Raman spectroscopy of nanocrystals in rock

447

ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2007, Vol. 43, No. 6, pp. 447–454. © Pleiades Publishing, Ltd., 2007.Original Russian Text © G.A. Sobolev, V.I. Vettegren, S.M. Kireenkova, V.B. Kulik, Yu.A. Morozov, A.I. Smul’skaya, 2007, published in Fizika Zemli, 2007, No. 6, pp. 7–14.

INTRODUCTION

The development of a modern theory of rock frac-ture requires information on the constitution and evolu-tionary formation mechanisms of structures differing insize from nanometers to thousands of kilometers.Whereas methods of investigating micro- and macro-structural features in rocks are well developed, thedevelopment of methods for studying objects ofnanometer sizes is only at its initial stage.

Nanoparticles exist in space, atmosphere, hydro-sphere, rocks, and magmas. In geological processes,nanoparticles can form during decomposition of solidsolutions in minerals and glasses, in submicron twin-ning of some crystals, during phase transitions from aliquid or gaseous state to a solid state, and in physicaland chemical weathering processes [Bogatikov, 2003;Gusev and Rempel, 2000; Petrov, 1986]. The applica-tion of spectroscopic and x-ray methods in mineralogygave rise to a new direction of research, nanomineralogy[

Nanomineralogy …

, 2005].

To detect nanoparticles in rock-forming minerals,we applied the Raman spectroscopy (RS) method. Themean free path

E

of a quantum of fundamental vibra-tions of atoms, the phonon, is known to vary in crystalsfrom ~1 nm to a few tens of nanometers. If the crystalsize

L

becomes close to or smaller than the mean freepath of a phonon

Λ

(

L

≤

Λ

), the impulse selection rules

q

= 0 are violated [Campbell and Fauchet, 1986; Gre-gora et al., 1994; Kozlovsky and Lang, 1992; Ma et al.,1998; Parker and Siegel, 1990; Bersani et al., 1998].For this reason, the phonons of the entire Brillouin zonebegin to contribute to the spectrum. As a result, RS

band maximums are displaced and become asymmet-ric. In the present work, this phenomenon is used todetect nanocrystals and to evaluate their sizes in a rock(PV-364 sandstone).

INSTRUMENTATION AND TECHNIQUE

The experiments were carried out with samples offine-lamellar arkosic sandstone (PV-364) from theRiphean turbidite sequences of the Srednii Peninsula atthe Barents Sea margin of the East European platform.

The rock, photographed with the help of an opticalmicroscope (Fig. 1), consists of clastic material (75%)and cement (25%). The former is represented by grainsof quartz and feldspar (~80%); mica (15%); and zircon,an ore mineral, and titanium dioxide

TiO

2

(on thewhole, 5%). Biotite and muscovite are replaced byhydromicas and chlorite. In the twinned grains of pla-gioclase and microcline, twin bands are a few micronswide. Zircon grains have sizes of 5–10

µ

m, and the tita-nium dioxide grains are up to 50

µ

m in size (on average,20

µ

m). Both clastic and cement interstitial

TiO

2

grainsare present. The cement is basal–porous hydromicas.The clasts are 5–90

µ

m in size (on average, 50

µ

m), andsome mica flakes reach 300

µ

m.

Rare interlayers 300–400

µ

m thick are enriched inmica, zircon, and

TiO

2

. Clusters of subparallel micasform schistosity, initiating the formation of a platy part-ing; slickensides a few fractions of a millimeter inthickness form on some surfaces of the latter. Such aslickenside is represented by a vitreous brown-mottledmass, isotropic and irregularly saturated with inclu-

Raman Spectroscopy of Nanocrystals in Rock

G. A. Sobolev

a

, V. I. Vettegren

b

, S. M. Kireenkova

a

, V. B. Kulik

b

, Yu. A. Morozov

a

, and A. I. Smul’skaya

a

a

Schmidt Institute of Physics of the Earth, Russian Academy of Sciences (RAS), Bol’shaya Gruzinskaya ul. 10, Moscow, 123995 Russia

b

Ioffe Physicotechnical Institute, RAS, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia

Received July 31, 2006

Abstract

—Nanocrystals were detected and identified in rocks by the method of Raman spectroscopy. Theexperiments showed that the Raman scattering spectra of fine-lamellar arkosic sandstone exhibit bands corre-sponding to lattice vibrations of anatase,

α

-quartz, and plagioclase. In all spectra of the rock, the bands are dis-placed towards high frequencies as compared with their position in spectra of single crystals and widen on thesame side. These results show that, in all of the studied places of the sample, the particles of anatase, quartz,and plagioclase have nanometer sizes, namely, of the order of 10 nm in anatase and quartz and about 20 nm inplagioclase. Moreover, in different places of the sample, not only the shape and position of the bands understudy but also their intensity vary, the latter being directly proportional to the concentration of nanocrystals.

PACS numbers: 91.60.Mk

DOI:

10.1134/S106935130706002X

448

IZVESTIYA, PHYSICS OF THE SOLID EARTH

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SOBOLEV et al.

sions up to a few microns in size; hydromicas, ore min-erals, and titanium dioxide are distinguishable in theinclusions.

The chemical composition of rock components(Table 1) was studied with an SX100 microprobe byN.N. Konankova at the Vernadsky Institute ofGeochemistry and Analytical Chemistry, RAS. As seenfrom Table 1, the rock minerals and vitreous mass ofthe slickenside are depleted in Ca and Na and areenriched in Ti. The vitreous mass contains 2.5–7.7 wt %of

TiO

2

. Some clasts are saturated with Ti-bearing epi-taxial intergrowths that are smaller than the microprobediameter (3

µ

m). These inclusions could not be diag-nosed but are visible with the microscope or manifestthemselves as an anomalously high

TiO

2

concentrationin quartz (76 wt %) or hydromica inclusions in theslickenside (up to 25 wt %). The microprobe analysisrevealed

TiO

2

, either pure or with an admixture of alu-minosilicate phase (84–93 wt % of

TiO

2

).

Figures 2 and 3 show photographs of a fragment ofthe micaceous

TiO

2

-enriched interlayer. The images areobtained in reflected electrons (Fig. 2) and in the char-acteristic radiation rays of titanium (Fig. 3) and display

TiO

2

grains in the rock. The grains coincide in shapewith cement pores and fill the space between clasts.

Interstitial grains are not typical of clastic

TiO

2

and arepossibly of authigenic origin.

To record the RS spectra, plates 3–4 mm thick weresawn from rock samples along the schistosity, acrossthe slickenside. Their surface was polished to decreasethe Rayleigh light scattering. To obtain the spectra, theplates were placed on an object table of a Ramalog 5spectrometer (Fig. 4). The spectra were excited by an

Ar

++

16508 (Spectra Physics) laser (

Λ

= 488.0 nm). Thelaser beam, when focused on the surface of the sample,gave a spot ~30

µ

m in diameter. In order to reduce theeffect of the local heating of the sample by the laserbeam, the minimum possible radiation power (0.1 W)was chosen.

The spectra were recorded in the photon-countingmode with a scattering angle of

≈

180°

. For accuratedetermination of band maximums, the radiation spectraof a neon lamp whose bands were used as datums weresimultaneously recorded. To reduce the influence of theinstrument function on the shape of bands, the spectralwidth of a slit was chosen equal to 2 cm

–1

. Since thebandwidth was at least four times larger than the slitspectral width, the spectra were not corrected in orderto take into account the instrument function. The deter-mination accuracy of the frequency of a maximum and

1011

1213

15

14

16

21

22

2019

1718

100

µ

m

Fig. 1.

Photograph of the PV-364 sandstone made with an optical polarizing microscope without an analyzer. The crosses are micro-probe analysis points, and the numbers enumerate the succession of measurements.


Vol. 43

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2007

RAMAN SPECTROSCOPY OF NANOCRYSTALS IN ROCK 449

the width of the study bands in the spectrum was noworse than 0.2 cm

–1

. The intensity of the laser light wasstabilized by an automatic electronic control schemeand was checked with the help of three independentdevices: a radiation power sensor in the laser powersupply unit, a Coherent Radiation meter of the opticalradiation power, and an FD-7k silicon diode. The accu-racy of maintaining the intensity of the photon flux irra-diating the sample at a given level was no worse than1% during the recording of the entire Raman scatteringspectrum.

For the purpose of reliable identification of theobserved RS bands, the spectra of single crystals of

anatase, rutile, quartz, and plagioclase were recordedconcurrently with the spectra of the rock samples.

RESULTS OF THE RS STUDY

Bands corresponding to the lattice vibrations of ana-tase,

α

-quartz, and plagioclase are present in the Ramanspectra of the rock (Fig. 5 and Table 2). It is significantthat the bands are displaced in all spectra toward highfrequencies as compared with their position in spectraof single crystals and widen on the same side (Fig. 6).These results show that, in all places examined in sam-ple PV-364, particles of anatase, quartz, and plagioclasehave nanometer sizes.

Table 1.

Chemical composition of PV-364 sandstone components

Measure-ment no. 1 2 3 4 5 6 7 8 9 10 11 12 13

Compo-nent Chlorite Hydromicas * Hydromicas Micro-

cline

SiO

2

27.49 45.47 20.03 48.58 33.51 39.68 40.77 51.8 21.98 54.04 43.47 38.89 65.27

TiO

2

0.45 0.3 22.75 0.12 7.68 9.62 1.72 0.46 25.46 0.05 0.0 0.25 0.02

Al

2

O

3

22.91 34.63 19.18 36.65 21.43 26.11 26.65 32.19 15.15 29.44 27.41 25.8 20.28

Cr

2

O

3

0.09 0.02 0.16 0.03 0.15 0.2 0.1 0.25 0.29 0.07 0.09 –0.01 0.04

FeO 23.11 3.61 17.27 3.74 11.44 7.13 6.2 4.0 17.62 6.46 16.92 15.25 0.1

MnO 0.01 0.04 3.02 0.02 0.67 0.28 0.63 0.02 3.21 0.02 0.03 0.05 0.0

MgO 15.28 0.69 1.44 1.0 4.3 3.36 2.24 3.17 1.94 3.28 5.0 7.08 0.03

CaO 0.1 0.01 1.41 0.04 0.89 0.36 0.38 0.06 1.24 0.19 0.16 0.19 0.0

Na

2

O 0.02 0.26 0.12 0.21 0.18 0.11 0.03 0.14 0.77 0.05 0.02 0.01 0.24

K

2

O 0.2 10.71 2.66 10.38 4.27 6.6 6.5 8.27 2.41 6.92 3.99 3.86 16.27

F –0.18 –0.02 0.29 0.0 –0.05 –0.11 0.25 0.32 0.17 0.16 0.19 0.06 –0.06

Total 89.65 95.72 88.34 100.77 84.52 93.44 85.47 100.68 90.24 100.69 97.27 91.42 102.25

14 15 16 17 18 19 20 21 22 23 24 25 26

Compo-nent Quartz Zircon TiO

2

TiO

2

Hydro-mica Chlorite Cement Micro-

cline ** Cement * ** Hydro-mica

SiO

2

99.76 37.53 1.65 2.52 47.57 29.01 50.4 65.05 20.28 35.82 39.71 4.67 46.27

TiO

2

0.02 –0.04 93.59 89.36 0.83 0.06 0.11 –0.03 76.05 0.14 2.53 84.19 1.12

Al

2

O

3

0.06 0.01 0.41 1.99 32.11 21.28 28.14 20.44 0.82 27.02 23.33 3.19 29.35

Cr

2

O

3

–0.01 –0.02 0.03 –0.01 –0.01 0.37 0.33 0.01 0.36 0.8 0.14 0.1 0.03

FeO 0.11 0.34 0.55 4.01 4.75 23.96 8.9 0.1 1.08 19.12 14.05 0.8 5.79

MnO 0.02 0.03 0.0 0.02 0.0 0.27 0.02 0.02 0.01 0.03 0.09 0.05 0.02

MgO 0.01 0.03 0.06 0.95 2.21 14.44 4.0 0.02 0.05 5.74 6.56 0.21 2.68

CaO 0.02 –0.04 0.12 0.03 –0.02 0.08 0.18 0.01 0.06 0.13 0.17 0.37 0.1

Na

2

O –0.01 –1.15 0.05 0.04 0.1 0.06 0.05 0.95 0.03 0.04 0.19 0.1 0.26

K

2

O 0.0 0.02 0.06 0.1 10.2 0.25 6.61 14.76 0.12 3.54 6.97 0.74 9.41

F –0.08 0.41 0.38 –0.21 0.51 0.28 0.31 0.13 –0.49 0.15 0.15 0.16 0.35

Total 100.0 38.36 96.9 99.0 98.29 90.06 99.08 101.48 98.85 92.52 93.89 94.58 95.38

* Isotropic substance of the slickenside.** Intergrowth of TiO

2

and a(n) (alumino)silicate phase.

450


Vol. 43

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SOBOLEV et al.

255

223

191

159

128

96

64

32

0100

µ

m BSEZ 15 kV

94

82

70

59

47

35

24

12

0100

µ

m TiKa 15 kV

Fig. 2.

Photograph of a thin section in reflected electrons.

Fig. 3.

Photograph of a thin section in Ti rays.

To determine the sizes of nanocrystals, the positionand shape of the most intense bands (143, ~460, and~510 cm

–1

) were analyzed. In the literature, the first

band is assigned to the

E

g

vibrations of the anatase lat-tice [Wu et al., 1999] (Fig. 6); the second band, to the

A

1

vibrations of the quartz lattice [De Boer et al., 1996;


Vol. 43

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2007


Umari et al., 2001] (Fig. 7); and the third band, to the

ν

S

(T–O–T)

vibrations, where T is the atom of silicon oraluminum in plagioclase crystals [Sharma et al., 1983](Fig. 8).

In the case of crystals of linear dimensions

L

≤

Λ

,the dependence of the Raman scattering intensity

I

(

ν

)

versus frequency

ν

takes the form [Tiong et al., 1984;Shen and Pollak, 1984]

, (1)

where

Γ

0

is the natural broadening of the band;

q

is thewave vector;

C

(0,

q

)

is the Fourier coefficient of thewave function

Ψ

'(q0, r) of a phonon,

(2)

r is the radius vector of a phonon; and q0 is the wavevector of the phonon with the vibration frequency ν0 ofan infinite single crystal.

I ν( ) C 0 q,( ) 2d3q

ν0 ν q( )–( )2 Γ0/2( )2+-----------------------------------------------------∫≅

C q0 q,( ) 1

2π( )3------------- Ψ ' q0 r,( ) iq0r–( )d3r;exp∫=

In the case of a spherical nanocrystal, we have

. (3)

According to [Zhang et al., 2000], the followingdependence of the frequency on the wave vector of thephonon q was accepted:

, (4)

where ∆ = 20 cm–1 and a is the mean interatomic dis-tance, equal to 0.38 nm for anatase [Zhang et al., 2000],0.5 nm for quartz [Levien et al., 1980], and 0.8 nm forplagioclase [Prewitt et al., 1976].

Using expressions (1)–(4), we found values of ν0and L that fit best the shape of the selected bands. Themean sizes of nanocrystals of anatase and quartz deter-mined in this sample vary from 5 to 9 nm depending ontheir position in the sample, and the size of plagioclasenanocrystals is approximately constant and amounts to~20 nm (Table 3).

Thus, the RS method has proven effective for detect-ing nanocrystals of anatase, quartz, and plagioclase in

C 0 q,( ) 2 q2L2–

16π2--------------–⎝ ⎠

⎛ ⎞exp=

ν q( ) ∆ 1 qa( )cos–[ ]=

1

2

3

To the device

500

0100

Intensity, arb. units

Frequency, cm–1

1000

1500

2000

2500

200 300 400 500 600 700 800

143 cm–1

394 cm–1 510 cm–1

460 cm–1 635 cm–1

Fig. 4. Layout of the experiment: (1) laser beam; (2) sam-ple; (3) mirror.

Fig. 5. Raman spectrum of sample PV-364 in the frequencyrange 100–1200 cm–1.

Table 2. Frequencies of band maximums ν in the Raman spectra of sample PV-364 and single crystals of anatase, quartz,and plagioclase

Sample Point no. ν, cm–1

PV-364 1 143.6 220 353 398.2 463.1 – 509.9 636

2 143.6 200 – 392.8 464.1 – 511.6 633

3 142.9 – – 398.8 463.2 477.4 507.7 635

4 142.4 220 353 395.7 463.3 476.9 507.4 635

Single crystal Plagioclase – – – – 456.8 480.1 508 –

Quartz – 205 353 391 462.9 – – –

Anatase 141.4 – – 393.7 – – 513.6 635

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IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 43 No. 6 2007

SOBOLEV et al.

sandstone PV-364 and estimating their sizes (from ~5to ~20 nm).

It was established that this method can also be usedfor estimating the internal stresses in nanocrystals.Actually, the ν0 frequencies were found to be lowerthan the vibration frequencies of single crystals by 0.5–1 cm–1. It is known that the vibration frequency of sin-gle crystals shifts with deformation of interatomicbonds [Vettegren et al., 1975]. A decrease in the ν0vibration frequency shows that the interatomic bonds innanocrystals of anatase, quartz, and plagioclase arestretched. An increase in the lattice spacing of small

particles in comparison with a massive single crystal isdue to two factors.

The first factor is a change in the environment ofatoms located on the surface of nanocrystals. For exam-ple, the absence of neighbors of surface atoms on ajuvenile surface in vacuum leads to a change in the dis-tances between surface atoms [Korsukov et al., 1998;Loffer, 1995; Gusev and Rempel, 2000]. A similareffect (a change in the interatomic distances) can alsobe caused by the adhesion bond between atoms of dif-ferent materials [Vettegren et al., 2005].

The second factor is a change in the spectrum andthe anharmonicity of lattice vibrations of one mineralnear the interface with another mineral or vacuum[Tabor et al., 1977; Vettegren et al., 2005]. Under theinfluence of these factors, the value of thermal expan-sion of the crystal lattice near the interface changes.

Further experiments are required to elucidate thequestion of which of the two factors makes the maincontribution to the observed effect of an increase in thelength of interatomic bonds.

Now, we estimate the value of a conventionalmechanical stress that could cause the stretching ofinteratomic bonds. It is known that the frequency shift∆ν of crystal vibrations due to hydrostatic compressionby a pressure Pef is described by the expression

∆ν = αPef,

where α is a mechanical spectroscopic coefficient. Thevalue of α amounts to 3.14 cm–1/GPa for the Eg vibra-tions of the anatase lattice [Sekiya et al., 2001; Lagarecand Desgreniers, 1995] and 0.081 cm–1/GPa for the A1vibrations of the quartz lattice [De Boer et al., 1996;Umari et al., 2001]. Since α does not depend on thedirection of the applied force (extension or compres-

0.12

0.06

0120 140 160 180


Frequency, cm–1

1

2

3

4

Fig. 6. Band of the Eg vibrations in the Raman spectra of ananatase single crystal (solid line) and sample PV-364. Thenumbers 1–4 refer to places in which the spectra wererecorded.

1

24

600

300

0


420 450 480Frequency, cm–1

0.2

0.1

0500 520

Intensity, arb. units3

4

Frequency, cm–1

Fig. 7. Band of the A1 vibrations in the Raman spectra of aquartz single crystal (solid line) and in the first (1), second(2), and fourth (4) places of sample PV-364.

Fig. 8. Band of the plagioclase νS(T–O–T) vibrations in theRaman spectra of a single crystal (solid line) and in the third (3)and fourth (4) places of sample PV-364.



sion), these values can be used for estimating tensilestresses that could cause the observed frequency shift inthe nanocrystals. We found that the value of suchstresses for anatase and α-quartz lies within the range90–200 MPa.

Moreover, our studies showed that not only theshape and position of the analyzed bands but also theirintensity, directly proportional to the concentration ofnanocrystals, vary throughout the sample. The concen-tration of nanocrystals was shown to vary by ~3 timesfor anatase, by an order of magnitude for quartz, and by~5 times for plagioclase.

RESULTS AND DISCUSSION

Our investigations showed that the goal of the detec-tion and identification of nanoparticles in a rock by theRS method was achieved and positive results wereobtained. The method itself is promising for solution ofmany problems related to nanoscale structures of thegeological medium and is usable for estimating thesizes and concentration of nanocrystals and deforma-tion of their interatomic bonds in various places of thesurface rock layer, ensuring a resolution equal to thediameter of the light beam (in our case ~30 µm).

We should note that the rock with the simplest andwell-known combination of characteristics of its miner-alogical composition and structure, as well as condi-tions of its formation and evolution, was chosen for ourexperiments.

At the same time, this work raises a number of prob-lems, and, in our opinion, the most important of them isto establish the rock development stage at which nano-particles are nucleated. The PV-364 detrital rockformed under the conditions of weathering, favorablefor the formation of nanoparticles. Moreover, it experi-enced diagenetic transformations at the illite level,involving incipient recrystallization of clastic andcement material and regeneration and segregation pro-cesses such as microtwinning of plagioclase and newformations of TiO2 in the intergranular space withexplicit morphological features of segregation concre-tions. Nucleation of nanoparticles is very probable atthis stage. Finally, the rock schistosity and slickensidesdue to gliding on lamination surfaces with the forma-tion of a TiO2-enriched vitreous substance clearly indi-cate a stress-induced impact on the rock. In this connec-tion, we should note that bands of rutile, the most stablemodification of TiO2, are absent in the Raman spectra.It is known that, at a temperature above 400 K [Wuet al., 1999] and at pressures of 1.5–2 GPa [Sekiyaet al., 2001; Lagarec and Desgreniers, 1995], nanocrys-tals of anatase are irreversibly transformed into rutile.In other words, the rock formed temperatures below400 K and at pressures no higher than 1.5 GPa.

The study of natural processes at a nanoscale levelrequires first of all the formulation of prospecting tasks.The study of the nature, mechanisms, and kinetics of

nanoscale initiation and fracture can be one such pros-pecting direction in geophysics. For this purpose, anumber of problems should be solved, including thefollowing:

choice of methods for detection of nanoparticles invarious natural objects under normal conditions;

recognition of the presence (or absence) of nanopar-ticles in natural materials under various P–T conditionsand deformation regimes;

specification of minerals in which nanoparticles aredetected in comparison with the genesis of mineral for-mation and the determination of their physical proper-ties;

identification of factors initiating the formation ofnanostructures in natural materials;

determination of the influence of time and size onthe properties of nanostructures in natural materials;

estimation of the effect of external stresses and tem-perature on the formation of or changes in nanostruc-tures in minerals and rocks;

elucidation of the possible use of nanostructures forthe estimation of the P–T conditions of their formationrelated, in particular, to earthquake source problems;and

classification of the geological conditions that arefavorable for the formation of nanostructures.

CONCLUSIONS

(1) The method of Raman spectroscopy can be usedfor detecting nanocrystals in rocks and evaluating theirsizes, as well as internal stresses and their concentra-tion in various parts of a sample.

(2) The sizes of nanocrystals in the sample studiedin this work are ~5–10 nm in anatase and quartz andabout 20 nm in plagioclase.

(3) A larger lattice spacing is characteristic ofnanocrystals of anatase, quartz, and plagioclase.

ACKNOWLEDGMENTS

This work was supported by program No. 6 of theDivision of Earth Sciences, Russian Academy of Sci-ences.

Table 3. Mean sizes of nanocrystals of anatase, quartz,and plagioclase in various places of sample PV-364

Crystal-lites Anatase Quartz Plagio-

clase

Point no. 1 2 3 4 1 2 3 3 4

L, nm 5.0 5.6 5.5 8.0 6.6 7.0 9.0 20 20

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SOBOLEV et al.

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Raman spectroscopy of nanocrystals in rock