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Raman spectroscopy of carbon and solid bitumens in sedimentary and metamorphic rocks
Spectrochimica Acta Part A 59 (2003) 2341 /2352 www.elsevier.com/locate/saa
Raman spectroscopy of carbon and solid bitumens in sedimentary and metamorphic rocks Jan Jehlicˇka a,*, Ondrˇej Urban a, Jan Pokorny´ b a
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 12843 Prague 2, Czech Republic b Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8, Czech Republic Received 15 June 2002; accepted 15 August 2002
Abstract Different types of carbonaceous matter from rocks display Raman spectral features which knowledge permits to obtain structural information of these materials. Application of Raman microspectroscopy to investigate kerogen, bitumen, fossils, highly carbonified amorphous carbon as well as graphite from different environments is reviewed. Differences in Raman spectra and structural differences between carbonaceous samples differing in their metamorphic history are discussed on the basis of new data. # 2003 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Carbon; Solid bitumen; Kerogen; Graphite
The chemical and isotopic composition and structural and microtextural features represent characteristic fingerprints of the organic matter in rocks. These data are indicative for the origin of
the organic matter. In addition, major information could be obtained concerning ulterior processes of transformation of carbonaceous matter. Similar data are very useful in the field of manufacture of carbonaceous materials. In this paper we present a review of applications of Raman spectroscopy to investigate natural carbonaceous matter from rocks. This technique is useful especially for:
* Corresponding author. Tel.: /420-221-95-1503; fax: / 420-221-95-1496. E-mail addresses: [email protected] (J. Jehlicˇka), [email protected] (J. Pokorny´).
/ determination of structural order of carbonaceous matter, / estimation of graphite crystallite size, / estimation of the presence of structural defects of carbonaceous matter.
1. Introduction 1.1. Why Raman spectroscopy
1386-1425/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00077-5
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These information are readily obtained for carbonaceous matter from different types of rocks, from sedimentary and especially from metamorphic rocks. Knowledge of specific minerals, rocks, of geological units is then completed and more complex reconstruction of geological processes is allowed. Few methods can be used to investigate the structural state of such carbons. Some of them are more sensitive to determine functional groups of mixtures of aliphatic and aromatic compounds of low carbonification grade (IR spectroscopy, NMR spectrometry). Others are more sensitive to investigate the carbon skeleton and the biperiodic / triperiodic structural transition. Only few analytical methods are able to characterize the structural order at the atomic scale (X-ray diffratometry, Raman microspectrometry and Transmission Electron Microscopy) and the microtexture i.e. mutual orientation in space of structural units (Transmission Electron Microscopy).
1.2. Raman spectrum of carbon The first scattering studies of graphite have been done by Tuinstra and Koenig [1]. Later, different authors modified the original peak assignments by these authors and for example Dresselhaus and Dresselhaus [2] and Ferrari and Robertson [3] summarize and review other interpretations of graphite Raman spectra. The Raman spectrum of crystalline graphite consists of a single strong first order line at 1582 cm 1 (designated as the G band) due to the E2g2 vibrational mode with weaker band at 42 cm 1 (E2g1) which is not resolvable from Rayleigh scattering. In the second-order spectrum of graphite the doublet G1? and G2? is situated at /2695 and /2735 cm 1 (S peak) and weak bands at /2450 and /3248 cm 1 [4]. The second-order Raman bands are assigned both to overtone scattering (2 /1360/2735 cm 1, the most intense, 2 /1620/3248 cm 1 a weak but sharp peak) and combination scattering (1620/830 /2450 cm 1, 1580/1355 /2950 cm 1).
For disordered carbons additional bands appear at /1355 cm 1 (D), /1620 cm 1 (D?) and / 2940 cm 1 (Dƒ). The second-order Raman bands at /2695 and /2735 cm 1 merge into a single symmetrical band G? band at /2710 cm 1 which is considerably broadened with increasing structural disorder. 1.3. Raman spectroscopy of artificial carbonaceous phases In 1985, Be´ny-Bassez and Rouzaud [4] published an important paper where results of structural investigation by Raman microspectrometry of four reference carbon types during high-temperature treatments (HTT) under inert gas were presented. Precursor artificial materials included anthracene, thin and thick carbon films and saccharose. These carbonaceous compounds are representatives of so-called graphitizing and nongraphitizing carbons. Using Raman microspectrometry it was possible to evidence different types of defects and to follow their variations during HTT. The evolution of carbonaceous materials throughout artificial HTT (carbonization and graphitization) is due to the elimination more or less rapid and more or less complete of different types of defects. This elimination allows the progressive rearrangement of the BSU. On the basis of these studies and using detailed sampling of carbonaceous matter of different metamorphic series, knowledge on the evolution of Raman spectra of carbonaceous matter has been progressively obtained. In this manner, combining such data also with other methods of structural investigation (XRD, TEM) changes at atomic and molecular level of natural carbons from rocks have been described. The in-plane graphite crystallite size (L) of polycrystalline materials can be deduced by comparing the intensity of the E2g mode at 1582 cm1 (IG) to the intensity of the disorder induced band at 1350 cm 1 (ID) L4:4IG =ID (in nm) Knight and White [5]. The effect of grain orientation on Raman spectrum has been discussed recently [6,7].
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The intensity IG/ID ratio depends not only on the degree of graphitization but also on the scattering geometry. Differences are observed for graphitic planes oriented in parallel or in perpendicular to the polarization orientation of the exciting laser beam. The effects of orientation on the Raman spectra require a precaution during the interpretation of spectra of carbonaceous samples. The first order Raman band of diamond appears at 1332 cm 1 which corresponds to the T2g zone center active mode [8]. Second-order Raman peaks are very weak, they appear at 2148, 2468 and 2653 cm 1 and correspond to overtones and combination bands [5,9].
2. Carbonaceous matter in sedimentary and metamorphic rocks In this part, some general features of the evolution of organic matter in the geological cycle are reviewed. We present most significant representatives of organic matter-rich sedimentary rocks. The carbonaceous matter of these environments could be transformed latter by metamorphic processes and its sampling and study permit us to decipher conditions of these transformations. It is useful to note that combination of geological processes in the long Earth history was rather complex. Studies of carbonaceous matter transformation especially from older geological units is then rather heavy deal. 2.1. Black shales Black shales and oil shales are organic rich laminates which formed in anoxic environments. Black shales contain a significant siliciclastic component, while inorganic carbonates may or may not be present. In oil shales, inorganic carbonates are dominant in addition to kerogen. Black shales are finely grained sedimentary rocks characterized by elevated content in organic carbon. Their black color could be caused, in some cases, by only 0.5% of Corg, in general organic carbon content varies between 0.5 and 10%. The major constituents of black shales include clay minerals, organic matter, quartz grains and even-
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tually other grains. The precursors of these rocks are products of marine sedimentation under euxinic conditions, under low content in oxygen and high organic matter input. High rate of clay sedimentation is necessary to permit the preservation of the organic matter. In such a general conditions the oxidation/destruction of the organic matter is prohibited (low oxygen content, quiet environment, rapid cover by fine clays) [10].
2.2. Coal Coal is a readily combustible organoclastic sedimentary rock composed mainly of lithified plant remains and containing more than 50% by weight of carbonaceous matter and inherent moisture. Coal represents accumulations of plant tissues in different states of preservation. Unaltered sedimentary material containing plant remains change progressively (these changes include physical, biochemical and chemical processes) during diagenesis and catagenesis. Series of coals of increased rank are then obtained, differing in structural and elemental properties. Humic coals pass through a peat stage at the site where plants grew. They are stratified, banded and contain lustrous brown /black material derived from the evolution of woody tissues. Sapropelic coals are not stratified macroscopically and are transformed during diagenesis as other organic rich sediments. They contain various amount of allochtonous organic and mineral material and also autochtonous sedimentary material including algae and spores. Macerals are basic organic constituents of coal that can be recognized under optical microscope. They are coalified remnants of various plant parts and tissues. Macerals differ from each other in their morphology, optical properties, chemical composition and hardness. The diagenetic transformation of peat into coal is called coalification, carbonification or organic maturation. Peats are changed into lignites, subbituminous, high, medium and low volatile bituminous coals, semi-anthracites and anthracites by the conjugated role of temperature and pressure. The rank of a coal depends upon the maximum
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temperature to which the coal was exposed during its geological history. This corresponds upon the maximum depth to which it has subsided into the crust. 2.3. Petroleum During diagenesis, there is continuous evolution of kerogen, reflecting increasing temperature and pressure during subsidence of sediments. Different functional groups are eliminated from buried organic material and numerous new compounds are generated. Low to medium molecular weight hydrocarbons are important constituents of such a mixture. These compounds are generated by the chemical and structural transformations of kerogen. During so-called primary migration they are transported through fine grained source rocks. The secondary migration include processes of transport and is terminated by collection of hydrocarbons in permeable porous reservoir rocks that represent a trap. Such rocks include clastic reservoir rocks-sandstones, siltstones and carbonates. Natural petroleum is a characteristic product of the diagenetic evolution s.l. of accumulations of organic matter [11]. 2.4. Organic matter evolution Most sediments, during deposition contain a mixture of inorganic detrital mineral grains, finegrained amorphous mineral constituents, organic matter and also living organisms. Depending upon the depositional environment, the porosity of the system is variously wetted and oxygenated. Such a system is extremely unstable. The organic matter is an effective reducing agent and is brought into intimate contact with fine-grained minerals. There is a tendency for organic matter to be oxidized by fine-grained minerals (sesquioxides of Fe and Al) and dissolved oxidizing agents. Two extreme situations can be imaged. In highly oxidizing continental or marine conditions (with slow rate of deposition) the organic matter may be oxidized to CO2 and H2O by aerobic bacteria. By contrast, rapid burial in a marine environment with reducing conditions on the interface between water and
sediment few centimeters deeper, permits the protection of the organic matter. The elemental composition of the organic matter-kerogen and coal changes during diagenesismaturation. Kerogen is defined as dispersed organic matter from sedimentary and metasedimentary rocks or as the insoluble part of the organic matter of rocks. The insolubility means here the insolubility in water, organic solvents, hydroxides and mineral acids (strong oxygen containing acids excluded). During sedimentary evolution the composition of each kerogen type change and there is a decrease in oxygen and hydrogen content. By high degree of alteration there is a possibility that the organic matter is converted to 100% of carbon. The diagenetic transformation of organic matter includes changes in higher plant accumulationspeats and in sediments occurring rather in marine environments in shales, characterized, in general, by the majority of allochtonous organic matter. This later case includes kerogen transformations and eventually the appearance of hydrocarbons. The diagenesis of organic matter s.l. is divisible into four stages. During eogenesis the role of pressure and temperature are subordinate and especially biochemical processes take place. This stage is also denominated as biochemical coalification. Biomolecules as lipids (fats, waxes, and steryl esters), carbohydrates (sugars, cellulose and chitin), proteins and lignins from different plant parts display different susceptibility to biochemical transformation. During this stage biopolymers depolymerize and their products react to form so called geopolymers: humic compounds and kerogen. These compounds are characterized by low solubility and high molecular weight. Humic compounds are soluble in alkali hydroxides (fulvic acids soluble at all pH, humic acids soluble in pH higher than 2), kerogen is insoluble in water, inorganic acids and organic solvents. An important product of this stage of diagenesis is gas, mainly methane. This biogenic gas forms locally economic accumulations both in coal seams and in surrounding sedimentary rocks. An other stage of diagenesis /catagenesis starts at temperatures about 40 /150 8C and pressures 3 / 15 MPa. During this stage coal rank change from
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sub-bituminous to anthracite. Coals and kerogen progressively increase in carbon content and decrease in volatiles. Aliphatic chains are cleaved and aromatic structures continue to increase in volume through condensation. The reflectance of vitrinite increases from about 0.2 to 2%. The amount of liquid hydrocarbon generated depends on the amount of liptinite. Significant amount of methane and carbon dioxide is liberated at this stage. Coalbed methane starts to be accumulated in coal strata. Liquid hydrocarbons are liberated from kerogen in this stage during socalled oil window. During the third stage of organic matter transformation, during metagenesis the structural ordering of the carbonaceous matter becomes higher. The dimension of aromatic clusters increases and methane are produced by the breaking of C /C bonds. Aliphatic molecules and hydrocarbons are cracked to methane. The reflectance of vitrinite increases from about 2 to 10%. The fourth stage of transformation is metamorphism, which is considered as a stage during which especially higher pressure and temperatures are reached and graphite or diamond could be seen as typical products.
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with increasing metamorphic grade [7,12/18]. However, the genesis of some Precambrian graphite deposits (on Sri Lanka, for example) is not clearly explained. It can be stressed-out that during regional metamorphic processes, the structure of carbonaceous matter improves resulting in graphite as end product. Low to moderate temperature conditions (200 /600 8C) superimposed to moderate to high pressures (5 /10 kbar) are required for graphite formation [17,19].
2.5.1. Regional metamorphism
2.5.1.2. Bitumens. Otherwise, when migration processes occur, mixtures of hydrocarbons, bitumens are formed in sedimentary or metasedimentary rocks or even on hydrothermal mineral deposits. These bitumens are first liquid, later, as influenced by different alteration processes (fluid migration, oxidation, thermal alteration) they can solidify. Their accumulations occur frequently in mineral or rock cracks and fissures, forming sometime vein type accumulations. Solid bitumens soluble in carbon disulfide are dessigned as bitumens s.s., those remaining insoluble as pyrobitumens [20,21]. Highly carbonized infusible solid bitumens occurring in mineral deposits are named anthraxolite, when it contains uranium and thorium, accompanying uranium mineralizations, it is designed as thucholite. Their transformation during metamorphic processes are different [22]. We will use the name solid bitumen as general term for transformed accumulated solid bitumens in sedimentary and metamorphic rocks. We will reserve the term asphaltic-like solid bitumen (ALSB) to identify solid bitumens characterized by low carbonization degree (as gilsonites have). On the contrary, hard solid bitumen (HSB) designates solid bitumens with low H/C ratio (cata-impsonites, for example).
2.5.1.1. Kerogens. Kerogens from different sedimentary rocks can be transformed during regional metamorphic processes to diverse polyaromatic pre-graphitic materials, graphitoids and in some conditions into graphite. The composition and the structure of many pre-graphitic materials occurring in regionally metamorphic rocks is relatively well-known, it has been shown that carbon becomes better organized into the graphite structure
2.5.2. Contact metamorphism It has been observed previously, that the transformation of carbonaceous matter during contact metamorphism could be different. This type of metamorphism is characterized, in general, by higher temperatures (up to about 1000 8C) depending on the type of magma and the dimension of volcanic body and low pressures comparing with regional metamorphism. Generally, oriented
2.5. Metamorphism Metamorphic conditions are necessary to transform carbonaceous residue into graphitoids (nonperfectly crystallized carbonaceous matter) or graphite. Elevated oriented pressure is especially necessary in addition to high temperatures to produce perfectly crystalline carbon modification-graphite.
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stress is much lower comparing with regional metamorphism. Such a metamorphism of carbonaceous matter has been described in places, where volcanic rocks intrude coal seams or black shale complexes [23 /25].
2.5.3. Raman spectroscopy of natural samples Numerous studies deal with the characterization of the structural ordering during natural process of graphitization. Raman spectroscopy has been applied previously to investigate coals, kerogens, or fossils, and more successfully other geological materials especially from metamorphic rocks [7,16,25 /30]. It is now well established that Raman spectrum of carbonaceous matter is sensitive to the degree of disorder or crystallinity of samples ranging from kerogens and coals to graphitoids and graphites from highly metamorphic environments. Numerous studies have demonstrated the usefulness of combining XRD and TEM with Raman microspectrometry [23 /25]. As noted previously, based on Raman calibrations established by [1,4] it was possible to access to estimates of La for any given carbonaceous matter. It was demonstrated that these characteristics of the carbonaceous matter evolution are independent on the type of the rock [7]. There is a possibility to deduce the degree of metamorphism and degree of crystallinity of carbonaceous matter of a metapelite from the La value derived from indirect Raman spectroscopic determination. Authors pointed-out that during the metamorphic evolution of carbonaceous matter the graphitization process is frequently identical for different marine or nonmarine precursor materials. This is especially well observable from the stage when three-dimensional crystalline order is established. Here, we present new results obtained from different metamorphic series of the Bohemian massif. We compare the impact of specific conditions of metamorphism on Raman spectra. Because of Precambrian age, these carbons have similar precursor organic matter. However, their actual structural state is different and reflects the role of pressure and temperature conditions of different metamorphic episodes.
3. Experimental 3.1. Samples 3.1.1. Highly carbonified kerogens The series of dispersed CM includes kerogens from black shales and schists from Zbecˇno, Klecany and Prachovice. In this study we focused on two Neoproterozoic localities, Klecany and Zbecˇno, which belong to volcano-sedimentary complex of Barrandian area, part of Bohemian massif (Czech republic). The primary sedimentary sequences of Barrandian area have been regionally metamorphosed by Cadomian orogeny and subsequent Variscan foldings (between 970 and 570 million years). The intensity of the regional metamorphism increases to the west. Zbecˇno belongs to the eastern area of the Bohemian massif, where shales are weakly regionally and contactly metamorphosed up to chlorite metamorphic zone. At Klecany the Proterozoic metasedimentary complex consists mostly of graywackes weakly regionally and contactly metamorphosed. As proposed by Fediuk (1993), the granitoid intrusion caused strong contact metamorphism of the sedimentary complex, reaching up to biotite metamorphic zone. Prachovice is situated in tectonically deformed Paleozoic part of ˇ elezne´ hory. Regional metamorphism belongs to Z Silurian period and the sedimentary sequence of black shales is metamorphosed in staurolite zone. 3.1.2. Graphites Lipnice n.S. is located in central massif of the Moldanubian pluton. Samples were collected from the erlane xenolites present in granite body of Melechov massif. Petrographically, variscian lateorogenic muscovite /biotite granites of Eisgarn type are predominant. Another similar epigenetic type of graphite was collected from Prˇibyslavice pegmatites. 3.1.3. Bitumens This group of samples represents highly carbonized and accumulated forms of organic matter of Precambrian age. At Klecany situated in eastern part of Neoproterozoic volcano /sedimentary
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complex, black and brittle solid bitumen (anthraxolite) was collected from mineralized hydrotermal veins formed during postmagmatic alteration of adjacent metasedimentary black shales caused by Hosˇtice granitoid body. Some of solid bitumens (SB) contain microscopic inclusions of uraniferous minerals. In contrary another sample of studied SB was present in the form of accumulation in the contact aureole of Neoproterozoic basaltic body.
3.2. Isolation procedures Kerogen was isolated from investigated shales and schists by dissolution of silicates and quartz. Rock powders were attacked first by 1 N HCl (for 3 h, at 80 8C), washed by distilled water and dissolved by a mixture of HCl/HF (1:3), (for 3 h, at 80 8C) and washed. This procedure was repeated twice. Powders were used for analysis. Investigation of solid bitumens and graphites has been realized on crude samples. Grains of about 3/5 mm were used for analysis. Due to the isotropic character of solid bitumens there was any defined orientation of the sample to the incident laser beam. For graphites, perpendicular orientation of 002 planes to the incident laser beam was used.
3.3. Instrument Micro-Raman analyses were performed on a multichannel Renishaw 2000 spectrometer coupled with a Peltier cooled CCD detector. Excitation was provided by the 514.5 nm line of a continuouswave 10 mW Ar-ion laser. The samples were scanned from 1000 to 3500 cm 1 at a spectral resolution of about 2 cm 1. The scanning parameter for each Raman spectrum was taken as 10 s and ten scans were summed for experimental run to provide a better signal-tonoise ratio. Multiple spot analyses on different areas of the same sample of CM provided similar spectra.
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4. Results 4.1. Raman spectroscopy of dispersed carbonaceous forms 4.1.1. Kerogens-graphitoids Raman spectra obtained on CM of kerogens from progressively metamorphosed black shales of the Barrandian area differ considerably (Fig. 1a). The evolution of structure is connected to the intensity of metamorphism. Dispersed CM from Zbecˇno has a lower degree of structural organization from series of studied kerogens, resulted from metamorphic conditions of chlorite metamorphic zone. Raman spectra show pronounced absorptions at about 1350 and 1580 cm 1. In numerically fitted Raman spectrum additional defect bands at 1217 and 1539 cm 1 are also present. Characteristic D1/G area ratio is around 1.79 and secondorder spectrum contains non intense S1 band. Dispersed CM from Klecany and Prachovice reveals a relatively high structural organization, similar to semigraphite The Raman spectra show pronounced absorptions at about 1350 and 1580 cm 1, but in fitted Raman spectra additional defect band at 1618 cm 1 is also observed. Characteristic area ratio D1/G is about 1.70 and 1.20 for Klecany and Prachovice sample, respectively. In the second-order spectrum of these CM intense S1-peak at around 2700 cm 1 and two weak overtones at 2930 and 3220 are present. The main second-order peak (S1 peak) is symmetric, i.e. no doublet can be resolved. High D1/G intensity ratio (0.80) and the absence of other D3 and D4 defect bands of the Prachovice CM confirm the highest degree of structural organization observed within the studied kerogens, related to staurolite metamorphic zone. 4.2. Raman spectroscopy of accumulated carbonaceous forms 4.2.1. Graphites Raman spectra of well crystallized graphite have been reported previously from many metamorphic units of the Bohemian massif especially from the Moldanubian of Precambrian age, Tepla´-Barrandian of Neoproterozoic age. Triperiodic structural
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Fig. 1. First and second-order Raman spectra of carbonaceous matter. (a) kerogens from Zbecˇno, Klecany and Prachovice, in order of increasing regional metamorphism; (b) graphite from Prˇibyslavice and Lipnice n. Sa´z. (c) Bitumen from Zbecˇno and Klecany.
order is documented by well-resolved first and second-order Raman spectra (Fig. 1b) with intense and sharp bands (example from Prˇibyslavice and Lipnice) (Table 1). Other examples from the Bohemian massif include graphites from highly metamorphic systems, from eclogite (Marianske´ la´zneˇ), granulite (Klet), erlane (Lipnice) and pegmatite.
Triperiodic structural order is documented by well resolved first and second-order Raman spectra. The results of Raman microspectroscopy analyses of graphite samples from Lipnice n.S. and Prachovice are presented on Fig. 1(b). First order Raman spectrum of both natural graphite samples contains a sharp and intense graphitic band at 1580 cm 1. Weak residual band D1 at
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Fig. 1 (Continued)
1351 cm 1 remains visible and characteristic parameters of these samples tabulated as D1/G intensity ratio valued around 0.15 /0.17 and D1/G area ratio around 0.27 /0.30. In the second-order spectrum sharp band S1 at about 2700 cm 1 is present and additional weak bands occur at 2951 and 3246 cm 1. 4.2.2. Solid bitumens Two different types of solid bitumens were studied (Fig. 1c). Raman spectrum of solid bitumen from Klecany indicates a very weak structural organization. The G band appears as a wide band at around 1600 cm 1. Characteristic D1/G area ratio and also intensity ratio are relatively low (1.42 and 0.69, respectively), which is caused by the defect bands D2 and D3 for the most part. The half-width ratio D1/G is high (about 2.1) and obtained second-order spectrum is not well developed. Raman spectrum of solid bitumen from Zbecˇno is quite similar to CM of adjacent kerogens (Fig. 1a). The crude Raman spectrum contains characteristic defect bands at about 1210, 1340 and 1530 cm1 The second-order spectrum bands of SB assigned as overtones are present at frequencies 2674, 2930 and 3140 cm 1. The area ratio D1/G is
relatively high (1.80), compared with SB from Klecany which is probably caused by remobilization processes. Structural evolution of SB from Zbecˇno, is in close correlation to precursor organic matter of bituminous character. This is similar to the case of shungite precursor from Precambrian metasedimentary complex of Karelia (Russia), previously studied by [22,29].
5. Discussion 5.1. Kerogen versus bitumen Raman spectra obtained on kerogens from progressively metamorphic series of Barrandian area differ considerably from spectra obtained on SB. Selected Raman spectra of kerogens from metasedimentary rocks from the Bohemian massif are reported on Fig. 1. This figure includes highly evolved kerogen samples from the eastern part of the Barrandian Neoproterozoic. They display slightly lower structural organization when compared with samples coming from higher metamorphic zones from the western part of this series [23].
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Table 1 Parameters of Raman spectra obtained from carbonaceous matter from Neoproterozoic units of the Bohemian massif (cm 1) Sample
Band D4 /1220
Band D1 /1350
Band D3 /1500
Band G /1580
Band D2 /1620
D1/G area
Position Intensity Width Position Intensity Width Position Intensity Width Position Intensity Width Position Intensity Width K
a
Zbecˇno Klecany Prachovice
1217 / /
189 / /
103 / /
1349 1354 1354
2964 3158 2326
79 58 46
1539 / /
278 / /
78 / /
1603 1587 1583
3089 2542 2882
42 42 30
/ 1618 1616
/ 714 658
/ 24 23
0.961 1.242 0.807
1.889 1.380 1.566
1.790 1.702 1.256
Lipnice n.S. Prˇibyslavice
/ /
/ /
/ /
1352 1352
658 539
30 38
/ /
/ /
/ /
1580 1580
3668 3548
17 21
1620 1622
161 116
9 9
0.179 0.151
1.728 1.813
0.309 0.274
SB a Zbecˇno Klecany
1207 1276
208 674
90 167
1341 1370
2331 1849
92 153
1530 1519
256 828
75 116
1601 1595
2993 2668
39 73
/ /
/ /
/ /
0.778 0.693
2.350 2.101
1.802 1.421
Sample
Band G?
Ga
Band /2950
Band /3240
Position Intensity Width Position Intensity Width Position Intensity Width Position Intensity Width K
Zbecˇno Klecany Prachovice
2691 2706 2708
424 1522 1514
201 73 78
/ / /
/ / /
/ / /
2937 2943 2946
644 493 407
144 109 83
3203 3228 3235
238 126 153
66 68 59
G
Lipnice n.S. 2689 Prˇibyslavice 2670
997 1208
56 63
2726 2730
1634 1311
37 38
2952 2950
323 189
101 79
3246 3250
338 318
23 25
SB
Zbecˇno Klecany
274 578
202 267
/ /
/ /
/ /
2925 2929
464 782
165 26
3200 3142
204 312
70 154
a
2674 2699
K, kerogen; G, graphite; SB, solid bitumen.
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D1/G D1/G intensity width
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The evolution of Raman characteristics is strongly connected to the intensity of metamorphism. It corresponds to the structural improvement of carbonaceous matter as visualized by TEM. For the less metamorphosed samples (eastern part, pumpellyite zone), the peak width of both Raman bands is about 60 /70 cm 1 and mean D/O ratio 1.3. When the metamorphic degree increases, the peak width of both the bands is considerably narrower. In the chlorite metamorphic zone (western part), the D/O ratio decreases to about 0.8. Major decrease of the D peak is observed on carbonaceous matter from biotite zone. A sharp O peak (width about 30 cm 1) dominates the first order spectrum and the D/O ratio is now only 0.4. However, weak band at about 1350 cm 1 is still present in the spectrum confirming remaining structural defects. In conclusion, changes in the Raman spectra of progressively regionally metamorphosed kerogens concern progressive disappearence of the 1350 cm 1 band, the diminution of the peak width of the E2g Raman band and the progressive decrease of the frequency of the E2g peak, from about 1590 cm 1 for low organized kerogens, to 1575 /1580 cm 1 for graphite. The macroscopic appearance as well as structural parameters obtained on solid bitumens from Klecany, HSBs from Mı´tov and shungites from Karelia by TEM, XRD and Raman spectroscopy are very similar to artificial glassy carbon. Such carbons prepared by a slow pyrolysis of furfuryl alcohol precursor, for example, display similar dimension of micropores as other samples of shungites (2 /10 nm). Their micro to mesoporosity is closed which is the major reason of their gas and liquid impermeability. However, Raman spectra of these samples differ considerably comparing to ALSBs or tar-like compounds [25]. The higher structural arrangement observed on kerogens and graphitoids comparing to SB is probably a result of different metamorphic processes (contact vs. regional). In the case of SB higher temperatures are to be expected for contact metamorphism than for regional one and lower pressures are in general considered for this type of metamorphism. In the absence of strong anisotropic constrains, related to tectonic pressures,
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only small Molecular Orientation Domains (MOD) are formed without preferential orientations. By contrast, in regional metamorphic processes (in the range of metamorphic zones investigated), temperatures can be included in the range 100/500 8C, but important anisotropic pressure can occur. In this case, characteristic preferential orientations of MOD are observed by TEM, due to the flattening of pores and progressive evolution toward graphitic lamellae. Similar graphitization process have been proposed for the evolution of anthracites during metamorphism [17,25]. When carbonaceous matter was only submitted to thermal effect of contact metamorphism, structural and microtextural evolutions are limited. Only weak evolutions were detected by Raman microspectrometry. The decrease of the width of the bands is in relation with the weak microtextural improvement, due to the release of different types of defects. When the carbonaceous matter evolved under the coupled effect of temperature and oriented pressure during regional metamorphism, graphitization develops as seen by TEM. Strong evolution of Raman spectra are brought out in this case (decrease of the band width, disappearance of the defect band). Such graphitization seems emphasized when carbon phases are in dispersed form between mineral lamellar particles. This dilution begins in the sedimentary rock (clay minerals) and can be found also in metamorphic equivalents (micas). In higher metamorphic zones, lamellar organo-mineral composites are formed. The effect of anisotropic pressure on such carbonaceous matter is in this case enhanced and graphite lamellae are frequently found as intergrowths with biotite.
6. Conclusions Raman spectroscopy reveals structural differences between samples of carbonaceous matter originating from similar precursor organic matter, which has been transformed by different metamorphic processes.
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Acknowledgements The work was supported by Grant Agency of the Charles University project Nr. 204/2000 B GEO. We are grateful for this funding.
References [1] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [2] M.S. Dresselhaus, G. Dresselhaus, in: M. Cardona, G. Guntherodt (Eds.), Light Scattering in Solids III, Springer, Berlin, 1982, p. 187. [3] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2002) 14095. [4] C. Be´nny-Bassez, J.-N. Rouzaud, Scanning Electron Microsc. 1 (1985) 119. [5] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [6] A. Wang, P. Dhamelincourt, J. Dubessy, D. Guerard, P. Landais, M. Lelaurain, Carbon 27 (1989) 218. [7] B. Wopenka, J.D. Pasteris, Am. Mineral. 78 (1993) 533. [8] R.P. Vidano, D.B. Fischbach, J.L. Wilis, T.M. Loehr, Solid State Commun. 39 (1981) 341. [9] S.A. Solin, Phys. Rev. Lett. 53 (1984) 2517. [10] J. Jehlicˇka, in: C.P. Marshall, R.W. Fairbridge (Eds.), Encyclopedia of Geochemistry, Kluwer Academic, Dordrecht, Boston, London, 1999. [11] B. Durand, G. Nicaise, Kerogen geochemistry, in: B. Durand (Ed.), Kerogen (Chapter 8), Technip, Paris, 1980, p. 36 (Chapter 8). [12] C.A. Landis, Contrib. Mineral. Petrol. 30 (1971) 34. [13] E.S. Grew, J. Geol. 82 (1974) 50.
