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Raman microprobe mineral identification
Pergamon 0892-6875(01)00175-3
Mineruls Engineering, Vol. 14, No. 12, pp. 1565-1577, 2001 0 2001 Published by Elsevier Science Ltd All rights reserved 0892-6875/01/$ - see front matter
RAMAN MICROPROBE MINERAL IDENTIFICATION*
G.A. HOPEP*,R. WOODSY and C.G. MUNCE’ (j/School of Science, Griffith University, Nathan, Queensland 4111, Australia 3 Cooperative Research Centre for microTechnology, Australia E-mail: [email protected] (Received 1 May 2001; accepted 20 June 2001)
ABSTRACT Raman spectroscopy and associated imaging techniques are generally non-destructive, and can be used to identify a wide range of minerals and gemstones. Raman is a sensitive technique which requires minimal sample preparation and can be used on massive specimens from lump ore to fine powders and liquids. Since glass and water are weak Raman scatterers, spectra from minerals can be obtained from samples in air, through glass, andfor immersed in water. Examples of the application of Raman spectroscopy to minerals of importance in mineral processing are discussed. These include: detection of minerals of light elements such as carbon and fluorine; distinguishing between polymorphs such as those of iron sulfides; characterizing sulfides containing minor elements such as iron in sphalerite; and the identification of silicate, oxide, and carbonate gangue minerals. The Raman microprobe also permits Raman imaging and mapping of surfaces and inclusions. Imaging of diamond is presented as an example of this technique. 0 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords Mineral processing,
Ore mineralogy,
Particle morphology,
Sulfide ores.
INTRODUCTION The identification and characterization of minerals is of fundamental importance in the development and operation of mining and mineral processing systems. Traditionally, such information has been provided by the mineralogist using an optical microscope. This conventional approach has now been augmented by the introduction of a number of instrumental techniques that provide elemental analysis of the mineral species. Electron imaging systems are now highly automated to provide elemental mapping of a number of individual mineral grains, and sophisticated computer software developed for presenting the data in a useful format. In Australia, CSIRO’s QEM-Scan is well established and JKMRC’s MLA has recently been developed; the value of these systems to the mineral processor is the subject of other papers in this volume. Laser Raman micro-spectroscopy provides an additional, non-destructive analytical technique to assist the mineralogist in identifying minerals in assemblages. It has the advantage that the sample needs no more preparation than that for conventional optical microscopic examination and that spectra can be rapidly
* Presented at Applied Mineralogy ‘01, Brisbane, Australia, March 2001
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recorded. In a Raman spectrometer, the sample under examination is irradiated with an intense source of monochromatic radiation, usually in the visible region of the spectrum, and the scattered light is analyzed by the spectrometer. Most of the scattered light, the Rayleigh line, has the same frequency as the incident laser beam, but a small fraction, the Raman component, has shifted frequencies due to interaction of photons with the sample that result in changes in the vibrational or rotational energy of chemical bonds in molecules or crystals and in the vibration of crystal lattices. The main optical problem in Raman spectroscopy is the separation of the weak Raman signal from the Rayleigh line. This can be achieved through the use of notch filters which also provide excellent optical throughput so that the extremely low Raman signal intensity that is scattered from a tiny volume of a weakly-scattering mineral can be detected. Thus, modern confocal Raman microprobes have a lateral resolution of - 1 urn and a depth resolution of - 2 urn. Spectrometers are also able to use a programmable microscope stage, thus enabling imaging and mapping of larger areas of mineral specimens in the same manner as with SEMs and electron microprobes. The Raman effect occurs when the electric field due to the light interacts with the electrons in the sample and it is the derived polarisability of the sample molecules or crystal structure that determines the Raman activity. This derived polarisability is related to the directional character of chemical bonds in the sample, thus the connection to vibrational spectroscopy. Unlike techniques based on electron microscopy, Raman spectroscopy is not critically dependent on the atomic mass of the elements in the mineral. Thus, compounds containing light elements can be characterized, for example those containing carbon hydrogen, carbon - nitrogen, metal - fluorine and boron - oxygen. Also, polymorphs of the same chemical composition can be distinguished and the influence of minor elements incorporated into mineral lattices investigated. Glass and water are weak Raman scatterers and, accordingly, spectra are easy to obtain from solid materials through glass and/or immersed in water. For materials, which are transparent at the wavelength of the laser being used, confocal measurements can be made to obtain spectra from selected regions inside the mineral sample, thus enabling the characterization of inclusions and artefacts. Furthermore, Raman spectra can be derived from species in solution and hence dissolved species in flotation and leach liquors can be characterized as well as solid compounds. The non-destructive nature of Raman spectroscopy makes it an important technique for the gemmologist as well as the mineralogist. It can be used to distinguish expensive stones from synthetic analogues and to identify treatments made to enhance colour or to fill fissures, Kiefert et al., (1999). In this paper, a number of examples are discussed on the application mineral chemistry.
of Raman spectroscopy
to aspects of
EXPERIMENTAL Griffith University has two Renishaw Raman spectrometers that are used for mineral identification, research on mineral processing, and related studies. The Renishaw Raman Spectrograph 100 (Multi Channel Compact Raman Analyser) has a rotary encoded grating stage, and an internal two stage Peltier cooled (-70” C) CCD detector. The spectral resolution is 5 cm- ‘, and the wavenumber reproducibility is 0.1 cm“. The incident radiation is conveyed through a fibre optic Raman probe from a Spectra Physics Argon ion laser of 514 nm excitation, or a Renishaw laser of 633 nm. The Renishaw 100 system is horizontally aligned, and is close to ideal for spectroelectrochemistry which requires a long working distance and controlled gas atmosphere in situations where oxygen can influence the reaction process, Woods et al., (2000). The sampled volume for this system is approximately 50 urn”. The second instrument is a Renishaw RM2000 Raman spectrometer of high resolution that has a computercontrolled stage. This instrument uses a conventional Leica UV metallurgical microscope and is equipped with a range of objectives, both visible and ultraviolet. At maximum magnification, the system can sample a minimum volume of about 1 urn”, limited by the diffraction of light and, typically, it is operated at 5 um3. Four laser frequencies are available, the red and green described above and a Kimmon HeCd laser that
Raman microprobe mineral identification
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radiation of both 325 and 448 nm. The instrument has a micron positionable programmable stage that can be used for mapping with any of the four laser frequencies and Raman imaging with 514 nm radiation.
