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There are many analytical methods to characterize different samples. Minerals are composed of many different elements, and some minerals have different classes based on their differing composition – despite being classed as the same mineral.
The classic example is feldspar, which exists in several different types, all with different characteristics, compositions and crystal types. There are many different characterization methods to determine the composition and characteristics of minerals that we’re going to look at here.
The simplest methods to identify a mineral are the geological field techniques that look at a mineral’s shape, luster, hardness, opacity, density, cleavage, fracture, color, and specific gravity—amongst other properties, which are distinguishable by hand. But these methods only show some of the physical characteristics of the mineral, and they can’t be used to give an accurate representation of the chemical composition and internal lattice crystal structure of the mineral.
To obtain this information, more advanced analytical characterization methods are needed, but because there are so many different methods that can be used, we often talk about them as a class of techniques—such as spectroscopy techniques, etc.
Different spectroscopy techniques can be used to probe the characteristics of minerals, with atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES), Raman spectroscopy, and Mössbauer spectroscopy being the most common.
Other lesser-used spectroscopic techniques include photon-induced X-ray emission spectroscopy (PIXE), photon-induced gamma emission spectroscopy (PIGE), infrared (IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy. Colorimetric methods can also be used in conjunction with a spectrophotometer in certain analysis scenarios.
The combination of all these spectroscopy techniques enables the elemental composition to be determined by identifying the elements in the mineral, as well as their relative ratios/concentration within the mineral. They can also be used to determine the different isotopic concentrations of each element in the mineral.
In terms of their characteristics, spectroscopy techniques can be used to identify the valence states within a mineral for analyzing the different mineral phases and crystallization process that the mineral undergoes, and for distinguishing between different mineralogical polymorphs.
Spectrometry is another class of techniques where there are a few options, with the most common being those that utilize the principles of inductively coupled plasma (ICP) and mass spectrometry (MS) methods, such as ICP-MS and laser ablation ICP-MS (LA-ICP-MS). Other spectrometry methods that can be used include secondary ion mass spectrometry (SIMS), thermal ionization mass spectrometry (TIMS), and multi-collector mass spectrometry.
Spectrometry methods are mostly used to determine the elemental composition of minerals and are widely used for minerals containing lighter elements—such as hydrogen, lithium, beryllium, etc. Spectrometry is better for identification purposes than for physical characteristics, but it can be used to date the mineral by analyzing the stable and radiogenic isotopes, to give an accurate age of the mineral.
Wet Chemical Methods
The typical wet chemical used is either colorimetric, volumetric or gravimetric, with the most common methods being titration with ethylenediaminetetraacetic acid (EDTA), redox titration reactions, gravimetric analysis, and colorimetric reactions.
Given the nature of the different methods, they rely on measuring specific weights or concentrations, and this primarily gives insights into the chemical composition and concentration of elements within the mineral, so are best used for mineral identification rather than for probing the properties of a mineral.
Overall, there are different methods to analyze minerals in different ways. There are a lot of methods that will analyze the elemental and isotopic concentrations, which can be used to understand how the mineral will behave. Some look at the relative concentrations and are used in a pure identification capacity.
To understand the physical properties of minerals, geological methods are some of the easiest to perform, as they indicate basic properties. However, to probe the intrinsic properties, the various spectroscopy methods offer the best scope and greatest amount of obtainable information.