[14] P.R. Buseck, B.-J. Huang, B. Miner, Org. Geochem. 12 (1988) 221. [15] G.-F. Wang, Lithos 22 (1989) 305. [16] J. Jehlicˇka, J.-N. Rouzaud, Org. Geochem. 16 (1990) 865. [17] M. Bonijoly, M. Oberlin, A. Oberlin, Int. J. Coal Geol. 1 (1982) 283. [18] O. Beyssac, J.-N. Rouzaud, B. Goffe´, F. Brunet, C. Chopin, Contrib. Mineral. Petrol. 143 (2002) 19. [19] A. Oberlin, Carbon 22 (1984) 521. [20] H. Jakob, Erdo¨l Kohle 6 (1967) 393. [21] H. Jacob, in: J. Parnell, P. Kucha, P. Landais (Eds.), Bitumens in Ore Deposits, Springer, Berlin, 1999, p. 53. [22] J. Jehlicˇka, Composition and structure of solid bitumens from the Prˇ´ıbram hydrothermal deposit, in: J. Pasˇava, K. ˇ a´k, B. Krˇ´ıbek (Eds.), Mineral Deposits: from their Origin Z to Environmental Impacts, Balkema, Rotterdam, 1995, p. 753. [23] J. Jehlicˇka, C. Be´ny, J.-N. Rouzaud, J. Raman Spectrosc. 28 (1997) 717. [24] B. Kwiecinska, Prace Mineralogiczne 9 (1967) 1. [25] J. Jehlicˇka, C. Be´ny, J. Mol. Struct. 480 (1999) 541. [26] O. Beyssac, J.N. Rouzaud, B. Goffe´, Contrib. Mineral. Petrol. 143 (2002) 19. [27] J.W. Valley, J.R. O’Neil, Geochim. Cosmochim. Acta 45 (1981) 411. [28] S. Duber, J.-N. Rouzaud, C. Blanche, C. Be´ny, D. Dumas, Extended Abstract of 21st Biennal Conference on Carbon, ACS, Buffalo, 1993, p. 316. [29] V.V. Kovalevski, P.R. Buseck, J.M. Cowley, Carbon 39 (2001) 243. [30] B. Krˇ´ıbek, J. Hrabal, P. Landais, J. Metamorph. Geol. 12 (1994) 493.
Raman spectroscopy of carbon and solid bitumens in sedimentary and metamorphic rocks Jan Jehlicˇka a,*, Ondrˇej Urban a, Jan Pokorny´ b a
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 12843 Prague 2, Czech Republic b Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8, Czech Republic Received 15 June 2002; accepted 15 August 2002
Abstract Different types of carbonaceous matter from rocks display Raman spectral features which knowledge permits to obtain structural information of these materials. Application of Raman microspectroscopy to investigate kerogen, bitumen, fossils, highly carbonified amorphous carbon as well as graphite from different environments is reviewed. Differences in Raman spectra and structural differences between carbonaceous samples differing in their metamorphic history are discussed on the basis of new data. # 2003 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Carbon; Solid bitumen; Kerogen; Graphite
The chemical and isotopic composition and structural and microtextural features represent characteristic fingerprints of the organic matter in rocks. These data are indicative for the origin of
the organic matter. In addition, major information could be obtained concerning ulterior processes of transformation of carbonaceous matter. Similar data are very useful in the field of manufacture of carbonaceous materials. In this paper we present a review of applications of Raman spectroscopy to investigate natural carbonaceous matter from rocks. This technique is useful especially for:
* Corresponding author. Tel.: /420-221-95-1503; fax: / 420-221-95-1496. E-mail addresses: [email protected] (J. Jehlicˇka), [email protected] (J. Pokorny´).
/ determination of structural order of carbonaceous matter, / estimation of graphite crystallite size, / estimation of the presence of structural defects of carbonaceous matter.
1. Introduction 1.1. Why Raman spectroscopy
1386-1425/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00077-5
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These information are readily obtained for carbonaceous matter from different types of rocks, from sedimentary and especially from metamorphic rocks. Knowledge of specific minerals, rocks, of geological units is then completed and more complex reconstruction of geological processes is allowed. Few methods can be used to investigate the structural state of such carbons. Some of them are more sensitive to determine functional groups of mixtures of aliphatic and aromatic compounds of low carbonification grade (IR spectroscopy, NMR spectrometry). Others are more sensitive to investigate the carbon skeleton and the biperiodic / triperiodic structural transition. Only few analytical methods are able to characterize the structural order at the atomic scale (X-ray diffratometry, Raman microspectrometry and Transmission Electron Microscopy) and the microtexture i.e. mutual orientation in space of structural units (Transmission Electron Microscopy).
1.2. Raman spectrum of carbon The first scattering studies of graphite have been done by Tuinstra and Koenig [1]. Later, different authors modified the original peak assignments by these authors and for example Dresselhaus and Dresselhaus [2] and Ferrari and Robertson [3] summarize and review other interpretations of graphite Raman spectra. The Raman spectrum of crystalline graphite consists of a single strong first order line at 1582 cm 1 (designated as the G band) due to the E2g2 vibrational mode with weaker band at 42 cm 1 (E2g1) which is not resolvable from Rayleigh scattering. In the second-order spectrum of graphite the doublet G1? and G2? is situated at /2695 and /2735 cm 1 (S peak) and weak bands at /2450 and /3248 cm 1 [4]. The second-order Raman bands are assigned both to overtone scattering (2 /1360/2735 cm 1, the most intense, 2 /1620/3248 cm 1 a weak but sharp peak) and combination scattering (1620/830 /2450 cm 1, 1580/1355 /2950 cm 1).
For disordered carbons additional bands appear at /1355 cm 1 (D), /1620 cm 1 (D?) and / 2940 cm 1 (Dƒ). The second-order Raman bands at /2695 and /2735 cm 1 merge into a single symmetrical band G? band at /2710 cm 1 which is considerably broadened with increasing structural disorder. 1.3. Raman spectroscopy of artificial carbonaceous phases In 1985, Be´ny-Bassez and Rouzaud [4] published an important paper where results of structural investigation by Raman microspectrometry of four reference carbon types during high-temperature treatments (HTT) under inert gas were presented. Precursor artificial materials included anthracene, thin and thick carbon films and saccharose. These carbonaceous compounds are representatives of so-called graphitizing and nongraphitizing carbons. Using Raman microspectrometry it was possible to evidence different types of defects and to follow their variations during HTT. The evolution of carbonaceous materials throughout artificial HTT (carbonization and graphitization) is due to the elimination more or less rapid and more or less complete of different types of defects. This elimination allows the progressive rearrangement of the BSU. On the basis of these studies and using detailed sampling of carbonaceous matter of different metamorphic series, knowledge on the evolution of Raman spectra of carbonaceous matter has been progressively obtained. In this manner, combining such data also with other methods of structural investigation (XRD, TEM) changes at atomic and molecular level of natural carbons from rocks have been described. The in-plane graphite crystallite size (L) of polycrystalline materials can be deduced by comparing the intensity of the E2g mode at 1582 cm1 (IG) to the intensity of the disorder induced band at 1350 cm 1 (ID) L4:4IG =ID (in nm) Knight and White [5]. The effect of grain orientation on Raman spectrum has been discussed recently [6,7].
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The intensity IG/ID ratio depends not only on the degree of graphitization but also on the scattering geometry. Differences are observed for graphitic planes oriented in parallel or in perpendicular to the polarization orientation of the exciting laser beam. The effects of orientation on the Raman spectra require a precaution during the interpretation of spectra of carbonaceous samples. The first order Raman band of diamond appears at 1332 cm 1 which corresponds to the T2g zone center active mode [8]. Second-order Raman peaks are very weak, they appear at 2148, 2468 and 2653 cm 1 and correspond to overtones and combination bands [5,9].
2. Carbonaceous matter in sedimentary and metamorphic rocks In this part, some general features of the evolution of organic matter in the geological cycle are reviewed. We present most significant representatives of organic matter-rich sedimentary rocks. The carbonaceous matter of these environments could be transformed latter by metamorphic processes and its sampling and study permit us to decipher conditions of these transformations. It is useful to note that combination of geological processes in the long Earth history was rather complex. Studies of carbonaceous matter transformation especially from older geological units is then rather heavy deal. 2.1. Black shales Black shales and oil shales are organic rich laminates which formed in anoxic environments. Black shales contain a significant siliciclastic component, while inorganic carbonates may or may not be present. In oil shales, inorganic carbonates are dominant in addition to kerogen. Black shales are finely grained sedimentary rocks characterized by elevated content in organic carbon. Their black color could be caused, in some cases, by only 0.5% of Corg, in general organic carbon content varies between 0.5 and 10%. The major constituents of black shales include clay minerals, organic matter, quartz grains and even-
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tually other grains. The precursors of these rocks are products of marine sedimentation under euxinic conditions, under low content in oxygen and high organic matter input. High rate of clay sedimentation is necessary to permit the preservation of the organic matter. In such a general conditions the oxidation/destruction of the organic matter is prohibited (low oxygen content, quiet environment, rapid cover by fine clays) [10].
