Micro-Raman spectroscopy of collotelinite, fusinite and macrinite

International Journal of Coal Geology 83 (2010) 415–422

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International Journal of Coal Geology

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Micro-Raman spectroscopy of collotelinite, fusinite and macrinite

A. Guedes a,⁎, B. Valentim a, A.C. Prieto b, S. Rodrigues a, F. Noronha a

a Centro de Geologia e Departamento de Geociências, Ambiente e Ordenamento do Território da Faculdade de Ciências, Universidade do Porto, 4169-007-Porto, Portugalb Departamento de Física de la Materia Condensada, Cristalografía y Mineralogía Facultad de Ciencias, Universidad de Valladolid, 47011-Valladolid, Spain

⁎ Corresponding author. Tel.: +351 220402489; fax:E-mail address: [email protected] (A. Guedes).

0166-5162/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.coal.2010.06.002

a b s t r a c t

Article history:Received 26 March 2010Received in revised form 1 June 2010Accepted 9 June 2010Available online 23 June 2010

Keywords:Raman spectroscopyCoalMaceralsRankPetrography

The Raman spectra and the Raman parameters have been correlated with changes in the structure of carbonmaterials, and most of the studies have revealed different development of the Raman spectrum.In the present study micro-Raman spectroscopy was conducted on coal bulk samples and on individual coalmacerals (collotelinite, fusinite, and macrinite) from a set of Penn State Coal Bank coals of increasing rank tostudy the variation of their spectral parameters with rank, and considering coal heterogeneity.The spectral parameters that better correlate with the increasing coal rank, for the coals studied are the fullwidth at half maximum of graphitic band (G: at ∼1580 cm−1), the position of disordered band (D: at∼1350 cm−1), and the integrated intensity ratio of the D band to G band (ID/IG). With increasing coal rank anarrower G band, a shift of D band to lower wavenumber, and an increase of integrated intensity ratio ID/IGare observed.For each coal, the Raman parameters obtained on fusinites and macrinites are similar and differ from thoseobtained on coal bulk samples and collotelinites.The variation of the Raman parameters with rank is very well reflected on the analyses of collotelinites.

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1. Introduction

Coals are sedimentary rocks, resulting from peat coalificationduring its burial history, and composed of organic and inorganicmatter. Therefore, when characterizing coal two characteristics mustbe considered: its rank (coalification degree) and its composition. Thecomplexity of coal becomes more evident at microscopic level sincethe organic constituents of coal include the maceral groups (humi-nite/vitrinite, inertinite, and liptinite), and the individual macerals inthose groups (Taylor et al., 1998). The coalification process of organicmatter has been from many years the subject of numerous studies,and it was found that during this process the structural chemistry ofcoal is progressively modified, for example the organic matterbecomes progressively more ordered to a certain extent (Bend,1992; Carpenter, 1988; Davidson, 2004; Given, 1984; Hatcher andClifford, 1997; van Krevelen, 1993).

One of the used means of studying changes in the structure oforganic matter during coalification and graphitisation includes Ramanspectroscopy. Several authors have correlated the Raman spectra andRaman parameters with changes in the structure of carbon materials,and most of the studies have revealed different development of thefirst- and second-order Raman spectra for organic matter in rocks

with varying degrees of coalification or graphitisation (Beyssac et al.,2003; Bustin et al., 1995; Cuesta et al., 1994, 1998; Ferrari andRobertson 2000, 2001; Green et al., 1983; Guedes et al. 2005; Jawhariet al., 1995; Rouzaud et al., 1983; Sadezky et al., 2005; Tuinstra andKoenig, 1970; Yoshida et al., 2006; Zerda et al., 2000). Other authorshave already applied this technique to conclude about the maturity ofcoals and other organic matter (Kelemen and Fang, 2001; Nestler etal., 2003; Quirico et al., 2005, 2009; Rantitsch et al., 2005) carrying outRaman analysis in bulk samples.

However, coal bulk analyses provide only general information notconsidering coal complexity and heterogeneity, and the impact ofeach coal component to the final result. However, coupling opticalmicroscopes to analytical equipment based on photons and electronsbeams, such as micro-probe, micro-FTIR, and micro-Raman, enablesdirect characterization of selected macerals (e.g. Bustin et al., 1996;Marques et al., 2009; Mastalerz and Bustin, 1993a,b, 1995, 1997;Ward and Gurba, 1999; Wilkins et al., 2002).

