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Analyzing minerals is a task that spans many scientific areas, from chemistry and materials science to geology/Earth science. Many minerals possess unique electromagnetic spectra, specific physiochemical characteristics, and will undergo a reaction under specific chemical conditions, and these traits enable minerals to be distinguished from each other, and from other substances in a sample. However, there are many different methods, from common wet chemistry techniques used in a lab to various types of analytical equipment. In this article, we will give a brief overview of the many methods that can be used to analyze minerals.
Wet Chemical Methods
There are a number of wet chemical methods that can be used to identify minerals by the way that they react with specific chemicals. The most common way to do this in a controlled and accurate manner is by titration. There are two common ways to detect the presence of minerals using titration methods, and these are by the minerals forming complexes with ethylenediaminetetraacetic acid (EDTA) and through redox reactions. It should be noted that minerals (or the mineral-containing sample) need to be ground (powdered) and solubilized first—but this is the case for almost all mineral analysis methods. Titrations that use EDTA are widely used for calcium-containing and sodium-containing minerals, but it can be used for many metallic minerals. EDTA is widely used because once all the metal ions from the mineral have complexed with the EDTA, the EDTA reacts with the indicator to signal the end of the reaction. In redox reaction titrations, the system is set up so that one of the half-reactions can be used as an end-point when the metal ions are either in a reduced or oxidized form—as each metal ion (and metal ion complexes) exhibits a distinct color in solution.
Another wet chemical method used is gravimetric analysis. Chemical reagents are added so that the mineral forms a known insoluble complex. This precipitate is then removed, dried, rinsed and weighed to determine the amount of mineral present using the chemical formula of the precipitate. However, this is a technique that can only be used with large amounts of sample and is not suitable for trace analysis measurements.
Spectroscopy and Spectrometry
There are many spectroscopy techniques that can be used to perform an analysis of minerals. Atomic absorption spectroscopy (AAS) is one example, and a solution containing the mineral is heated to high temperatures so that it becomes atomized and subjected to electromagnetic radiation. A controlled flame linked to a detector is then used to capture the wavelengths of lights not absorbed by the mineral, which is used to deduce the wavelengths that were absorbed by the mineral. This then enables the type of material, and its concentration, to be identified.
Methods that rely on inductively coupled plasma (ICP) and mass spectrometry (MS) methods, including ICP-MS and laser ablation ICP-MS (LA-ICP-MS), can be used to determine the elemental composition of a mineral. ICP-MS causes the atoms in the mineral to be atomized and form a plasma, and then the MS component separates the elements by their mass-to-charge ratio which enables each element in the mineral to be identified. The amount of each element is also recorded, and this is used to back out the concentration of the mineral. LA-ICP-MS differs slightly in that it uses a laser to ablate the mineral before it gets formed into a plasma. Other types of MS that can be used include secondary ion mass spectrometry (SIMS), thermal ionization mass spectrometry (TIMS), and multi-collector mass spectrometry.
Atomic emission spectroscopy (AES) is another spectroscopy method that can be used. Rather than measuring the absorption of electromagnetic wavelengths (like in AAS), it’s a method that measures the emission of wavelengths from a mineral sample after its electrons have been excited. The wavelengths emitted are characteristic for each element which enables the elemental composition to be determined, and the intensity of each emitted wavelength backs out the concentration of each element. ICP-AES can also be used and uses the ICP component to turn the mineral sample into a plasma and the AES component again records the wavelength emitted by each element.
Ramen and Mössbauer spectroscopy are two methods which are beneficial when it comes to mineral analyses. Mössbauer spectroscopy offers an efficient way of understanding the valence states within minerals and has become a known method for analyzing the properties of mineral phases and rock crystallization processes. On the other hand, Raman spectroscopy offers a way of identifying minerals and mineral phases and can be used to distinguish between different polymorphs—something which is not possible with other techniques.
Colorimetric methods can also be employed, where a specific reagent is used to react with the mineral and can be quantified by using a spectrophotometer to measure the absorbance of a solution at a designated wavelength.
There are other methods which are not as widely used but are applicable to certain situations. For example, neutron activation analysis (NAA) can be used to determine the concentration of elements within a mineral but needs to be close to a nuclear reactor to perform the analysis. Therefore, it is not applicable to all scenarios. Ion selective electrodes (ISEs) have some use within the food industry for seeing how minerals bind to foodstuff, as the ISEs are only responsive to free ions and not ones that have complexed with other agents. Electron probe micro-analysis (EMPA) is a method that relies on the emission of X-rays after the electrons in the elements of the mineral have been excited, and the specific emitted wavelengths are used to identify the elemental composition of the mineral and the intensity is used to determine the concentration of each element within the mineral.
Additionally, other methods that can be used to analyze minerals include X-ray diffraction (XRD), X-ray fluorescence, photon-induced X-ray emission spectroscopy (PIXE), photon-induced gamma emission spectroscopy (PIGE), infrared (IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, and scanning electron microscopy (SEM), but some are used to a lesser degree than others.
Sources and Further Reading