2.2. Coal Coal is a readily combustible organoclastic sedimentary rock composed mainly of lithified plant remains and containing more than 50% by weight of carbonaceous matter and inherent moisture. Coal represents accumulations of plant tissues in different states of preservation. Unaltered sedimentary material containing plant remains change progressively (these changes include physical, biochemical and chemical processes) during diagenesis and catagenesis. Series of coals of increased rank are then obtained, differing in structural and elemental properties. Humic coals pass through a peat stage at the site where plants grew. They are stratified, banded and contain lustrous brown /black material derived from the evolution of woody tissues. Sapropelic coals are not stratified macroscopically and are transformed during diagenesis as other organic rich sediments. They contain various amount of allochtonous organic and mineral material and also autochtonous sedimentary material including algae and spores. Macerals are basic organic constituents of coal that can be recognized under optical microscope. They are coalified remnants of various plant parts and tissues. Macerals differ from each other in their morphology, optical properties, chemical composition and hardness. The diagenetic transformation of peat into coal is called coalification, carbonification or organic maturation. Peats are changed into lignites, subbituminous, high, medium and low volatile bituminous coals, semi-anthracites and anthracites by the conjugated role of temperature and pressure. The rank of a coal depends upon the maximum
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temperature to which the coal was exposed during its geological history. This corresponds upon the maximum depth to which it has subsided into the crust. 2.3. Petroleum During diagenesis, there is continuous evolution of kerogen, reflecting increasing temperature and pressure during subsidence of sediments. Different functional groups are eliminated from buried organic material and numerous new compounds are generated. Low to medium molecular weight hydrocarbons are important constituents of such a mixture. These compounds are generated by the chemical and structural transformations of kerogen. During so-called primary migration they are transported through fine grained source rocks. The secondary migration include processes of transport and is terminated by collection of hydrocarbons in permeable porous reservoir rocks that represent a trap. Such rocks include clastic reservoir rocks-sandstones, siltstones and carbonates. Natural petroleum is a characteristic product of the diagenetic evolution s.l. of accumulations of organic matter [11]. 2.4. Organic matter evolution Most sediments, during deposition contain a mixture of inorganic detrital mineral grains, finegrained amorphous mineral constituents, organic matter and also living organisms. Depending upon the depositional environment, the porosity of the system is variously wetted and oxygenated. Such a system is extremely unstable. The organic matter is an effective reducing agent and is brought into intimate contact with fine-grained minerals. There is a tendency for organic matter to be oxidized by fine-grained minerals (sesquioxides of Fe and Al) and dissolved oxidizing agents. Two extreme situations can be imaged. In highly oxidizing continental or marine conditions (with slow rate of deposition) the organic matter may be oxidized to CO2 and H2O by aerobic bacteria. By contrast, rapid burial in a marine environment with reducing conditions on the interface between water and
sediment few centimeters deeper, permits the protection of the organic matter. The elemental composition of the organic matter-kerogen and coal changes during diagenesismaturation. Kerogen is defined as dispersed organic matter from sedimentary and metasedimentary rocks or as the insoluble part of the organic matter of rocks. The insolubility means here the insolubility in water, organic solvents, hydroxides and mineral acids (strong oxygen containing acids excluded). During sedimentary evolution the composition of each kerogen type change and there is a decrease in oxygen and hydrogen content. By high degree of alteration there is a possibility that the organic matter is converted to 100% of carbon. The diagenetic transformation of organic matter includes changes in higher plant accumulationspeats and in sediments occurring rather in marine environments in shales, characterized, in general, by the majority of allochtonous organic matter. This later case includes kerogen transformations and eventually the appearance of hydrocarbons. The diagenesis of organic matter s.l. is divisible into four stages. During eogenesis the role of pressure and temperature are subordinate and especially biochemical processes take place. This stage is also denominated as biochemical coalification. Biomolecules as lipids (fats, waxes, and steryl esters), carbohydrates (sugars, cellulose and chitin), proteins and lignins from different plant parts display different susceptibility to biochemical transformation. During this stage biopolymers depolymerize and their products react to form so called geopolymers: humic compounds and kerogen. These compounds are characterized by low solubility and high molecular weight. Humic compounds are soluble in alkali hydroxides (fulvic acids soluble at all pH, humic acids soluble in pH higher than 2), kerogen is insoluble in water, inorganic acids and organic solvents. An important product of this stage of diagenesis is gas, mainly methane. This biogenic gas forms locally economic accumulations both in coal seams and in surrounding sedimentary rocks. An other stage of diagenesis /catagenesis starts at temperatures about 40 /150 8C and pressures 3 / 15 MPa. During this stage coal rank change from
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sub-bituminous to anthracite. Coals and kerogen progressively increase in carbon content and decrease in volatiles. Aliphatic chains are cleaved and aromatic structures continue to increase in volume through condensation. The reflectance of vitrinite increases from about 0.2 to 2%. The amount of liquid hydrocarbon generated depends on the amount of liptinite. Significant amount of methane and carbon dioxide is liberated at this stage. Coalbed methane starts to be accumulated in coal strata. Liquid hydrocarbons are liberated from kerogen in this stage during socalled oil window. During the third stage of organic matter transformation, during metagenesis the structural ordering of the carbonaceous matter becomes higher. The dimension of aromatic clusters increases and methane are produced by the breaking of C /C bonds. Aliphatic molecules and hydrocarbons are cracked to methane. The reflectance of vitrinite increases from about 2 to 10%. The fourth stage of transformation is metamorphism, which is considered as a stage during which especially higher pressure and temperatures are reached and graphite or diamond could be seen as typical products.
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with increasing metamorphic grade [7,12/18]. However, the genesis of some Precambrian graphite deposits (on Sri Lanka, for example) is not clearly explained. It can be stressed-out that during regional metamorphic processes, the structure of carbonaceous matter improves resulting in graphite as end product. Low to moderate temperature conditions (200 /600 8C) superimposed to moderate to high pressures (5 /10 kbar) are required for graphite formation [17,19].
2.5.1. Regional metamorphism
2.5.1.2. Bitumens. Otherwise, when migration processes occur, mixtures of hydrocarbons, bitumens are formed in sedimentary or metasedimentary rocks or even on hydrothermal mineral deposits. These bitumens are first liquid, later, as influenced by different alteration processes (fluid migration, oxidation, thermal alteration) they can solidify. Their accumulations occur frequently in mineral or rock cracks and fissures, forming sometime vein type accumulations. Solid bitumens soluble in carbon disulfide are dessigned as bitumens s.s., those remaining insoluble as pyrobitumens [20,21]. Highly carbonized infusible solid bitumens occurring in mineral deposits are named anthraxolite, when it contains uranium and thorium, accompanying uranium mineralizations, it is designed as thucholite. Their transformation during metamorphic processes are different [22]. We will use the name solid bitumen as general term for transformed accumulated solid bitumens in sedimentary and metamorphic rocks. We will reserve the term asphaltic-like solid bitumen (ALSB) to identify solid bitumens characterized by low carbonization degree (as gilsonites have). On the contrary, hard solid bitumen (HSB) designates solid bitumens with low H/C ratio (cata-impsonites, for example).