provides
RESULTS AND DISCUSSION Light elements Carbon
The presence of naturally-floatable carbonaceous shales in ores results in complications in the flotation separation of sulfide minerals. Normally, the carbonaceous gangue is either removed in a prefloat, or a depressant (e.g., hydroxy ethyl cellulose) added to retain this material in the tails. Such procedures are carried out in the mines in the Mt Isa Inlier, where carbonaceous gangue occurs in association with pyrite, Woodcock and Hamilton, (1993). Carbons also need special treatment in the flotation of complex lead-zinc deposits in India, Kalyan et al., (2000) and of copper-moly ores in China, Jian et al., (1993). Raman spectroscopy can assist in investigating such ores since it can characterize carbon species and identify carbon in sulfide mineral assemblages. Figure 1 shows Raman spectra from different allotropes of carbon and of coal. Crystalline diamond is an excellent Raman scatterer and has a single phonon band at 1332 cm-‘. In the case of graphite, the sp2 bonded carbon spectrum obtained using 5 14 nm radiation is resonance enhanced, and displays two bands, the major band is the E2s fundamental transition and is generally referred to as the G peak. This is the large 1580 cm-’ band in the graphite spectrum of figure 1. The polycrystalline graphite used to obtain this spectrum also exhibits another band at 1360 cm-’ referred to as the D peak. In studies of the formation of diamond-like carbon, Gilkes et al. (1997) were able to identify sp’ carbon in the presence of the sp2 carbon, by using UV excitation. The tetrahedral carbon in their deposited films could not be clearly identified using 5 14 nm excitation due to the greater Raman cross section of the sp2 D peak at this wavelength, even though the sp3 species comprised 80% of the deposit.
ZnS concentrate
\
graphite diamond
8 co Q Ei 8 8 F
r
0 r
CD
Raman shift / cm-’
Fig. 1 Raman spectra from diamond, graphite, coal, and a zinc concentrate containing carbonaceous gangue.
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The spectrum for black coal presented in figure 1 displays the bands characteristic of sp* bonded carbon. The poor crystallinity of this sample is reflected in the relative size of the D peak to the G peak which increases with disorder. The carbon bands are broad as expected from a composite graphitic material such as natural coal. Figure 1 also presents a spectrum from a zinc cleaner concentrate derived from the Mt Isa Inlier. The presence of carbonaceous gangue in this concentrate is clearly evident from the appearance of bands at 1610 cm-’ and - 1360 cm-‘. There are no bands displayed by sphalerite itself in this region (v.i.). The bands at wavenumbers below 720 cm-r in the spectrum arise from the sphalerite in the concentrate. Solution species Raman spectroscopy can also determine carbonate in flotation pulps as well as identify carbonate gangue minerals. Figure 2 compares a Raman spectrum from dissolved carbonate with those from calcite and cerussite.
carbonate solution
cerussite
Raman shift / cm-’ Fig.2
Raman spectra from calcite, synthetic cerussite, a dilute potassium and a zinc concentrate slurry containing carbonate in the pulp.
carbonate
aqueous
solution,
The spectrum for calcite shows an intense band at 1098 cm-’ arising from the symmetric stretching vibration of the carbonate group. There are also weak bands arising from other carbonate vibrations, the assymetric stretch near 1400 cm-’ and the in-plane bending mode at 700 cm-‘. The strong band at 300 cm-’ is due to the lattice vibrations of the calcite crystal. Calcite is an important optically active crystal and has been extensively studied. The internal vibrational modes have been calculated by a number of methods and the data has been discussed by White (1974). The spectrum from a synthetic cerussite in Fig. 2 is similar to that for calcite, even though this compound crystallizes with the aragonite structure. This is primarily due to the Raman active bands emanating from the carbonate. The major band positions are shifted, the one corresponding to the symmetric stretching vibration of the carbonate group is shifted by - 44 cm-’ to 1054 cm-‘. The aragonite structure has a lower
Ramao microprobe mineral identification
than that of calcite, and this results in the splitting of the cerussite assymetric near 1440 cm-’ into two bands which have increased intensity compared to calcite. symmetry
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carbonate
stretch
The solution spectrum of saturated potassium carbonate gives the carbonate ion spectrum which is similar to that obtained from the solid. However, the spectrum derived from a dilute solution of potassium carbonate displays a broad band at 1097 cm-’ due to the stretching vibration of the carbonate group. It is much broader than that obtained from the crystalline species as a result of a range of interactions of carbonate ions with water. Weak, broad bands are also evident from other carbonate vibrations at wavenumbers similar to the carbonate minerals. The broad band at 571 cm-’ is probably due to the formation of some bicarbonate or bicarbonate-like hydrogen bonding in the carbonate solution. Figure 2 also shows a spectrum obtained from a slurry of a zinc concentrate derived from the Mt Isa Inlier. The bands characteristic of dissolved carbonate/bicarbonate are apparent as well as those from lattice vibrations in sphalerite. Fluorine Fluorine impurities in metal sulfide concentrates cause problems in smelting since they result in the formation of hydrofluoric acid that damages smelter vessels as well as introducing environmental concerns. The two major sources of fluorine in the concentrate are the common fluoride minerals such as fluorite and apatite, and silicates and aluminosilicates containing fluoride substitutions such as talc and chlorite. It is possible to remove minerals such as talc and apatite during flotation, but fluorite and fluorine-containing chlorite are difficult to separate from the metal sulfide values. It is often necessary to leach concentrates to remove fluorine before smelting; for example, such a procedure is carried out on both the lead and zinc concentrates at the BHP Cannington mine in Queensland. Fluorite and chlorite give distinctive Raman spectra, as shown in Fig. 3. The chlorite spectrum in this figure was obtained from an inclusion within a large natural quartz crystal utilizing the confocal properties of the Raman spectrometer. Figure 3 also shows a sample of sphalerite containing fluorite is instantly recognizable from the spectrum.
fluorite from the Mt Isa Inlier. The presence
of the
sphalerite containing fluorite
Fig.3 Raman spectra from chlorite, fluorite, and a specimen of zinc sulfide in ore containing
fluorite.
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G. A. Hope et (11.
Polymorphs Boron nitride is not a mineral found in nature, but is included here to demonstrate the ability to identify a compound composed of light elements, and to distinguish between different polymorphs. White BN is isomorphous with graphite, whereas the black form has a similar structure to diamond. Figure 4 shows Raman spectra of these two polymorphs.
Fig.4 Raman spectra from cubic and hexagonal boron nitride. The spectrum from the hexagonal polymorph shows a single band at - 1365 cm-’ which is similar to the graphite homologue. The cubic sphalerite (diamond-like) form gives rise to two bands at 1309 cm-’ and 1064 cm-‘; these bands are the fundamental longitudinal optical (LO) mode and the transverse optical (TO) mode, respectively. Iron sulfides are ubiquitous in sulfide mineral deposits and the aim in mineral processing is generally to reject these species. The dimorphs of Fe!& the cubic form, pyrite, and the orthorhombic, marcasite, have somewhat different flotation properties, Sutherland and Wark, (1955). Figure 5 shows that Raman spectroscopy can readily distinguish between these two types of Fe&. Both dimorphs display two major bands, at 377 cm-’ and 342 cm-’ for pyrite, and at 384 cm-’ and at 321 cm-’ for marcasite. These values are close to those reported previously, Mernagh and Trudu, (1993). The similarity between the modes at the higher wavenumbers for these dimorphs led Lutz and Muller (1991) to conclude that they arise from the S-S stretching vibration of the disulfide group which exists in these crystal structures. The efficient separation of pentlandite from pyrrhotite is important in minimizing SOz emissions in the smelting operation. In the copper-nickel mines operated by Falconbridge and Into in the Sudbury basin, the first pyrrhotite rejection stage involves magnetic separation. Pyrrhotite exists with a range of iron/sulfur ratios and also with hexagonal and monoclinic crystal lattices. Only the former structure is magnetic and hence the monoclinic form proceeds entirely to the flotation stages. Raman spectroscopy does not provide
Raman microprobe
mineral identification
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the same definitive identification of these two forms of pyrrhotite, and there appears to be a significant degree of variation in the spectra obtained from the non-magnetic pyrrhotite ores we have examined. However, from our Raman examination of the samples we observe that, if a spectrum is obtained, the pyrrhotite is the non-magnetic variety, the form which is a major problem in the processing of pentlandite. The lack of Raman activity of magnetic pyrrhotite could explain why Mernagh and Trudu (1993) did not observe a spectrum from the two mineral samples they examined.
marcasite
00000000~~ 00000000 Q)lclnC)rQ) rrrrr
t-V)C)r
Raman shift I cm-’ Fig.5 Raman spectra from pyrite and marcasite, (left) 100 - 2000 cm-‘, (right) 280 - 460 cm’.
Mineral orientation For crystals with asymmetric geometry, the interaction of the laser radiation is dependent on the angle of incidence of the light beam in relation to the crystal lattice and the polarisation of the incident beam. Thus, the Raman spectrum obtained depends on the crystal orientation. The polarisation of the scattered radiation is an important result of the Raman effect for particular crystal structures. Figure 6 shows Raman spectra for rutile in which the laser (of fixed polarisation) is incident on the sample parallel to, and at right angles to, the c axis of the crystal. 1
anatase 1 rutiie
parallel --.rutile perpendicular
Fig.6
/n”“\.
L-I
Raman spectra of anatase and of rutile oriented with the crystal c-axis parallel and perpendicular to the laser beam.
G. A. Hope et al.
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The bands are at the same wavenumbers for both orientations since the crystal structure is the same. The dependence on orientation occurs in the case of rutile because of the asymmetry of the crystal structure. The mineral has a very high polarisability and exhibits strong dipole interactions. In the Raman spectrum the major bands are all derived from oxygen displacements, and correspond to the following vibrations of the rutile space group; Es 447 cm“, Ai, 612 cm-’ and BQ 826 cm-‘. The band positions, intensities and polarisation reflect the directional nature of the oxygen vibrations in the rutile lattice, Beattie and Gilson, (1969) where there is a considerable dependence of the stretching character on lattice direction. Figure 6 also shows the Raman spectrum from anatase, a polymorph of rutile. Both forms of titanium dioxide have a tetragonal crystal lattice, but with different axial ratios. Three bands are apparent for anatase and they occur at 639 cm-i, 516 cm-’ and 397 cm-i which readily enables identification of the two polymorphs. Sulfides Many sulfide minerals are Raman active and give characteristic spectra that can be used for identification. Sulfide structures can be considered as closed-packed sulfur atoms with the metal atoms arranged in the tetrahedral and octahedral holes. Group theory analysis of the lattice vibrations can be carried out to calculate Raman modes. A spectrum for natural sphalerite with the fundamental, combination and two phonon bands identified from the data reported by Krauzman, (1968) is shown in Fig. 7.
LO 34e
LA overtone
TO+TA
216
396 TO+LA
,
I
419
LO 2 phonon 668
I LO+TO
TO
637
, TO 2 phonon
n
TA
296 272
613
331
’
I
I
t
Raman shift / cm-’ Fig.7
Raman spectrum of natural sphalerite with band assignments; LO is longitudinal optical mode, TO is transverse optical mode, LA is longitudinal acoustic mode, 2 phonon denotes that two phonons are involved in the transition.
The sphalerite structure consists of a cubic closed packed lattice of sulfur atoms with zinc atoms situated in tetrahedral holes. Sphalerite has only one formula unit in the Bravais space cell and hence the first order spectrum should consist of only one mode. However, since there is no centre of inversion in sphalerite, the first order spectrum is split by the crystal field into a longitudinal optical (LO) mode, in which atomic displacements are oriented parallel to the wavevector of the incoming light, and a transverse optical (TO)
Raman microprobe mineral identification
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mode in which the atomic vibrations are perpendicular. The LO band for sphalerite in Fig. 7 appears at 348 cm-’ and the TO band at 272 cm-‘. Acoustic modes also exist at corresponding lower energy levels in both longitudinal (LA) and transverse (TA) modes. As expected from group theory, galena, with a symmetrical sodium chloride structure, does not produce a first order Raman spectrum. Adsorption of the energy of the laser radiation results in oxidation of this mineral to form lead oxysulfates which are Raman active. Shapter et al. (2000) suggested that the appearance of spectra for lead oxysulfates permit an easy identification of galena in complex mineral ore samples. Raman spectroscopy has also been used to identify elemental sulfur and metal polysulfides as products of oxidation of pyrite, Zhu et al., (1992) and of nickel matte and heazlewoodite, Zhu et al., (1997). Minor elements Most sulfide minerals contain minor elements incorporated in their crystal structure. Iron can be substituted for zinc in the sphalerite lattice up to -20 % and this results in a large increase in cell volume. The lattice parameter increases linearly with the content of iron and other substituted cations, such as manganese and cadmium, Craig and Scott, (1974). Figure 8 shows Raman spectra from pure zinc sulfide and from four natural sphalerites with a range of iron content.
lO%Fe
6 % Fe
E
-
5
2 % Fe
0 % Fe
650
550
:
450
350
250
Raman shift / cm-’ Fig.8 Raman spectra of sphalerites with different degrees of iron substitution.
G. A. Hope et nl.
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It can be seen from Fig. 8 that the Raman spectrum of sphalerite varies with its iron content. Iron is a strongly polarising atom and when it is incorporated (substituted) into the sphalerite lattice it modifies the vibrational properties of the crystal. Mernagh and Trudu (1993) reported that the presence of a small amount of iron in the structure of sphalerite does not affect the frequencies of the main sphalerite band, but enhances the intensity of the second-order spectrum. The spectra in Fig. 8 agree with this observation. However, the intensity of the second-order spectra is seen to diminish again when the iron content is increased to values 2 6 90. Mernagh and Trudu (1993) also noted that the TO mode became more intense when the mineral contained iron. This is also apparent in Fig. 8. Furthermore, the ratio of the intensities of the LO and TO bands in Fig. 8 can be seen to decrease continuously as the iron content is increased. Additional studies on a range of sphalerite surfaces, the iron content of which has been accurately from determined, are required to ascertain if the iron content of sphalerite could be determined measurements of this ratio. Gangue minerals In developing, optimizing, and monitoring mineral processing, the identity of gangue minerals is important, as well as that of the valuable components. Raman spectroscopy provides a means of identifying many minerals. Spectra from a number of important gangue minerals have already been presented in this work. Figure 9 presents Raman spectra from other silicate gangue minerals that occur in sulfide ores, viz., quartz, rhodonite and talc. It can be seen that these three gangue minerals give rise to characteristic spectra that can be used for their identification.
