Raman Spectroscopy of Minerals, Crystals and inorganic Salts

Abstract: in this article we describe the application of Raman spectroscopy (in backscattering configuration) to the study of minerals, crystals and a series of inorganic compounds.

Minerals

Raman spectroscopy, being non-destructive and requiring short measurement times, a low amount of material and no sample preparation, can be used for the analysis and study of minerals and gems. The standard way to perform the analysis with Raman spectroscopy is the comparison of the “spectral fingerprint” of the sample with the spectra of standard minerals. In addition to the recognition of the chemical species, Raman spectroscopy, in the context of solid state physics, is also used to characterize materials, and more specifically to investigate their crystalline structure or crystallinity.

Barite

Barite is a barium mineral belonging to the celestine group; consists of barium sulphate (BaSO₄), has a relative density of 4.5 and a hardness of 3 on the Mohs scale. It is a crystallizing solid in the orthorhombic system, it is generally colorless or white in its pure state, it can also be colored in yellow or blue. The Raman spectrum is shown in figure 1, in which the peak at about 950 cm^-1 due to the sulphate anion is evident.

Fig. 1 – Barite Raman spectrum

Quartz

Quartz (silicon dioxide, SiO₂) is the second most abundant mineral in the earth’s crust (about 12% of its volume) after feldspar. Quartz has a crystalline structure consisting of silicon-oxygen tetrahedra joined together for the 4 vertices to form right or left spirals. In some crystals the left or right crystalline parts coexist to form the twins. Hardness is 7 on the Mohs scale. Habitus (the shape) is a hexagonal prism with the faces of two rhombohedra at the top arranged in such a way as to form a hexagonal bipyramid. Quartz is a material with remarkable chemical stability and is resistant to acids except hydrofluoric acid. It has high hardness, mechanical strength and heat resistance. The quartz has no flaking. Some physical properties of quartz crystals are piezoelectricity and pyroelectricity, which is the ability to electrically polarize the opposite faces of the crystal following mechanical deformation caused by compression or after heating. From the optical point of view, quartz has a high transmissibility in the visible and especially in the ultraviolet.

Our Raman analysis was made on a sample of crystalline quartz and on a sample of amorphous quartz (glass cuvette), the spectra are shown in figure 2. The peak at about 500 cm^-1 is evident, present only in the crystalline sample. Raman spectra of crystalline and amorphous solids of the same chemical composition can be significantly different, mainly due to the presence or absence of spatial order and long-range translational symmetry. Amorphous solids can be considered as a set of units of the same chemical composition, but with variable bond angles and lengths depending on the interactions of the chemical bond with the nearest neighbors. There is no order in their arrangement in space. The result is the enlargement or complete disappearance of the well-defined bands that are instead observed in crystalline solids. These narrow bands correspond to the interaction of electromagnetic radiation with the phonons of the crystal lattice.

Fig. 2 – Amorphous and crystalline quartz Raman spectra

Titanium dioxide

Titanium dioxide, also known as titania, is a chemical compound that comes in the form of a colorless, almost white, crystalline powder; has the chemical formula TiO₂. TiO₂ in nature is present in two main different crystalline forms: rutile and anatase (in the image on the left). Rutile is the most common form: each titanium atom is octahedral surrounded by six oxygen atoms; anatase has a tetragonal structure, more elongated than that of rutile. The two crystalline phases of titanium dioxide are easily distinguished by Raman spectroscopy. In the images of figures 3 and 4 we report the Raman spectra of the two crystalline forms, and in figure 5 the comparison between the two spectra where the difference between the Raman maxima in the two crystalline configurations is clearly seen.

Fig. 3 – Powder titanium dioxide Raman spectrum – rutile crystal structure

Fig. 4 – Anatase crystal Raman spectrum (titanium dioxide)


Fig. 5 – Comparison of anatase and rutile Raman spectra

Calcite

Calcite is a mineral consisting of neutral calcium carbonate (CaCO₃) belonging to the homonymous group. One of the most varied minerals in terms of shape and color, it tends to be rhombohedral in nature, but also scalenohedral, tabular and prismatic. It often has a phenomenon of fluorescence when subjected to ultraviolet rays, with red, yellow, pink and blue colors and can also be thermoluminescent. It is completely soluble in hydrochloric acid, with a typical and lively effervescence. The phenomenon of birefringence (used in the construction of the Nicol prism, one of the first polarizers) is typical of this mineral. The Raman spectrum is shown in figure 6, in which the peak at about 1070 cm^-1 due to the carbonate anion is evident.

Fig. 6 – Calcite Raman spectrum

Fluorite

fluorite, also called fluorine or spatofluor, is a very common mineral composed of calcium fluoride (CaF₂). It is the most important of the fluorinated minerals. The structure can be described as a face-centered cubic lattice of Ca²⁺ ions, with all the tetrahedral cavities occupied by F⁻ ions. Fluorine thus has a tetrahedral 4 coordination, calcium with a cubic 8 coordination. Some types, when exposed to ultraviolet rays, show a conspicuous phenomenon of fluorescence, a phenomenon that takes its name from the mineral. The Raman spectrum is shown in figure 7, the fluorite crystal shows an evident peak at about 330 cm^-1, superimposed on a continuum due to the glass matrix.

Fig. 7 – Fluorite Raman spectrum

Realgar

Realgar is a mineral, arsenic sulfide (As₄S₄). It is composed of 29.9% sulfur and 70.1% arsenic. Partially soluble in acids including nitric acid, better in aqua regia. Samples must be kept away from light because the crystals disintegrate easily when exposed, due to the content of arsenolite, orpiment, pararealgar and other photosensitive arsenic sulphides. The Raman spectrum is shown in figure 8, the realgar crystal shows an evident peak at about 360 cm^-1, and a minor maximum at about 700 cm^-1.

Fig. 8 – Realgar Raman spectrum

Tourmaline

Tourmalines are a group of minerals belonging to the class of silicates, order of cyclosilicates. The mineral, or properly group of minerals, consists of a wide range of isomorphic mixtures with highly variable chemical-physical characteristics: these are fluoriferous borosilicates of sodium, calcium, magnesium, iron and aluminum. The crystal belongs to the trigonal system, it is prismatic, very elongated, vertically striated and sometimes with unequal development at the two ends of the vertical axis. Its color depends on the chemical composition and may not be uniform in the crystal. The Raman spectrum is shown in figure 9, the tourmaline mineral shows peaks at about 3400-3600 cm^-1, due to the stretching of the O-H bonds, and other peaks at lower frequencies.

Fig. 9 – Tourmaline Raman spectrum

Ulexite

Ulexite is a mineral, a hydrated sodium and calcium borate (NaCaB₅O₆(OH)₆·5(H₂O)). The acicular crystals behave like optical fibers, totally transmitting the image through them. A fragment in which two surfaces parallel to each other and perpendicular to the fibrousness are smoothed, shows a particular transparency: resting the lower surface on a writing, the letters appear on the upper one as if they floated on it or were projected on it like a screen. For this reason, ulexite is commonly called television stone in the United States. The Raman spectrum is shown in figure 10, the ulexite mineral shows peaks at about 3400 cm^-1, due to the stretching of the O-H bonds, and other evident peaks at lower frequencies, in particular an intense peak at 1000 cm^-1.

Fig. 10 – Ulexite Raman spectrum

MonoCrystals

A single crystal (or single crystal solid) is a material in which the crystal lattice is continuous and uninterrupted throughout the sample, with no grain boundaries, which can have significant effects on the physical and electrical properties of the material. The intensity of Raman diffusion depends very much on the degree of crystallinity of the sample, Raman spectroscopy is therefore able to investigate the crystalline phase of the sample and to distinguish between crystalline and amorphous samples. As we have already described in the paragraph relating to the analysis of crystalline and amorphous quartz, the Raman spectrum of a crystalline sample is generally characterized by one or more particularly narrow Raman bands, often located in the lower part of the spectrum, less than 1000 cm^-1.

Bismuth Germanate (BGO)

Bismuth Germanium Oxide or Bismuth Germanium is an inorganic chemical compound of bismuth, germanium and oxygen. More commonly the term refers to the compound with the chemical formula Bi₄Ge₃O₁₂ (BGO), with the crystalline structure of cubic evlitin, used as a scintillator.

Fig. 11 – BGO monocrystal Raman spectrum

LYSO

Lutetium-yttrium oxyorthosilicate, also known as LYSO, is an inorganic chemical compound used primarily as a scintillator crystal for gamma radiation detection. Its chemical formula is Lu_2(1-x)Y_2xSiO₅.