2.5.1.1. Kerogens. Kerogens from different sedimentary rocks can be transformed during regional metamorphic processes to diverse polyaromatic pre-graphitic materials, graphitoids and in some conditions into graphite. The composition and the structure of many pre-graphitic materials occurring in regionally metamorphic rocks is relatively well-known, it has been shown that carbon becomes better organized into the graphite structure
2.5.2. Contact metamorphism It has been observed previously, that the transformation of carbonaceous matter during contact metamorphism could be different. This type of metamorphism is characterized, in general, by higher temperatures (up to about 1000 8C) depending on the type of magma and the dimension of volcanic body and low pressures comparing with regional metamorphism. Generally, oriented
2.5. Metamorphism Metamorphic conditions are necessary to transform carbonaceous residue into graphitoids (nonperfectly crystallized carbonaceous matter) or graphite. Elevated oriented pressure is especially necessary in addition to high temperatures to produce perfectly crystalline carbon modification-graphite.
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stress is much lower comparing with regional metamorphism. Such a metamorphism of carbonaceous matter has been described in places, where volcanic rocks intrude coal seams or black shale complexes [23 /25].
2.5.3. Raman spectroscopy of natural samples Numerous studies deal with the characterization of the structural ordering during natural process of graphitization. Raman spectroscopy has been applied previously to investigate coals, kerogens, or fossils, and more successfully other geological materials especially from metamorphic rocks [7,16,25 /30]. It is now well established that Raman spectrum of carbonaceous matter is sensitive to the degree of disorder or crystallinity of samples ranging from kerogens and coals to graphitoids and graphites from highly metamorphic environments. Numerous studies have demonstrated the usefulness of combining XRD and TEM with Raman microspectrometry [23 /25]. As noted previously, based on Raman calibrations established by [1,4] it was possible to access to estimates of La for any given carbonaceous matter. It was demonstrated that these characteristics of the carbonaceous matter evolution are independent on the type of the rock [7]. There is a possibility to deduce the degree of metamorphism and degree of crystallinity of carbonaceous matter of a metapelite from the La value derived from indirect Raman spectroscopic determination. Authors pointed-out that during the metamorphic evolution of carbonaceous matter the graphitization process is frequently identical for different marine or nonmarine precursor materials. This is especially well observable from the stage when three-dimensional crystalline order is established. Here, we present new results obtained from different metamorphic series of the Bohemian massif. We compare the impact of specific conditions of metamorphism on Raman spectra. Because of Precambrian age, these carbons have similar precursor organic matter. However, their actual structural state is different and reflects the role of pressure and temperature conditions of different metamorphic episodes.
3. Experimental 3.1. Samples 3.1.1. Highly carbonified kerogens The series of dispersed CM includes kerogens from black shales and schists from Zbecˇno, Klecany and Prachovice. In this study we focused on two Neoproterozoic localities, Klecany and Zbecˇno, which belong to volcano-sedimentary complex of Barrandian area, part of Bohemian massif (Czech republic). The primary sedimentary sequences of Barrandian area have been regionally metamorphosed by Cadomian orogeny and subsequent Variscan foldings (between 970 and 570 million years). The intensity of the regional metamorphism increases to the west. Zbecˇno belongs to the eastern area of the Bohemian massif, where shales are weakly regionally and contactly metamorphosed up to chlorite metamorphic zone. At Klecany the Proterozoic metasedimentary complex consists mostly of graywackes weakly regionally and contactly metamorphosed. As proposed by Fediuk (1993), the granitoid intrusion caused strong contact metamorphism of the sedimentary complex, reaching up to biotite metamorphic zone. Prachovice is situated in tectonically deformed Paleozoic part of ˇ elezne´ hory. Regional metamorphism belongs to Z Silurian period and the sedimentary sequence of black shales is metamorphosed in staurolite zone. 3.1.2. Graphites Lipnice n.S. is located in central massif of the Moldanubian pluton. Samples were collected from the erlane xenolites present in granite body of Melechov massif. Petrographically, variscian lateorogenic muscovite /biotite granites of Eisgarn type are predominant. Another similar epigenetic type of graphite was collected from Prˇibyslavice pegmatites. 3.1.3. Bitumens This group of samples represents highly carbonized and accumulated forms of organic matter of Precambrian age. At Klecany situated in eastern part of Neoproterozoic volcano /sedimentary
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complex, black and brittle solid bitumen (anthraxolite) was collected from mineralized hydrotermal veins formed during postmagmatic alteration of adjacent metasedimentary black shales caused by Hosˇtice granitoid body. Some of solid bitumens (SB) contain microscopic inclusions of uraniferous minerals. In contrary another sample of studied SB was present in the form of accumulation in the contact aureole of Neoproterozoic basaltic body.
3.2. Isolation procedures Kerogen was isolated from investigated shales and schists by dissolution of silicates and quartz. Rock powders were attacked first by 1 N HCl (for 3 h, at 80 8C), washed by distilled water and dissolved by a mixture of HCl/HF (1:3), (for 3 h, at 80 8C) and washed. This procedure was repeated twice. Powders were used for analysis. Investigation of solid bitumens and graphites has been realized on crude samples. Grains of about 3/5 mm were used for analysis. Due to the isotropic character of solid bitumens there was any defined orientation of the sample to the incident laser beam. For graphites, perpendicular orientation of 002 planes to the incident laser beam was used.
3.3. Instrument Micro-Raman analyses were performed on a multichannel Renishaw 2000 spectrometer coupled with a Peltier cooled CCD detector. Excitation was provided by the 514.5 nm line of a continuouswave 10 mW Ar-ion laser. The samples were scanned from 1000 to 3500 cm 1 at a spectral resolution of about 2 cm 1. The scanning parameter for each Raman spectrum was taken as 10 s and ten scans were summed for experimental run to provide a better signal-tonoise ratio. Multiple spot analyses on different areas of the same sample of CM provided similar spectra.
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4. Results 4.1. Raman spectroscopy of dispersed carbonaceous forms 4.1.1. Kerogens-graphitoids Raman spectra obtained on CM of kerogens from progressively metamorphosed black shales of the Barrandian area differ considerably (Fig. 1a). The evolution of structure is connected to the intensity of metamorphism. Dispersed CM from Zbecˇno has a lower degree of structural organization from series of studied kerogens, resulted from metamorphic conditions of chlorite metamorphic zone. Raman spectra show pronounced absorptions at about 1350 and 1580 cm 1. In numerically fitted Raman spectrum additional defect bands at 1217 and 1539 cm 1 are also present. Characteristic D1/G area ratio is around 1.79 and secondorder spectrum contains non intense S1 band. Dispersed CM from Klecany and Prachovice reveals a relatively high structural organization, similar to semigraphite The Raman spectra show pronounced absorptions at about 1350 and 1580 cm 1, but in fitted Raman spectra additional defect band at 1618 cm 1 is also observed. Characteristic area ratio D1/G is about 1.70 and 1.20 for Klecany and Prachovice sample, respectively. In the second-order spectrum of these CM intense S1-peak at around 2700 cm 1 and two weak overtones at 2930 and 3220 are present. The main second-order peak (S1 peak) is symmetric, i.e. no doublet can be resolved. High D1/G intensity ratio (0.80) and the absence of other D3 and D4 defect bands of the Prachovice CM confirm the highest degree of structural organization observed within the studied kerogens, related to staurolite metamorphic zone. 4.2. Raman spectroscopy of accumulated carbonaceous forms 4.2.1. Graphites Raman spectra of well crystallized graphite have been reported previously from many metamorphic units of the Bohemian massif especially from the Moldanubian of Precambrian age, Tepla´-Barrandian of Neoproterozoic age. Triperiodic structural
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Fig. 1. First and second-order Raman spectra of carbonaceous matter. (a) kerogens from Zbecˇno, Klecany and Prachovice, in order of increasing regional metamorphism; (b) graphite from Prˇibyslavice and Lipnice n. Sa´z. (c) Bitumen from Zbecˇno and Klecany.
order is documented by well-resolved first and second-order Raman spectra (Fig. 1b) with intense and sharp bands (example from Prˇibyslavice and Lipnice) (Table 1). Other examples from the Bohemian massif include graphites from highly metamorphic systems, from eclogite (Marianske´ la´zneˇ), granulite (Klet), erlane (Lipnice) and pegmatite.
Triperiodic structural order is documented by well resolved first and second-order Raman spectra. The results of Raman microspectroscopy analyses of graphite samples from Lipnice n.S. and Prachovice are presented on Fig. 1(b). First order Raman spectrum of both natural graphite samples contains a sharp and intense graphitic band at 1580 cm 1. Weak residual band D1 at
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Fig. 1 (Continued)
1351 cm 1 remains visible and characteristic parameters of these samples tabulated as D1/G intensity ratio valued around 0.15 /0.17 and D1/G area ratio around 0.27 /0.30. In the second-order spectrum sharp band S1 at about 2700 cm 1 is present and additional weak bands occur at 2951 and 3246 cm 1. 4.2.2. Solid bitumens Two different types of solid bitumens were studied (Fig. 1c). Raman spectrum of solid bitumen from Klecany indicates a very weak structural organization. The G band appears as a wide band at around 1600 cm 1. Characteristic D1/G area ratio and also intensity ratio are relatively low (1.42 and 0.69, respectively), which is caused by the defect bands D2 and D3 for the most part. The half-width ratio D1/G is high (about 2.1) and obtained second-order spectrum is not well developed. Raman spectrum of solid bitumen from Zbecˇno is quite similar to CM of adjacent kerogens (Fig. 1a). The crude Raman spectrum contains characteristic defect bands at about 1210, 1340 and 1530 cm1 The second-order spectrum bands of SB assigned as overtones are present at frequencies 2674, 2930 and 3140 cm 1. The area ratio D1/G is
relatively high (1.80), compared with SB from Klecany which is probably caused by remobilization processes. Structural evolution of SB from Zbecˇno, is in close correlation to precursor organic matter of bituminous character. This is similar to the case of shungite precursor from Precambrian metasedimentary complex of Karelia (Russia), previously studied by [22,29].
5. Discussion 5.1. Kerogen versus bitumen Raman spectra obtained on kerogens from progressively metamorphic series of Barrandian area differ considerably from spectra obtained on SB. Selected Raman spectra of kerogens from metasedimentary rocks from the Bohemian massif are reported on Fig. 1. This figure includes highly evolved kerogen samples from the eastern part of the Barrandian Neoproterozoic. They display slightly lower structural organization when compared with samples coming from higher metamorphic zones from the western part of this series [23].
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Table 1 Parameters of Raman spectra obtained from carbonaceous matter from Neoproterozoic units of the Bohemian massif (cm 1) Sample
Band D4 /1220
Band D1 /1350
Band D3 /1500
Band G /1580
Band D2 /1620
D1/G area
Position Intensity Width Position Intensity Width Position Intensity Width Position Intensity Width Position Intensity Width K
a
Zbecˇno Klecany Prachovice
1217 / /
189 / /
103 / /
1349 1354 1354
2964 3158 2326
79 58 46
1539 / /
278 / /
78 / /
1603 1587 1583
3089 2542 2882
42 42 30
/ 1618 1616
/ 714 658
/ 24 23
0.961 1.242 0.807
1.889 1.380 1.566
1.790 1.702 1.256
Lipnice n.S. Prˇibyslavice
/ /
/ /
/ /
1352 1352
658 539
30 38
/ /
/ /
/ /
1580 1580
3668 3548
17 21
1620 1622
161 116
9 9
0.179 0.151
1.728 1.813
0.309 0.274
SB a Zbecˇno Klecany
1207 1276
208 674
90 167
1341 1370
2331 1849
92 153
1530 1519
256 828
75 116
1601 1595
2993 2668
39 73
/ /
/ /
/ /
0.778 0.693
2.350 2.101
1.802 1.421
Sample
Band G?
Ga
Band /2950
Band /3240
Position Intensity Width Position Intensity Width Position Intensity Width Position Intensity Width K
Zbecˇno Klecany Prachovice
2691 2706 2708
424 1522 1514
201 73 78
/ / /
/ / /
/ / /
2937 2943 2946
644 493 407
144 109 83
3203 3228 3235
238 126 153
66 68 59
G
Lipnice n.S. 2689 Prˇibyslavice 2670
997 1208
56 63
2726 2730
1634 1311
37 38
2952 2950
323 189
101 79
3246 3250
338 318
23 25
SB
Zbecˇno Klecany
274 578
202 267
/ /
/ /
/ /
2925 2929
464 782
165 26
3200 3142
204 312
70 154
a
2674 2699
K, kerogen; G, graphite; SB, solid bitumen.
J. Jehlicˇka et al. / Spectrochimica Acta Part A 59 (2003) 2341 /2352
D1/G D1/G intensity width
J. Jehlicˇka et al. / Spectrochimica Acta Part A 59 (2003) 2341 /2352
The evolution of Raman characteristics is strongly connected to the intensity of metamorphism. It corresponds to the structural improvement of carbonaceous matter as visualized by TEM. For the less metamorphosed samples (eastern part, pumpellyite zone), the peak width of both Raman bands is about 60 /70 cm 1 and mean D/O ratio 1.3. When the metamorphic degree increases, the peak width of both the bands is considerably narrower. In the chlorite metamorphic zone (western part), the D/O ratio decreases to about 0.8. Major decrease of the D peak is observed on carbonaceous matter from biotite zone. A sharp O peak (width about 30 cm 1) dominates the first order spectrum and the D/O ratio is now only 0.4. However, weak band at about 1350 cm 1 is still present in the spectrum confirming remaining structural defects. In conclusion, changes in the Raman spectra of progressively regionally metamorphosed kerogens concern progressive disappearence of the 1350 cm 1 band, the diminution of the peak width of the E2g Raman band and the progressive decrease of the frequency of the E2g peak, from about 1590 cm 1 for low organized kerogens, to 1575 /1580 cm 1 for graphite. The macroscopic appearance as well as structural parameters obtained on solid bitumens from Klecany, HSBs from Mı´tov and shungites from Karelia by TEM, XRD and Raman spectroscopy are very similar to artificial glassy carbon. Such carbons prepared by a slow pyrolysis of furfuryl alcohol precursor, for example, display similar dimension of micropores as other samples of shungites (2 /10 nm). Their micro to mesoporosity is closed which is the major reason of their gas and liquid impermeability. However, Raman spectra of these samples differ considerably comparing to ALSBs or tar-like compounds [25]. The higher structural arrangement observed on kerogens and graphitoids comparing to SB is probably a result of different metamorphic processes (contact vs. regional). In the case of SB higher temperatures are to be expected for contact metamorphism than for regional one and lower pressures are in general considered for this type of metamorphism. In the absence of strong anisotropic constrains, related to tectonic pressures,
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only small Molecular Orientation Domains (MOD) are formed without preferential orientations. By contrast, in regional metamorphic processes (in the range of metamorphic zones investigated), temperatures can be included in the range 100/500 8C, but important anisotropic pressure can occur. In this case, characteristic preferential orientations of MOD are observed by TEM, due to the flattening of pores and progressive evolution toward graphitic lamellae. Similar graphitization process have been proposed for the evolution of anthracites during metamorphism [17,25]. When carbonaceous matter was only submitted to thermal effect of contact metamorphism, structural and microtextural evolutions are limited. Only weak evolutions were detected by Raman microspectrometry. The decrease of the width of the bands is in relation with the weak microtextural improvement, due to the release of different types of defects. When the carbonaceous matter evolved under the coupled effect of temperature and oriented pressure during regional metamorphism, graphitization develops as seen by TEM. Strong evolution of Raman spectra are brought out in this case (decrease of the band width, disappearance of the defect band). Such graphitization seems emphasized when carbon phases are in dispersed form between mineral lamellar particles. This dilution begins in the sedimentary rock (clay minerals) and can be found also in metamorphic equivalents (micas). In higher metamorphic zones, lamellar organo-mineral composites are formed. The effect of anisotropic pressure on such carbonaceous matter is in this case enhanced and graphite lamellae are frequently found as intergrowths with biotite.
6. Conclusions Raman spectroscopy reveals structural differences between samples of carbonaceous matter originating from similar precursor organic matter, which has been transformed by different metamorphic processes.
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Acknowledgements The work was supported by Grant Agency of the Charles University project Nr. 204/2000 B GEO. We are grateful for this funding.
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