infrared carbonate rock chemistry characterization
Nasrullah Zaini is a PhD student in the department of Earth Systems Analysis (ESA). His supervisor is prof.dr. F.D. van der Meer from the faculty of Geo-Information Science and Earth Observation (ITC).
This thesis presents an analytical approach for characterizing carbonate rock chemistry, especially for analyzing the chemical compositions of carbonate rocks for cement raw materials. The shortwave infrared (SWIR) spectroscopy and laboratory-based hyperspectral imagery or hyperspectral imaging techniques are combined with geochemical data analysis to estimate mineral chemistries and their compositions in carbonate rock samples.
Carbonate rocks are one of the important natural resources for construction materials and the cement industry. The uses of carbonate rocks or limestones as the main component in the raw mix for making cement clinker depend highly on the rocks’ chemical compositions. The carbonate rocks are formed by a mosaic of minerals mostly containing calcium carbonate (CaCO3) or calcite and calcium magnesium carbonate (CaMg(CO3)2) or dolomite. These sedimentary rocks in nature are also composed of complex geologic mixtures that exist in the form of intimate mixtures, grain size variations, weathered constituents, and alteration products. Those mixtures can create a major obstacle in analyzing mineralogical and chemical compositions of the rocks.
Conventional analytical methods are well established for characterizing mineral chemistry of carbonate rocks, as well as to analyze and control the chemical compositions of the cement raw materials and products. However, the majority of these traditional methods involve a labor-intensive and time consuming process for sample preparation and analysis. Therefore, there is a need for a robust and reproducible approach for characterization and chemical quality control of carbonate rocks that satisfies the industry standard. Spectroscopy provides a non-destructive technique and can be used outdoors for determining mineralogy and chemical information of carbonate rocks based on their spectral feature characteristics.
In the context of this study, the infrared spectroscopy and laboratory-based hyperspectral imaging (imaging spectroscopy) methods were used to analyze mineral chemistries of carbonate rocks that are suitable for cement raw materials. This was done by combining spectroscopic parameters with geochemical characteristics to estimate mineralogical and chemical compositions of carbonate rocks.
The first study presented in this thesis analyzed the effects of grain size and carbonate mineral mixtures on spectral absorption feature characteristics of calcite and dolomite in the shortwave infrared (SWIR) (features at 2.3 and 2.5 μm) and thermal infrared (TIR) (features at 11.5 and 14 μm) wavelength regions. Spectral analysis showed that varying grain sizes and carbonate mineral contents in the synthetic samples influenced spectral reflectance values and absorption feature characteristics. Absorption band positions of pure and mixed calcite and dolomite in the SWIR and TIR regions for both features were displaced slightly as observed in previous studies. The band positions of calcite and dolomite varied relative to grain size only in the TIR region. These positions shifted to longer wavelengths for the feature at 11.5 μm and to shorter wavelengths for the feature at 14 μm from fine to coarse grain size. The wavelength positions of calcite-dolomite mixtures in the SWIR and TIR regions were determined by the quantity of calcite and dolomite in the sample.
Characterization of carbonate rock chemistry using laboratory-based SisuCHEMA hyperspectral imagery was demonstrated. Several spectral recognition approaches, such as wavelength position, spectral angle mapper (SAM) and linear spectral unmixing (LSU) were used to derive the chemical composition and the relative abundance of carbonate minerals from the spectral data of hyperspectral images by applying spectral endmembers of the carbonate synthetic samples established in the first study. Results showed that chemical composition (Ca-Mg ratio) of carbonate minerals at a pixel (e.g., sub-grain) level can be extracted from the image pixel spectra using these spectral analysis methods. For the image shortwave infrared (SWIR) spectra, the wavelength position approach was found to be sensitive to all compositional variations of carbonate mineral mixtures when compared to the SAM and LSU approaches. The correlation between geochemical characteristics and spectroscopic parameters also revealed the presence of these carbonate mixtures with various chemical compositions in the rock samples.
The application of SWIR spectroscopy as a quality control technique for the mineral chemistry analysis of Portland cement-grade limestone was investigated. The spectroscopic properties of SWIR reflectance spectra, such as wavelength position and depth of absorption feature and geochemical characteristics of limestone samples were used to identify and estimate the abundance and composition of carbonate and clay minerals on the rock surfaces by following the approaches and results of the first two studies. The depth of the carbonate (CO3) and Al-OH absorption features are linearly correlated with the contents of CaO and Al2O3 in the samples, respectively. Variations in the wavelength position of CO3 and Al-OH absorption features are related to changes in the chemical compositions of the samples. The results showed that the dark gray and light gray limestone samples are better suited for manufacturing Portland cement clinker than the dolomitic limestone samples. The results indicate that SWIR spectroscopy is an alternative approach for the chemical quality control of cement raw materials.
The research provides an alternative approach for estimating mineral chemistries and compositions of carbonate rocks using SWIR spectroscopy and laboratory-based hyperspectral imaging methods. The findings of this research can be used to complement the conventional analytical approaches for chemical quality control of carbonate rocks in the cement industries.