Characterization of the sulphate mineral coquimbite, a secondary iron sulphate from Javier Ortega mine, Lucanas Province, Peru – Using infrared, Raman spectroscopy and thermogravimetry Ray L. Frost a,⇑ ,Z ˇ eljka Z ˇ igovec ˇki Gobac b , Andrés López a , Yunfei Xi a , Ricardo Scholz c , Cristiano Lana c , Rosa Malena Fernandes Lima d a School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia b Institute of Mineralogy and Petrography, Department of Geology, Faculty of Science, University of Zagreb, Horvatovac 95, 10000 Zagreb, Croatia c Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35400-00, Brazil d Mining Engineering Department, School of Mines, Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35400-00, Brazil highlights  We have studied the mineral coquimbite.  Using SEM with EDX, thermal analytical techniques and Raman and infrared spectroscopy.  Chemical formula was determined as ðFe 3þ 1:37 ; Al 0:63 Þ P 2:00 ðSO 4 Þ 3  9H 2 O.  Thermal analysis showed a total mass loss of 73.4% on heating to 1000 °C. article info Article history: Received 19 November 2013 Received in revised form 30 January 2014 Accepted 30 January 2014 Available online 3 February 2014 Keywords: Coquimbite Amarantite Raman spectroscopy Sulphate Infrared spectroscopy abstract The mineral coquimbite has been analysed using a range of techniques including SEM with EDX, thermal analytical techniques and Raman and infrared spectroscopy. The mineral originated from the Javier Ortega mine, Lucanas Province, Peru. The chemical formula was determined as ðFe 3þ 1:37 ; Al 0:63 Þ P 2:00 ðSO 4 Þ 3  9H 2 O. Thermal analysis showed a total mass loss of 73.4% on heating to 1000 °C. A mass loss of 30.43% at 641.4 °C is attributed to the loss of SO 3 . Observed Raman and infrared bands were assigned to the stretching and bending vibrations of sulphate tetrahedra, aluminium oxide/hydroxide octahedra, water molecules and hydroxyl ions. The Raman spectrum shows well resolved bands at 2994, 3176, 3327, 3422 and 3580 cm 1 attributed to water stretching vibrations. Vibrational spectroscopy combined with thermal analysis provides insight into the structure of coquimbite. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Coquimbite is an iron (III) sulphate mineral of ideal formula Fe 3þ 2 ðSO 4 Þ 3  9H 2 O, which derives its name from the Coquimbo re- gion, Chile [1]. Coquimbite is found as secondary mineral devel- oped in the oxidized portions of weathering iron sulphide deposits in arid regions, although it could be also found associated with fumarolic activity [1]. The mineral was first described from Tierra Amarilla near Copiapó, Chile [2–5], and other occurrences were reported in Quetena, Chuquicamata, Alcaparrosa, Chile; Con- cepción mine, Huelva, Spain; Skouriotisa, Cyprus; Rammelsberg, Hartz, Germany, among others [1,3,6–8]. Coquimbite was recently found in Peru, at the Javier Ortega mine, Lucanas Province, Departmento Ayacucho [9]. Main ore min- eral in Javier Ortega mine is chalcopyrite and secondary ore miner- als are sphalerite and galena [9]. Coquimbite occurs in the oxidized portions of this iron sulphide deposit, together with other second- ary sulphate minerals like alunogen, chalcanthite, copiapite, halo- trichite, jarosite, krausite, römerite and with sulphur. Coquimbite in Javier Ortega mine occurs in well developed violet crystals up to 5 cm long [9]. Coquimbite is a hexagonal mineral [10,11] with a = 10.922(9), c = 17.084(14) Å, space group P  31c and Z =4 [12]. Ideal formula of this mineral is Fe 3þ 2 ðSO 4 Þ 3  9H 2 O, but there is often a replace- ment of Fe 3+ by Al 3+ , so the formula of this mineral should be writ- ten Fe 3þ 2x Al x ðSO 4 Þ 3  9H 2 O [6,12]. There are three outstanding features of the coquimbite structure. The dominant structural http://dx.doi.org/10.1016/j.molstruc.2014.01.086 0022-2860/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +61 731382407; fax: +61 731381804. E-mail address: r.frost@qut.edu.au (R.L. Frost). Journal of Molecular Structure 1063 (2014) 251–258 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc