Hounslow and Maher (1999): p.139
Laboratory Procedures for Quantitative Extraction and Analysis of Magnetic Minerals from Sediments Mark W. Hounslow and Barbara A. Maher Centre for Environmental Magnetism and Palaeomagnetism, Geography, Lancaster University, Bailrigg, Lancaster, UK. LA1 4YB. E-mail:
[email protected],
[email protected]
In: Walden, J., Oldfield, F., Smith, J. (eds.) Environmental Magnetism, a practical guide. Quaternary Research Association, Technical Guide No. 6., p139-164, 1999. Contents 9.1
Introduction
9.2
Rationale and overview of magnetic mineral extraction and analysis procedures Disaggregation Fractionation into clastic grain sizes Magnetic Extraction Mineral grain analysis
9.3
Magnetic mineral Extraction Magnetic extraction for fine fractions (38µm fractions Magnetic extraction of paramagnetic components Quantification of extraction efficiencies Calculation of fractionated mineral magnetic properties and contributions
9.4
X-Ray Diffraction Background Instrumental considerations Sample preparation Identifying minerals in magnetic extracts Semi-quantitative X-ray diffraction analysis Peak intensity measurement Mineral abundance
9.5 Optical microscopy Introduction Grain mounts Polished sections 9.6 Scanning electron microscopy Introduction
Hounslow and Maher (1999): p.140 Elemental analysis on the SEM: X-ray spectrometry Observations on the SEM Analysis of flat polished specimens in the SEM 9.7 Analytical or Transmission electron microscopy (AEM/TEM) Background Specimen considerations Elemental analysis on the AEM: X-ray Spectrometry Electron diffraction: identifying the mineralogy of the material Collection of observations on the AEM 9.8 Differential iron extractions 9.9 Mössbauer analysis Background Mössbauer Parameters Appendix 9.1: Disaggregation, Dispersion of Samples and Carbonate Dissolution Ultrasonic Disaggregation Dissolution of Carbonate Appendix 9.2: Preparation of Reagents Pre-filtering of Contaminants from Laboratory Reagents Buffered Acetic Acid Mix Deflocculant NaCl Solution (Flocculant) Appendix 9.3: Centrifuging Sample Suspensions Appendix 9.4: The Magnetic Mineral Extraction Procedures The EMP Extract Procedure Removal of the Ultrafine Extract, EMPT Measurements and Calculations Magnetic Extraction Procedure for Grains >38µm High Gradient Magnetic Extraction Appendix 9.5. Sample Preparation for XRD and Microscopy XRD Slides Grain Mounts Polished Sections of Extracts and Sediments Preparation of Samples for Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Analysis (EDXA) Preparation of Samples for Transmission Electron Microscopy (TEM)
Hounslow and Maher (1999): p.141
9.1
Introduction
The palaeomagnetic and mineral magnetic properties of rocks, soils, sediments, dusts and organic materials often arise from the presence of very small amounts of ferrimagnetic minerals, such as magnetite and maghemite. These minerals carry strong, positive magnetic susceptibilities and acquire induced and remanent magnetisations. In some cases, the magnetic properties of natural materials are determined by the presence of canted antiferromagnetic minerals, such as haematite and goethite, which carry a weak positive susceptibility and weak but stable remanent magnetisations. Occasionally dominant are paramagnetic minerals (for example, Fe-rich silicates and carbonates), which have a weak positive susceptibility but acquire no magnetic remanence. Mineral magnetic measurements can provide rapid and often distinctive characterisation of sample magnetic properties, with the possibility of discrimination of high-resolution environmental changes. However, any interpretation of changing mineral magnetic and palaeomagnetic properties in terms of environmental and climatic factors depends heavily on identification of the origins of the magnetic minerals, whether authigenic, allogenic, or diagenetic. Currently, unique and diagnostic identification of mineral composition, morphology and grain size is not obtainable from mineral magnetic data alone. Independent mineralogical analyses are required to complement the magnetic characterisations. Insights into the origins of the magnetic minerals in a sample are gained by examination of their morphology, grain size, chemical composition and their mineral associates. Mineralogical examination of magnetic grains in situ in a sample is sometimes possible (by optical or scanning electron microscopy), particularly when the grains are abundant and larger than ~1µm (e.g. Walker et al., 1981; Turner & Ixer, 1984; Lu et al., 1994). Such in situ examination provides information on mineralogy and also mineral inter-relationships. Where magnetic grains of sub-micrometre size are present, analytical electron microscopy (AEM) can be applied to sample sections thinned by ion-beam milling (Geissman et al., 1988); however, this method has currently most usefully been applied to rock samples with high magnetic mineral concentrations. Given a sample of magnetically-enhanced soil, characterised by moderately high magnetic susceptibility (~300. 10-8 m3 kg-1), frequency-dependent susceptibility (~10% of χLF), and χARM values (~20. 10-6 m3 kg-1), and with a value of ~0.4 for Wohlfarth’s R, in situ examination of the magnetic minerals would be difficult, as the magnetic data indicate the presence of well-dispersed, ultrafine (10µm)
B X
A[2] A[3]
B[2] X
[4]
A: Method will provide most relevant and easily obtained information. B: May provide additional relevant information. X: Unlikely to provide additional significant information. [1],[2],[3]…..sequential order in which to perform observations on extract.