Raman shift / cm-’ Fig.9 Raman spectra of three common silicate gangue minerals: quartz, rhodonite and talc. Raman imaging The Raman spectrometer can be used for the analysis of extended areas in two formats. Firstly, point to point mapping, based on a grating selected band position can be applied using the programmable microscope stage and utilizing the autofocus computer routine. This provides relatively rapid information on the distribution of band intensity across the surface of a sample. Several characteristic bands can be selected to provide composite maps of (mineral) band intensity. The area analysed using mapping can be
Ramaomicroprobemineral identification
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large, but acquisition time is a direct function of the number of points sampled and experiment times can become rather long with large memory requirements.
The second technique for obtaining information on Raman band intensity for area of a sample is to use Raman imaging. In this technique the Raman scattered light is filtered using an appropriately tuned band pass filter and the light collected on the CCD camera chip. The camera then is acting as a Raman detector where each component in the array collects the Raman signal emanating from the relevant section of the sample. Figure 10a shows a digital camera white light image for a characteristic feature on the surface of a natural diamond. This optical image has the same scale in both x and y directions, and this is given by the length of the bar on the micrograph. In Fig. lob we present the Raman image of the same area of the sample. The collection time was 100 seconds. The Raman image of the diamond surface can be easily recognised as the same as the optical micrograph, however it has different x and y scaling, and is only derived from the diamond Raman emission band. The band intensity is dependent on the orientation of the surface and most of the physical structure appears in the intensity map. Regions of surface contamination appear black, and flaws or surface inclusions would also be reduced in intensity. Raman imaging generally requires strong Raman signals to be of practical use while mapping is generally more sensitive, but slower and more computer intensive.
(a)
(b) Fig. 10
Image of a natural diamond surface, (a) white light optical image, (b) the corresponding Raman filter image obtained at 1330 cm -I.
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G. A. Hope et ~1.
CONCLUSIONS The Raman microprobe provides a powerful approach to the identification of minerals and gangue materials in sulfide ores. It has a unique application in the rapid identification of samples comprised of light elements such as carbon, fluorine, boron, oxygen, and nitrogen. The technique is sensitive to the crystal structure of the sample and can provide useful information on the presence of various allotropes and polymorphs. In the case of carbonaceous materials, the Raman spectrum can determine its presence in concentrates and is capable of identifying the type of material from the appropriate spectral database. In conjunction with the optical microscope it can provide the mineralogist with rapid confirmation or identification of mineral species in prospecting samples and also processing plant fractions.
ACKNOWLEDGEMENTS The authors acknowledge microTechnology..
the support
of the Australian
Research
Grants
Scheme
and the CRC for
REFERENCES Beattie, I.R. and Gilson, T.R., Oxide phonon spectra. J. Chem. Sot. A, 1969,2322-2327. Craig, J.R. and Scott, S.D., Sulfide phase equilibria. In Sulfide Mineralogy, ed., P.H. Ribbe, Mineralogical Society of America, Washington, DC, 1974, pp. CS l-l 10. Gilkes, K.W.R., Sands, H.S., Batchelder, D.N., Robertson, J. and Milne, W.I., Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy. Appl. Phys. Left., 1997, 70,1980-1982.
Jian, Z., Jiaxiang,
from graphite complex and its mechanism. In Congress, Sydney, Australia, Vol 4, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 1993, pp. 949-952. Kalyan, D.K., Rao, T.K., Pradip, Rangarayan, V., Sharma, N.K., Gaur, R.K., and Kohad, V.P., Reduction of graphite in lead concentrate during flotation of a complex lead-zinc ore from Raypura-Dariba, India. In Proceedings of the XXI International Mineral Processing Congress, ed., P. Massacci, Elsevier, Amsterdam, 2000, p. B8a 138-145. Kiefert, L., Hanni, H.A., Chalain, J-P. and Weber, W., Identification of filler substances in emeralds by infrared and Raman spectroscopy, J. Gemmol., 1999,26,501-520. Krauzman, M., Spectre Raman du second ordre de la blende ZnS. C.R. Acad.Sci.Paris, Ser. B., 1968, 1224-1226. Lutz, H.D. and Muller, B., Lattice vibrational spectra, LXVIII. Single-crystal Raman spectra of marcasitetype iron chalcogenides and pnictides, FeXz (X=S, Se, Te; P, As, Sb). Phys. Chem. Miner., 1991, 14, Proceedings
D. and Lin C., Separating
of the XVIII
International
molybdenite
Mineral
Processing
265-268.
Mernagh, T.P. and Trudu, A.G., A laser Raman microprobe study of some geologically important sulphide minerals. Chemical Geology, 1993, 103, 113-127. Shapter. J.G., Brooker, M.H. and Skinner, W.M., Observation of the oxidation of galena using Raman Spectroscopy. International Journal of Mineral Processing, 2000,60, 199-211. Sutherland, K.L. and Wark I.W., Principles of Flotation, Australasian Institute of Mining and Metallurgy, Melbourne, Victoria, 1955. White, W.B., The anhydrous oxide minerals. In The infrared spectra of minerals, ed., V.C. Farmer, Mineralogical Society, London, 1974 pp. 227-284. Woodcock, J.T. and Hamilton, J.K., Australasian Mining and Metallurgy, The Sir Maurice Mawby Memorial Volume, Monograph No. 19, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 1993. Woods, R., Hope, G.A. and Watling, K, Surface enhanced Raman scattering spectroscopic studies of the adsorption of flotation collectors. Minerals Engineering, 2000, 13, 345-356. Zhu, X., Li, J., Bodily, D.M. and Wadsworth, M.E., Transpassive oxidation of pyrite. In Proceedings ofthe International Symposium on Electrochemistry in Mineral and Metal Processing Ill, eds. R. Woods and P.E. Richardson, PC 92-17, Electrochem. Sot., Pennington, NJ, 1992, pp.391-409.
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Zhu, X., Wadsworth, M.E. and Woods, R., Electrochemical kinetics of the anodic dissolution of nickel matte and synthetic Ni3S2. In Nickel-Cobalt 97, Volume I: Hydrometallurgy and Refining of Nickel and Cobalt (Eds., W.C. Cooper and I. Mihaylov) Canadian Institute of Metallurgy, Canada, 1997, pp. 153167.
