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Laser Raman Spectroscopy & its applications

Laser Raman spectroscopy uses a monochromatic laser to interact with molecular vibrational modes in a sample, shifting the laser energy down (Stokes) or up (anti-Stokes) through inelastic scattering. It is most useful for fingerprinting chemical species of which minerals and vein forming fluids are of immediate relevance to all geological investigations and researches. Although Laser Raman spectroscopy (LRS) is most sensitive to covalent bonds with little or no natural dipole moment, as in carbon compounds, it also effectively identifies partially covalent bonds with dipole moments which characterize most known minerals. However, the system, by its basic principle, is ineffective for chemical compounds or minerals that have purely electrovalent or metallic structure.

A Renishaw in Via Reflex Laser Raman Microscopies operative at the Central Petrological Laboratory, Geological Survey of India. It is a high-sensitivity system with integrated research grade microscope, enabling high-resolution confocal measurements and supports multiple lasers (540 nm and 785 nm), with automatic software switching of excitation wavelength. The other components of the spectroscope include holographic notch filter (to eliminate the elastic or Rayleigh scatter), lens systems, gratings for resolution and a charge- coupled device (CCD. The Raman spectrum collected consists of a sequence of peaks in a wave number vs intensity diagram. Each peak is associated with a different bond in the molecule. This pattern, unique to each substance, is searched against an established library database and the best correlated sample/mineral/fluid is considered the most likely match.

Renishaw in Via Reflex Laser Raman Internal principle of LRS microscope Raman Spectroscopic profile for olivine Microscope

Sample Preparation for LRS

 Laser Raman Spectroscope can easily and effectively operate on a flat surface of a mineral. Such a flat surface includes a cleavage plane or crystal face of a single crystal; a wafer, a cut and polished surface as in gemstone, a polished block of rock (sufficiently small so as to fit on the stage of the microscope) or a thin section.  A thin polished section or a polished section can give excellent results.  The sample should not be covered (as with a cover slip) or coated (like gold or carbon coating as done in SEM or EPMA).  If the sample has already been subjected to study under SEM or EPMA and there exists a layer of coating on the sample, then for best results the coating has to be removed not simply by mechanical rubbing with organic solvent but rather more effectively by repolishing.  Also carbonate samples should not be Alizarin treated as the dye has its own strong LRS signature.

Procedure for LRS booking

 GSI officers from Regional Laboratories or from all State Units, who are interested in using the LRS, are requested to send a letter (through proper channel) to the Director, Central Petrological Laboratories, CHQ requesting a time slot.  The numbers of samples and the type of samples needs to be mentioned in the letter.  Also required is the e-mail and Fax contacts of the officer concerned.  The allotted time slot will be sent to the officer via both e-mail and Fax.  Also it is always preferred that the officer be personally present during the study of his/ her samples.  The knowledge of the officer on his / her specific requirements, textural controls etc. help in both speed and accuracy of the analyses and also helps in minimizing office expenditure.

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Applications of LRS in Geological Investigations

A. Identification of minerals:

Raman Spectroscope is an easy, cost effective and non destructive application for identification of mineral phases that cannot easily be identified under microscope. The following are only a few examples of minerals that can be successfully identified with the help of LRS. i. Minerals in Bauxite and Laterite :

Bauxite is a heterogeneous naturally occurring material with principal constituent minerals gibbsite (Al2O3.3H2O), boehmite (Al2O3.H2O or γ-AlO(OH)) and diaspore (α-AlO(OH)), the later having the same composition as boehmite, but is denser and harder. These are mixed with the two iron oxides goethite and haematite, the clay mineral kaolinite and small amounts of anatase TiO2. These phases are often difficult to identify separately in hand specimen or under microscope. EPMA cannot effectively separate them as they are all compositionally very similar. However, Laser Raman Spectroscopy, which depends on bond structure and is independent of mineral composition, has been shown to have distinct signatures for each of these phases and thus can be effectively used in phase identification.

Garnierite group of minerals, the Ni-Mg-bearing hydrous phyllosilicates found associated with Ni- laterite, include micron scale serpentine, talc, chlorite, smectite, sepiolite etc. Each of these phases can be identified by their separate LRS signatures ii. Iron Ore minerals:

The iron oxide minerals like magnetite (Fe3O4), hematite (Fe2O3), goethite(FeO(OH)), maghemite (Fe2O3, γ-Fe2O3), kenomagnetite (tetrahedral Fe3O4)are all structurally different and can be easily identified by their distinct LRS signatures. iii. Carbonate phases :

Carbonates provide a wealth of information about sedimentary environments and ancient climates, as well as being important in the formation of oil reservoirs; cement industry etc. The mineralogy of ooids is usually either calcite or aragonite, which correspond to climatic greenhouse and icehouse conditions: being able to distinguish the two is therefore important in interpreting the environmental significance of a particular rock formation.