Micro-Raman spectroscopy (MRS) of coal allows obtaining theRaman spectrum of a very small volume of material, with a lateralresolution higher than 1 μm. Since the technique uses a microscope itpermits to carry punctual analysis in specific components such as themacerals. Although Raman spectroscopy was carried out by Zerda etal. (1981) in maceral concentrates, MRS was used by Wilkins et al.(2002) measuring liptinite, vitrinite and inertinite fluorescenceintensities and, by Marques et al. (2009) in vitrinite from high rankcoals. A detailed summary concerning the analysis of coal by Ramanspectroscopy is given in Potgieter-Vermaak et al. (2010).

416 A. Guedes et al. / International Journal of Coal Geology 83 (2010) 415–422

The present work aims to study the evolution of Raman spectraand Raman parameters on coal bulk samples and individual coalmacerals (collotelinite, fusinite, and macrinite) from a set of PennState Coal Bank coals of increasing rank, considering coal heteroge-neity. The MRS analysis of individual coal macerals, in particular ofcollotelinite would allow for the correlation of these data withcollotelinite reflectance.

2. Materials and methods

Nine Penn State Coal Bank coals were used in this study(Table 1) corresponding to a set of vitrinite-rich (N79 vol.% vitriniteon a mineral matter free basis) coals of increasing rank from Lowrank A (sub-bituminous coal) to High rank A (anthracite A) (ISO,2005).

Raman analyses are performedwith air objectives and the contrastis lower than using oil immersion objectives. Therefore, shape anddistinct reflectance were important reasons for choosing collotelinite,fusinite, and macrinite. Another important factor is that colloteliniteand fusinite are, respectively, the most representative macerals of thevitrinite and inertinite macerals groups. Macrinite has the advantageof being easily identified with air objectives, and is still a poorlyunderstood maceral.

The Raman experiments were performed on polished sectionsprepared for organic petrography (ISO 7404-5, 2009) using a JOBIN-YVON LABRAM spectrometer. A He–Ne laser was used giving amonochromatic red light of 632.8 nm at a power of 2 mW, with anapproximated irradiance on the sample of 200 kW cm−2. Ramanspectra were measured with a density filter to avoid thermaldecomposition of samples by the laser.

The ×100 objective (0.95 numerical aperture) lens of an Olympusoptical microscope was used to focus the laser beam on the sampleand also to collect the scattered radiation. A highly sensitive CCDcamera was used to collect the Raman spectra. Extended scans from1000 to 1800 cm−1 for the first-order Raman spectrum and from 2500to 3500 cm−1 for the second-order Raman spectrum were performedon each sample.

Micro-Raman analyses were both conducted in grains of bulk coalsamples and in specific macerals: collotelinite, fusinite, and macrinite.For each bulk sample five different grains were analysed and forpolished samples five collotelinite, fusinite and macrinite macerals,totalling twenty analyses, were carried on each sample. Coal maceralswhere identified, in the polished sections using the incident lightmicroscope coupled to the Raman spectrometer, on the basis of theiroptical properties and morphology according to the ICCP classifica-tion and redefinition (ICCP, 1963, 1971, 1975, 1998, 2001; Sýkorováet al., 2005), and Hower et al. (2009).

In order to determine the precise frequencies, bandwidths and therelative intensities of the bands, the spectra were deconvoluted.

Table 1Origin, rank, proximate, ultimate and petrographic analysis of coals.

Origin Sample Storage % Rr Rank (ISO 11760, 2005) Moist. Ash

wt.% wt.% (db)

Penn StateCoal Bank

Decs-24 Argon 0.42 Low A 13.20 13.39Decs-18 0.50 Medium D 6.81 12.25Decs-23 0.69 Medium C 2.00 9.44Decs-12 0.81 Medium C 2.40 10.25Decs-14 0.95 Medium C 1.46 10.52Decs-3 1.19 Medium B 1.10 5.37Decs-19 1.60 Medium A 1.01 4.60PSOC-1515 2.30 High C 2.44 29.17Decs-21 4.22 High A 3.99 11.15

Rr: vitrinite mean random reflectance; V, I, L: vitrinite, inertinite, liptinite.

Adequate fits to the experimental data were obtained using a mixedGaussian–Lorentzian curve-fitting procedure in a Labspec program ofDilor–Jobin Yvon. All the analytical and calculation procedures wereconducted according to Beyssac et al. (2003).

2.1. Raman spectra

Raman spectroscopy is an important tool for material character-ization. Its utility in the characterization of solids is due to its highsensitivity to the lattice vibrations and properties of the crystal. Onstudying carbon material it is known that the two crystalline forms ofcarbon, diamond and graphite, are characterized by two Ramanbands at 1332 cm−1 and 1581 cm−1, respectively (Nemanich andSolin, 1979; Wang et al. 1994). Since Raman spectrum is quitesensitive to lattice order breakdown Raman spectroscopy shouldprovide information about graphite and the disordered carbonpresent.

In the first-order Raman spectrum of graphite, the E2g2 vibrationmode (polyaromatic structures) with D4

6h crystal symmetry occurs ataround 1580 cm−1 (G band). For poorly organized material, addi-tional bands appear around 1150 cm−1, 1350 cm−1, 1500 cm−1, and1620 cm−1. The 1350 cm−1 band (D band) is known as the disorderor defect band. The origin of this band has been related to disorderallowed zone edge modes of microcrystalline graphite due to thepresence of impurities, structural defects and tetrahedral carbon andits disorder is reflected by a shift in the position and by a shoulder at1620 cm−1. Its intensity has been reported to be inversely propor-tional to the graphite grain size.

Both the 1150 cm−1 and 1500 cm−1 bands appear only in verypoorly organized carbonaceous material. Last, the 1620 cm−1 bandforms a shoulder on the G band. In perfect graphite, this component isabsent while in very poorly organized material, the two bands cannotbe resolved and then a single broad band occurs at around 1600 cm−1

(Beny-Bassez and Rozaud, 1985; Beyssac et al., 2002, 2003; Lee, 2004;Tuinstra and Koenig, 1970; Wopenka and Pasteris, 1993). In thesecond-order spectrum several features appear around 2400 cm−1,2700 cm−1, 2900 cm−1 and 3300 cm−1, attributed to overtone orcombination scattering (Nemanich and Solin, 1979). The most visibleone, near 2700 cm−1, splits into two bands in well-crystallizedgraphite. According to Lespade et al. (1984), this splitting occurswhenthe carbonaceous matter acquires a triperiodic organization. TheRaman parameters of these bands, frequency, full width at halfmaximum (FWHM) and intensity, are very sensitive to structuraldisturbances by electronic configuration sp2–sp3 changes in thecarbon bonds (C–C) and consequently their presence are used toestimate the structural properties of the carbon material. Animportant summary of the deconvolution of the Raman spectrumand the assignments of the different Raman bands is given in Li(2007).

Volatile matter C H N St O diff. V I L

wt.% (daf) wt.% (db) wt.% (daf) Vol.% (mmf)

47.14 76.26 5.30 1.32 6.38 10.74 90 7 346.93 79.08 5.80 1.44 4.80 8.89 86 6 843.53 81.95 5.63 1.49 4.27 6.66 79 14 740.13 83.32 5.69 1.37 1.25 8.37 83 9 836.42 85.20 5.46 1.45 2.01 5.87 89 8 328.73 87.37 5.88 1.74 0.69 4.32 94 6 019.19 89.87 4.90 1.14 0.78 3.31 90 10 011.92 88.07 3.91 1.13 0.82 6.07 91 9 05.08 90.33 4.01 0.80 0.57 4.30 87 13 0

Fig. 1. Example of the bands and band parameters of a band profile obtained on coal bulk sample (Decs-21). The solid line is the fitting curve. FWHM-G: Full Width at Half Maximumof G band; IG: G band integrated intensity; ID: D band integrated intensity.

417A. Guedes et al. / International Journal of Coal Geology 83 (2010) 415–422

3. Results and discussion

The general spectra obtained on bulk samples and on collotelinite,fusinite andmacrinite from the coal set reveal not only the presence ofthe usual bands G and D on the first-order Raman spectrum, but alsothe presence of weaker additional bands at around 1090 cm−1,1190 cm−1, 1260 cm−1, 1450 cm−1, 1520 cm−1, and 1680 cm−1

(Figs. 1–2). No second-order spectrum was obtained either on thebulk samples or macerals from the coal set studied.