Correspondence on papers published in Minerals Engineering is invited by e-mail to [email protected]
Mineruls Engineering, Vol. 14, No. 12, pp. 1565-1577, 2001 0 2001 Published by Elsevier Science Ltd All rights reserved 0892-6875/01/$ - see front matter
RAMAN MICROPROBE MINERAL IDENTIFICATION*
G.A. HOPEP*,R. WOODSY and C.G. MUNCE’ (j/School of Science, Griffith University, Nathan, Queensland 4111, Australia 3 Cooperative Research Centre for microTechnology, Australia E-mail: [email protected] (Received 1 May 2001; accepted 20 June 2001)
ABSTRACT Raman spectroscopy and associated imaging techniques are generally non-destructive, and can be used to identify a wide range of minerals and gemstones. Raman is a sensitive technique which requires minimal sample preparation and can be used on massive specimens from lump ore to fine powders and liquids. Since glass and water are weak Raman scatterers, spectra from minerals can be obtained from samples in air, through glass, andfor immersed in water. Examples of the application of Raman spectroscopy to minerals of importance in mineral processing are discussed. These include: detection of minerals of light elements such as carbon and fluorine; distinguishing between polymorphs such as those of iron sulfides; characterizing sulfides containing minor elements such as iron in sphalerite; and the identification of silicate, oxide, and carbonate gangue minerals. The Raman microprobe also permits Raman imaging and mapping of surfaces and inclusions. Imaging of diamond is presented as an example of this technique. 0 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords Mineral processing,
Ore mineralogy,
Particle morphology,
Sulfide ores.
INTRODUCTION The identification and characterization of minerals is of fundamental importance in the development and operation of mining and mineral processing systems. Traditionally, such information has been provided by the mineralogist using an optical microscope. This conventional approach has now been augmented by the introduction of a number of instrumental techniques that provide elemental analysis of the mineral species. Electron imaging systems are now highly automated to provide elemental mapping of a number of individual mineral grains, and sophisticated computer software developed for presenting the data in a useful format. In Australia, CSIRO’s QEM-Scan is well established and JKMRC’s MLA has recently been developed; the value of these systems to the mineral processor is the subject of other papers in this volume. Laser Raman micro-spectroscopy provides an additional, non-destructive analytical technique to assist the mineralogist in identifying minerals in assemblages. It has the advantage that the sample needs no more preparation than that for conventional optical microscopic examination and that spectra can be rapidly
* Presented at Applied Mineralogy ‘01, Brisbane, Australia, March 2001
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recorded. In a Raman spectrometer, the sample under examination is irradiated with an intense source of monochromatic radiation, usually in the visible region of the spectrum, and the scattered light is analyzed by the spectrometer. Most of the scattered light, the Rayleigh line, has the same frequency as the incident laser beam, but a small fraction, the Raman component, has shifted frequencies due to interaction of photons with the sample that result in changes in the vibrational or rotational energy of chemical bonds in molecules or crystals and in the vibration of crystal lattices. The main optical problem in Raman spectroscopy is the separation of the weak Raman signal from the Rayleigh line. This can be achieved through the use of notch filters which also provide excellent optical throughput so that the extremely low Raman signal intensity that is scattered from a tiny volume of a weakly-scattering mineral can be detected. Thus, modern confocal Raman microprobes have a lateral resolution of - 1 urn and a depth resolution of - 2 urn. Spectrometers are also able to use a programmable microscope stage, thus enabling imaging and mapping of larger areas of mineral specimens in the same manner as with SEMs and electron microprobes. The Raman effect occurs when the electric field due to the light interacts with the electrons in the sample and it is the derived polarisability of the sample molecules or crystal structure that determines the Raman activity. This derived polarisability is related to the directional character of chemical bonds in the sample, thus the connection to vibrational spectroscopy. Unlike techniques based on electron microscopy, Raman spectroscopy is not critically dependent on the atomic mass of the elements in the mineral. Thus, compounds containing light elements can be characterized, for example those containing carbon hydrogen, carbon - nitrogen, metal - fluorine and boron - oxygen. Also, polymorphs of the same chemical composition can be distinguished and the influence of minor elements incorporated into mineral lattices investigated. Glass and water are weak Raman scatterers and, accordingly, spectra are easy to obtain from solid materials through glass and/or immersed in water. For materials, which are transparent at the wavelength of the laser being used, confocal measurements can be made to obtain spectra from selected regions inside the mineral sample, thus enabling the characterization of inclusions and artefacts. Furthermore, Raman spectra can be derived from species in solution and hence dissolved species in flotation and leach liquors can be characterized as well as solid compounds. The non-destructive nature of Raman spectroscopy makes it an important technique for the gemmologist as well as the mineralogist. It can be used to distinguish expensive stones from synthetic analogues and to identify treatments made to enhance colour or to fill fissures, Kiefert et al., (1999). In this paper, a number of examples are discussed on the application mineral chemistry.
of Raman spectroscopy
to aspects of
EXPERIMENTAL Griffith University has two Renishaw Raman spectrometers that are used for mineral identification, research on mineral processing, and related studies. The Renishaw Raman Spectrograph 100 (Multi Channel Compact Raman Analyser) has a rotary encoded grating stage, and an internal two stage Peltier cooled (-70” C) CCD detector. The spectral resolution is 5 cm- ‘, and the wavenumber reproducibility is 0.1 cm“. The incident radiation is conveyed through a fibre optic Raman probe from a Spectra Physics Argon ion laser of 514 nm excitation, or a Renishaw laser of 633 nm. The Renishaw 100 system is horizontally aligned, and is close to ideal for spectroelectrochemistry which requires a long working distance and controlled gas atmosphere in situations where oxygen can influence the reaction process, Woods et al., (2000). The sampled volume for this system is approximately 50 urn”. The second instrument is a Renishaw RM2000 Raman spectrometer of high resolution that has a computercontrolled stage. This instrument uses a conventional Leica UV metallurgical microscope and is equipped with a range of objectives, both visible and ultraviolet. At maximum magnification, the system can sample a minimum volume of about 1 urn”, limited by the diffraction of light and, typically, it is operated at 5 um3. Four laser frequencies are available, the red and green described above and a Kimmon HeCd laser that
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radiation of both 325 and 448 nm. The instrument has a micron positionable programmable stage that can be used for mapping with any of the four laser frequencies and Raman imaging with 514 nm radiation.
