Mineral and trace element composition of the Lokpanta oil shales in the Lower Benue Trough, Nigeria Oluwadayo O. Sonibare a,⇑ , Dorrit E. Jacob a , Colin R. Ward b , Stephen F. Foley a a Department of Geosciences and Earth System Research Centre, University of Mainz, Becherweg 21, 55099 Mainz, Germany b School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia article info Article history: Received 13 May 2010 Received in revised form 26 April 2011 Accepted 27 April 2011 Available online 10 May 2011 Keywords: Lokpanta Oil shale Mineralogy Trace elements Lower Benue Trough abstract The concentrations of minerals and trace elements in the Lokpanta oil shale from the Lower Benue Trough, Nigeria have been determined by X-ray diffraction (XRD) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), respectively. X-ray diffraction data were evaluated using the SIROQUANT™ interactive data processing system based on Rietveld interpretation methods. A new method of trace element determination in oil shale, involving LA-ICP-MS analysis of glass beads prepared by fusing oil shale ash on an iridium strip heater was used, and the accuracy of the method was assessed by including a standard shale reference material (SGR-1b) in the analysis program. The minerals in the raw oil shales are mainly quartz, calcite and clay minerals, with the latter being represented by kaolinite and interstratified illite/smectite. Ashes of the oil shale samples prepared at 815 °C have quartz and (in some cases) illite as the dominant mineral phases, along with a significant proportion of amorphous materials. The Lokpanta oil shales are highly enriched in some potentially haz- ardous trace elements, including V, Cr and Ni, when compared with oil shales from other deposits around the world. The results obtained for the trace elements in the reference material show that the LA-ICP-MS method described in this study is very accurate and precise for the determination of a wide range of trace elements in oil shales. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Oil shale is a natural fine-grained sedimentary rock that yields a significant proportion of oil or liquid hydrocarbon upon pyrolysis. It can be burnt directly as low-grade fuel for power generation and heating purposes, and also provides chemical feedstock for the manufacturing of products such as adhesives, resins and plas- tics [1]. In Nigeria, Mid-Cretaceous oil shale deposits have been re- ported in the Lokpanta area of the Abakaliki Anticlinorium, a depocenter in the Lower Benue Trough [2]. The proven reserves amount to about 5.76 Â 10 9 tonnes [3]. In recent times, there has been an increasing interest in the utilization of these oil shale deposits as a source of energy. Knowledge about the mineralogy and trace element distribution in the raw oil shale and the ash pro- duced after retorting is highly important for various exploitation programs. It is well known that mineralogical composition has an effect on the utilization processes for oil shale. Certain minerals exhibit specific catalytic effects in gasification and liquefaction during oil shale retorting [4]. Another problem often associated with oil shale exploitation is the release of toxic elements into the environment during oil shale retorting, or from leaching of spent shale dumps [5–7]. Different aspects of the Lokpanta oil shale, including thermal and geochemical characterization and economic evaluation have been studied [3,8]. However, no detailed quantitative evaluation of the minerals and trace elements in the oil shale has been done to date. In the present study the mineralogy and trace element composition of the Lokpanta oil shales were investigated on a quantitative basis using interactive Rietveld based X-ray diffrac- tion and laser ablation-ICP-MS, respectively. 2. Quantitative X-ray diffractometry X-ray diffraction (XRD) analysis is an important tool for mineral identification, and has been used for the determination of minerals in oil shales and coals [9–15]. Identification of minerals is usually based on the investigation of powdered raw samples, or of residues obtained from low temperature ashing [16,17]. XRD has generally been regarded as inadequate for quantitative determination of minerals in coal, due to the effects of variation in mineral crystal- linity, preferred orientation in the sample mount, and differential 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.04.037 ⇑ Corresponding author. Present address: Petroleum and Environmental Geo- chemistry Research Group, Chemistry Department, University of Ibadan, Nigeria. Tel.: +234 8080 627 459; fax: +234 2 8103043. E-mail addresses: sonibaredayo@yahoo.com, oo.sonibare@mail.ui.edu.ng (O.O. Sonibare). Fuel 90 (2011) 2843–2849 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel