MÖSSBAUER SPECTRA AND ELECTRON EXCHANGE IN TOURMALINE AND STAUROLITE

(1)

HAL Id: jpa-00216691

https://hal.archives-ouvertes.fr/jpa-00216691

Submitted on 1 Jan 1976

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

MÖSSBAUER SPECTRA AND ELECTRON

EXCHANGE IN TOURMALINE AND STAUROLITE

R. Scorzelli, E. Baggio-Saitovitch, J. Danon

To cite this version:

(2)

IN TOURMALINE AND STAUROLITE

R. B. SCORZELLI, E. BAGGIO-SAITOVITCH and

J.

DANON

Centro Brasileiro de Pesquisas Fisicas Rio de Janeiro, Brazil

Resume.

-

Les spectres Mossbauer de la tourmaline et de la staurotide presentent, B la temp&- rature ambiante, deux doublets de Fez+. Le doublet interne de plus faible intensite augmente avec la temperature et par l'irradiation avec des Blectrons de 2 MeV. Ce doublet disparait reversiblement aux basses temperatures (infkrieures a 77 K). Ceci a 6t6 interpret6 comme une consCquence de 1'Bchange d'un electron entre le Fez+ et la lacune dans le rkseau.

Abstract.

-

The Mossbauer spectra of tourmaline and staurolite presents at room temperature two doublets of Fez+. The inner doublet is less intense and increases with temperature and by irradiation with 2 MeV electrons. This doublet vanishes reversibly at low temperatures (below

77 K). This behaviour has been interpreted as due to an electron exchange between Fez+ and the vacancy.

The tourmaline mineral is a silicate which can be found with different chemical composition whose general formula is NaR,Al,B,Si,O,(OH, F), [I]. As indicate by X-ray structural studies, in this complex crystal structure there are two distorted octahedral sites : the first contains Mg or Mn which form trigonal planes perpendicular to cc c )) axes ; the second site is

occupied by A13'. In both sites the octahedral coordi- nation is made by four 0- and two OH- groups [2] resulting in cis and trans forms. The Fez + is a substitutional impurity in these systems and can in principle be found in both octahedral sites. Strong pleochroic bands at about 14 000 cm-I are present in the optical spectra of tourmaline as well as in others silicates (vivianite, biotite, olivine, etc.), which have been attributed to electron charge transfer between iron ions [3].

The staurolite mineral has a crystal structure cha- racteristic of a monoclinic spacial group and the accepted chemical formula is HzFe,AI,,Si,04,. The iron ions (Fez +) are tetrahedrally coordinated to four oxygens while the A13 + ions occupy three non equiva- lent octahedral sites 141.

Extensive studies by Mossbauer spectroscopy have been performed in both tourmaline and staurolite, and it has been difficult to interpret the spectra in terms of the available structural information [5, 6, 7, 8, 91.

In the present work we investigated these minerals by Mossbauer spectroscopy in a large temperature range. We have also studied the electron irradiation and thermal decomposition effects, as described in previous investigations [lo].

1. Experimental.

-

The samples studied are

natural minerals, brazilian tourmaline of Schorl type, which came from the state of Minas Gerais, and a staurolite which comes from Araxri, Minas Gerais. The experiments have been performed with poly- cristalline samples in order to avoid anisotropic effects, except when there was special interest in studying the single crystal samples. The spectra were recorded with a sinusoidal drive coupled to a Hewlett Packard multichannel analyser of 1 024 channels. A 3701145 IBM computer was used for folding and analysing the data with a lorentzian least square fitting.

The 4.2 K spectra have been taken with both

source and absorber in liquid helium. For the other temperatures the source was kept at room temperature. The measurement between 6 K and 300 K was made in an Oxford cryostat. The irradiation was performed in an electron linear accelerator with a 2 MeV electron beam with the sample kept in a liquid nitrogen cryostat. The source was a 23 mC of Co5' in Copper and the isomer shift data are given with respect to this source.

2. Results. - We have studied crystals of black tourmaline (around 10 % Fe) and green tourmaline

(< 5

%

Fe). The Mossbauer spectra of the black tourmaline (Fig. 1) have been fitted with two doublets of Fez' whose parameters are listed in table I. The large line width of the inner doublet indicates a strong inhomogeneity for this site. In all samples investigated the amount of Fe3+ was found to be less than 1 %.

5 1

(3)

C6-802 R. B. SCORZELLI, E. BAGGIO-SAITOVITCH AND J. DANON

-

4.0 - 2 0 0 20 40

V E L O C I T Y ( m m / s )

Re. 1.

-

Mossbauer spectra of black tourmaline at several temperatures.

The spectrum of the green tourmaline (< 5

% Fe)

shows only the external doublet, which corresponds

to Fe2+ with parameters 6 = 0.86 mm/s and

AEQ.= 2.39 mm/s. In general the area of the inner doublet was found to be larger in samples with rela- tively high iron concentration.

Figure 1 shows the spectrum of black tourmaline at different temperatures. It can be seen that on lowering the temperature the intensity of the inner doublet decreases and at low temperatures the spectrum consists only of the external doublet. This behaviour is reversible, the inner doublet reappearing at higher temperatures.

The temperature variation of the area of the inner doublet is illustrated in figure 2, along with the temperature dependence of the quadrupole splitting of

FIG. 2. - Upper : Temperature variation of the area of the inner doublet of tourmaline. Lower : Temperature variation of the quadrupole splittings of the outer (AQl) and inner (AQz) doublets

in tourmaline.

both doublets. The parallel variation of the doublets eliminates the possibility of supperposition of the lines, which would explain the disappearance of the inner doublet.

The area of the inner doublet varies markedly in the temperature range investigated. In order to obtain some insight into the mechanism of this temperature dependence we have calculated from an Arrhenius plot the activation energy of the process, which was found to be about 0.01 eV. This low value is typical of the range of electron exchange phenomena and precludes any process of site exchange or mouvement of heavier particles, for which a much higher activation energy is required.

The electron irradiation experiments with the two kinds of tourmalines have shown that the radiation effects are present only in the tourmaline that origi- naly exibited the inner doublet. The Mossbauer spectra obtained with a black tourmaline submitted

Mossbauer parameters of black tourmaline 6 , (1. S.), AEQ,, L, = L, (line widths) external doublet ;

(4)

to increasing doses of radiation with the 2 MeV electron beam are illustrated in figure 3. It can be seen that the major effect of the irradiation is the increase in the area of inner doublet (Table 11).

A small oxidation takes place only at very high doses (7 hours of irradiation with 15 PA).

IRRADIATED I h 2 9 8 K IRRADIATED 2 h 2 9 8 K IRRADIATED 4 h 2 9 8 K IRRADIATED 6h 2 9 8 K VELOCITY (rnm/s)

FIG. 3. - Mossbauer spectra of electron irradiated black tourmaline.

The fitting of the spectrum of the non irradiated sample was possible without imposing any ties ;

but this was not feasible with the irradiated samples, in which the broadening of the lines due to the irradiation effect is very great. The annealing of tourmaline in Hz atmosphere leads to a decrease of line width but there is no change in the area corresponding to the inner doublet. The behaviour of the inner doublet with temperature remains after irradiation,

and below 77 K the spectrum of the irradiated sample consists only of the external doublet.

Heating the tourmaline above room temperature also increases the intensity of the inner doublet. At about 800 K a slight oxidation of the sample was observed. It is interesting to observe that the Fe3+ species formed does not desappear at low temperatures, as indicated by a spectrum of the heated sample at 4.2 K.

In order to get a better understanding of the previous results we investigated the Mossbauer spectra of staurolite, which shows at toom temperature a spectrum similar to that of the tourmaline, with the advantage of presenting a larger area corresponding to the inner doublet [ll]. The crystal structure of both silicates are very different but the symmetry of the sites are quite similar and in both cases we have the possibility of finding Fez+ in A13+ sites, as has been shown in neutron diffraction studies of staurolite [12].

The Mossbauer spectrum at room temperature presented two doublets assigned to Fez

+.

The external doublet is the most intense and of smallest line width. The inner doublet is composed of very large lines which can be assigned to a superposition of lines. The spectrum did not indicate the presence of Fe3+. The large line width of the inner doublet would be in agreement with the hypothesis that the inner doublet corresponds to Fe2+ in the three differents A13+ sites. The Mossbauer spectra of staurolite shown in figure 4 shows the interesting behaviour observed with tourmaline ; at lower temperatures the inner doublet vanishes, and reappears at higher. temperatures. The parameters derived from the fitting of the spectra are given in table 111.

The fitting of these spectra is difficult because of the large line widths, which are frequent in all minerals (generally higher than 0.30 mm/s). We could fit the spectra with three doublets but the narrow line widths thus obtained are not expected to occur in mineral systems. The spectra have been fitted for 4 lines assigned to two doublets of Fe2+ up to the tempera- ture of 45 K. Below this temperature the fittings were made for two lines taking into account only one doublet of Fez+.

From the temperature variation of the area of the

Mossbauer parameters of electron irradiated tourmaline

Irradiation Time 61 A E ~ , 6 2 AEe L I

(5)

C6-804 R. B. SCORZELLI, E. BAGGIO-SAITOVITCH AND J. DANON

VELOCITY ( r n m / s )

FIG. 4. - Mossbauer spectra of staurolite at several temperatures.

inner doublet we have also evaluated the activation energy of the process, which was found to be of the same order of magnitude as that obtained with the tourmaline.

The irradiation and heating of staurolite minerals leads also to an increase of the inner doublet, and to a small oxidation. In this case it is possible to fit the spectra with 3 doublets, one corresponding to Fe3+. The irradiation effect is higher than in tourmaline,

the alterations occuring at comparatively smaller dosis.

The behaviour of the irradiated staurolite at low temperatures (Fig. 5) is similar to that found in tourmaline. At 77 K we observe only the external Fe2+ doublet superimposed on a Fe3+ doublet, which arises from oxidation by irradiation.

The Mossbauer spectra at 4.2 K of staurolite shows magnetic ordering as can be seen in figure 6. This interesting result can be correlated with previous findings of antiferromagnetic ordering of iron ions in silicate minerals [13]. The possibility of paramagnetic

V E L O C I T Y ( r n m / s ) FIG. 5.

-

Mossbauer spectra of electron irradiated staurolite as

a function of temperature.

Mossbauer parameters of staurolite

(6)

relaxation seems to be excluded from single crystals experiments which show a marked orientation dependence of the magnetic hyperfine spectra of staurolite at 4.2 K. (5. R. Regnard, Personal Communication).

; 380 W C = l a 372 0 5 5 364 3 0 0

3. Discussion. - The Mossbauer spectra of the tourmaline and staurolite samples investigated present Fez+ ions in at least two different sites. A straigh- foward interpretation is to assign the internal doublet to Fe2+ substitutional in A13' sites. In both minerals this site is the more distorted one and can present an inhomogeneous environment.

However it is difficult to explain the temperature behaviour of the internal doublet on the basis of a permanent distortion of the substitutional Fe2+ site. The low activation energy value obtained for the formation of this site suggests that an electron exchange mechanism is responsible for the appearance of the inner doublet spectrum. Such charge transfer phenomena have been invoked before in tourmalines and

- _.;.:! _{. . .}_.

: ...,. ... are used to explain optical properties of these minerals.

. . . . : .z: _.,

:._ A similar charge transfer can occur in our systems,

-

but not envolving Fe3+ which was found to be absent

.. .

. . in the samples investigated.

-

: . . . . . .

... .

.: . , .:. . ... : The substitution of Fe2+ in A13+ sites requires a . . ..:

... .. _.. .

_.

_{. .}

.. _{:. :..._,_}..: . :

.

vacancy of negative ion (possibly OH-) to achieve

. _.. , _{. .}. . .

. . charge compensation. These vacancies are electrophilic

.. . and tend to capture electrons. At lower temperatures

the electrons are localized at the vacancies but with increasing temperature they delocalize between the

- vacancies and the Fe2+ sites. The electron exchange

In a fluctuacting electric field gradient if one assumes that the transition probabilities from two states is proportional to the equilibrium population of these states, the variation of the quadrupole splitting with the temperature is given by [9, 141 :

- 4 4 - 2 2 o 2 2 4 4 between the Fez+ ion and the vacancy leads at higher

VELOCITY (rnrnlrl temperatures to relaxation spectrum which corres-

ponds to a decrease of the quadrupole splitting at the

FIG. 6.

-

Mossbauer spectrum of staurolite at 4.2 K. _{Fe2+ site.}

where u = E,/kT, E, being the activation energy of the process.

Using the parallel variation of AE,, and AEQ, from figure 2 it is possible to fit the termal variation of the quadrupole splittings in tourmaline with an activation energy of about 0.03 eV which compares with the value calculated from the thermal variation of the intensity of the inner doublet spectrum.

The increase in the intensity of the internal peaks due to the irradiation and the thermal effects can be easily understood in the framework of this model since both create vacancies in the lattice and as a consequence increase the number of Fe2+ ions which participate in the electron exchange process.

References

[I] DEER, W. A., HOWIE, R. A. and ZUSSMAN, J., (( An Intro- duction to the Rock-Forming Minerals (Longmans, Green and Co. Ltd. London) 1969.

[2] TOWSEND, M. G., J. Phys. Chem. Sol. 31 (1970) 2481. [3] WILKINS, R. W. T., FARRELL, E. F. and NAIMAN, C. S.,

J. Phys. Chem. Sol. 30 (1969) 43.

[4] NARAY-SZAB~, I. and SASVARI, K., Acta Crystall. 11 (1958) 862.

[5] HERMON, E., SIMKIN, D., DONNAY, G. and MUIR, W. B.,

Proc. Int. Conf. Mossbauer Spectroscopy, Israel 1972. [6] ZHELUDEV, I. S., BELOV, V. F., PILNEV, V. G., BELOV, A. F.,

Puoc. Int. Conf Mossbauer Spectroscopy, Bendor 1974. [7] BURNS, ROGER G., Can. J. Spectuosc. 17 (1972) 51-9.

[8] NYUSSIK, G. N., PLATONOV, Y. M., A. N., IZV. Akad. Nauk. SSSR, Ser. Geol. 2 (1970) 146-50.

[9] POLLACK, H. and BRUYNEEL, W., Proc. Int. Con$ Moss- bauer Spectroscopy (Cracow-Poland) 1975, p. 427. [lo] DRAGO, V., SAITOVITCH, E. B. and DANON, J., PYOC. Int. Conf Mossbauer Spectroscopy, Cracow 1975, p. 225. [ l l ] B A N C R O ~ , G. M., MADDOCK, A. G. and BURNS, R. G.,

Ceochim. et Cosmoch. Acta, 31 (1967) 2219.

[12] TAKEUCHI, Y., AIKAWA, N. and YAMAMOTO, T., 2. Kristoll.

136 (1972) S. 1-22.

[13] BORG, R. 5. and BORG, I. Y., Pmc. Int. Conf. Mossbauer Spectroscopy (Cracow-Poland) 1975, p. 167.