provides
RESULTS AND DISCUSSION Light elements Carbon
The presence of naturally-floatable carbonaceous shales in ores results in complications in the flotation separation of sulfide minerals. Normally, the carbonaceous gangue is either removed in a prefloat, or a depressant (e.g., hydroxy ethyl cellulose) added to retain this material in the tails. Such procedures are carried out in the mines in the Mt Isa Inlier, where carbonaceous gangue occurs in association with pyrite, Woodcock and Hamilton, (1993). Carbons also need special treatment in the flotation of complex lead-zinc deposits in India, Kalyan et al., (2000) and of copper-moly ores in China, Jian et al., (1993). Raman spectroscopy can assist in investigating such ores since it can characterize carbon species and identify carbon in sulfide mineral assemblages. Figure 1 shows Raman spectra from different allotropes of carbon and of coal. Crystalline diamond is an excellent Raman scatterer and has a single phonon band at 1332 cm-‘. In the case of graphite, the sp2 bonded carbon spectrum obtained using 5 14 nm radiation is resonance enhanced, and displays two bands, the major band is the E2s fundamental transition and is generally referred to as the G peak. This is the large 1580 cm-’ band in the graphite spectrum of figure 1. The polycrystalline graphite used to obtain this spectrum also exhibits another band at 1360 cm-’ referred to as the D peak. In studies of the formation of diamond-like carbon, Gilkes et al. (1997) were able to identify sp’ carbon in the presence of the sp2 carbon, by using UV excitation. The tetrahedral carbon in their deposited films could not be clearly identified using 5 14 nm excitation due to the greater Raman cross section of the sp2 D peak at this wavelength, even though the sp3 species comprised 80% of the deposit.
ZnS concentrate
\
graphite diamond
8 co Q Ei 8 8 F
r
0 r
CD
Raman shift / cm-’
Fig. 1 Raman spectra from diamond, graphite, coal, and a zinc concentrate containing carbonaceous gangue.
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The spectrum for black coal presented in figure 1 displays the bands characteristic of sp* bonded carbon. The poor crystallinity of this sample is reflected in the relative size of the D peak to the G peak which increases with disorder. The carbon bands are broad as expected from a composite graphitic material such as natural coal. Figure 1 also presents a spectrum from a zinc cleaner concentrate derived from the Mt Isa Inlier. The presence of carbonaceous gangue in this concentrate is clearly evident from the appearance of bands at 1610 cm-’ and - 1360 cm-‘. There are no bands displayed by sphalerite itself in this region (v.i.). The bands at wavenumbers below 720 cm-r in the spectrum arise from the sphalerite in the concentrate. Solution species Raman spectroscopy can also determine carbonate in flotation pulps as well as identify carbonate gangue minerals. Figure 2 compares a Raman spectrum from dissolved carbonate with those from calcite and cerussite.
carbonate solution
cerussite
Raman shift / cm-’ Fig.2
Raman spectra from calcite, synthetic cerussite, a dilute potassium and a zinc concentrate slurry containing carbonate in the pulp.
carbonate
aqueous
solution,
The spectrum for calcite shows an intense band at 1098 cm-’ arising from the symmetric stretching vibration of the carbonate group. There are also weak bands arising from other carbonate vibrations, the assymetric stretch near 1400 cm-’ and the in-plane bending mode at 700 cm-‘. The strong band at 300 cm-’ is due to the lattice vibrations of the calcite crystal. Calcite is an important optically active crystal and has been extensively studied. The internal vibrational modes have been calculated by a number of methods and the data has been discussed by White (1974). The spectrum from a synthetic cerussite in Fig. 2 is similar to that for calcite, even though this compound crystallizes with the aragonite structure. This is primarily due to the Raman active bands emanating from the carbonate. The major band positions are shifted, the one corresponding to the symmetric stretching vibration of the carbonate group is shifted by - 44 cm-’ to 1054 cm-‘. The aragonite structure has a lower
Ramao microprobe mineral identification
than that of calcite, and this results in the splitting of the cerussite assymetric near 1440 cm-’ into two bands which have increased intensity compared to calcite. symmetry
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carbonate
stretch
The solution spectrum of saturated potassium carbonate gives the carbonate ion spectrum which is similar to that obtained from the solid. However, the spectrum derived from a dilute solution of potassium carbonate displays a broad band at 1097 cm-’ due to the stretching vibration of the carbonate group. It is much broader than that obtained from the crystalline species as a result of a range of interactions of carbonate ions with water. Weak, broad bands are also evident from other carbonate vibrations at wavenumbers similar to the carbonate minerals. The broad band at 571 cm-’ is probably due to the formation of some bicarbonate or bicarbonate-like hydrogen bonding in the carbonate solution. Figure 2 also shows a spectrum obtained from a slurry of a zinc concentrate derived from the Mt Isa Inlier. The bands characteristic of dissolved carbonate/bicarbonate are apparent as well as those from lattice vibrations in sphalerite. Fluorine Fluorine impurities in metal sulfide concentrates cause problems in smelting since they result in the formation of hydrofluoric acid that damages smelter vessels as well as introducing environmental concerns. The two major sources of fluorine in the concentrate are the common fluoride minerals such as fluorite and apatite, and silicates and aluminosilicates containing fluoride substitutions such as talc and chlorite. It is possible to remove minerals such as talc and apatite during flotation, but fluorite and fluorine-containing chlorite are difficult to separate from the metal sulfide values. It is often necessary to leach concentrates to remove fluorine before smelting; for example, such a procedure is carried out on both the lead and zinc concentrates at the BHP Cannington mine in Queensland. Fluorite and chlorite give distinctive Raman spectra, as shown in Fig. 3. The chlorite spectrum in this figure was obtained from an inclusion within a large natural quartz crystal utilizing the confocal properties of the Raman spectrometer. Figure 3 also shows a sample of sphalerite containing fluorite is instantly recognizable from the spectrum.
fluorite from the Mt Isa Inlier. The presence
of the
sphalerite containing fluorite
Fig.3 Raman spectra from chlorite, fluorite, and a specimen of zinc sulfide in ore containing
fluorite.
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Polymorphs Boron nitride is not a mineral found in nature, but is included here to demonstrate the ability to identify a compound composed of light elements, and to distinguish between different polymorphs. White BN is isomorphous with graphite, whereas the black form has a similar structure to diamond. Figure 4 shows Raman spectra of these two polymorphs.
