Laboratory spectroscopic characterization and classification of natural kaolin-group minerals

Introduction: Kaolin-group minerals include kaolinite (Al2[Si2O5](OH)4), dickite (Al2[Si2O5](OH)4), halloysite (Al2[Si2O5](OH)4·2-4H2O), and nacrite (Al2[Si2O5](OH)4), which mainly formed by hydrothermal and weathering processes on Earth, and their formation environment is diverse (e.g. sedimentary, altered crustal rocks) [1]. They have also been found on Mars at Nili Fossae, Mawrth Vallis, Abrabia Terra and Leighton Crater [2]. The presense of smectite and carbonate near the kaolinite is observed on Mars [2]. Importantly, it can be the key tracer for the ancient aqueous alteration and pale environment of Mars. The current researches about kaolin-group minerals on Mars mainly uses visible and near infrared (VNIR) orbital data (e.g. CRISM). In this work, we studied some natural minerals by VNIR, Laser-induced breakdown spectroscopy (LIBS), Laser Raman spectroscopy and X-ray diffraction (XRD), then analyzed the spectral charactrtistics of kaolin-group minerals and rocks. Moreover, all the data sets would be helpful for data interpretation of future Mars explorations. Samples: Four kaolin-group minerals, two kaolinite clay minerals and three mixture samples (Table 1) were prepared in our study and all samples’s partical size is less than 38 μm. We performed XRD measurements to determine the phases. The halloysite is divided into two series, one was dried for 6 hours at 105 °C in an oven before the spectral measurements. Considering the presence of smectite and carbonate near the kaolinite outcrop, and the bedrock is basaltic on Mars, we prepared binary mixtures of kaolinite with smectite, calcite and andesite basalt, respectively, to mimic the Martian surface mineral assemblages. Experiments: The laboratory set-up of LIBS used a pulsed Nd:YAG laser at 1064 nm, 5 Hz pulse, ~91 mJ/pulse energy on target, and an integration time of 1 ms. Introducing the martian gas simulant (CO2) at ~7 mbar before LIBS experiment. LIBS spectra were collected from five different points of the target, and 30 shots per point. All LIBS spectra were averaged to generate five spectra for a single sample (Fig. 1). LIBS spectra were processed by denoising and continuum removal, and the elements were determined according to the ChemCam quick element search tool (C-QuEST) [3] based on data sets from National Institute of Standards and Technology (NIST) and ChemCam team. We used the FieldSpecR 4 Hi-Res VNIR spectrometer (Analytical Spectral Devices., Inc), and VNIR spectra ranging from 350 to 2500 nm (Fig. 2) were obtained for the <38 μm paticle size fraction of all samples. The Raman spectra were obtained by inVia ○R Raman System using the 532 nm green laser. In addition, XRD measurements were performed to the samples for determining the phases of samples.