MÖSSBAUER ANALYSIS OF TALC AND CHLORITE

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MÖSSBAUER ANALYSIS OF TALC AND CHLORITE

C. Blaauw, G. Stroink, W. Leiper

To cite this version:

C. Blaauw, G. Stroink, W. Leiper. MÖSSBAUER ANALYSIS OF TALC AND CHLORITE. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-411-C1-412. �10.1051/jphyscol:19801159�. �jpa-00219645�

JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au n ^O 1, Tome 41, janvier 1980, page C1-411

K~SSBAUER ANALYSIS OF TALC AND CHLORITE

C. Blaauw, G . Stroink and W . Leiper

Physics department, DaZhousie U n i v e r s i t y H a Z i f m , Nova S c o t i a , Cimada B3H 3J5.

1. Introduction

Trioctahedral sheetsil icates have a 1 ayered structure with sheets of SiOk tetrahedra bonded to one (e.g., chrysotile) or two (e.g., talc) sheets of MgO6 octahedra. Such composite sheets may be held together by either van der Waals forces or by ions such as Na

+

, K+, ca2+, ~ a [I]. ~ + In this paper we discuss' 57Fe Mossbauer measurements of several samples of talc and chlorite. They are compared with previous measurements of brucite and chrysotile.

2. Experimental

Eight samples of each mineral were measured by Missbauer spectroscopy at room temperature (RT).

Two samples of each were a1 so measured at 1 iquid nitrogen temperature (LNT)

.

An X-ray analysis gave no evidence of phase impurities in any of the samples

.

No significant differences could be measured for the isomer shift (IS) and quadrupole

splitting (QS) of the different samples for either talc or chlorite, although the Fe content of the samples, as determined by an electron microprobe analysis, varied from 2% to 20% of the Mg content of the samples. We also measured, at LNT, a sample of chrysotile, which had previously been measured at RT. Numerical results are given in Table 1.

All spectra of talc could be analysed in terms of one doublet corresponding to a high spin Fez', whereas in the chlorites some Fe3' was also present, generally consisting of about 4% of the iron in the sample. This amount of Fe3+ did not give a doublet of sufficient intensity for an accurate determination of its spectral parameters, and we shall ignore it in the remainder of this pa per.

3. Spectral asymmetry

In all spectra, some asymmetry existed in the

2

+

relative intensity of the two lines of the Fe doublet. The intensity ratio varied between 1.2 and 2.3 for different samples. In our samples the talc and chlorite flakes lay predominantly in the plane of the absorber. Symmetry considerations dictate that the principal axis of the electric field gradient (EFG) is perpendicular to the flakes, so that for most of the crystallites the principal axis is parallel to the incident y-rays.

Under those conditions an asymmetry of the line intensities is expected.

In previous work [21 we argued that in brucite the ground state of the system would be an AIg- singlet, which means that the 6th 3d electron of the ferrous ion is in a dZ2 orbital. Because the local symmetry features are the same, we expect the same result to be true for all the sheetsilicates considered here.

The quadrupole coupling constant e2qQ is expected to be negative, because Q is positive for the excited state of 5 7 ~ e and q is negative for the

3 1

dZ2 orbital. This means that the ( 2 - ⁺

*

^-)

l 2 transition has a lower energy than the (* - 2 +

*

²⁾

transition of the 57Fe nucleus. Because for small angles between the EFG principal axis and the

3 1

y-rays the ( + 2 +

*

^-) ₂ transition will have the larger intensity, we expect this line to be the low velocity line in the Mb'ssbauer absorption spectrum, in agreement with the experimental results.

4. Isomer shift and quadrupole splitting

At a given temperature, the Fe2" IS essentially the same, reflecting the fact that the near

neighbouring environment of the Mg-sites is similar in all silicates considered here. For the Fez' QS, differing values reflect small differ- ences in the environment of the Fez' ions in the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801159

c1-412 JOURNAL DE PHYSIQUE

TABLE 1 . Fe Mossbauer Parameters of Some Trioctahedral S h e e t s i l i c a t e s . All values are in mm/sec.

IS a r e r e l a t i v e t o metallic iron.

Mineral Brucite Chrysotile

Talc Chlorite

References C 21

80K: t h i s work;

300K: C31 This work This work

d i s t i n c t minerals. We note t h a t t h e difference in the QS between LNT and RT i s s i g n i f i c a n l t y l a r g e r in t a l c than i t i s in t h e other sheet s i l i c a t e s . If the change in t h e QS with temperature i s s o l e l y due t o a repopulation of the ~ e " energy l e v e l s i t i s possible t o c a l c u l a t e from the data the

spl i t t i n g 6 between t h e lowest A and E energy

'9 9

levels in the present configuration 121. W find a e value of A = (900 2 1.00) K f o r t a l c , as opposed t o values of t h e order of 1100 K f o r the other s i l i c a t e s . Physically, the difference may be related t o the smaller average Fe-0 distance reported f o r t a l c (1.88 A , 141) a s compared t o values reported f o r t h e other s i l i c a t e s discussed here (2.05-2.10 A, [5,6] ) .

5. S i t e designations

The spectral linewidths in t a l c correspond t o those normally found with our spectrometer and indicate t h a t there i s only one doublet. Thus, the d.ifference between M1 and M2 octahedral s i t e s -in t h i s material does not r e s u l t in measurable

differences in t h e corresponding Mossbauer double%.

2

+

In t h e c h l o r i t e s , s l i g h t l y broadened Fe l i n e s were observed. One possible i n t e r p r e t a t i o n i s t h a t a small difference between M1 and M2 s i t e spectra i s present here. However, because t h e c h l o r i t e s t r u c t u r e consists of a l t e r n a t e layers of t a l c and brucite, ~ e may a1 so be present in t h e ~ + brucite layer octahedral s i t e s . No d e f i n i t e con- clusions can be drawn about t h e r e l a t i v e occu- pation of these s i t e s , but t h e QS measured i s much closer t o t h a t in t a l c than t o t h a t in brucite, suggesting t h a t t h e t a l c r a t h e r than t h e

6. Acknowledgments

Samples of t a l c and c h l o r i t e were provided by Dr. M. Z e n t i l l i of the Department o f Geology a t Dalhousie University. This work was supported f i n a n c i a l l y by t h e National Science and Engineering Research Council of Canada.

7 : References

[ I ] Dana, J . D . , Manual of Mineralogy. 19th ed., John Wiley & Sons, New York (1977).

[ 21 Blaauw, C . , Stroink, G., Leiper, W . and Z e n t i l l i , M . , Phys. S t a t . Sol.

b92

^(1979),

in press.

C31 Blaauw, C . , Stroink, G., Leiper, W. and Z e n t i l l i , M., Can. Mineral. (1979), i n press.

[4] Hendricks, S. B . , 2 . Krist.

99

(1938), 264.

C51 Wicks, F. J . and Whittaker, E . J . W . , Can.

Mineral

. ¹³

(1975), 227.

[ 6 ] Shirozu, H . and Bailey, S. W., Am. Mineral. 50

(1965), 868.

brucite layer octahedral s i t e s a r e occupied by

~ e ~ ' .