Fig.4 Raman spectra from cubic and hexagonal boron nitride. The spectrum from the hexagonal polymorph shows a single band at - 1365 cm-’ which is similar to the graphite homologue. The cubic sphalerite (diamond-like) form gives rise to two bands at 1309 cm-’ and 1064 cm-‘; these bands are the fundamental longitudinal optical (LO) mode and the transverse optical (TO) mode, respectively. Iron sulfides are ubiquitous in sulfide mineral deposits and the aim in mineral processing is generally to reject these species. The dimorphs of Fe!& the cubic form, pyrite, and the orthorhombic, marcasite, have somewhat different flotation properties, Sutherland and Wark, (1955). Figure 5 shows that Raman spectroscopy can readily distinguish between these two types of Fe&. Both dimorphs display two major bands, at 377 cm-’ and 342 cm-’ for pyrite, and at 384 cm-’ and at 321 cm-’ for marcasite. These values are close to those reported previously, Mernagh and Trudu, (1993). The similarity between the modes at the higher wavenumbers for these dimorphs led Lutz and Muller (1991) to conclude that they arise from the S-S stretching vibration of the disulfide group which exists in these crystal structures. The efficient separation of pentlandite from pyrrhotite is important in minimizing SOz emissions in the smelting operation. In the copper-nickel mines operated by Falconbridge and Into in the Sudbury basin, the first pyrrhotite rejection stage involves magnetic separation. Pyrrhotite exists with a range of iron/sulfur ratios and also with hexagonal and monoclinic crystal lattices. Only the former structure is magnetic and hence the monoclinic form proceeds entirely to the flotation stages. Raman spectroscopy does not provide
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the same definitive identification of these two forms of pyrrhotite, and there appears to be a significant degree of variation in the spectra obtained from the non-magnetic pyrrhotite ores we have examined. However, from our Raman examination of the samples we observe that, if a spectrum is obtained, the pyrrhotite is the non-magnetic variety, the form which is a major problem in the processing of pentlandite. The lack of Raman activity of magnetic pyrrhotite could explain why Mernagh and Trudu (1993) did not observe a spectrum from the two mineral samples they examined.
marcasite
00000000~~ 00000000 Q)lclnC)rQ) rrrrr
t-V)C)r
Raman shift I cm-’ Fig.5 Raman spectra from pyrite and marcasite, (left) 100 - 2000 cm-‘, (right) 280 - 460 cm’.
Mineral orientation For crystals with asymmetric geometry, the interaction of the laser radiation is dependent on the angle of incidence of the light beam in relation to the crystal lattice and the polarisation of the incident beam. Thus, the Raman spectrum obtained depends on the crystal orientation. The polarisation of the scattered radiation is an important result of the Raman effect for particular crystal structures. Figure 6 shows Raman spectra for rutile in which the laser (of fixed polarisation) is incident on the sample parallel to, and at right angles to, the c axis of the crystal. 1
anatase 1 rutiie
parallel --.rutile perpendicular
Fig.6
/n”“\.
L-I
Raman spectra of anatase and of rutile oriented with the crystal c-axis parallel and perpendicular to the laser beam.
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The bands are at the same wavenumbers for both orientations since the crystal structure is the same. The dependence on orientation occurs in the case of rutile because of the asymmetry of the crystal structure. The mineral has a very high polarisability and exhibits strong dipole interactions. In the Raman spectrum the major bands are all derived from oxygen displacements, and correspond to the following vibrations of the rutile space group; Es 447 cm“, Ai, 612 cm-’ and BQ 826 cm-‘. The band positions, intensities and polarisation reflect the directional nature of the oxygen vibrations in the rutile lattice, Beattie and Gilson, (1969) where there is a considerable dependence of the stretching character on lattice direction. Figure 6 also shows the Raman spectrum from anatase, a polymorph of rutile. Both forms of titanium dioxide have a tetragonal crystal lattice, but with different axial ratios. Three bands are apparent for anatase and they occur at 639 cm-i, 516 cm-’ and 397 cm-i which readily enables identification of the two polymorphs. Sulfides Many sulfide minerals are Raman active and give characteristic spectra that can be used for identification. Sulfide structures can be considered as closed-packed sulfur atoms with the metal atoms arranged in the tetrahedral and octahedral holes. Group theory analysis of the lattice vibrations can be carried out to calculate Raman modes. A spectrum for natural sphalerite with the fundamental, combination and two phonon bands identified from the data reported by Krauzman, (1968) is shown in Fig. 7.
LO 34e
LA overtone
TO+TA
216
396 TO+LA
,
I
419
LO 2 phonon 668
I LO+TO
TO
637
, TO 2 phonon
n
TA
296 272
613
331
’
I
I
t
Raman shift / cm-’ Fig.7
Raman spectrum of natural sphalerite with band assignments; LO is longitudinal optical mode, TO is transverse optical mode, LA is longitudinal acoustic mode, 2 phonon denotes that two phonons are involved in the transition.
The sphalerite structure consists of a cubic closed packed lattice of sulfur atoms with zinc atoms situated in tetrahedral holes. Sphalerite has only one formula unit in the Bravais space cell and hence the first order spectrum should consist of only one mode. However, since there is no centre of inversion in sphalerite, the first order spectrum is split by the crystal field into a longitudinal optical (LO) mode, in which atomic displacements are oriented parallel to the wavevector of the incoming light, and a transverse optical (TO)
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mode in which the atomic vibrations are perpendicular. The LO band for sphalerite in Fig. 7 appears at 348 cm-’ and the TO band at 272 cm-‘. Acoustic modes also exist at corresponding lower energy levels in both longitudinal (LA) and transverse (TA) modes. As expected from group theory, galena, with a symmetrical sodium chloride structure, does not produce a first order Raman spectrum. Adsorption of the energy of the laser radiation results in oxidation of this mineral to form lead oxysulfates which are Raman active. Shapter et al. (2000) suggested that the appearance of spectra for lead oxysulfates permit an easy identification of galena in complex mineral ore samples. Raman spectroscopy has also been used to identify elemental sulfur and metal polysulfides as products of oxidation of pyrite, Zhu et al., (1992) and of nickel matte and heazlewoodite, Zhu et al., (1997). Minor elements Most sulfide minerals contain minor elements incorporated in their crystal structure. Iron can be substituted for zinc in the sphalerite lattice up to -20 % and this results in a large increase in cell volume. The lattice parameter increases linearly with the content of iron and other substituted cations, such as manganese and cadmium, Craig and Scott, (1974). Figure 8 shows Raman spectra from pure zinc sulfide and from four natural sphalerites with a range of iron content.
lO%Fe
6 % Fe
E
-
5
2 % Fe
0 % Fe
650
550
:
450
350
250
Raman shift / cm-’ Fig.8 Raman spectra of sphalerites with different degrees of iron substitution.
G. A. Hope et nl.
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It can be seen from Fig. 8 that the Raman spectrum of sphalerite varies with its iron content. Iron is a strongly polarising atom and when it is incorporated (substituted) into the sphalerite lattice it modifies the vibrational properties of the crystal. Mernagh and Trudu (1993) reported that the presence of a small amount of iron in the structure of sphalerite does not affect the frequencies of the main sphalerite band, but enhances the intensity of the second-order spectrum. The spectra in Fig. 8 agree with this observation. However, the intensity of the second-order spectra is seen to diminish again when the iron content is increased to values 2 6 90. Mernagh and Trudu (1993) also noted that the TO mode became more intense when the mineral contained iron. This is also apparent in Fig. 8. Furthermore, the ratio of the intensities of the LO and TO bands in Fig. 8 can be seen to decrease continuously as the iron content is increased. Additional studies on a range of sphalerite surfaces, the iron content of which has been accurately from determined, are required to ascertain if the iron content of sphalerite could be determined measurements of this ratio. Gangue minerals In developing, optimizing, and monitoring mineral processing, the identity of gangue minerals is important, as well as that of the valuable components. Raman spectroscopy provides a means of identifying many minerals. Spectra from a number of important gangue minerals have already been presented in this work. Figure 9 presents Raman spectra from other silicate gangue minerals that occur in sulfide ores, viz., quartz, rhodonite and talc. It can be seen that these three gangue minerals give rise to characteristic spectra that can be used for their identification.