Mineral Grain Analysis The magnetic extractions are only a small part of the process of finding the minerals responsible for the magnetic properties, since identification of the phases extracted is the ultimate goal. A problem for quantitative estimation of the amounts of magnetic minerals is that they are spread through several extracts, and have a wide variety of grain sizes so that it is not possible to "view" all the minerals using the same observing instrument. The EME, EMP and EHG extracts can be resolved using the SEM, and optical microscopy, whereas the EMPT extract and smaller grains in the EMP and EHG extracts, require AEM. Table 9.5 summarises the observation procedures most relevant to each type of magnetic extract. A means of ' unifying'these various microscope observations is through the use of Xray diffraction (XRD), which has the capability to determine the mineralogy and semi-quantitative amounts of the various minerals in the extracts. The mineralogy of the extracts is likely to be a complex mixture of diamagnetic, paramagnetic and ferrimagnetic minerals. A sequence of analysis from the lowest resolution instrument to the highest often provides the clearest insight:Step 1: XRD of extracts, preferably using an instrument with a monochromatic radiation source and the capability to analyse small amounts of sample (section 9.4). Step 2: Optical microscopy of the magnetic extracts. This provides: a) quantification of the abundance of opaque Fe-Ti oxides, b) an initial idea of the grain size of the ferrimagnetic particles and c) a cross-check on the XRD results. This stage could also include observation of the oxides using polished grains mounts, to examine the internal structure of the grains and, if the grains are sufficiently large, an idea of their composition (Section 9.5). Step 3: Having thus obtained initial information on the types and abundance of minerals present in the extracts, SEM observations and energy-dispersive x-ray analyses (EDXA) can next be utilised (section 9.6). From these observations can be ascertained: a) the morphology of particles, b) the semi-quantitative chemical composition of these phases, c) grain size, and d) by now, a good grasp of the origins of the larger magnetic grains, and processes which may have modified them. Details
Hounslow and Maher (1999): p.147 of their internal structure can also be gained by examining the polished sections using the backscatter mode of the SEM. Step 4: AEM observations and EDXA analysis of the EMPT and EMP extracts. These observations on the 38µm) and HG extraction (with reduced field ~100mT) of the fine fraction (combined with rock magnetic measurements) will be most effective in isolating these grains.
Magnetic extraction for fine fractions ( 38)
+ RT
eq. 9.6
The % contribution of the coarse fraction (i.e. >38µm, or 63-250µm etc.) to the bulk magnetic property of the sample can also be calculated. Let RT and R>38 represent the mass-specific magnetic property for the bulk sample and the >38 fraction, respectively, before magnetic extraction. WT and W>38 are the dry weights of the bulk sample (before sieving) and of the >38µm fraction, respectively. Then the % contribution (e.g. SIRM>38/SIRMT) is:
R > 38
=
W > 38 WT RT
*100%
eq. 9.7
9.4 X-ray Diffraction Background X-ray diffraction methods are commonly used for routine identification of mineral components in soils, sediments and rocks, but particularly clay minerals in sediments and soils. Useful texts on this subject are Hardy & Tucker (1988) and Brown & Brindley (1980). A more detailed introduction to the physics and methodology of X-ray diffraction is given by Klug & Alexander (1974). Addressed here are the specific problems and techniques associated with analysing magnetic extracts. When a focused X-ray beam is directed at a crystalline specimen, the aligned planes of molecules in the crystal will diffract the X-rays, at the appropriate angle given by Braggs Law, nλ= 2d sinθ, where λ is the wavelength of the incident radiation, d (the d-spacing) is the spacing of the lattice plane, and θ is half the angle between the incident and diffracted X-ray beam. A typical X-ray diffraction trace, like that in Figure 9.4, is built up by moving the radiation detector through various angles of θ, perhaps from ~3o 2θ to ~60o 2θ. The various peaks in intensity obtained are positioned
Hounslow and Maher (1999): p.150 according to the various spacings of the lattice planes (d-spacing) of the mineral concerned. The spacing of lattice planes in crystalline earth materials can vary from ~20Å, which plot on the left of the trace at low 2θ angles, to