The coal bulk samples are characterized by a D band atwavenumber position ranging from 1328 cm−1 to 1387 cm−1. Thehighest values were observed for sample Decs-18 (0.50% Rr) and thelowest for sample Decs-21 (4.22% Rr). The FWHMvalues of D band areconcentrated between 90 and 100 cm−1.

G band position is around 1584–1599 cm−1, with the highestvalues obtained on sample Decs-21. The G band FWHM values variedbetween 42 and 97 cm−1, with the highest values observed forsample Decs-21 and the lowest on sample Decs-18 (Table 2).

The ID/IG intensity area ratio values varied between 0.62 and 2.19(samples Decs-18 and Decs-21, respectively) and a general increasewith increasing rank is observed (Table 2).

For Raman parameters of the coal macerals (Table 2), collotelinitespresented D band positions from 1325 cm−1 to 1388 cm−1, similar tothose observed on coal bulk samples. However, the highest values oncollotelinites were observed on sample Decs-24 (0.42% Rr), and thelowest on sample Decs-21 (4.22% Rr). The FWHM values ofcollotelinites D band varied between 61 cm−1 and 100 cm−1.

G band position is around 1585–1605 cm−1 for collotelinites.However, contrarily to the coal bulk samples the highest values were

obtained on sample Decs-24. The FWHM values of G band variedbetween 37 and 99 cm−1, with the minimum and maximum valuescorresponding to the samples Decs-21 and Decs-24, respectively. TheID/IG intensity area ratio values varied between 0.37 and 2.67[obtained on samples Decs-12 (0.81% Rr) and Decs-21, respectively].

The fusinites showed D band positions from 1326 cm−1 to1362 cm−1 (obtained on samples Decs-21 and Decs-24, respectively),and FWHM values from 85 cm−1 to 102 cm−1. The G band positionwavenumbers are scattered between 1588 and 1607 cm−1 and theFWHM values varied between 38 cm−1 and 80 cm−1, with the lowestvalues obtained on sample Decs-21. The lowest value of ID/IG ratiowas obtained on sample Decs-18 (0.74) and the highest on sampleDecs-21 (2.30).

Finally, in the macrinites the D band appears approximatelybetween 1324 cm−1 and 1358 cm−1 (samples Decs-21 and Decs-24,respectively) with an FWHM of 85–100 cm−1. The G band with anFWHM ranging from 41 cm−1 (Decs-21) to 82 cm−1 (Decs-12−0.81% Rr), is present around 1590–1603 cm−1, with the highestvalue obtained on sample Decs-21 (4.22% Rr). The ID/IG intensity arearatio shows values between 0.73 (for sample Decs-12) and 2.52 (forsamples Decs-12 and Decs-21, respectively).

The plot of the mean values of Raman parameters from the coalbulk samples and the different macerals in each coal studied (Figs. 3and 4) revealed differences between the maceral groups, and alsobetween the macerals and the respective coal bulk. However, fusiniteand macrinite macerals show similar parameters.

This similarity could be related with the fact that both maceralshave, fundamentally, their origin on the same original material(Hower et al., 2009), and both resulting from decay paths (fusinite:

Fig.

2.Re

presen

tative

Raman

spectraob

tained

onbu

lksamples,collotelin

ites,fus

inites

andmacrinitesfrom

theco

alsstud

ied.

(A)Decs-21

:4.22

%Rr;(B

)Decs-3:

1.19

%Rr;(C

)Decs-18

:0.50

%Rr).

418 A. Guedes et al. / International Journal of Coal Geology 83 (2010) 415–422

Table 2Raman parameters obtained on coal bulk samples and coal macerals from the set ofcoals studied.

Sample Analysis Rr% w-D(cm−1)

FWHM-D(cm−1)

w-G(cm−1)

FWHM-G(cm−1)

ID/IG

Decs-24 Coal 0.42 1378 100 1592 89 1.041382 100 1593 95 1.081374 100 1591 83 1.02

Collotelinite 1379 89 1600 87 0.721388 98 1605 99 0.851372 82 1595 72 0.48

Fusinite 1350 100 1596 70 1.051362 100 1601 77 1.281336 99 1593 59 0.89

Macrinite 1349 98 1597 70 1.051358 100 1599 76 1.361346 90 1595 67 0.88

Decs-18 Coal 0.50 1376 95 1592 89 0.821387 100 1596 97 0.891365 90 1589 81 0.62

Collotelinite 1365 99 1596 70 1.281380 100 1605 89 2.421347 92 1591 52 0.67

Fusinite 1340 100 1600 62 1.231349 100 1607 76 1.461335 100 1594 55 0.74

Macrinite 1344 100 1594 74 0.901349 100 1595 75 0.971339 100 1592 72 0.83

Decs-23 Coal 0.69 1376 100 1590 89 1.081380 100 1593 91 1.161374 99 1584 84 0.97

Collotelinite 1368 90 1591 79 0.671375 100 1596 96 0.981354 61 1587 60 0.45

Fusinite 1337 99 1599 65 1.261349 100 1602 70 1.561329 96 1596 54 1.06

Macrinite 1341 99 1597 68 1.131346 100 1600 74 1.251334 98 1594 60 0.90

Decs-12 Coal 0.81 1368 100 1594 81 1.041382 100 1598 87 1.171336 100 1593 70 0.86

Collotelinite 1359 87 1591 72 0.771369 100 1596 79 1.221344 66 1589 63 0.37

Fusinite 1340 100 1598 68 1.021349 100 1602 75 1.191335 100 1594 60 0.78

Macrinite 1343 98 1595 69 1.041354 100 1600 82 1.521333 95 1590 54 0.73

Decs-14 Coal 0.95 1361 99 1595 79 1.161367 100 1599 85 1.291354 96 1588 70 1.02

Collotelinite 1359 94 1589 78 0.811367 100 1593 92 1.111350 75 1585 68 0.58

Fusinite 1338 99 1598 62 1.231345 100 1602 71 1.581331 94 1594 53 0.86

Macrinite 1345 100 1598 64 1.131345 100 1598 64 1.131345 100 1598 64 1.13

Decs-3 Coal 1.19 1366 99 1591 82 0.991369 100 1593 85 1.061361 97 1590 76 0.92

Collotelinite 1354 98 1591 69 1.071359 100 1594 77 1.421347 83 1587 56 0.74

Fusinite 1338 100 1594 68 1.131351 102 1600 80 1.681328 99 1588 51 0.77

Macrinite 1347 96 1596 64 1.281350 100 1597 67 1.531345 85 1595 61 1.17

(continued on next page)

Table 2 (continued)

Sample Analysis Rr% w-D(cm−1)

FWHM-D(cm−1)

w-G(cm−1)

FWHM-G(cm−1)

ID/IG

Decs-19 Coal 1.60 1355 100 1592 80 1.081359 100 1595 87 1.271352 99 1589 73 0.92

Collotelinite 1350 97 1590 68 1.031356 100 1594 78 1.301341 88 1587 60 0.70

Fusinite 1339 100 1601 61 1.291344 100 1603 62 1.631333 99 1596 53 1.12

Macrinite 1337 100 1600 58 1.491340 100 1601 58 1.601335 100 1599 58 1.38

PSOC-1515 Coal 2.30 1346 100 1593 63 1.361347 100 1594 67 1.571345 100 1593 59 1.19

Collotelinite 1347 94 1593 55 1.171355 100 1596 58 1.571342 84 1589 52 0.96

Fusinite 1337 98 1600 57 1.291344 100 1603 64 1.731330 91 1597 52 1.07

Macrinite 1338 100 1598 56 1.461338 100 1598 56 1.461338 100 1598 56 1.46

Decs-21 Coal 4.22 1330 98 1598 43 1.751331 100 1599 45 2.191328 92 1597 42 1.39

Collotelinite 1328 95 1598 39 1.851331 100 1600 42 2.671325 89 1595 37 1.40

Fusinite 1329 95 1603 45 1.671332 100 1604 49 2.301326 85 1601 38 1.31

Macrinite 1328 100 1602 43 2.091332 100 1603 45 2.521324 100 1600 41 1.79

Rr: vitrinite mean randon reflectance; w-D: wavenumber of D band; FWHM-D: Fullwidth at half maximum of D band; w-G: wavenumber of G band; FWHM-G: full widthat half maximum of G band, ID/IG: intensity ratio of the bands. For the Ramanparameters — bold: mean values; italic: maximum values; normal text: minimumvalues.

419A. Guedes et al. / International Journal of Coal Geology 83 (2010) 415–422

wildfires; macrinite: wildfires, microbial degradation, or oxidation bycirculating water) before or during peatification (Hower et al., 2009;Scott and Glasspool, 2007; Taylor et al., 1998). Microprobe and micro-FTIR studies of macerals (Bustin et al., 1996; Mastalerz and Bustin,

Fig. 3. D band vs. G band position (mean values) for the bulk coal samples andrespective macerals.

Fig. 6. Position of G band (mean values) vs. rank for the bulk coal samples andrespective macerals.

Fig. 4. Full width at half maximum (FWHM) vs. position of G band (mean values) for thebulk coal samples and respective macerals.

420 A. Guedes et al. / International Journal of Coal Geology 83 (2010) 415–422

1993a,b, 1997; Ward and Gurba, 1999) also corroborate this, sincecarbon and oxygen contents of fusinite and macrinite macerals arealmost the same.

The Raman parameters obtained on coal bulk samples show moreaffinity with the collotelinite than with the group of fusinite–macrinite. This is a result of coals studied being vitrinite-rich.

Relatively to the variation of Raman parameters with theincreasing coalification of the coals studied (Figs. 5–8), it is observedthat generally the G band becomes narrower with increasing rank, forboth coal bulk samples and respective macerals (Fig. 5). The sametrend was described by Kelemen and Fang (2001), Quirico et al.(2005) and Marques et al. (2009).

A shift in this band to higher wavenumber is only observed onsamples PSOC-1515 and Decs-21, the highest rank samples of thestudied set (Fig. 6). With increasing of coal rank a shift of D bandto lower wavenumber occurs on both coal bulk and collotelinite(Fig. 7). The same trend was already described by Kelemen andFang (2001), Nestler et al. (2003) and Quirico et al. (2005).Kelemen and Fang (2001) and Quirico et al. (2005, 2009) alsoobserve a variation of the full width at half maximum of this band.However, in the present work no trend was observed for theFWHM of D band.

A general increase of integrated intensity ID/IG ratio with theincreasing of coal rank is observed on both coal bulk samples andmacerals (Fig. 8). The increase in this parameter was reported byMarques et al. (2009) for high rank coals. However, a decrease in this

Fig. 5. Full width at half maximum of G band (FWHM-G) (mean values) vs. coal rank (%Rr) for the bulk coal samples and respective macerals.

ratio was also found (Kelemen and Fang, 2001). This could result fromthe fact that the existence of other minor bands in the spectrum werenot reported and thus leading to different spectra deconvolution,since the use of these minor bands found in the spectra allows for thereduction of area of the main bands (G and D) in carrying out thespectral curve fitting.

Finally, the variation of the Raman parameters with rank is betterreflected on the analyses of collotelinite than for bulk samples,macrinite, and fusinite (Figs. 5–8).

4. Conclusions

The general spectra, obtained on bulk samples, and on collotelinite,fusinite and macrinite from the coal set, revealed the presence of theusual bands G and D on the first-order Raman spectrum, and thepresence of weaker additional bands at around 1090 cm−1, 1190 cm−1,1260 cm−1, 1450 cm−1, 1520 cm−1, and 1680 cm−1. No second-orderRaman spectra were obtained on this set of Penn State coals.

For each coal studied, Raman parameters obtained on colloteliniteare similar to those obtained on coal bulk samples. Most probably, thisis due to the fact that the coal set is vitrinite-rich and the highestprobability of analysing vitrinite grains on a random analysis. TheRaman parameters obtained on fusinites and macrinites are similarfor each coal, but differ from those obtained on respective coal bulksamples and collotelinite.

The spectral parameters that better correlate with the increasingcoal rank, for the coals studied are the full width at half maximumof Gband, the position of D band, and the ID/IG ratio. With increasing coal

Fig. 7. Position of D band (mean values) vs. rank for the bulk coal samples andrespective macerals.

Fig. 8. ID/IG intensity area ratio (mean values) vs. rank for the bulk coal samples andrespective macerals.

421A. Guedes et al. / International Journal of Coal Geology 83 (2010) 415–422

rank a narrower G band, a shift of D band to lower wavenumber, andan increase of ID/IG ratio are observed.

The variation of the Raman parameters with rank is very wellreflected on the analyses of collotelinite.

Acknowledgements

This work has been supported by projects POCI/CLI/60557/2004and PPCDT/CLI/60557/2004 of Fundação para a Ciência e a Tecnologia(Portugal). We would like to thank Dr. Walter Pickel and theanonymous reviewer for helpful manuscript reviews.

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