Raman shift / cm-’ Fig.9 Raman spectra of three common silicate gangue minerals: quartz, rhodonite and talc. Raman imaging The Raman spectrometer can be used for the analysis of extended areas in two formats. Firstly, point to point mapping, based on a grating selected band position can be applied using the programmable microscope stage and utilizing the autofocus computer routine. This provides relatively rapid information on the distribution of band intensity across the surface of a sample. Several characteristic bands can be selected to provide composite maps of (mineral) band intensity. The area analysed using mapping can be
Ramaomicroprobemineral identification
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large, but acquisition time is a direct function of the number of points sampled and experiment times can become rather long with large memory requirements.
The second technique for obtaining information on Raman band intensity for area of a sample is to use Raman imaging. In this technique the Raman scattered light is filtered using an appropriately tuned band pass filter and the light collected on the CCD camera chip. The camera then is acting as a Raman detector where each component in the array collects the Raman signal emanating from the relevant section of the sample. Figure 10a shows a digital camera white light image for a characteristic feature on the surface of a natural diamond. This optical image has the same scale in both x and y directions, and this is given by the length of the bar on the micrograph. In Fig. lob we present the Raman image of the same area of the sample. The collection time was 100 seconds. The Raman image of the diamond surface can be easily recognised as the same as the optical micrograph, however it has different x and y scaling, and is only derived from the diamond Raman emission band. The band intensity is dependent on the orientation of the surface and most of the physical structure appears in the intensity map. Regions of surface contamination appear black, and flaws or surface inclusions would also be reduced in intensity. Raman imaging generally requires strong Raman signals to be of practical use while mapping is generally more sensitive, but slower and more computer intensive.
(a)
(b) Fig. 10
Image of a natural diamond surface, (a) white light optical image, (b) the corresponding Raman filter image obtained at 1330 cm -I.
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G. A. Hope et ~1.
CONCLUSIONS The Raman microprobe provides a powerful approach to the identification of minerals and gangue materials in sulfide ores. It has a unique application in the rapid identification of samples comprised of light elements such as carbon, fluorine, boron, oxygen, and nitrogen. The technique is sensitive to the crystal structure of the sample and can provide useful information on the presence of various allotropes and polymorphs. In the case of carbonaceous materials, the Raman spectrum can determine its presence in concentrates and is capable of identifying the type of material from the appropriate spectral database. In conjunction with the optical microscope it can provide the mineralogist with rapid confirmation or identification of mineral species in prospecting samples and also processing plant fractions.
ACKNOWLEDGEMENTS The authors acknowledge microTechnology..
the support
of the Australian
Research
Grants
Scheme
and the CRC for
REFERENCES Beattie, I.R. and Gilson, T.R., Oxide phonon spectra. J. Chem. Sot. A, 1969,2322-2327. Craig, J.R. and Scott, S.D., Sulfide phase equilibria. In Sulfide Mineralogy, ed., P.H. Ribbe, Mineralogical Society of America, Washington, DC, 1974, pp. CS l-l 10. Gilkes, K.W.R., Sands, H.S., Batchelder, D.N., Robertson, J. and Milne, W.I., Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy. Appl. Phys. Left., 1997, 70,1980-1982.
Jian, Z., Jiaxiang,
from graphite complex and its mechanism. In Congress, Sydney, Australia, Vol 4, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 1993, pp. 949-952. Kalyan, D.K., Rao, T.K., Pradip, Rangarayan, V., Sharma, N.K., Gaur, R.K., and Kohad, V.P., Reduction of graphite in lead concentrate during flotation of a complex lead-zinc ore from Raypura-Dariba, India. In Proceedings of the XXI International Mineral Processing Congress, ed., P. Massacci, Elsevier, Amsterdam, 2000, p. B8a 138-145. Kiefert, L., Hanni, H.A., Chalain, J-P. and Weber, W., Identification of filler substances in emeralds by infrared and Raman spectroscopy, J. Gemmol., 1999,26,501-520. Krauzman, M., Spectre Raman du second ordre de la blende ZnS. C.R. Acad.Sci.Paris, Ser. B., 1968, 1224-1226. Lutz, H.D. and Muller, B., Lattice vibrational spectra, LXVIII. Single-crystal Raman spectra of marcasitetype iron chalcogenides and pnictides, FeXz (X=S, Se, Te; P, As, Sb). Phys. Chem. Miner., 1991, 14, Proceedings
D. and Lin C., Separating
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Mernagh, T.P. and Trudu, A.G., A laser Raman microprobe study of some geologically important sulphide minerals. Chemical Geology, 1993, 103, 113-127. Shapter. J.G., Brooker, M.H. and Skinner, W.M., Observation of the oxidation of galena using Raman Spectroscopy. International Journal of Mineral Processing, 2000,60, 199-211. Sutherland, K.L. and Wark I.W., Principles of Flotation, Australasian Institute of Mining and Metallurgy, Melbourne, Victoria, 1955. White, W.B., The anhydrous oxide minerals. In The infrared spectra of minerals, ed., V.C. Farmer, Mineralogical Society, London, 1974 pp. 227-284. Woodcock, J.T. and Hamilton, J.K., Australasian Mining and Metallurgy, The Sir Maurice Mawby Memorial Volume, Monograph No. 19, Australasian Institute of Mining and Metallurgy, Parkville, Victoria, 1993. Woods, R., Hope, G.A. and Watling, K, Surface enhanced Raman scattering spectroscopic studies of the adsorption of flotation collectors. Minerals Engineering, 2000, 13, 345-356. Zhu, X., Li, J., Bodily, D.M. and Wadsworth, M.E., Transpassive oxidation of pyrite. In Proceedings ofthe International Symposium on Electrochemistry in Mineral and Metal Processing Ill, eds. R. Woods and P.E. Richardson, PC 92-17, Electrochem. Sot., Pennington, NJ, 1992, pp.391-409.
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Zhu, X., Wadsworth, M.E. and Woods, R., Electrochemical kinetics of the anodic dissolution of nickel matte and synthetic Ni3S2. In Nickel-Cobalt 97, Volume I: Hydrometallurgy and Refining of Nickel and Cobalt (Eds., W.C. Cooper and I. Mihaylov) Canadian Institute of Metallurgy, Canada, 1997, pp. 153167.
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