfate was left open and remains up to now a matter of speculation. Some authors expressed doubts that the β-FeOOH model
is correct for schwertmannite at all (Majzlan and Myneni, 2008) and even a goethite like structural element was proposed (Loan et al., 2004, Loan et al., 2005).
The porous channel-structure of schwertmannite created attention due to its sorp- tion capacities for heavy metals or toxic complexes such as chromate and arsenate (e. g. Waychunas et al. 1995, Webster et al. 1998, Carlson et al. 2002, Regenspurg and Peier 2005, Burton et al. 2010). The studies that use synthetic schwertmannite use dierent ways to obtain it and the subsequent phase identication is usually performed by XRD. Due to the close-to-amorphous nature of schwertmannite the majority of diractograms in literature exhibit a strikingly bad signal to noise ratio. For an understanding of the bacterial inuence on the precipitation of schwert- mannite TEM samples of pure cultures of Leptospirillum ferrooxidans EHS 2 were analysed. To answer the question whether schwertmannite is indeed a structurally distinct single-phase material we synthesized schwertmannite materials by dier- ent biotic and abiotic pathways. The products were quantitatively analysed using Rietveld-analysis on XRD data showing a high signal/noise ratio. Furthermore tem- perature dependent Mössbauer spectroscopy from 4.2 K to room temperature was conducted.