Carbonate minerals such as calcite, dolomite and aragonite have similar optical properties and distinguishing them is often difficult. Alizarin Red test can successfully demarcate some phases but fail to distinguish the others. Optical techniques also require the preparation of thin sections which is time-consuming and consequently expensive. Methods such as X-ray diffraction lack spatial resolution (in most cases being restricted to examination of crushed rock or single crystals extracted from a sample) and electron microscopy techniques require polished and carbon- coated surfaces and is still not useful for calcite and aragonite as these two phases are polymorphs and therefore cannot be distinguished by compositional dataalone.

Laser Raman Spectroscope has distinct and well identified set of peak positions for each of these carbonate minerals and can hence be effectively applied for mineral identification in small limestone blocks and thin sections. iv. Sulphides

Sulfides (chalcopyrite, bornite, sphalerite, pyrite, marcasite, cubanite, pentlanadite, covellite,arsenopyrite) are weaker Raman scatters compared to silicates, carbonates and sulfate minerals. They have distinctive Raman bands in the ∼300–500 Δcm−1 region. Raman spectra from these mineral species are very consistent in band position and normalized band intensity albeit rather overlapping. A few sulphides, such as galena and pyrrhotite, do not exhibit a first-order Raman spectrum due to their structural symmetry and Raman spectra could not be obtained from some of the metal-excess groups of sulphides because of their many metallic characteristics. However, good quality Raman spectra are possible from most sulphides and sulphosalts. However, the sulphide phases have distinct optical properties and can be easily identified through reflected light petrography. v. Tin and Tungsten minerals

The tin-tungsten minerals can be identified through petrographic study. But LRS provides an easier approach to mineral identification confirmation. The two wolframite end members ferberite (FeWO) and hubnerite (MnWO) have nearly identical structure and therefore identical peak positions in LRS. However studied signatures for scheelite

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(CaWO), stolzite (PbWO) and tungstite (WO8.H2O) are available and they vary sufficiently in details so as to allow LRS to identify these phases separately.

Cassiterite (SnO2), stannite (Cu2FeSnS4), the rare mineral hemusite (Cu6SnMoS8) andkesterite (Cu2.04(Zn0.84Fe0.18)Sn1.01S4) have their distinct LRS signatures. However considerable degree of overlap between signatures of cernite (Cu2CdSnS4) and stannite make their distinction impossible under LRS. vi. Manganese minerals

Manganese oxides, oxyhydroxides, carbonates and silicates constitute a large family of minerals. A few examples include manganosite (MnO), hausmannite (Mn2+Mn3+2O4), pyrolusite (MnO2),bixbyite (Mn2O3), ramsdellite (Mn4+O2), jacobsite (Mn2+Fe3+2O4), hollandite (Ba(Mn4+6Mn3+2)O16), manganite (Mn3+O(OH)), allactite (Mn7(AsO4)2(OH)8), birnessite (Mn-oxyhydroxide),romanechite ((Ba,H2O)2(Mn4+,Mn3+)5O10), braunite (Mn2+Mn3+6SiO12), neltnerite (CaMn3+6O8(SiO4)), rhodochrosite (MnCO3) etc.These minerals have very similar optical properties which makes it extremely difficult to separate them through reflectance and transmission microscopy. Also due to large variations in oxidation states of manganese among the oxide minerals, identification of such phases with electron microprobe may not always be reliable.However, there exists extensive LRS data on the peak shifts depending on these mineral structures. Therefore LRS is the most reasonable and cost effective approach for manganese mineral identification. vii. Manganese nodules / Polymetallic nodules

Polymetallic nodules, also called manganese nodules were discovered at the end of the 19th century in the Kara Sea, in the Arctic Ocean off Siberia (1868). During the scientific expeditions of the H.M.S. Challenger (1872–76), they were found to occur in most oceans of the world. The important constituents of polymetallic nodules include todorokite [(Na,Ca,K,Ba,Sr)1-x(Mn,Mg,Al)6O12·3-4H2O], whichis a complex hydrous manganese oxide mineral,vernadite [(Mn4+,Fe3+,Ca,Na)(O,OH)2·nH2O] and buserite (Na4Mn14O27·21H2O). These phases are frequently present as alternate micron scale discontinuous lamellae. LRS can prove be a method for identification of such constituent phases as each of these minerals have separate set of Stoke’s shift values. viii. Aluminosilicate polymorphs

The aluminosilicate polymorphs,kyanite, sillimanite and andalusite,have distinct optical properties and crystal habits and are thus easily recognisable in transmitted light microscopy. However, as it often happens in medium to low grade pelites, later alteration or retrograde metamorphism often leads to partial to complete replacement of the aluminosilicate phases. At times, only patches of the original minerals are preserved within lower grade phyllosilicate aggregates. Under such circumstances, identification of the aluminosilicate concerned becomes difficult with straight forward petrographic approach. But proper identification of the Al2SiO5 polymorph is absolutely essential both for understanding the geothermobarometric conditions and for determination of beneficiation processes. EPMA study is no alternative since the concerned phases are polymorphs and have same chemical formula. However, LRS gives distinct signatures for each of these phases and can thus be used for their identification in problematic situations. ix. Carbon allotropes

There are a wide variety of different carbon nanostructures, however they all have a few basic things in common. First, all of these materials are predominantly made up of pure carbon, and as such can be called carbon allotropes. The range of these materials starts with the well known allotropes of diamond and graphite, and continues on to encompass fullerenes, graphene and more complex structures such as carbon nanotubes that re finding increasingly important applications in a variety of industries. From a molecular perspective, these materials are all entirely composed of C-C bonds, although the orientation of these bonds is different in the different materials and therefore, to characterize their molecular structure in a meaningful manner, it is necessary to have a technique which is highly sensitive to even slight changes in orientation of C-C bonds.

For diamond, where the material consists of highly uniform C-C bonds in a tetrahedral crystal structure, the Raman spectrum is very simple. It consists of only a single band at 1332 cm-1 because all of the bonds in the crystal are of the same orientation and strength resulting in a single vibrational frequency. In contrast, the small crystal size of nanocrystalline diamond results in a finite-size effect in which the lattice is somewhat distorted. This is manifested in the Raman spectrum by a slightly downshifted tetrahedral sp3 band. The additional band at 1620 cm-1 and the shoulders on the 1620 cm-1 and tetrahedral sp3 band are also indicative of sp2 bonded carbon that represents surface defect modes, which would be insignificant in larger diamond crystals. Finally, the very broad band around 500 cm-1 is indicative of some amorphous sp3 bonded carbon.

Thegraphite spectrum has several bands in the spectrum and the main band has shifted from 1332 cm-1 in diamond to 1580-82 cm-1 in graphite. The reason for this is that graphite is composed of sp2 bonded carbon in planar sheets in which the bond energy of the sp2 bonds is higher than the sp3 bonds of diamond.

When comparing Raman spectra of graphene and graphite, at first glance the spectra look very similar. This is not too surprising as graphite is just stacked graphene. However, there are some significant differences,the most obvious being that the band at 2690-2700 cm-1, which is known as the G' band, is much more intense than the G band

NCEGR, GSI, KOLKATA in graphene compared to graphite. You may have heard the G' band referred to as the 2D band; both 2D and G' are accepted names for this band.

The main feature in the C60 fullerenespectrum is a relatively sharp line at around 1462 cm-1, known as the pentagonal pinch mode. In contrast, the spectrum of C70 fullereneis littered with numerous bands. This is due to a reduction in molecular symmetry which results in more Raman bands being active.

Carbon nanotubes (SWCNT, DWCNT and their superset MWCNT) are cylindrical carbon tubes having distinct LRS signatures. x. Gemstones

Raman spectroscopy is particularly useful in identifying and characterizing gemstones that cannot be subjected to cutting, polishing and EPMA studies. It has been used to distinguish between gemstone and its simulant, to identify gems that are set in jewellery / objects of historical importance, to make some estimation on gem composition from comparing acquired data with available experimental data, to identify inclusions within gems, to characterize fluid inclusions in gems and to understand nature of treatments in some treated gems. Some experimental works on possible differences in LRS signatures between natural and synthetic gems are being presently carried out.

B. Identification of vein forming fluids through fluid inclusion study

Laser Raman Spectroscope can be ideally used, in conjunction with heating cooling stage for fluid inclusion study, to identify and characterize the fluid inclusions in minerals. Fluid inclusions are microscopic entrapments of liquid and gas within crystals. As ore minerals often form from a liquid or in an aqueous medium, tiny blebs of that liquid can become trapped within the crystal structure or in healed fractures within a crystal. These small inclusions range in size from 0.1 to 1 mm and are usually only visible in detail by microscopic study. Fluids play an important role in geological processes and fluid inclusion study helps in understanding of ore genetic processes.

Laser Raman Spectroscope can penetrate through a transparent crystal / wafer into the trapped fluid inclusion. Such inclusions can be monophase, biphase or polyphase. The individual phases within the inclusion can be identified by focussing the laser beam on each of them separately and receiving the corresponding signatures. The knowledge about the nature of ore forming fluids and the trapped salts and resultant salinity can be used successfully for ore genetic interpretation. C. Identification of Inclusions in transparent gem variety minerals

The LRS is a completely non-destructive process. The laser beam can penetrate a transparent mineral without causing any harm to its structure or composition. Thus the laser is used to penetrate gem variety minerals / crystals and collect spectrum from mineral inclusions inside the gemstone. Such data can be used for mineral inclusion identification from inside gemstones which on its turn helps in understanding of gem genesis and therefore further gemstone prospecting.

D. Shocked Minerals

LRS peak position, FWHM, symmetricity etc. depend on molecular vibration, which on its turn, is affected by lattice deformation. Thus LRS has been successfully used for study of shock metamorphic polymorphs in meteorites and impact craters.

(Subrata Chakraborti) Director, CPL