MÖSSBAUER STUDY OF NATURAL CRYSTALS OF STAUROLITE

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MÖSSBAUER STUDY OF NATURAL CRYSTALS OF

STAUROLITE

J. Regnard

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, 1976, Dtcembre page C6-797

J. R. REGNARD

Centre dYEtudes Nucleaires de Grenoble DRF/Groupe d'Interactions Hyperfines 85X-38041 Grenoble Cedex, France

Rbsumb.

-

Des echantillons monocristallins et en poudre de staurotide ont kt6 eiudiks par spectromktrie Mossbauer B T = 300, 77 et 4,2 K. Contrairement aux resultats antkrieurs obtenus par diffraction des rayons X et par spectromktrie Mossbauer, on observe B 300 K la presence de trois doublets quadrupolaires qui correspondent B trois sites de Fez+ distincts. A partir des intensites des raies Mossbauer obtenues lorsque les rayons y se propagent selon les axes a , b ou c du cristal, on dkduit la direction de I'axe principal du gradient de champ electrique en ce qui concerne le site tetraedrique du Fez+ : cet axe se situe dans le plan (b, c) B environ 55O de l'axe c. A 4,2 K les spectres Mossbauer indiquent la presence d'un ordre magnktique oh les spins sont presque parall6les

B I'axe b du cristal. Par spectromktrie Mossbauer on determine une temperature de transition magnktique de 6 f 1 K.

Abstract. - Mossbauer experiments have been carried out on powder and crystal samples of staurolite. Contrary to the previous X-ray and Mossbauer results, we observe at 300 K the presence of three different quadrupole doublets corresponding to three distinct Fez+ sites. From the intensity of the Mossbauer lines obtained when the y-rays are parallel to a, b or c axes of the crystal, we can deduce that the principal axis of the electric field gradient for the Fez+ tetrahedral site is situated in the (b, c) plane at about 55" from the c axis. At 4.2 K the Mossbauer spectra indicate the presence of a magnetic order with the spins roughly parallel to the b axis. The ordering temperature is found to be 6

-+

1 K.

1. Introduction.

-

Many silicates have been studied by Mossbauer spectroscopy [I] but generally in powdered form and a t room temperature. We report here a preliminary study of Mossbauer absorption for monocrystals of staurolite from Locmine (BRI- TANNY). This mineral crystallises either as large monocrystals (Fig. la), or in cruciform twins of two types : one in which the two individuals cross a t nearly 900 (Fig. lb), the other in which they cross at nearly 600. The ideal formula proposed for this natural silicate is HFe2Al,Si4024 [2]. The symmetry is

monoclinic (pseudo-orthorhombic) with two molecules per cell (a = 7.82

A,

b = 16.52

A,

c = 5.63

A,

P

= 900). 2. Results.

-

The chemical analysis of the crystals of staurolite (brown colour) was done by emission spectroscopy and atomic absorption spectrophoto- metry. The analyses of our specimens are listed in table I together with the average composition from previous results [3-51 and the ideal composition. We remark that the SiO, and Al,O, content is some- what different compared with the other specimens

FIG. 1. - Monocrystal (a) and 90° cruciform twin (b) of staurolite from Britanny. The direction of the crystallographic axes are shown in (a).

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C6-798 J. R. REGNARD

Chemical analyses of dzyerent staurolite specimens. The content of the diferent oxydes are given in weight per cent.

(1) This work (2) average composition from refe- rences [3-51.

(3) Composition from the ideal formula

H 2 0 . 9(A120,). 8(Si02). 4(Fe0). Oxide component

-

SiO, TiO, A1203 FeO

+

e(Fe,O,) MnO MgO H z 0

and the ideal formula. The percentage of 40

%

for A1203 and 45

%

for SiO, was confirmed by a complementary gravimetric analysis and it seems that the composition of staurolite from BRITANNY is slightly different from the others at least for the two major metallic elements. The proportion of FeO in our crystals (11.3

%)

is a little smaller than the usual concentration of 13-14

%.

VELOCITY (mmls)

FIG. 2.

-

Mossbauer spectra of staurolite powder at1300zand 4.2 K. For this figure and the following the solid lines are-least-

square fits of the data.

The three crystallographic axes a, b and c were located by X-ray diffraction : as shown in figure l a ,

the c axis (growth axis of the monocrystal) is perpendicular to the lozenge plane where the- a and b axes are along the short diagonal and the long diagonal respectively.

I

-2 .1 0 1 2 3 -3 -2 -1 0 1 2 3

VELOCITY (mmls)

FIG. 3.

-

Mossbauer spectra of staurolite crystal obtained at 300 K and 4.2 K with the y-ray propagation direction oriented

successively parallel to the a, b, c axes.

Mossbauer experiments have been carried out on powder samples of staurolite and on 0.3 mm crystal slices cut perpendicular to the a, b, c axes. The spectra of powder samples and of oriented crystal slices a t 300 K are given in figures 2 and 3. The 300 K powder spectrum can be fitted with three symmetric quadrupole doublets corresponding to the Fez+ charge state and a small doublet for Fe3+ (2-3

%

of the total absorption). The parameter values of these ferrous doublets are given in the table 11. The Mossbauer spectra obtained

Parameter values given by the best least-square Jits of the 300 K powder spectrum. 6 (mm/s) is the isomer shift relative to iron metal,

r

(mm/s) the hafl width of the lines, 2 E (mmls) the value of the quadrupole splitting and I

(%)

the relative intensity of the diferent doublets. 6

r

2 E I Fez+

-

(mmls) - (mmls)

-

(mmls)

-

(%)

-

Doublet I 0.94f 0.01 0.21rt0.01 2.38jI0.02 60rt2 (lines 1 and 6) Doublet I1 0.95h0.02 0.20% 0.02 1.80f 0.04 16&3 (lines 2 and 5)

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with the y-ray propagation direction parallel to the a, b, c axes of the crystal (Fig. 3) are composed of three asymmetric doublets in the same positions, as for the powder, but the relative intensities of each pair of lines depend on the angle between the y-ray direction and the principal axis of the electric field gradient (e. f. g.). The relative intensities of the lines are given in table 111. Concerning the doublet I, due

Relative intensities of the Mossbauer lines when the y-ray propagation direction is successively parallel to the a, b, c axis of the staurolite crystal. Pairs of

lines 1-6, 2-5, 3-4 constitute the I, 11 and III doublets

respectively (1 + 6 corresponds to increasing energy)

1 1 1 2 1 3 1 4

-

1 5 1 6

y / a 4 0 f l 12% 1.5 14f1.5 9f.l 7 1 1 18f 1

y / / b 20f 1 8 f l 9 f 1 1311.5 9f 1 41f.1

Y / / C 3 3 f 1 6 f 1 10&l lOfl l l f l 30f 1

to the majority Fez+ site, one can see that the ratio of the intensity of the lines 1 and 6 depends strongly on the orientation of the crystal :

Furthermore the 300 K powder and y

//

c spectra

I1

are very similar, and

-

(y

//

c) is very close to unity.

I6

For the two other orientations, the value of this ratio is inverted.

As shown on figure 4, the crystal spectrum (y

//

c) at 77 K is composed mainly of a broad quadrupole

VELOCITY (mmls

FIG. 4. - Mijssbauer spectrum of staurolite crystal at 77 K with the orientation y // c.

doublet due to the I contribution (6 = 1.06 mm/s,

r

= 0.26 mm/s, 2 E = 2.79 mm/s, I = 80

%).

The

intensity of the inner doublets decreases clearly with temperature (Fig. 4) suggesting that the three different doublets overlap partially to give a single broad doublet. At 8 K, the contributions of the I1 and I11 doublets still exist, but are very weak.

At 4.2 K (Fig. 2 and 3), the Mossbauer spectra indicate the presence of magnetic order. The value of the magnetic hyperfine field is found to be of the order of 160 kOe at 4.2 K. The magnetic ordering temperature has been measured to be 6 _+ 1 K.

3. Discussion.

-

Concerning the 300 K Mossbauer results, we ,observe here three different quadrupole splittings instead of one or two in the previous stu- dies [3, 61. The parameters for the largest doublet are similar to those given in ref. 131. Contrary to the first X-ray results [7], Fez+ ions occur in three distinct coordination sites in the staurolite structure : the principal pair of peaks ( I = 60

%)

is attributed to Fez+ in the iron site (tetrahedral site). The others two can be attributed to Fez+ in the two lowest symmetry sites (very distorted octahedral sites) such as Al(3A) or Al(3B) in which Fe2+ substitutes for A13+ [4]. The marked difference with the previous Mossbauer works comes from the contribution of the I1 and I11 doublets (about twice larger'in this work). The fact that the iron site is only 60

%

occu- pied by Fez+ would suggest that some of the SiO, in excess of the ideal formula is also found on this site.

The intensity values of the lines of the principal doublet (I) are obtained by least-square fits of the spectra (Table 111) for the three distinct orientations of the crystal. From these values, one can obtain information about the direction of the principal axis of the e. f. g. If

el,

8,, 8, are the angles between the a, b, c axes respectively and the z direction of the e. f. g. principal axis system, we can derive quite consistent values of the three angles :

(i) from the similarity between the 300 K powder and y

//

c spectra confirmed by the ratio

one can deduce that sin2 8, N 213 and cos2 83 N 113 (mean values of the powder case). That means that the angle between the c axis and the principal axis of the e. f. g. is 9, cz 5 5 O ;

(ii) the angular dependence of the intensity of the lines of a quadrupole doublet is given by 3/2(1

+

cos2 8) for the

+

312 +

+

112 transition and 1

+

(312) sin2 8 for the

+

112 +

+

112 transition. Using the values 11/16 for y / / a and y

//

b, we have : (a) in the case

e2qQ

>

0 where

2

+

3 sin2

el

2

+

3 sin2 8,

N 2.2 ; N 0.5,

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C6-800 J. R. REGNARD we obtain

8, N 900 and 82

-

28" ; (b) in the case eZqQ

c

0 where

we obtain 8,

-

24O and 8,

-

900. Since in case (a) the angles 8, and 8, should be complementary and in case (b) the same applies for 0, and 8,, the situation (a) where e2 qQ is positive seems to be more appro- priate. This result is in agreement with the positive sign of the quadrupole interaction found in the magnetic phase (Fig. 2) by using the diagrams given by kiindig [8]. Then the direction of the principal axis of the e. f. g. for the ~ e tetrahedral site is given ~ ' by the three angles 8,

-

900, 8,

-

280, 8,

-

550 and is situated in the (b, c) plane at about 550 from the c axis.

At 77 K, the spectrum of staurolite crystal obtained with y

//

c is mainly composed of a broadened quadrupole doublet (80

%)

and the contribution of the inner doublets (20

%)

is reduced by a factor of 2 from 300 K. These doublets have been attributed to Fez' ions substituted in Al(3A) and Al(3B) sites as suggested by ref. [3]. The substitution of A13+ by Fez+ ions in these sites creates a negative vacancy in the neigh- bourhood of Fe2'. The charge compensation can be done in three ways : (i) by the substitution of A13+ by Ti4+ or Si4+ on a neighbouring site ; (ii) by the presence of a OH- group substituting O = in the octahedron surrounding the Fez' ion ; (iii) by the trapping of electrons in the negative vacancy. In the

case (ii) the two inner quadrupole doublets can be due to two definite substituted A1 sites and to a different location of OH- groups near the site. In the case (iii) Scorzelli et al. [9] have suggested that the intensity variation with temperature of the inner peaks are due to a time dependent electron exchange phenomena and not to a definite site. At the present time, it is difficult to distinguish between these three possibilities, although the SiO, content suggests that the charge compensation is satisfied by the presence of Si4+ on a neighbouring A1 or Fe site.

The last interesting point is the appearance at 4.2 K of a magnetic order : this material is one of the rare examples of a silicate mineral with magnetic order at helium temperature. For the moment we have not completely analysed the 4.2 K powder and crystal spectra, where the quadrupole interaction has the same order of magnitude that the magnetic interaction. From the intensities at 4.2 K of the y

//

b and y

//

c spectra, it seems that the spin directions are roughly parallel to the b axis and consequently in the same direction as the principal axis of the e. f. g. The value

6 K of the magnetic ordering temperature is consistent with the large distance between iron atoms

(Fe,

-

Fe, 6.8 i%; Fe,

-

Fe,*

--

3.4

A)

and the low iron content of the mineral.

Acknowledgments.

-

The author wishes to thank Dr. 5, M. D. Coey and Dr. 5. Chappert whose help and advice made this work possible. The author is also indebted to Dr. Laugier for the X-ray orientation analysis of the crystals.

References

[I] RACLAVSKY, K. Proceedings of the 5th Int. Con$ on Moss- [6] BANCROFT, G. M., MADDOCK, A. G. and BURNS, R. G.

bauer Spectroscopy, Bratislava, part 2 (1973) 347 and Geochem. Cosmochen. Acta 31 (1967) 2219.

references therein. _{[7] NARAY-SZABO,}_{I. and SASVARI,}_{K., Acta Crystall.}₁₁₍₁₉₅₈₎

121 WYCKOFP, R. W: G., Crystal structures 2d edition vol. 4 OL?

(1968) 184. O W L .

[3] TAKEUCHI, Y., AIKAwA, N. and YAMAMWO, T., 2. ~ ~ j181 K ~ I G , ~ - W., Nuc~. Instrum. and ~ e t h . 48 (1967) 219.

tall. 136 (1972) 1. 191 SCORZELLI, R. B., SAITOVITCH, E. B. and DANON, J., Pro-

[4] SMITH, J. V., Amer. Mineral. 53 (1968) 1139. ceedings of this International Conference on Moss- [5] JUURINEN, A., Ann. Acad. Sci. Fenn., Ser. A I11 Geol. 47 bauer spectroscopy, Corfou (1976). J. Physique Colloq.