High-resolution three-dimensional imaging of coal using microfocus X-ray computed tomography, with special reference to modes of mineral occurrence Alexandra Golab a , Colin R. Ward b, ⁎, Asep Permana b, 1 , Paul Lennox b , Pieter Botha c a Digitalcore Pty Limited, 73 Northbourne Avenue, Canberra, ACT, 2600, Australia b School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, 2052, Australia c FEI Australia Pty Limited, 27 Mayneview Street, Milton, Qld, 4064, Australia abstract article info Article history: Received 28 November 2011 Received in revised form 15 March 2012 Accepted 10 April 2012 Available online 5 May 2012 Keywords: Mineral matter X-ray tomography, petrifactions Siderite nodules Fracture fillings Australia Samples of coal from the Sydney and Bowen Basins of eastern Australia have been imaged at high resolution using a large-field, 3D microfocus X-ray computed tomography (μCT) system, with special but not exclusive attention to evaluating the modes of occurrence of the mineral matter within the coal. The samples imaged were 110 mm, 25 mm, 19 mm, 10 mm, and 4 mm in size, yielding voxel dimensions of 54, 30, 12, 6, and 3 μm respectively. Data collection was carried out using a helical stage, providing images with > 2000 2 voxels in the horizontal (X–Y) plane and up to 3500 voxels high. Three-dimensional image blocks derived from the scans were examined as cross-sections along orthogonal planes and as perspective images, manipulated to be viewed from any angle. Imaging after saturating the coal with X-ray attenuating brine was also carried out to highlight the distribution of connected micro-pores and cleats, and improve the detail of features seen within the samples. Features evaluated within the coals included the size and three-dimensional distribution of siderite nodules, and different types of mineral infillings in petrifactions of maceral components. Individual macerals could also be identified within the coal, based partly on X-ray density and partly on the associated porosity and structure. In some cases high-resolution images enabled the nature of individual plant particles to be identi- fied within the coal samples. Mineral-filled cleats and open fractures were also evaluated, including the origin of radiating fracture patterns around siderite nodules in vitrinite. In some cases several generations of cleat and/or fractures could be distinguished, and the sequence of their formation and infilling was interpreted. Complementary analyses of the mineral matter in the samples were carried out using X-ray diffraction, as well as examination of polished sections by optical microscopy examination. Images obtained from the μCT scans were also registered against SEM–EDX and QemSCAN images of polished sections prepared from the same samples after scanning, providing a more definitive basis for identifying the different components and for integrating μCT data with results from other petrographic and electron microscope studies. © 2012 Elsevier B.V. All rights reserved. 1. Introduction X-ray micro-computed tomography (μCT) is a technique that yields high-resolution three-dimensional X-ray images of solid opaque objects quickly and non-destructively (Carlson et al., 2003). It is similar to medical CT scanning, but carried out on a smaller scale and with greatly increased resolution (down to 2 μm is possible). Use of such imaging is of value in a variety of applications, including examination of clastic (Golab et al., 2010) and carbonate (Arns et al., 2005b) reservoir rocks in petroleum geology, as well as three-dimensional studies of coal (Mazumder et al., 2006), paper (Roberts et al., 2003), biomaterials (Knackstedt et al., 2006), bones (Zezabe et al., 2005), and volcanic ash (Ersoy et al., 2010), and materials for palaeontology (Long et al., 2006), soil science, meteoritics, and geotechnics (Ketcham and Carlson, 2001; Mees et al., 2003). Digital 3D imaging of core material at the pore scale using μCT, for example, allows improved understanding of petrophysical response, multiphase flow properties and geological heterogeneity in petroleum reservoir evaluations (e.g., Arns et al., 2005a). The intensity recorded in the pixels (2D) and voxels (3D) obtained in μCT analysis represents the relative radio density, or relative atten- uation, of X-rays through individual segments of the imaged material (Novelline, 1997). The X-ray attenuation of any material is a function of both the electron density and effective atomic number of the species (Van Geet et al., 2001b). Within the tomogram, the X-ray opac- ity of the material in each individual segment determines its brightness, allowing a three-dimensional image to be reconstructed from sections viewed at different angles. Voids are usually represented as black in such images due to their low X-ray opacity, Fe-bearing minerals are usu- ally light grey or white due to high X-ray opacity, and Si- and Al-bearing International Journal of Coal Geology 113 (2013) 97–108 ⁎ Corresponding author. Tel.: + 61 4 0245 1594. E-mail address: c.ward@unsw.edu.au (C.R. Ward). 1 Present address: Ministry of Energy and Mineral Resources, Jalan Diponegoro 57, Bandung, Indonesia. 0166-5162/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2012.04.011 Contents lists available at SciVerse ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo