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Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials J Theo Kloprogge, The University of Queensland, Brisbane, QLD, Australia ã 2017 Elsevier Ltd. All rights reserved.
Introduction Infrared (IR) and Raman spectroscopy have become mainstream techniques not only to identify minerals and inorganic minerals but also to study reactions, such as phase changes, as a function of changes in temperature and pressure. The main advantage of these two techniques especially IR and Raman microscopy is that they are non-destructive, in comparison to other commonly used techniques such as X-ray diffraction or optical microscopy. The identification of minerals generally starts by visual examination. Based on properties such as color, crystal habitus, hardness, luster, cleavage, etc. a first identification of many rock-forming minerals is possible. It becomes more difficult for the more rare minerals that are mostly present in only minor amounts and often as small crystals. In many of these cases visual examination even by an experienced mineralogist does not give a definite answer, and other analytical techniques are needed in order to arrive at a positive identification. The classical petrologist or mineralogist will start with the preparation of thin sections of approximately 30 mm thickness of the rocks sampled in the field for examination under the optical microscope. Optical properties such as color, pleochroism, refractive index, birefringes, etc. allow minerals to be identified. However, this method, though cheap, is rather timeconsuming and depends strongly on the experience of the geologist. Alternatively, minerals can be identified by their crystal structure by X-ray diffraction and chemical composition by methods such as electron microprobe analysis. Raman and IR spectroscopy can unambiguously identify minerals and inorganic materials. Raman spectroscopy is the only technique that can even identify the purity and physical form of elements, such as carbon, germanium, sulfur, and silicon. Despite the fact that both techniques in their simplest form have been around for many decades, it is only in more recent years that applications in areas such as high temperature/high pressure studies of phase changes, archeology, art, and gemology, etc. have been developed. This chapter will attempt to highlight some of these recent developments in IR and Raman spectroscopy.
Basic Identification of Minerals Minerals can be described as naturally occurring, inorganic crystalline materials. They can be classified based on their chemical characteristics. In general the following eight classes can be identified: (1) elements, (2) sulfides and related compounds, (3) halides, (4) oxides and hydroxides, (5) carbonates, nitrates, borates, (6) sulfates, tungstates, chromates, and molybdates, (7) phosphates, arsenates, and vanadates, and (8) silicates.
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
An essential part of this grouping is based on the occurrence of various characteristic anionic groups. Each of these groups is reflected as specific bands in either or both the IR and Raman spectra and function as fingerprints for specific minerals. The characteristic wavenumber regions of the various characteristic groups are listed in Table 1. It has to be kept in mind that division of the regions for the different anionic groups is only very general, that each mineral has its own characteristic pattern, and that in specific cases the situation can be much more complex. For example, distortion of a symmetric anionic group in the crystal structure can result in a splitting of the Raman bands. A good example of these is some of the carbonate minerals. The unperturbed carbonate ion is a planar triangle with point symmetry D3h. Group theory analysis of the carbonate molecule predicts four normal modes (Table 2) from which two are IR and Raman active while one is IR active only and one is Raman active only. Fig. 1A shows the symmetric stretch u1 for a series of common carbonates that belong to the calcite group (calcite, magnesite, rhodochrosite, siderite, and smithsonite) plus aragonite and dolomite. Calcite is trigonal with space group R3c, D63d and with two CaCO3 units in the primitive rhombohedral cell. The carbonate group is strongly covalent and is expected to show vibrational modes close to those of the free ion. However, there are two carbonate groups in the primitive cell and thus two similar sets of internal vibrations. These two sets are constrained by the factor group symmetry to vibrate either in phase or in anti-phase to each other with respect to the center of symmetry. Thus the internal modes form a Raman-active set coupled in phase and an IR-active set coupled anti-phase. The 1407 cm1 band associated with the u3 is the strongest feature in the IR spectrum (Fig. 1E) but is very weak in the Raman spectrum (Fig. 1B). The symmetric stretching mode u1 is the strongest band in the Raman spectrum and is normally extremely sharp. Other clearly observable bands are the u4 at 713 cm1 (Fig. 1C) and the lattice modes u13 at 287 cm1 and u14 at 151 cm1 (Fig. 1D). When the Ca2þ cation is replaced by other intermediate-size divalent cations, for example, Mg, Fe, Zn, Co, Mn, etc., the calcite structure remains intact. The effect of the different metals on the IR and Raman spectra is limited and only minor shifts are observed. In contrast, the polymorph of calcite, the mineral aragonite is orthorhombic with space group Pmcn, D16 2h , with four ACO3 formula units per unit cell. The perturbations of the low symmetry aragonite structure produce three effects on the molecular modes of the CO2 3 ion. First of all there is a relaxation of the selection rules that permits u1 to become active in the IR and u2 in the Raman (Fig. 1C). Secondly, the degeneracy is lifted and u2 and u4 each split into two components (site group splitting) (Fig. 1E). Finally, because of the interactions between the four carbonate molecules in the unit cell, each internal mode appears four times.
http://dx.doi.org/10.1016/B978-0-12-409547-2.12154-7
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Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Characteristic wavenumber regions for various anionic groups
Table 1
Anionic group Sulfides Hydroxides Oxides Carbonates Nitrates
4000–3000 cm1
1600–1200 cm1
1200–800 cm1
1
800–600 cm1
<400 cm1
<500 cm1
1
4000–3000 cm
600–400 cm1
1200–600 cm 1
1
1550–1300 cm
890–800 cm 1050 cm1 850–800 cm1 900–600 cm1 1250–900 cm1 950–750 cm1 950–800 cm1 950–750 cm1 1150–900 cm1 900–770 cm1 900–700 cm1 1200–800 cm1
1350–1250 cm1
Borates Sulfates Tungstates Chromates Molybdates Phosphates Arsenates Vanadates Silicates
<1200 cm1 700–670 cm1 770–715 cm1 680–570 cm1
450–250 cm1 500–350 cm1 450–250 cm1 600–400 cm1 400–350 cm1 400–300 cm1
Normal modes of the CO2 3 ion
Table 2 Mode
Symmetric stretch Out-of-plane bend Asymmetric stretch In-plane bend
u1 u2 u3 u4
Symmetry
Selection rules
Wavenumber (cm1)
A10 00 A2 0 E E0
Raman IR IR þ Raman IR þ Raman
1063 879 1415 680
(A)
(B)
n1
n3 (g)
(g)
(f)
Raman intensity (AU)
Raman intensity (AU)
(f)
(e)
(d)
(e)
(d)
(c) (c)
(b)
(b)
(a) (a) 1060
1070
1080 1090 1100 1110 Wavenumber (cm−1)
1120
1130
1340
1360
1380
1400 1420 1440 Wavenumber (cm−1)
1460
1480
Fig. 1 Raman spectra of various carbonates (a ¼ calcite, b¼ aragonite, c ¼ dolomite, d¼ magnesite, e ¼ rhodochrosite, f ¼ siderite, g¼ smithsonite) in (A) 1000–1200 cm1 region, (B) 1300–1500 cm1 region,
(D)
(C)
n4 (g) (g) (f)
(f)
Raman intensity (AU)
Raman intensity (AU)
(e)
(e)
(d)
(d) (c)
(c) (b) (b) (a)
(a)
650
700
750
800
850
900
130
180
Wavenumber (cm−1)
230 280 Wavenumber (cm−1)
(E)
n3
n2 n4 (g)
IR intensity (AU)
(f)
(e)
(d)
(c)
n1
(b)
(a) 600
800
1000
1200
1400
1600
Wavenumber (cm−1) Fig. 1 (Continued) (C) 600–900 cm1 region, (D) 100–400 cm1 region, and (E) infrared spectra in the 600–1700 cm1 region.
330
270
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
IR and Raman Spectroscopy of Mineral Pigments in Art IR microscopy and Raman microscopy, spectroscopy using fiber-optic probes, and more recently also mobile and handheld Raman spectrometers have become of interest to study objects of arts such as medieval texts or paintings. Several of the pigments used are based on naturally occurring or synthetic minerals (Table 3). A great advantage of these vibrational spectroscopic methods here is that it can deal with not only the organic components but also the inorganic components. However, Raman spectroscopy is often favored based on the smaller scattering intensity of water and hydroxyl groups in the Raman effect, especially in ancient, hydrated biomaterials such as linens and cottons. For paintings and manuscripts it has the advantage of accessibility of the low wavenumber region (<500 cm1), which cannot be observed with standard midIR spectroscopy, but which gives important information with regards to identification of various pigments. For art historians it is important to learn about the materials and their provenance in order to get an understanding of ancient techniques, to attribute a certain manuscript or painting to a certain artist or workshop, to possibly date the material and to distinguish fakes from real artwork. The common mineral pigments can, similarly to Table 1, be divided into different groups based on their chemical composition. Sulfides such as greenockite, realgar, cinnabar, and orpiment (Fig. 2) show all their characteristic bands below 500 cm1. The oxides form a good example where potential fake art can be differentiated from much older art. For synthetic pigments such as the TiO2 polymorphs, anatase and rutile, the dates of first production are well documented. Rutile has been used since 1923 and anatase since 1947, which means that the observation of one of those pigments in a disputed Renaissance painting clearly indicates a forgery. Fig. 3A shows the Raman spectra of anatase and rutile in the region below 900 cm1, clearly showing two distinct bands at 392 and 516 cm1 for anatase and at 442 and 605 cm1 for rutile. In the case of colored pigments one has to be careful in the choice of the laser wavelength because of absorption of the scattered light by the sample. In general the best choice is to use a laser excitation line with a wavelength that is outside that of the contour of the electronic absorption bands of the pigment. An unwanted side effect of using laser light is the change in pigment structures due to localized heating. A good example of this can be found in the group of lead oxides. Minium (red lead, Pb3O4) can decompose to massicot (PbO) when using a 514.5-nm laser but the minium will remain intact when using a 632.8-nm laser (Fig. 3B). As shown earlier in this paper the carbonates can easily be recognized by the vibrational modes of the CO2 3 group in the crystal structure. Besides the already discussed calcite, the major carbonate pigments include green malachite, blue azurite, and white hydrocerussite (lead white) (Fig. 3C). Hydrocerussite shows a sharp and strong carbonate u1 at 1042 cm1, azurite shows the same band at 1094 cm1, while malachite exhibits two bands with maxima at 1099 and 1052 cm1. The second band at 1052 cm1 is associated with the OH-bending mode. The u2 in azurite can be observed as a maximum at 763 cm1, the same band is much weaker in malachite at
750 cm1. For the carbonate group, IR bands are observed at 1095 cm1 (u1), 834 and 816 cm1 (u2), 1430 and 1419 cm1 (u3), and 764 and 739 cm1 (u4). For azurite the bands at 3453 and 3427 cm1 have been assigned as the O–H stretching mode with the O–H bending modes found at 1035 and 952 cm1. Malachite displays two hydroxyl stretching bands at 3474 and 3404 cm1 while O–H out-of-plane bending modes are found at 1045 and 875 cm1. IR bands for the Cu–O stretching modes are observed at 495 and 400 cm1, while Cu–O bending modes occur at 455 and 345 cm1. Bands at 305 and 240 cm1 are assigned to the O–Cu–OH-bending modes. The Raman out-of-plane bending modes are found at 194 and 170 cm1. The sulfates such as barite, gypsum, and posjnakite are characterized by four different sulfate vibrational modes, which can be split depending on the site symmetry of the sulfate group in the crystal structure. The free anion shows u1 at 983 cm1, u2 at 450 cm1, u3 at 1105 cm1, and u4 at 611 cm1. All four vibrational modes of the sulfate anions (u1, u2, u3, and u4) are Raman active, the symmetric stretching modes u1 giving rise to the strongest bands in the spectra (Fig. 3D). On the other hand, the u1 mode gives rise to a weak IR band at 1000 cm1. In the case of gypsum spectra it is difficult to assign unequivocally the band(s) resulting from the u2 sulfate mode. In the region where it is to be expected no band characteristic for this mode is observed. Actually, a broad band is found at 458 cm1 in the IR spectra of the gypsum samples but it is assigned to water libration modes (Fig. 4A). The bands originating from the water molecules are found in the IR spectra with the OH-stretching modes at 3552, 3492, and 3404 cm1 (Fig. 4B) and the corresponding bending mode has two bands at 1685 and 1620 cm1 (unexpected because there is only one type of water molecules), which can easily be explained as a consequence of correlation-field effects (vibrational interaction of identical oscillators) (Fig. 4B). For barite, as in gypsum, as a result of the lower local symmetry of the SO2 anions in the solid state, the IR-inactive (for Td 4 symmetry) u1 and u2 modes become active with a very weak IR band at 982 cm1, that is, in the region where the u1 mode is expected. The u2 appears as two bands at 470 and 439 cm1. The u3 and u4 modes are strong in the IR with bands at 1205, 1136, 1100, and 1042 cm1 and at 641, 633, and 614 cm1, respectively. Due to the broadness of the bands it is not always possible to observe all bands and commonly only three maxima for the u3 and two for u4 are observed. Pseudomalachite is a phosphate mineral. The free PO3 4 group shows the u1 around 970 cm1, the u2 around 360 cm1, the u3 around 1080 cm1, and finally the u4 around 510 cm1. In pseudomalachite these bands are observed in the IR at 953 cm1, 450 and 422 cm1, 1096 and 1025 cm1, and 615, 555, 530, and 482 cm1, respectively. In addition the pseudomalachite structure also contains hydroxyl groups, which are evident by the OH stretching modes around 3582, 3435, and 3390 cm1 and the corresponding bending modes at 1390 and 1325 cm1. Crocoite is a mineral that contains the chromate anion as its characteristic group. Free CrO2 4 shows, similar to sulfate, the u1 around 830 cm1, u2 around 330 cm1, u3 around 765 cm1, and u4 around 330 cm1, but others have reported different values such as 847, 348, 884, and 368 cm1,
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Table 3
271
Mineral (natural and synthetic) pigments used in art
Name
Chemical formula
Color
Azurite Lazurite Posnjakite Cuprorivaite Magnetite Goethite (ochre) Hematite (sienna, burnt ¼ mix þ clay) Greenockite Atacamite Celadonite/glauconite mix (green earth) Malachite Pseudomalachite Eskolaite Litharge Realgar Minium Cinnabar (vermillion) Cadmoselite (cadmium red) Anatase Baryte Calcite (chalk, whiting) Gypsum Kaolin (mainly kaolinite) Rutile Hydrocerussite (lead white) Sphalerite þ baryte (lithophone Greenockite (cadmium yellow) Crocoite Massicot Goethite þ clay þ silica (ochre) Orpiment
Cu3(CO3)2(OH)2 Na6Ca2(Al6Si6O24)(SO4,S,S2,S3,Cl,OH)2 Cu4(SO4)(OH)6H2O CaCu[Si4O10] Fe2þFe3þ 2 O4 FeOOH Fe2O3 þ clay CdS Cu2(OH)3Cl K(Mg,Fe2þ)Fe3þ(Si4O10)(OH)2 þ (K,Na)(Mg,Fe2þ,Fe3þ)(Fe3þ,Al)(Si,Al)4O10(OH)2 Cu2(CO3)(OH)2 Cu5(PO4)2(OH)4 Cr2O3 PbO As4S4 Pb3O4 HgS CdSe TiO2 BaSO4 CaCO3 CaSO42H2O Al2(Si2O5)(OH)4 TiO2 Pb3(CO3)2(OH)2 ZnS þ BaSO4 CdS PbCrO4 PbO FeOOH þ clay þ SiO2 As2S3
Blue Blue Blue Blue Black Brown/orange Brown/orange Brown/orange Green Green Green Green Green Red Red Red Red Red White White White White White White White White Yellow Yellow Yellow Yellow Yellow
Raman intensity (AU)
Realgar
Orpiment
Cinnabar
Greenockite
100
200
300
400
500
600
Wavenumber (cm−1) Fig. 2 Raman spectra of selected sulfides: greenockite, cinnabar, orpiment, and realgar in the region 100–700 cm1.
700
272
(B)
(A)
Raman intensity (AU)
Raman intensity (AU)
Rutile
Massicot
Anatase 100
200
300
400
500
600
Wavenumber (cm−1)
700
800
900
Litharge 125
225
325
425
525
625
725
825
Wavenumber (cm−1)
(D)
(C)
Crocoite Malachite
Azurite
Raman intensity (AU)
Raman intensity (AU)
Pseudomalachite
Gypsum
Barite Hydrocerussite Posnjakite 120
320
520
720
Wavenumber (cm−1)
920
1120
100
300
500
700
900
1100
Wavenumber (cm−1)
Fig. 3 Raman spectra of selected pigments: (A) anatase and rutile in the 100–900 cm1 region, (B) litharge, massicot, and minium in the 125–900 cm1 region, (C) hydrocerussite, azurite, and malachite in the 120–1200 cm1 region, and (D) posnjakite, barite, gypsum, pseudomalachite, and crocoite in the 100–1200 cm1 region.
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Minium
(B)
Absorbance (AU)
Barite
Gypsum 400
Fig. 4
600
800
1000
1200
Wavenumber (cm−1)
1400
1600
1800
3000
3100
3200
3300
3400
3500
3600
Wavenumber (cm−1)
Infrared spectra of (A) the sulfates gypsum and barite in the region 400–1800 cm1 and (B) gypsum in the hydroxyl stretching region 3000–4000 cm1.
3700
3800
3900
4000
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Absorbance (AU)
(A)
273
274
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
respectively. For crocoite all the allowed vibrations are Raman active, but only the T2 symmetry species are IR active. All degeneracies are removed in crocoite and the resulting nine vibrational modes are all Raman and IR active. In the IR spectrum stretching bands have been found at 885, 855, and 825 cm1 and a weak broad band, presumably due to the bending modes, at 388 cm1. The Raman spectrum of crocoite shows three bands in the CrO stretching region at 856, 841, and 825 cm1. The band at 841 cm1 is the most intense and is assigned to the u1 symmetric stretching vibration, while the other two bands occur as weak shoulders on either side. The Raman spectrum of crocoite shows a series of bands at 402, 379, 360, and 330 cm1. One probable assignment is that the first three bands at 402, 379, and 360 cm1 are attributable to the u4 modes and that the band at 330 cm1 to the u2 mode.
IR and Raman Spectroscopy in Gemology IR and Raman spectroscopy have become routine spectroscopic techniques in gemological laboratories in the last 25 þ years. Since vibrational spectroscopy, in particular IR and Raman microscopy, fiber-optic, handheld Raman, and attenuated total reflection IR, is a non-destructive technique it is ideal to identify not only gem minerals, in particular rare gem minerals, exotic stimulants, or varieties of unusual appearance, but also their inclusions. One of the major advantages of these different methods is that the gemstone can be studied even when it is cut and set in a jewel. These inclusions may provide information with regards to modifications such as heat treatment, impregnation with organic material to hide imperfections such as cracks. Lately not only the positions of the Raman bands for identification but also the properties of the lines themselves, in particular line width has become of interest as this provides information related to properties such as grain size, metamictization (eg, in zircon), etc. Not all materials respond well to the Raman laser, in other words are poor Raman scatterers, such as black oxides, opal, or cubic zirconia. Also the effects of fluorescence or the sharp lines due to the 546 nm mercury emission from nearby fluorescent lighting can be misinterpreted. It is quite easy to determine by classic gemological techniques whether a gemstone is a garnet, a tourmaline, etc., but it is much more difficult to determine its precise composition within their solid solutions. However, this can be achieved by vibrational spectroscopy. Fig. 5A–C provides the Raman and IR spectra of some of the most common gem minerals. In the low wavenumber region, also known as the fingerprint region, it is clear that each mineral has its own characteristic set of bands and the gemstones can easily be distinguished from each other using either Raman or IR spectroscopy, though the differences are more pronounced in the Raman spectra. In addition some of the minerals contain hydroxyl groups and/or water, such as topaz, turquoise, and zoisite (gem varieties: pink thullite and heat treated deep blue tanzanite), which can be observed in the IR in the hydroxyl stretching region between 2500 and 4000 cm1 (Fig. 5C). Topaz shows a sharp band around 3649 cm1, the strength of which is affected by the amount of F for OH substitution in the crystal structure. Turquoise contains both hydroxyl groups and crystal water
with bands at 3508, 3463, 3446, 3295, and 3090 cm1. The first three relatively sharp bands are associated with hydroxyl groups, while the remaining very broad bands are associated with the OH-stretching modes of water. Synthetic turquoise can be distinguished from natural turquoise because of their smaller grain size, which is reflected in the Raman spectra as much broader bands. In particular the phosphate bands are well resolved in natural turquoise, whereas in synthetic turquoise these bands tend to be broad, less intense, and less resolved. The garnets are used as gemstones in a wide range of colors and often a name like almandine, andradite, grossular, pyrope, etc. is given based on the color only. This often does not conform to the chemical composition that mineralogists have given to each of these garnet minerals. In addition, except for synthetic garnets, all garnets are solid solutions. Vibrational spectroscopy can help in distinguishing the different solid solutions as the spectra change as a function of the garnet composition (Fig. 5D and E, Table 4). The systematic shift in band positions gives a qualitative or even a quantitative measure of their composition. For the garnets a first distinction is made on the elements in the positions X and Y in the general formula X3Y2(SiO4)3. The first group all have Al in the Y position and is commonly known as the pyralspite group (pyrope with X ¼ Mg, almandine with X ¼ Fe, and spessartine with X ¼ Mn), while the second group all have Ca in the X position and is known as the ugrandite group (uvaroite with Y ¼ Cr, grossular with Y ¼ Al, and andradite with Y ¼ Fe). A similar effect of the changes in chemical compositions of the tourmaline group minerals such as dravite [Na(Mg3) Al6(Si6O18)(BO3)3(OH)3(OH)], elbaite [Na(Li1.5Al1.5)Al6(Si6 O18)(BO3)3(OH)3(OH)], liddicoatite [Ca(Li2Al)Al6(Si6O18) (BO3)3(OH)3(OH)], and schorl [Na(Fe2þ 3 )Al6(Si6O18)(BO3)3 (OH)3(OH)] is shown in Fig. 5F and G. Though at first sight not as clear as for the garnets, here also changes in the spectra can be observed with the changes of the different substitutions in the tourmaline structure. Diamond shows a single sharp band in the Raman spectrum at 1332 cm1 in comparison to the synthetic cubic zirconia (often sold as a cheaper replacement for diamond) that shows a distinctive broad band around 620 cm1 plus several other bands and can easily be identified by Raman spectroscopy (Fig. 6A). Zircon often contains trace amounts of radioactive elements in its crystal structure that over time can result in radiation damage and the crystalline structure changes to an amorphous state while the outer crystal shape is retained. This is in geology known as metamictization. The degree of metamictization can easily be seen in the Raman spectrum. Where a well-crystalline zircon shows strong and sharp bands, a metamict zircon only shows very broad and much weaker bands (Fig. 6B).
Heating/Cooling Stage Raman Spectroscopy These days heating/cooling stages for IR and Raman microscopes can be easily obtained commercially. These stages allow for heating/cooling through circulation of heated or cooled liquid or gas around a sample cell. Because of the increased optical path and the spherical aberration of the cell window,
(A)
(B) Zoisite Zoisite
Turquoise Turquoise
Forsterite
Chrysoberyl
IR intensity (AU)
Raman intensity (AU)
Jadeite
Jadeite Forsterite Chrysoberyl Beryl
Beryl
Topaz
Topaz
150
350
550
750
950
1150
1350
400
500
600
700
800
900
1000
1100
1200
1300
1400
Wavenumber (cm−1)
Wavenumber (cm−1)
(D)
(C)
Uvarovite
Turquoise
Spessartine Raman intensity (AU)
IR intensity (AU)
Zoisite
Pyrope
Grossular
Andradite
Topaz Almandine 2500
2700
2900
3100
3300
Wavenumber (cm−1)
3500
3700
3900
150
250
350
450
550
650
750
850
950
1050
Wavenumber (cm−1)
275
Fig. 5 Infrared and Raman spectra of selected gemstones: (A) Raman spectra of corundum, topaz, beryl, chrysoberyl, forsterite, jadeite, turquoise, and zoisite in the 150–1400 cm1, (B) infrared spectra of the same minerals in the region 400–1400 cm1, (C) infrared spectra of topaz, turquoise, and zoisite in the hydroxyl stretching region 2500–4000 cm1, (D) Raman spectra of various garnets in the region 150–1100 cm1, (Continued)
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Corundum
Corundum
276
(F)
(E)
Pyrope
Grossular
Raman intensity (AU)
IR intensity (AU)
Spessartine
Liddicoatite
Elbaite
Andradite
Dravite
Almandine 400
500
600
700
800
900
1000
1100
1200
Wavenumber (cm−1)
150
250
350
450
550
650
750
850
950
1050
1150
Wavenumber (cm−1)
(G)
IR intensity (AU)
Schorl
Elbaite
Dravite
400
600
800
1000
1200
1400
Wavenumber (cm−1)
Fig. 5 (Continued) (E) infrared spectra of the same garnets in the region 400–1200 cm1, (F) Raman spectra of selected tourmalines in the region 150–1200 cm1, (G) infrared spectra of the same tourmalines in the region 400–1500 cm1.
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Schorl
Uvarovite
Table 4
Differentiation of garnets by their IR and Raman spectra in the region below 1000 cm1
IR 980–940 cm1 Raman 900–925 cm1 Pyralspite group
Position of IR band in 980–890 cm1 region Raman band in 870–925 cm1 region
No IR band near 640–620 cm1 Single IR band near 340 cm1 Raman band near 358 cm1 D(897–877 cm1 bands) ¼ 26–27 cm1 IR band near 630 cm1 IR bands in the region of 350–310 cm1 Raman bands in the region 325–365 cm1 D(885–865 cm1 bands) ¼ 19–21 cm1
Pyrope
Almandine or spessartine
No IR band in the region 360–300 cm1 Raman bands near 370–375 cm1 Grossular or uvarovite
IR 920–890 cm1 Raman 870–875 cm1 Ugrandite group Two IR bands in the region of 360–300 cm1 Two Raman bands in the region of 345–370 cm1
(A)
Andradite
Two Raman bands near 862 and 925 cm1
IR bands near 962, 897, and 877 cm1 Raman bands near 856 and 914 cm1 ¼ Almandine IR bands near 950, 885, and 865 cm1 Raman bands near 840 and 902 cm1 ¼ Spessartine IR bands near 915, 960, and 840 cm1 Raman bands near 820, 844 and 878 cm1 ¼ Grossular IR bands near 900, 840, and 830 cm1 Raman bands near 820, 873, and 887 cm1 ¼ Uvarovite Raman bands near 811, 836, and 871 cm1
(B)
Metamict zircon
Raman intensity (AU)
Raman intensity (AU)
Cubic zirconia
Zircon
Diamond
150
350
550 750 950 Wavenumber (cm−1)
1150
1350
150
350
550 750 Wavenumber (cm−1)
950
1150
Fig. 6 Raman spectra of (A) diamond and synthetic cubic zirconia in the region 150–1500 cm1, and (B) zircon and metamict zircon in the region 150–1200 cm1.
278
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Inner hydroxyl
Raman intensity (AU)
Inner-surface hydroxyls
3580
3600
3620
3640
3660
3680
3700
3720
3740
Wavenumber (cm−1)
Fig. 7 Raman spectra at room temperature (bottom) and after cooling to liquid nitrogen (top) temperature of kaolinite in the hydroxyl stretching region 3580–3740 cm1.
spectra are noisier with decreased intensity and require longer accumulation times. Cooling is often used to obtain better quality spectra as bands tend to sharpen and shift. In the case of overlapping bands this can result in the possibility of observing each band individually. A good example is the clay mineral kaolinite, which shows five overlapping hydroxyl stretching bands in the IR and Raman spectra. Compared to the room temperature spectrum the inner hydroxyl band at 3621 cm1 shifted slightly to 3618 cm1 upon cooling. The two inner-surface hydroxyl bands at 3670 and 3685 cm1 shifted significantly to higher wavenumbers at 3979 and 3694 cm1 with a broad shoulder at 3707 cm1. The remaining inner-surface hydroxyl band at 3652 cm1 splits into two components at 3660 cm1 and a shoulder at 3663 cm1 (Fig. 7). Calcium oxalate occurs either as the dihydrate or the monohydrate (two rare minerals, weddellite and whewelite). Weddellite can be found as authogenic crystals in sea floor mud. It also has been reported in peat bearing sediments and in calcite bearing lacustrine sediments. It occurs with whewellite, urea, phosphammite, and aphthitalite. Together, whewellite and weddellite are the most common renal calculi (kidney stones). Whewellite shows a characteristic series of phase changes upon heating and is often used for teaching purposes as an example or as a standard in thermal analysis. The decomposition of whewellite occurs in three distinct steps (Fig. 8): 1. dehydration around 115 C CaC2 O4 2H2 O ! CaC2 O4 þ 2H2 O ðgasÞ 2. decomposition to calcium carbonate around 425 C
CaC2 O4 ! CaCO3 þ CO ðgasÞ 3. decomposition to calcium oxide around 600 C CaCO3 ! CaO þ CO2 ðgasÞ The Raman spectra clearly show changes between 150 C and 200 C and again between 450 C and 500 C. The CO stretching vibration for oxalate in solution is observed at 1496 cm1 and occurs for calcium oxalate around 1475 cm1 at room temperature. No bands are observed in the IR spectrum at this temperature, which means that the structure of calcium oxalate is centrosymmetric. Mild heating to 50 C causes two bands to appear in the Raman spectrum. These bands are similar to those observed for whewellite, the partially dehydrated form of weddellite. The bands shift to 1488 and 1462 cm1 in the 150 C spectrum (Fig. 9), that is, there is a slight red shift on heating. At temperatures above 150 C the bands are observed at 1478 and 1466 cm1. These bands correspond to the symmetric stretching modes of anhydrous calcium oxalate. At temperatures above 450 C a single band is now observed at 1480 cm1 corresponding to the CO symmetric stretching mode of calcium carbonate. In addition a small band at 1628 cm1 is observed in the room temperature spectrum, which can be attributed to the antisymmetric stretching vibration. This band should not be observed in the Raman spectrum, but distortion of the structure probably allowed the observation of this Raman forbidden band. Aqueous oxalate shows the IR-active antisymmetric stretching mode at 1600 cm1. This band is observed at 1650 cm1 at 50 C and shifts to 1647 cm1 with further heating. Around 450 C this band disappears and a new very broad band becomes visible
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
279
100 CaC2O4.2H2O -> CaC2O4 + 2 H2O(gas)
90
Weight (%)
80 70
CaC2O4 −> CaCO3 + CO(gas)
60 50
CaCO3 −> CaO + CO2(gas)
40 30 30
90
150
210
270
330
390
450
510
570
630
690
750
Temperature (°C)
Fig. 8 Thermogravimetric analysis (TGA) decomposition profile of whewellite in the temperature range 30–800 C showing the three steps of (1) dehydration, (2) decomposition to calcium carbonate, and (3) decomposition of the calcium carbonate to calcium oxide.
around 1618 cm1. The final step of the decomposition of calcium carbonate to calcium oxide is at a temperature above the upper limit of the Raman thermal stage and can therefore not be observed.
IR Emission Spectroscopy
Fig. 9 Heating stage Raman spectra of whewellite in the range 1300–1700 cm1 from 50 C to 550 C.
IR emission spectroscopy forms a new and valuable spectroscopic method that can be applied in situ during heat treatment of a sample. The technique of measuring discrete vibrational frequencies emitted by thermally excited molecules has not been widely used for the study of inorganic materials and minerals. The major advantages of infrared emission spectra (IES) are sample analysis in situ at increasing temperature and no difficult sample preparation other than grinding it to submicron particle size. The normal single-beam reference spectrum obtained during the recording of an IR spectrum is actually the emission spectrum of the instrument source modified by the instrument response function. Transmission spectra are obtained by ratioing the signals in the absence and presence of the sample. In the same way emission spectra can be obtained by ratioing the emission signal of a sample to that of a reference, normally a blackbody source that emits radiation according to Planck’s formula (ie, a continuum). In the normal course of events three sets of spectra are obtained. Firstly the black body radiation (actually gray body radiation from graphite) over the temperature ranges of interest. Secondly the sample holder radiation is obtained for the same temperatures and finally the spectra of a thin layer of sample on the holder are obtained. The emittance spectra can then be calculated for each temperature by subtracting the single-beam spectra of the holder from that of the holderþ sample and the result ratioed to the single-beam spectra of the gray body. Earlier in the paragraph on heating stage Raman spectroscopy the example of whewellite, CaC2O42H2O, was shown. Fig. 10 shows the same experiment using infrared emission spectroscopy. The spectra at low temperatures do not distinctly show
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Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
75°C 100°C 125°C 150°C 200°C 250°C 300°C 350°C Emissivity
375°C 400°C 425°C 450°C 475°C 500°C 525°C 550°C 575°C 600°C 625°C 650°C 650
850
1050
1250
1450
1650
1850
Wavenumber (cm−1) Fig. 10 Infrared emission spectra (IES) of whewellite in the range 650–1850 cm1 from 75 C to 650 C at 25 C increments.
the band associated with the OH-bending modes of the crystal water around 1645 cm1 as it is obscured by the u(antisymmetric C]O) mode around 1620 cm1. This makes it hard to observe the dehydration step. Between 1900 and 650 cm1 the oxalate is characterized by a broad band around 1640 cm1 plus a sharper band around 1325 cm1, which can be ascribed to the u(symmetric C]O) mode. The weaker band around 788 cm1 can be ascribed to the d(O–C–O) mode. Some significant changes can be observed when the sample is heated to above 375 C. No weight changes are observed in the thermogravimetric analysis (TGA) (Fig. 8) indicating that the changes are associated with a phase change. This is well known for the calcium oxalate monohydrate where the dehydrated a-CaC2O4 transitions to b- and even g-CaC2O4 between 350 C and 400 C. A new band at 762 cm1 has been assigned to these two new phases. Another drastic change in the spectra can be observed between 400 C and 450 C, when the CaC2O4 decomposes to CaCO3. The typical oxalate bands disappear and instead the carbonate bands appear around 712 (u4), 874 (u2), and 1413 cm1 (u3 with a second band around 1471 cm1). All these bands have shifted as a result of the high temperature compared to the IR of CaCO3 at room temperature. The final decomposition to CaO takes place around 600–650 C as evidenced by the disappearance of the carbonate bands. The temperatures at which the different reactions are observed occur at slightly higher temperatures than in the TGA associated with the fact that the reactions take some time. It is not possible to observe the spectrum of the CaO as its bands are in the far-IR region below 400 cm1.
See also: AFM and Raman Spectroscopy, Applications in Cellular Imaging and Assays; ATR and Reflectance IR Spectroscopy, Applications; Chromatography-IR, Applications; Chromatography-IR, Methods and Instrumentation; Computational Methods and Chemometrics in Near Infrared Spectroscopy; Far Infrared Spectroscopy Applications; Femtosecond Stimulated Raman Spectroscopy; Forensic Science, Applications of IR Spectroscopy; Forensic Science, Applications of Raman Spectroscopy to Fiber Analysis; FT-IR and Raman Spectroscopies, Polymorphism Applications; FTIR Spectroscopy of Aqueous Solutions; FT-Raman Spectroscopy, Applications; Gas Phase Raman Scattering: Methods and Applications in the Energy Industry; Gemstone Analysis by Spectroscopy; Geology and Mineralogy Applications of Atomic Spectroscopy; High Resolution Gas Phase IR Spectroscopy Applications; High Resolution Gas Phase IR Spectroscopy Instrumentation; High-Pressure IR; IR and Raman Spectroscopies, Matrix Isolation Studies; IR and Raman Spectroscopies of Inorganic, Coordination and Organometallic Compounds; IR and Raman Spectroscopies, Polymer Applications; IR and Raman Spectroscopies, Studies of Hydrogen Bonding and Other Physicochemical Interactions; IR and Raman Spectroscopies, The Study of Art Works; IR and Raman Spectroscopy, Industrial Applications; IR, Biological Applications; IR, Medical Science Applications; IR Spectral Group Frequencies of Organic Compounds; IR Spectrometers; IR Spectroscopic Data Processing; IR Spectroscopy Sample Preparation Methods; IR Spectroscopy, Soil Analysis Applications; IR Spectroscopy, Surface Studies; IR Spectroscopy, Theory; Luminescence Spectroscopy, Inorganic Condensed Matter Applications; Near-Infrared Spectra,
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Interpretation; Near-Infrared Spectroscopy; Near-IR Spectrometers; NIR FT-Raman; NMR Spectroscopy, 29Si; NMR Spectroscopy, Heteronuclei, B, Al, Ga, In, Tl; NMR Spectroscopy, Heteronuclei, Ge, Sn, Pb; Nonlinear Raman Spectroscopy, Applications; Nonlinear Raman Spectroscopy, Instruments; Nonlinear Raman Spectroscopy, Theory; Protein Structure Analysis by CD, FTIR, and Raman Spectroscopies; Raman and Infrared Microspectroscopy; Raman Optical Activity, Applications; Raman Optical Activity, Macromolecule and Biological Molecule Applications; Raman Optical Activity, Small Molecule Applications; Raman Optical Activity, Spectrometers; Raman Optical Activity, Theory; Raman Spectrometers; Raman Spectroscopy, Biochemical Applications; Raman Spectroscopy, Medical Applications: A New Look Inside Human Body With Raman Imaging; Raman Spectroscopy, Soil Analysis Applications; Rayleigh Scattering and Raman Effect, Theory; Resonance Raman Applications; Spatially Offset Raman Spectroscopy; Spatially Resolved IR; Structural Chemistry Using NMR Spectroscopy, Inorganic Molecules; Surface-Enhanced Raman Optical Activity (SEROA); Surface-Enhanced Raman Scattering (SERS), Applications; Surface-Enhanced Raman Scattering (SERS) Biochemical Applications; Time-Resolved Raman Spectroscopy; Transmission Raman: Methods and Applications; UV-Visible Absorption and Circular Dichroism Spectroscopy, Inorganic Chemistry Applications; Vibrational CD, Applications; Vibrational CD Spectrometers; Vibrational CD, Theory and Application to Determination of Absolute Configuration; Vibrational CD, Theory; Vibrational, Rotational and Raman Spectroscopy, Historical Perspective; Vibrational Spectroscopy Applications in Drugs Analysis; Vibrational Spectroscopy, Flame and Temperature Measurement; Vibrational Spectroscopy in Food Processing; X-Ray Diffraction, Studies of Inorganic Compounds and Minerals.
Further Reading Spectroscopic Methods in Mineralogy. Beran A and Libowitzky E (eds.) (2004) 1st ed. Budapest: Eo¨tvo¨s University Press. Crane MJ, Leverett P, Shaddick LR, Williams PA, Kloprogge JT, and Frost RL (2001) The PbCrO4-PbSO4 System and Its Mineralogical Significance. Neues Jb. Mineral. Monat. 2001(11): 505–519. de Oliveira LFC, Edwards HGM, Frost RL, Kloprogge JT, and Middleton PS (2002) Caput Mortuum: Spectroscopic and Structural Studies of an Ancient Pigment. Analyst 127: 536–541.
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Raman Spectroscopy Applied to Earth Sciences and Cultural Heritage. Dubessy J, Caumon M-C, and Rull F, et al. (eds.) (2012) 1st ed. London: The Mineralogical Society of Great Britain and Ireland. The Infrared Spectra of Minerals. Farmer VC (ed.) (1974) London: The Mineralogical Society of Great Britain and Ireland. Frost RL and Kloprogge JT (2001) Towards a Single Crystal Raman Spectrum of Kaolinite at 77 K. Spectrochim. Acta A 57(1): 163–175. Frost RL, Ruan H, and Kloprogge JT (2000) Application of Infrared Emission Spectroscopy to the Study of Natural and Synthetic Inorganic Materials. (www.ijvs. com)Internet J. Vib. Spectro. 4(1): 5. Frost RL, Martens WN, Kloprogge JT, and Crane MA (2002) Raman Spectroscopic Study of Cinnabar (HgS), Realgar (As4S4), and Orpiment (As2S3) at 298 and 77K. Neues Jb. Mineral. Monat. 10: 469–480. Frost RL, Martens WN, Kloprogge JT, and Williams PA (2002) Raman Spectroscopy of the Basic Copper Chloride Minerals Atacamite and Paratacamite; Implications for the Study of Copper, Brass and Bronze Objects of Archaeological Significance. J. Raman Spectrosc. 33(10): 801–806. Frost RL, Williams PA, Martens WN, and Kloprogge JT (2002) Raman Spectroscopy of the Polyanionic Copper(II) Minerals Buttgenbachite and Connelite: Implications for Studies of Ancient Copper Objects and Bronzes. J. Raman Spectrosc. 33: 752–757. Frost RL, Martens WN, Rintoul L, Mahmutagic E, and Kloprogge JT (2001) Raman Spectroscopic Study of Azurite and Malachite at 298 and 77K. J. Raman Spectrosc. 33: 252–259. Frost RL, Edwards HGH, Duong L, Kloprogge JT, and Martens WN (2002) Raman Spectroscopic and SEM Study of Cinnabar from Herod’s Palace and Its Likely Origin. Analyst 127(2): 293–296. The Application of Vibrational Spectroscopy to Clay Minerals and Layered Double Hydroxides. Kloprogge JT, et al. (ed.) (2005) 1st ed. Aurora, CO: The Clay Minerals Society. Kloprogge JT and Frost RL (2000) Raman Microscopy at 77 K of Natural Gypsum CaSO4∙2H2O. J. Mater. Sci. Lett. 19(3): 229–231. Kloprogge JT, Frost RL, and Lack D (2000) Raman Microscopic Study at 300 K and 77 K of Some Pegmatite Minerals from the Iveland-Evje Area, Aust-Agder, Southern Norway. Spectrochim. Acta A 56(3): 501–513. Kloprogge JT, Visser D, Martens WN, Duong LV, and Frost RL (2003) Identification by Raman Microscopy of Magnesian Vivianite Formed from Fe2þ, Mg, Mn2þ and PO3 4 Leached from Metal and Bone Fragments in a Roman Camp Near Fort Vechten, Utrecht, The Netherlands. Neth. J. Geosci. 82(2): 209–214. Martens WN, Frost RL, Kloprogge JT, and Williams PA (2003) A Raman Spectroscopic Study of the Basic Copper Sulphates – Implications for Copper Corrosion and ‘Bronze Disease’. J. Raman Spectrosc. 34: 145–151. Reddy BJ, Frost RL, Martens WN, Weier ML, and Kloprogge JT (2006) Spectroscopic Characterization of Mn-Rich Tourmalines. Vib. Spectrosc. 44: 42–49. Ross SD (1972) Inorganic Infrared and Raman Spectra, 1st ed. Maidenhead, Berkshire: McGraw-Hill. Smith E and Dent G (2005) Modern Raman Spectroscopy: A Practical Approach, 1st ed. Chichester, West Sussex: John Wiley & Sons.
Introduction Infrared (IR) and Raman spectroscopy have become mainstream techniques not only to identify minerals and inorganic minerals but also to study reactions, such as phase changes, as a function of changes in temperature and pressure. The main advantage of these two techniques especially IR and Raman microscopy is that they are non-destructive, in comparison to other commonly used techniques such as X-ray diffraction or optical microscopy. The identification of minerals generally starts by visual examination. Based on properties such as color, crystal habitus, hardness, luster, cleavage, etc. a first identification of many rock-forming minerals is possible. It becomes more difficult for the more rare minerals that are mostly present in only minor amounts and often as small crystals. In many of these cases visual examination even by an experienced mineralogist does not give a definite answer, and other analytical techniques are needed in order to arrive at a positive identification. The classical petrologist or mineralogist will start with the preparation of thin sections of approximately 30 mm thickness of the rocks sampled in the field for examination under the optical microscope. Optical properties such as color, pleochroism, refractive index, birefringes, etc. allow minerals to be identified. However, this method, though cheap, is rather timeconsuming and depends strongly on the experience of the geologist. Alternatively, minerals can be identified by their crystal structure by X-ray diffraction and chemical composition by methods such as electron microprobe analysis. Raman and IR spectroscopy can unambiguously identify minerals and inorganic materials. Raman spectroscopy is the only technique that can even identify the purity and physical form of elements, such as carbon, germanium, sulfur, and silicon. Despite the fact that both techniques in their simplest form have been around for many decades, it is only in more recent years that applications in areas such as high temperature/high pressure studies of phase changes, archeology, art, and gemology, etc. have been developed. This chapter will attempt to highlight some of these recent developments in IR and Raman spectroscopy.
Basic Identification of Minerals Minerals can be described as naturally occurring, inorganic crystalline materials. They can be classified based on their chemical characteristics. In general the following eight classes can be identified: (1) elements, (2) sulfides and related compounds, (3) halides, (4) oxides and hydroxides, (5) carbonates, nitrates, borates, (6) sulfates, tungstates, chromates, and molybdates, (7) phosphates, arsenates, and vanadates, and (8) silicates.
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
An essential part of this grouping is based on the occurrence of various characteristic anionic groups. Each of these groups is reflected as specific bands in either or both the IR and Raman spectra and function as fingerprints for specific minerals. The characteristic wavenumber regions of the various characteristic groups are listed in Table 1. It has to be kept in mind that division of the regions for the different anionic groups is only very general, that each mineral has its own characteristic pattern, and that in specific cases the situation can be much more complex. For example, distortion of a symmetric anionic group in the crystal structure can result in a splitting of the Raman bands. A good example of these is some of the carbonate minerals. The unperturbed carbonate ion is a planar triangle with point symmetry D3h. Group theory analysis of the carbonate molecule predicts four normal modes (Table 2) from which two are IR and Raman active while one is IR active only and one is Raman active only. Fig. 1A shows the symmetric stretch u1 for a series of common carbonates that belong to the calcite group (calcite, magnesite, rhodochrosite, siderite, and smithsonite) plus aragonite and dolomite. Calcite is trigonal with space group R3c, D63d and with two CaCO3 units in the primitive rhombohedral cell. The carbonate group is strongly covalent and is expected to show vibrational modes close to those of the free ion. However, there are two carbonate groups in the primitive cell and thus two similar sets of internal vibrations. These two sets are constrained by the factor group symmetry to vibrate either in phase or in anti-phase to each other with respect to the center of symmetry. Thus the internal modes form a Raman-active set coupled in phase and an IR-active set coupled anti-phase. The 1407 cm1 band associated with the u3 is the strongest feature in the IR spectrum (Fig. 1E) but is very weak in the Raman spectrum (Fig. 1B). The symmetric stretching mode u1 is the strongest band in the Raman spectrum and is normally extremely sharp. Other clearly observable bands are the u4 at 713 cm1 (Fig. 1C) and the lattice modes u13 at 287 cm1 and u14 at 151 cm1 (Fig. 1D). When the Ca2þ cation is replaced by other intermediate-size divalent cations, for example, Mg, Fe, Zn, Co, Mn, etc., the calcite structure remains intact. The effect of the different metals on the IR and Raman spectra is limited and only minor shifts are observed. In contrast, the polymorph of calcite, the mineral aragonite is orthorhombic with space group Pmcn, D16 2h , with four ACO3 formula units per unit cell. The perturbations of the low symmetry aragonite structure produce three effects on the molecular modes of the CO2 3 ion. First of all there is a relaxation of the selection rules that permits u1 to become active in the IR and u2 in the Raman (Fig. 1C). Secondly, the degeneracy is lifted and u2 and u4 each split into two components (site group splitting) (Fig. 1E). Finally, because of the interactions between the four carbonate molecules in the unit cell, each internal mode appears four times.
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Characteristic wavenumber regions for various anionic groups
Table 1
Anionic group Sulfides Hydroxides Oxides Carbonates Nitrates
4000–3000 cm1
1600–1200 cm1
1200–800 cm1
1
800–600 cm1
<400 cm1
<500 cm1
1
4000–3000 cm
600–400 cm1
1200–600 cm 1
1
1550–1300 cm
890–800 cm 1050 cm1 850–800 cm1 900–600 cm1 1250–900 cm1 950–750 cm1 950–800 cm1 950–750 cm1 1150–900 cm1 900–770 cm1 900–700 cm1 1200–800 cm1
1350–1250 cm1
Borates Sulfates Tungstates Chromates Molybdates Phosphates Arsenates Vanadates Silicates
<1200 cm1 700–670 cm1 770–715 cm1 680–570 cm1
450–250 cm1 500–350 cm1 450–250 cm1 600–400 cm1 400–350 cm1 400–300 cm1
Normal modes of the CO2 3 ion
Table 2 Mode
Symmetric stretch Out-of-plane bend Asymmetric stretch In-plane bend
u1 u2 u3 u4
Symmetry
Selection rules
Wavenumber (cm1)
A10 00 A2 0 E E0
Raman IR IR þ Raman IR þ Raman
1063 879 1415 680
(A)
(B)
n1
n3 (g)
(g)
(f)
Raman intensity (AU)
Raman intensity (AU)
(f)
(e)
(d)
(e)
(d)
(c) (c)
(b)
(b)
(a) (a) 1060
1070
1080 1090 1100 1110 Wavenumber (cm−1)
1120
1130
1340
1360
1380
1400 1420 1440 Wavenumber (cm−1)
1460
1480
Fig. 1 Raman spectra of various carbonates (a ¼ calcite, b¼ aragonite, c ¼ dolomite, d¼ magnesite, e ¼ rhodochrosite, f ¼ siderite, g¼ smithsonite) in (A) 1000–1200 cm1 region, (B) 1300–1500 cm1 region,
(D)
(C)
n4 (g) (g) (f)
(f)
Raman intensity (AU)
Raman intensity (AU)
(e)
(e)
(d)
(d) (c)
(c) (b) (b) (a)
(a)
650
700
750
800
850
900
130
180
Wavenumber (cm−1)
230 280 Wavenumber (cm−1)
(E)
n3
n2 n4 (g)
IR intensity (AU)
(f)
(e)
(d)
(c)
n1
(b)
(a) 600
800
1000
1200
1400
1600
Wavenumber (cm−1) Fig. 1 (Continued) (C) 600–900 cm1 region, (D) 100–400 cm1 region, and (E) infrared spectra in the 600–1700 cm1 region.
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Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
IR and Raman Spectroscopy of Mineral Pigments in Art IR microscopy and Raman microscopy, spectroscopy using fiber-optic probes, and more recently also mobile and handheld Raman spectrometers have become of interest to study objects of arts such as medieval texts or paintings. Several of the pigments used are based on naturally occurring or synthetic minerals (Table 3). A great advantage of these vibrational spectroscopic methods here is that it can deal with not only the organic components but also the inorganic components. However, Raman spectroscopy is often favored based on the smaller scattering intensity of water and hydroxyl groups in the Raman effect, especially in ancient, hydrated biomaterials such as linens and cottons. For paintings and manuscripts it has the advantage of accessibility of the low wavenumber region (<500 cm1), which cannot be observed with standard midIR spectroscopy, but which gives important information with regards to identification of various pigments. For art historians it is important to learn about the materials and their provenance in order to get an understanding of ancient techniques, to attribute a certain manuscript or painting to a certain artist or workshop, to possibly date the material and to distinguish fakes from real artwork. The common mineral pigments can, similarly to Table 1, be divided into different groups based on their chemical composition. Sulfides such as greenockite, realgar, cinnabar, and orpiment (Fig. 2) show all their characteristic bands below 500 cm1. The oxides form a good example where potential fake art can be differentiated from much older art. For synthetic pigments such as the TiO2 polymorphs, anatase and rutile, the dates of first production are well documented. Rutile has been used since 1923 and anatase since 1947, which means that the observation of one of those pigments in a disputed Renaissance painting clearly indicates a forgery. Fig. 3A shows the Raman spectra of anatase and rutile in the region below 900 cm1, clearly showing two distinct bands at 392 and 516 cm1 for anatase and at 442 and 605 cm1 for rutile. In the case of colored pigments one has to be careful in the choice of the laser wavelength because of absorption of the scattered light by the sample. In general the best choice is to use a laser excitation line with a wavelength that is outside that of the contour of the electronic absorption bands of the pigment. An unwanted side effect of using laser light is the change in pigment structures due to localized heating. A good example of this can be found in the group of lead oxides. Minium (red lead, Pb3O4) can decompose to massicot (PbO) when using a 514.5-nm laser but the minium will remain intact when using a 632.8-nm laser (Fig. 3B). As shown earlier in this paper the carbonates can easily be recognized by the vibrational modes of the CO2 3 group in the crystal structure. Besides the already discussed calcite, the major carbonate pigments include green malachite, blue azurite, and white hydrocerussite (lead white) (Fig. 3C). Hydrocerussite shows a sharp and strong carbonate u1 at 1042 cm1, azurite shows the same band at 1094 cm1, while malachite exhibits two bands with maxima at 1099 and 1052 cm1. The second band at 1052 cm1 is associated with the OH-bending mode. The u2 in azurite can be observed as a maximum at 763 cm1, the same band is much weaker in malachite at
750 cm1. For the carbonate group, IR bands are observed at 1095 cm1 (u1), 834 and 816 cm1 (u2), 1430 and 1419 cm1 (u3), and 764 and 739 cm1 (u4). For azurite the bands at 3453 and 3427 cm1 have been assigned as the O–H stretching mode with the O–H bending modes found at 1035 and 952 cm1. Malachite displays two hydroxyl stretching bands at 3474 and 3404 cm1 while O–H out-of-plane bending modes are found at 1045 and 875 cm1. IR bands for the Cu–O stretching modes are observed at 495 and 400 cm1, while Cu–O bending modes occur at 455 and 345 cm1. Bands at 305 and 240 cm1 are assigned to the O–Cu–OH-bending modes. The Raman out-of-plane bending modes are found at 194 and 170 cm1. The sulfates such as barite, gypsum, and posjnakite are characterized by four different sulfate vibrational modes, which can be split depending on the site symmetry of the sulfate group in the crystal structure. The free anion shows u1 at 983 cm1, u2 at 450 cm1, u3 at 1105 cm1, and u4 at 611 cm1. All four vibrational modes of the sulfate anions (u1, u2, u3, and u4) are Raman active, the symmetric stretching modes u1 giving rise to the strongest bands in the spectra (Fig. 3D). On the other hand, the u1 mode gives rise to a weak IR band at 1000 cm1. In the case of gypsum spectra it is difficult to assign unequivocally the band(s) resulting from the u2 sulfate mode. In the region where it is to be expected no band characteristic for this mode is observed. Actually, a broad band is found at 458 cm1 in the IR spectra of the gypsum samples but it is assigned to water libration modes (Fig. 4A). The bands originating from the water molecules are found in the IR spectra with the OH-stretching modes at 3552, 3492, and 3404 cm1 (Fig. 4B) and the corresponding bending mode has two bands at 1685 and 1620 cm1 (unexpected because there is only one type of water molecules), which can easily be explained as a consequence of correlation-field effects (vibrational interaction of identical oscillators) (Fig. 4B). For barite, as in gypsum, as a result of the lower local symmetry of the SO2 anions in the solid state, the IR-inactive (for Td 4 symmetry) u1 and u2 modes become active with a very weak IR band at 982 cm1, that is, in the region where the u1 mode is expected. The u2 appears as two bands at 470 and 439 cm1. The u3 and u4 modes are strong in the IR with bands at 1205, 1136, 1100, and 1042 cm1 and at 641, 633, and 614 cm1, respectively. Due to the broadness of the bands it is not always possible to observe all bands and commonly only three maxima for the u3 and two for u4 are observed. Pseudomalachite is a phosphate mineral. The free PO3 4 group shows the u1 around 970 cm1, the u2 around 360 cm1, the u3 around 1080 cm1, and finally the u4 around 510 cm1. In pseudomalachite these bands are observed in the IR at 953 cm1, 450 and 422 cm1, 1096 and 1025 cm1, and 615, 555, 530, and 482 cm1, respectively. In addition the pseudomalachite structure also contains hydroxyl groups, which are evident by the OH stretching modes around 3582, 3435, and 3390 cm1 and the corresponding bending modes at 1390 and 1325 cm1. Crocoite is a mineral that contains the chromate anion as its characteristic group. Free CrO2 4 shows, similar to sulfate, the u1 around 830 cm1, u2 around 330 cm1, u3 around 765 cm1, and u4 around 330 cm1, but others have reported different values such as 847, 348, 884, and 368 cm1,
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Table 3
271
Mineral (natural and synthetic) pigments used in art
Name
Chemical formula
Color
Azurite Lazurite Posnjakite Cuprorivaite Magnetite Goethite (ochre) Hematite (sienna, burnt ¼ mix þ clay) Greenockite Atacamite Celadonite/glauconite mix (green earth) Malachite Pseudomalachite Eskolaite Litharge Realgar Minium Cinnabar (vermillion) Cadmoselite (cadmium red) Anatase Baryte Calcite (chalk, whiting) Gypsum Kaolin (mainly kaolinite) Rutile Hydrocerussite (lead white) Sphalerite þ baryte (lithophone Greenockite (cadmium yellow) Crocoite Massicot Goethite þ clay þ silica (ochre) Orpiment
Cu3(CO3)2(OH)2 Na6Ca2(Al6Si6O24)(SO4,S,S2,S3,Cl,OH)2 Cu4(SO4)(OH)6H2O CaCu[Si4O10] Fe2þFe3þ 2 O4 FeOOH Fe2O3 þ clay CdS Cu2(OH)3Cl K(Mg,Fe2þ)Fe3þ(Si4O10)(OH)2 þ (K,Na)(Mg,Fe2þ,Fe3þ)(Fe3þ,Al)(Si,Al)4O10(OH)2 Cu2(CO3)(OH)2 Cu5(PO4)2(OH)4 Cr2O3 PbO As4S4 Pb3O4 HgS CdSe TiO2 BaSO4 CaCO3 CaSO42H2O Al2(Si2O5)(OH)4 TiO2 Pb3(CO3)2(OH)2 ZnS þ BaSO4 CdS PbCrO4 PbO FeOOH þ clay þ SiO2 As2S3
Blue Blue Blue Blue Black Brown/orange Brown/orange Brown/orange Green Green Green Green Green Red Red Red Red Red White White White White White White White White Yellow Yellow Yellow Yellow Yellow
Raman intensity (AU)
Realgar
Orpiment
Cinnabar
Greenockite
100
200
300
400
500
600
Wavenumber (cm−1) Fig. 2 Raman spectra of selected sulfides: greenockite, cinnabar, orpiment, and realgar in the region 100–700 cm1.
700
272
(B)
(A)
Raman intensity (AU)
Raman intensity (AU)
Rutile
Massicot
Anatase 100
200
300
400
500
600
Wavenumber (cm−1)
700
800
900
Litharge 125
225
325
425
525
625
725
825
Wavenumber (cm−1)
(D)
(C)
Crocoite Malachite
Azurite
Raman intensity (AU)
Raman intensity (AU)
Pseudomalachite
Gypsum
Barite Hydrocerussite Posnjakite 120
320
520
720
Wavenumber (cm−1)
920
1120
100
300
500
700
900
1100
Wavenumber (cm−1)
Fig. 3 Raman spectra of selected pigments: (A) anatase and rutile in the 100–900 cm1 region, (B) litharge, massicot, and minium in the 125–900 cm1 region, (C) hydrocerussite, azurite, and malachite in the 120–1200 cm1 region, and (D) posnjakite, barite, gypsum, pseudomalachite, and crocoite in the 100–1200 cm1 region.
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Minium
(B)
Absorbance (AU)
Barite
Gypsum 400
Fig. 4
600
800
1000
1200
Wavenumber (cm−1)
1400
1600
1800
3000
3100
3200
3300
3400
3500
3600
Wavenumber (cm−1)
Infrared spectra of (A) the sulfates gypsum and barite in the region 400–1800 cm1 and (B) gypsum in the hydroxyl stretching region 3000–4000 cm1.
3700
3800
3900
4000
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Absorbance (AU)
(A)
273
274
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
respectively. For crocoite all the allowed vibrations are Raman active, but only the T2 symmetry species are IR active. All degeneracies are removed in crocoite and the resulting nine vibrational modes are all Raman and IR active. In the IR spectrum stretching bands have been found at 885, 855, and 825 cm1 and a weak broad band, presumably due to the bending modes, at 388 cm1. The Raman spectrum of crocoite shows three bands in the CrO stretching region at 856, 841, and 825 cm1. The band at 841 cm1 is the most intense and is assigned to the u1 symmetric stretching vibration, while the other two bands occur as weak shoulders on either side. The Raman spectrum of crocoite shows a series of bands at 402, 379, 360, and 330 cm1. One probable assignment is that the first three bands at 402, 379, and 360 cm1 are attributable to the u4 modes and that the band at 330 cm1 to the u2 mode.
IR and Raman Spectroscopy in Gemology IR and Raman spectroscopy have become routine spectroscopic techniques in gemological laboratories in the last 25 þ years. Since vibrational spectroscopy, in particular IR and Raman microscopy, fiber-optic, handheld Raman, and attenuated total reflection IR, is a non-destructive technique it is ideal to identify not only gem minerals, in particular rare gem minerals, exotic stimulants, or varieties of unusual appearance, but also their inclusions. One of the major advantages of these different methods is that the gemstone can be studied even when it is cut and set in a jewel. These inclusions may provide information with regards to modifications such as heat treatment, impregnation with organic material to hide imperfections such as cracks. Lately not only the positions of the Raman bands for identification but also the properties of the lines themselves, in particular line width has become of interest as this provides information related to properties such as grain size, metamictization (eg, in zircon), etc. Not all materials respond well to the Raman laser, in other words are poor Raman scatterers, such as black oxides, opal, or cubic zirconia. Also the effects of fluorescence or the sharp lines due to the 546 nm mercury emission from nearby fluorescent lighting can be misinterpreted. It is quite easy to determine by classic gemological techniques whether a gemstone is a garnet, a tourmaline, etc., but it is much more difficult to determine its precise composition within their solid solutions. However, this can be achieved by vibrational spectroscopy. Fig. 5A–C provides the Raman and IR spectra of some of the most common gem minerals. In the low wavenumber region, also known as the fingerprint region, it is clear that each mineral has its own characteristic set of bands and the gemstones can easily be distinguished from each other using either Raman or IR spectroscopy, though the differences are more pronounced in the Raman spectra. In addition some of the minerals contain hydroxyl groups and/or water, such as topaz, turquoise, and zoisite (gem varieties: pink thullite and heat treated deep blue tanzanite), which can be observed in the IR in the hydroxyl stretching region between 2500 and 4000 cm1 (Fig. 5C). Topaz shows a sharp band around 3649 cm1, the strength of which is affected by the amount of F for OH substitution in the crystal structure. Turquoise contains both hydroxyl groups and crystal water
with bands at 3508, 3463, 3446, 3295, and 3090 cm1. The first three relatively sharp bands are associated with hydroxyl groups, while the remaining very broad bands are associated with the OH-stretching modes of water. Synthetic turquoise can be distinguished from natural turquoise because of their smaller grain size, which is reflected in the Raman spectra as much broader bands. In particular the phosphate bands are well resolved in natural turquoise, whereas in synthetic turquoise these bands tend to be broad, less intense, and less resolved. The garnets are used as gemstones in a wide range of colors and often a name like almandine, andradite, grossular, pyrope, etc. is given based on the color only. This often does not conform to the chemical composition that mineralogists have given to each of these garnet minerals. In addition, except for synthetic garnets, all garnets are solid solutions. Vibrational spectroscopy can help in distinguishing the different solid solutions as the spectra change as a function of the garnet composition (Fig. 5D and E, Table 4). The systematic shift in band positions gives a qualitative or even a quantitative measure of their composition. For the garnets a first distinction is made on the elements in the positions X and Y in the general formula X3Y2(SiO4)3. The first group all have Al in the Y position and is commonly known as the pyralspite group (pyrope with X ¼ Mg, almandine with X ¼ Fe, and spessartine with X ¼ Mn), while the second group all have Ca in the X position and is known as the ugrandite group (uvaroite with Y ¼ Cr, grossular with Y ¼ Al, and andradite with Y ¼ Fe). A similar effect of the changes in chemical compositions of the tourmaline group minerals such as dravite [Na(Mg3) Al6(Si6O18)(BO3)3(OH)3(OH)], elbaite [Na(Li1.5Al1.5)Al6(Si6 O18)(BO3)3(OH)3(OH)], liddicoatite [Ca(Li2Al)Al6(Si6O18) (BO3)3(OH)3(OH)], and schorl [Na(Fe2þ 3 )Al6(Si6O18)(BO3)3 (OH)3(OH)] is shown in Fig. 5F and G. Though at first sight not as clear as for the garnets, here also changes in the spectra can be observed with the changes of the different substitutions in the tourmaline structure. Diamond shows a single sharp band in the Raman spectrum at 1332 cm1 in comparison to the synthetic cubic zirconia (often sold as a cheaper replacement for diamond) that shows a distinctive broad band around 620 cm1 plus several other bands and can easily be identified by Raman spectroscopy (Fig. 6A). Zircon often contains trace amounts of radioactive elements in its crystal structure that over time can result in radiation damage and the crystalline structure changes to an amorphous state while the outer crystal shape is retained. This is in geology known as metamictization. The degree of metamictization can easily be seen in the Raman spectrum. Where a well-crystalline zircon shows strong and sharp bands, a metamict zircon only shows very broad and much weaker bands (Fig. 6B).
Heating/Cooling Stage Raman Spectroscopy These days heating/cooling stages for IR and Raman microscopes can be easily obtained commercially. These stages allow for heating/cooling through circulation of heated or cooled liquid or gas around a sample cell. Because of the increased optical path and the spherical aberration of the cell window,
(A)
(B) Zoisite Zoisite
Turquoise Turquoise
Forsterite
Chrysoberyl
IR intensity (AU)
Raman intensity (AU)
Jadeite
Jadeite Forsterite Chrysoberyl Beryl
Beryl
Topaz
Topaz
150
350
550
750
950
1150
1350
400
500
600
700
800
900
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1100
1200
1300
1400
Wavenumber (cm−1)
Wavenumber (cm−1)
(D)
(C)
Uvarovite
Turquoise
Spessartine Raman intensity (AU)
IR intensity (AU)
Zoisite
Pyrope
Grossular
Andradite
Topaz Almandine 2500
2700
2900
3100
3300
Wavenumber (cm−1)
3500
3700
3900
150
250
350
450
550
650
750
850
950
1050
Wavenumber (cm−1)
275
Fig. 5 Infrared and Raman spectra of selected gemstones: (A) Raman spectra of corundum, topaz, beryl, chrysoberyl, forsterite, jadeite, turquoise, and zoisite in the 150–1400 cm1, (B) infrared spectra of the same minerals in the region 400–1400 cm1, (C) infrared spectra of topaz, turquoise, and zoisite in the hydroxyl stretching region 2500–4000 cm1, (D) Raman spectra of various garnets in the region 150–1100 cm1, (Continued)
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Corundum
Corundum
276
(F)
(E)
Pyrope
Grossular
Raman intensity (AU)
IR intensity (AU)
Spessartine
Liddicoatite
Elbaite
Andradite
Dravite
Almandine 400
500
600
700
800
900
1000
1100
1200
Wavenumber (cm−1)
150
250
350
450
550
650
750
850
950
1050
1150
Wavenumber (cm−1)
(G)
IR intensity (AU)
Schorl
Elbaite
Dravite
400
600
800
1000
1200
1400
Wavenumber (cm−1)
Fig. 5 (Continued) (E) infrared spectra of the same garnets in the region 400–1200 cm1, (F) Raman spectra of selected tourmalines in the region 150–1200 cm1, (G) infrared spectra of the same tourmalines in the region 400–1500 cm1.
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Schorl
Uvarovite
Table 4
Differentiation of garnets by their IR and Raman spectra in the region below 1000 cm1
IR 980–940 cm1 Raman 900–925 cm1 Pyralspite group
Position of IR band in 980–890 cm1 region Raman band in 870–925 cm1 region
No IR band near 640–620 cm1 Single IR band near 340 cm1 Raman band near 358 cm1 D(897–877 cm1 bands) ¼ 26–27 cm1 IR band near 630 cm1 IR bands in the region of 350–310 cm1 Raman bands in the region 325–365 cm1 D(885–865 cm1 bands) ¼ 19–21 cm1
Pyrope
Almandine or spessartine
No IR band in the region 360–300 cm1 Raman bands near 370–375 cm1 Grossular or uvarovite
IR 920–890 cm1 Raman 870–875 cm1 Ugrandite group Two IR bands in the region of 360–300 cm1 Two Raman bands in the region of 345–370 cm1
(A)
Andradite
Two Raman bands near 862 and 925 cm1
IR bands near 962, 897, and 877 cm1 Raman bands near 856 and 914 cm1 ¼ Almandine IR bands near 950, 885, and 865 cm1 Raman bands near 840 and 902 cm1 ¼ Spessartine IR bands near 915, 960, and 840 cm1 Raman bands near 820, 844 and 878 cm1 ¼ Grossular IR bands near 900, 840, and 830 cm1 Raman bands near 820, 873, and 887 cm1 ¼ Uvarovite Raman bands near 811, 836, and 871 cm1
(B)
Metamict zircon
Raman intensity (AU)
Raman intensity (AU)
Cubic zirconia
Zircon
Diamond
150
350
550 750 950 Wavenumber (cm−1)
1150
1350
150
350
550 750 Wavenumber (cm−1)
950
1150
Fig. 6 Raman spectra of (A) diamond and synthetic cubic zirconia in the region 150–1500 cm1, and (B) zircon and metamict zircon in the region 150–1200 cm1.
278
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Inner hydroxyl
Raman intensity (AU)
Inner-surface hydroxyls
3580
3600
3620
3640
3660
3680
3700
3720
3740
Wavenumber (cm−1)
Fig. 7 Raman spectra at room temperature (bottom) and after cooling to liquid nitrogen (top) temperature of kaolinite in the hydroxyl stretching region 3580–3740 cm1.
spectra are noisier with decreased intensity and require longer accumulation times. Cooling is often used to obtain better quality spectra as bands tend to sharpen and shift. In the case of overlapping bands this can result in the possibility of observing each band individually. A good example is the clay mineral kaolinite, which shows five overlapping hydroxyl stretching bands in the IR and Raman spectra. Compared to the room temperature spectrum the inner hydroxyl band at 3621 cm1 shifted slightly to 3618 cm1 upon cooling. The two inner-surface hydroxyl bands at 3670 and 3685 cm1 shifted significantly to higher wavenumbers at 3979 and 3694 cm1 with a broad shoulder at 3707 cm1. The remaining inner-surface hydroxyl band at 3652 cm1 splits into two components at 3660 cm1 and a shoulder at 3663 cm1 (Fig. 7). Calcium oxalate occurs either as the dihydrate or the monohydrate (two rare minerals, weddellite and whewelite). Weddellite can be found as authogenic crystals in sea floor mud. It also has been reported in peat bearing sediments and in calcite bearing lacustrine sediments. It occurs with whewellite, urea, phosphammite, and aphthitalite. Together, whewellite and weddellite are the most common renal calculi (kidney stones). Whewellite shows a characteristic series of phase changes upon heating and is often used for teaching purposes as an example or as a standard in thermal analysis. The decomposition of whewellite occurs in three distinct steps (Fig. 8): 1. dehydration around 115 C CaC2 O4 2H2 O ! CaC2 O4 þ 2H2 O ðgasÞ 2. decomposition to calcium carbonate around 425 C
CaC2 O4 ! CaCO3 þ CO ðgasÞ 3. decomposition to calcium oxide around 600 C CaCO3 ! CaO þ CO2 ðgasÞ The Raman spectra clearly show changes between 150 C and 200 C and again between 450 C and 500 C. The CO stretching vibration for oxalate in solution is observed at 1496 cm1 and occurs for calcium oxalate around 1475 cm1 at room temperature. No bands are observed in the IR spectrum at this temperature, which means that the structure of calcium oxalate is centrosymmetric. Mild heating to 50 C causes two bands to appear in the Raman spectrum. These bands are similar to those observed for whewellite, the partially dehydrated form of weddellite. The bands shift to 1488 and 1462 cm1 in the 150 C spectrum (Fig. 9), that is, there is a slight red shift on heating. At temperatures above 150 C the bands are observed at 1478 and 1466 cm1. These bands correspond to the symmetric stretching modes of anhydrous calcium oxalate. At temperatures above 450 C a single band is now observed at 1480 cm1 corresponding to the CO symmetric stretching mode of calcium carbonate. In addition a small band at 1628 cm1 is observed in the room temperature spectrum, which can be attributed to the antisymmetric stretching vibration. This band should not be observed in the Raman spectrum, but distortion of the structure probably allowed the observation of this Raman forbidden band. Aqueous oxalate shows the IR-active antisymmetric stretching mode at 1600 cm1. This band is observed at 1650 cm1 at 50 C and shifts to 1647 cm1 with further heating. Around 450 C this band disappears and a new very broad band becomes visible
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
279
100 CaC2O4.2H2O -> CaC2O4 + 2 H2O(gas)
90
Weight (%)
80 70
CaC2O4 −> CaCO3 + CO(gas)
60 50
CaCO3 −> CaO + CO2(gas)
40 30 30
90
150
210
270
330
390
450
510
570
630
690
750
Temperature (°C)
Fig. 8 Thermogravimetric analysis (TGA) decomposition profile of whewellite in the temperature range 30–800 C showing the three steps of (1) dehydration, (2) decomposition to calcium carbonate, and (3) decomposition of the calcium carbonate to calcium oxide.
around 1618 cm1. The final step of the decomposition of calcium carbonate to calcium oxide is at a temperature above the upper limit of the Raman thermal stage and can therefore not be observed.
IR Emission Spectroscopy
Fig. 9 Heating stage Raman spectra of whewellite in the range 1300–1700 cm1 from 50 C to 550 C.
IR emission spectroscopy forms a new and valuable spectroscopic method that can be applied in situ during heat treatment of a sample. The technique of measuring discrete vibrational frequencies emitted by thermally excited molecules has not been widely used for the study of inorganic materials and minerals. The major advantages of infrared emission spectra (IES) are sample analysis in situ at increasing temperature and no difficult sample preparation other than grinding it to submicron particle size. The normal single-beam reference spectrum obtained during the recording of an IR spectrum is actually the emission spectrum of the instrument source modified by the instrument response function. Transmission spectra are obtained by ratioing the signals in the absence and presence of the sample. In the same way emission spectra can be obtained by ratioing the emission signal of a sample to that of a reference, normally a blackbody source that emits radiation according to Planck’s formula (ie, a continuum). In the normal course of events three sets of spectra are obtained. Firstly the black body radiation (actually gray body radiation from graphite) over the temperature ranges of interest. Secondly the sample holder radiation is obtained for the same temperatures and finally the spectra of a thin layer of sample on the holder are obtained. The emittance spectra can then be calculated for each temperature by subtracting the single-beam spectra of the holder from that of the holderþ sample and the result ratioed to the single-beam spectra of the gray body. Earlier in the paragraph on heating stage Raman spectroscopy the example of whewellite, CaC2O42H2O, was shown. Fig. 10 shows the same experiment using infrared emission spectroscopy. The spectra at low temperatures do not distinctly show
280
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
75°C 100°C 125°C 150°C 200°C 250°C 300°C 350°C Emissivity
375°C 400°C 425°C 450°C 475°C 500°C 525°C 550°C 575°C 600°C 625°C 650°C 650
850
1050
1250
1450
1650
1850
Wavenumber (cm−1) Fig. 10 Infrared emission spectra (IES) of whewellite in the range 650–1850 cm1 from 75 C to 650 C at 25 C increments.
the band associated with the OH-bending modes of the crystal water around 1645 cm1 as it is obscured by the u(antisymmetric C]O) mode around 1620 cm1. This makes it hard to observe the dehydration step. Between 1900 and 650 cm1 the oxalate is characterized by a broad band around 1640 cm1 plus a sharper band around 1325 cm1, which can be ascribed to the u(symmetric C]O) mode. The weaker band around 788 cm1 can be ascribed to the d(O–C–O) mode. Some significant changes can be observed when the sample is heated to above 375 C. No weight changes are observed in the thermogravimetric analysis (TGA) (Fig. 8) indicating that the changes are associated with a phase change. This is well known for the calcium oxalate monohydrate where the dehydrated a-CaC2O4 transitions to b- and even g-CaC2O4 between 350 C and 400 C. A new band at 762 cm1 has been assigned to these two new phases. Another drastic change in the spectra can be observed between 400 C and 450 C, when the CaC2O4 decomposes to CaCO3. The typical oxalate bands disappear and instead the carbonate bands appear around 712 (u4), 874 (u2), and 1413 cm1 (u3 with a second band around 1471 cm1). All these bands have shifted as a result of the high temperature compared to the IR of CaCO3 at room temperature. The final decomposition to CaO takes place around 600–650 C as evidenced by the disappearance of the carbonate bands. The temperatures at which the different reactions are observed occur at slightly higher temperatures than in the TGA associated with the fact that the reactions take some time. It is not possible to observe the spectrum of the CaO as its bands are in the far-IR region below 400 cm1.
See also: AFM and Raman Spectroscopy, Applications in Cellular Imaging and Assays; ATR and Reflectance IR Spectroscopy, Applications; Chromatography-IR, Applications; Chromatography-IR, Methods and Instrumentation; Computational Methods and Chemometrics in Near Infrared Spectroscopy; Far Infrared Spectroscopy Applications; Femtosecond Stimulated Raman Spectroscopy; Forensic Science, Applications of IR Spectroscopy; Forensic Science, Applications of Raman Spectroscopy to Fiber Analysis; FT-IR and Raman Spectroscopies, Polymorphism Applications; FTIR Spectroscopy of Aqueous Solutions; FT-Raman Spectroscopy, Applications; Gas Phase Raman Scattering: Methods and Applications in the Energy Industry; Gemstone Analysis by Spectroscopy; Geology and Mineralogy Applications of Atomic Spectroscopy; High Resolution Gas Phase IR Spectroscopy Applications; High Resolution Gas Phase IR Spectroscopy Instrumentation; High-Pressure IR; IR and Raman Spectroscopies, Matrix Isolation Studies; IR and Raman Spectroscopies of Inorganic, Coordination and Organometallic Compounds; IR and Raman Spectroscopies, Polymer Applications; IR and Raman Spectroscopies, Studies of Hydrogen Bonding and Other Physicochemical Interactions; IR and Raman Spectroscopies, The Study of Art Works; IR and Raman Spectroscopy, Industrial Applications; IR, Biological Applications; IR, Medical Science Applications; IR Spectral Group Frequencies of Organic Compounds; IR Spectrometers; IR Spectroscopic Data Processing; IR Spectroscopy Sample Preparation Methods; IR Spectroscopy, Soil Analysis Applications; IR Spectroscopy, Surface Studies; IR Spectroscopy, Theory; Luminescence Spectroscopy, Inorganic Condensed Matter Applications; Near-Infrared Spectra,
Infrared and Raman Spectroscopy of Minerals and Inorganic Materials
Interpretation; Near-Infrared Spectroscopy; Near-IR Spectrometers; NIR FT-Raman; NMR Spectroscopy, 29Si; NMR Spectroscopy, Heteronuclei, B, Al, Ga, In, Tl; NMR Spectroscopy, Heteronuclei, Ge, Sn, Pb; Nonlinear Raman Spectroscopy, Applications; Nonlinear Raman Spectroscopy, Instruments; Nonlinear Raman Spectroscopy, Theory; Protein Structure Analysis by CD, FTIR, and Raman Spectroscopies; Raman and Infrared Microspectroscopy; Raman Optical Activity, Applications; Raman Optical Activity, Macromolecule and Biological Molecule Applications; Raman Optical Activity, Small Molecule Applications; Raman Optical Activity, Spectrometers; Raman Optical Activity, Theory; Raman Spectrometers; Raman Spectroscopy, Biochemical Applications; Raman Spectroscopy, Medical Applications: A New Look Inside Human Body With Raman Imaging; Raman Spectroscopy, Soil Analysis Applications; Rayleigh Scattering and Raman Effect, Theory; Resonance Raman Applications; Spatially Offset Raman Spectroscopy; Spatially Resolved IR; Structural Chemistry Using NMR Spectroscopy, Inorganic Molecules; Surface-Enhanced Raman Optical Activity (SEROA); Surface-Enhanced Raman Scattering (SERS), Applications; Surface-Enhanced Raman Scattering (SERS) Biochemical Applications; Time-Resolved Raman Spectroscopy; Transmission Raman: Methods and Applications; UV-Visible Absorption and Circular Dichroism Spectroscopy, Inorganic Chemistry Applications; Vibrational CD, Applications; Vibrational CD Spectrometers; Vibrational CD, Theory and Application to Determination of Absolute Configuration; Vibrational CD, Theory; Vibrational, Rotational and Raman Spectroscopy, Historical Perspective; Vibrational Spectroscopy Applications in Drugs Analysis; Vibrational Spectroscopy, Flame and Temperature Measurement; Vibrational Spectroscopy in Food Processing; X-Ray Diffraction, Studies of Inorganic Compounds and Minerals.
Further Reading Spectroscopic Methods in Mineralogy. Beran A and Libowitzky E (eds.) (2004) 1st ed. Budapest: Eo¨tvo¨s University Press. Crane MJ, Leverett P, Shaddick LR, Williams PA, Kloprogge JT, and Frost RL (2001) The PbCrO4-PbSO4 System and Its Mineralogical Significance. Neues Jb. Mineral. Monat. 2001(11): 505–519. de Oliveira LFC, Edwards HGM, Frost RL, Kloprogge JT, and Middleton PS (2002) Caput Mortuum: Spectroscopic and Structural Studies of an Ancient Pigment. Analyst 127: 536–541.
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Raman Spectroscopy Applied to Earth Sciences and Cultural Heritage. Dubessy J, Caumon M-C, and Rull F, et al. (eds.) (2012) 1st ed. London: The Mineralogical Society of Great Britain and Ireland. The Infrared Spectra of Minerals. Farmer VC (ed.) (1974) London: The Mineralogical Society of Great Britain and Ireland. Frost RL and Kloprogge JT (2001) Towards a Single Crystal Raman Spectrum of Kaolinite at 77 K. Spectrochim. Acta A 57(1): 163–175. Frost RL, Ruan H, and Kloprogge JT (2000) Application of Infrared Emission Spectroscopy to the Study of Natural and Synthetic Inorganic Materials. (www.ijvs. com)Internet J. Vib. Spectro. 4(1): 5. Frost RL, Martens WN, Kloprogge JT, and Crane MA (2002) Raman Spectroscopic Study of Cinnabar (HgS), Realgar (As4S4), and Orpiment (As2S3) at 298 and 77K. Neues Jb. Mineral. Monat. 10: 469–480. Frost RL, Martens WN, Kloprogge JT, and Williams PA (2002) Raman Spectroscopy of the Basic Copper Chloride Minerals Atacamite and Paratacamite; Implications for the Study of Copper, Brass and Bronze Objects of Archaeological Significance. J. Raman Spectrosc. 33(10): 801–806. Frost RL, Williams PA, Martens WN, and Kloprogge JT (2002) Raman Spectroscopy of the Polyanionic Copper(II) Minerals Buttgenbachite and Connelite: Implications for Studies of Ancient Copper Objects and Bronzes. J. Raman Spectrosc. 33: 752–757. Frost RL, Martens WN, Rintoul L, Mahmutagic E, and Kloprogge JT (2001) Raman Spectroscopic Study of Azurite and Malachite at 298 and 77K. J. Raman Spectrosc. 33: 252–259. Frost RL, Edwards HGH, Duong L, Kloprogge JT, and Martens WN (2002) Raman Spectroscopic and SEM Study of Cinnabar from Herod’s Palace and Its Likely Origin. Analyst 127(2): 293–296. The Application of Vibrational Spectroscopy to Clay Minerals and Layered Double Hydroxides. Kloprogge JT, et al. (ed.) (2005) 1st ed. Aurora, CO: The Clay Minerals Society. Kloprogge JT and Frost RL (2000) Raman Microscopy at 77 K of Natural Gypsum CaSO4∙2H2O. J. Mater. Sci. Lett. 19(3): 229–231. Kloprogge JT, Frost RL, and Lack D (2000) Raman Microscopic Study at 300 K and 77 K of Some Pegmatite Minerals from the Iveland-Evje Area, Aust-Agder, Southern Norway. Spectrochim. Acta A 56(3): 501–513. Kloprogge JT, Visser D, Martens WN, Duong LV, and Frost RL (2003) Identification by Raman Microscopy of Magnesian Vivianite Formed from Fe2þ, Mg, Mn2þ and PO3 4 Leached from Metal and Bone Fragments in a Roman Camp Near Fort Vechten, Utrecht, The Netherlands. Neth. J. Geosci. 82(2): 209–214. Martens WN, Frost RL, Kloprogge JT, and Williams PA (2003) A Raman Spectroscopic Study of the Basic Copper Sulphates – Implications for Copper Corrosion and ‘Bronze Disease’. J. Raman Spectrosc. 34: 145–151. Reddy BJ, Frost RL, Martens WN, Weier ML, and Kloprogge JT (2006) Spectroscopic Characterization of Mn-Rich Tourmalines. Vib. Spectrosc. 44: 42–49. Ross SD (1972) Inorganic Infrared and Raman Spectra, 1st ed. Maidenhead, Berkshire: McGraw-Hill. Smith E and Dent G (2005) Modern Raman Spectroscopy: A Practical Approach, 1st ed. Chichester, West Sussex: John Wiley & Sons.