Fig. 12 – LYSO monocrystal Raman spectrum

Lithium niobate

Lithium niobate (LiNbO₃) is an not-natural salt consisting of niobium, lithium and oxygen of niobic acid. Its single crystals are an important material for optical waveguides, cell phones, piezoelectric sensors, optical modulators, and various other linear and non-linear optical applications. Lithium niobate is sometimes referred to by the trade name linobate. Lithium niobate is a colorless solid and is insoluble in water. It belongs to the space group R3c (group No. 161) and has a trigonal crystal system, which lacks inversion symmetry and exhibits ferroelectricity, Pockels effect, piezoelectricity, photoelasticity and non-linear optical polarizability. Lithium niobate has negative uniaxial birefringence which is slightly dependent on crystal stoichiometry and temperature.

Fig. 13 – Lithium niobate Raman spectrum

Copper sulphate

Copper sulphate is a ternary salt, it is a chemical compound based on copper, sulfur and oxygen with the formula CuSO₄. This salt exists in different forms depending on the degree of hydration. The anhydrous form, CuSO₄, is pale green or greyish white, while the more common pentahydrate form, CuSO₄·5H₂O, is bright blue. The Raman spectrum is shown in figure 14, in which the peak at about 950 cm^-1 due to the sulphate anion is evident.

Fig. 14 – Copper sulphate Raman spectrum

Inorganic and organic salts in powder or solution

Raman spectroscopy is also useful for the analysis of inorganic and organic salts in the form of powders and in aqueous solution. The Raman bands obtained from the powders are often very evident and rather narrow. Bandwidth is a measure of the degree of crystallinity. When salts are dissolved in water, Raman peaks usually decrease in intensity (until they disappear in the baseline for low concentrations) and increase their width.

Indium nitrate

Indium nitrate in powder and aqueous solution, chemical formula In(NO₃)₃ · H₂O. The peak at about 1000 cm^-1 is evident in both the powder and aqueous solution samples.

Fig. 15 – Indium nitrate Raman spectrum, powder and water solution

Sodium bicarbonate

Sodium hydrogen carbonate is a sodium salt of carbonic acid, of the formula NaHCO₃. It is commonly known as sodium bicarbonate, a disused name by the IUPAC, or even just ‘bicarbonate’. In nature, as well as being frequently dissolved in surface and underground waters, it is rarely present as a mineral, generally in the form of efflorescences, encrustations and concretions in evaporitic deposits.

Fig. 16 – Sodium bicarbonate Raman spectrum

Sodium thiosulfate

Sodium thiosulfate pentahydrate is the sodium salt of thiosulfuric acid (Na₂S₂O₃ · 5H₂O). At room temperature it appears as a colorless odorless solid. After it has melted and allowed to cool, it remains liquid unless a crystallization seed is immersed in it. In aqueous solution the Raman peaks are very small compared to the baseline level.

Fig. 17 – Sodium thiosulfate, powder and water solution

Citric acid

Citric acid is a solid, colorless substance, a tricarboxylic acid, with the brute formula C₆H₈O₇, the structural formula is represented in the image on the side. It is soluble in water over a wide pH range. Although citric acid is one of the most common acids in plant organisms and a metabolic product of aerobic organisms, when it is solid or in concentrated solution it must be handled with caution. It is found in traces in fruit, especially of the genus Citrus: lemon juice can contain up to 3-4% and orange 1%. It is also present in woods, mushrooms, tobacco, wine and even milk. The Raman spectrum shown in figure 18 shows many bands among which we can identify those due to the stretching of the O-H bond.

Fig. 18 – Powdered citric acid Raman spectrum

Urea

Urea is a chemical compound with the formula CO(NH₂)₂ and molar mass 60.06 g/mol; under normal conditions it appears as a colorless crystalline solid; it is the diamide of carbonic acid. In all tetrapods, with the exception of birds and some reptiles, it is the substance by which the nitrogenous products of metabolism are eliminated from the body. The Raman spectrum shown in figure 19 shows the characteristic peak at about 1000 cm^-1 due to the stretching of the N-C-N bond.

Fig. 19 – Powdered urea Raman spectrum

Conclusions

Our DIY Raman spectrometer, in backscattering configuration, has allowed the successful application of the Raman spectroscopy technique to the study of minerals, crystals and inorganic salts.

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