AN 57Fe MÖSSBAUER STUDY OF THE HEAT TREATMENT OF ALIBATES FLINT

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AN 57Fe MÖSSBAUER STUDY OF THE HEAT TREATMENT OF ALIBATES FLINT

C. Jones, M. Dombsky, A. Skinner

To cite this version:

C. Jones, M. Dombsky, A. Skinner. AN 57Fe MÖSSBAUER STUDY OF THE HEAT TREAT- MENT OF ALIBATES FLINT. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-481-C2-482.

�10.1051/jphyscol:19792167�. �jpa-00218542�

JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3, Tome 40, mars 1979, page C2-481

5 7

AN

C.H.W. Jones, M. Dombsky and A.F. Skinner

Department of chenristry, Simon Fraser University, Burnaby, B r i t i s h CoZwnbia, V5A 15'6, Canada.

Xdsumd.- Les effets de recuit sur des Qchantillons d'Alibates flint ont 6t6 Qtudids par spectrosco- pie MEssbauer du 5 7 ~ e . Pour des recuits 1 temperature dgale ou supdrieure 1 27,5OC, les spectres, enregistrgs 1 la tempdrature ambiante ou 1 celle de l'azote liquide, prQsentent deux modifications : i) une d6composition thermique de a-Fe00H conduisant 2 a-Fe203 et ii) une variation apparente de la fraction d'effet sans recul pour la contribution aux spectres de a-Fe203. Ces rdsultats prdliminai- res suggsrent que cette technique peut Stre utile pour pr6ciser les conditions de recuit d'dchan- tillons archEologiques B base dtAlibates.

Abstract.- The effect on the 5 7 ~ e MEssbauer spectrum of heating samples of Alibates flint has been studied. On heating at or above 275OC, two changes in the 5 7 ~ e spectrum, subsequently recorded at room temperature or liquid nitrogen temperature, were observed : i) the thermal decomposition of cl-Fe00H in the sample to yield a-Fe203, and ii) an apparent change in the recoil-free fraction of the a-Fe203 component. These preliminary results suggest that this technique may find some use in determining to what extend artifacts made from Alibates flint,and which may be of archaeological interest, have been heated by prehistoric peoples in the past.

I. Introduction.- For some time archaeologists have been interested in the extent to which prehistoric peoples heat-treated chert or flint prior to sha- ping it into tools. A number of techniques have been used in atte:n?ting to establish the extent of this practice, including thermoluminescence /l/, elec- tron diffraction / 2 / , and X-ray diffraction / 3 / . In the present paper the effect of pre-heating on the 5 7 ~ e Mijssbauer spectrum of Alibates flint from Texas has been studied. This mineral was chosen for study because it was widely used by prehistoric peoples of the area in the manufacture of tools and implements.

2. Results and Discussion.- Samples taken for stu- dy were thin slivers of material (1 to 2 cm2 and ca. 114 mn~ thick) obtained by knapping from the fresh face of a much larger piece of flint. Six samples from three different sources of Alibates flint were investigated. Some samples were washed with dilute nitric acid and distilled water and yielded precisely the same spectra as samples which had not been washed. The samples studied varied

from dark brown to pinkish red. All gave similar 5 7 ~ e ~lijssbauer spectra following similar thermal treatment.

The 5 7 ~ e Mijssbauer spectra were recorded at room temperature and at liquid nitrogen temperatu- re and the two were essentially the same for any one sample, differing only in % resonance absorp- tion.

Figure 1 shows a typical set of spectra ob- tained at liquid nitrogen temperature for a sample

which was heated for 14 hs. first at 180°c, then at 275'~, and finally at 525'~.

1

-

5 '

l- !a

HEATED 275%

-10 -6 -2 +2 +6 +l0

VELOCITY (mrnls) 2-

Ffg. 1 : " ~ e ~assbauer spectra recorded at liquid nltrogen temperature for a sample of Alibates flint which had been heated for 14 hs. at 180, 275, and 525OC.

HEATED 525%

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792167

C2-482 JOURNAL DE PHYSIQUE In the unheated sample, three iron species can be

identified; i) a-Fe203, the domir.ant cordponent

,

ii)an overlapping magnetic hyperfine split compo- nent of low intensity which may be assigned to a-FeOOH, and iii) a broad irresolved absorption of low intensity at the centre of the spectrum, cor- responding to non-magnetic iron. Following heating at or above 275°C for 14hs., a-FeOOH was no longer observed in the spectrum while the a-Fe203 absorp- tion shobed a considerable increase in intensity.

This may be ascribed to the thermal decomposition.

2 a-FeOOH

+

which is known t 3 occur between 200 and 300'~ in naturally occurring goethite minerals 141.

However the increase in intensity of the a-Fe203 absorption zas too large to be explained solely by the decomposition of the small amount of a-FeOOH originally present. A detailed study of another sample showed that on heating at 240°C the absorption due to a-Fez03 was again observed to increase by a factor of two in intensity, while that due to a-FeOOH did not change significantly.

On heating at a higher temperature the a-Fe00H was then observed to decompose. The former effect may arise from a change in the crystallinity of the impurity sites in the flint, and hence a change in the MSssbauer recoil-free fraction, when the sample is heated. Electron microscopy /2/ has given clear evidence of such changes in samples of chert hea- ted to 200 to 800°C.

3. Conclusions.- On heating samples of Alibates flint in the range 200 to 300°c, two effects are observed. Firstly, there is an apparent change in the recoil free fraction of the a-Fe203 coinponent on heating at ca. 240'~ and, secondly, on heating at a higher temperature a-Fe00H present thermally decomposes to a-Fe203.

A major question arises in the possibleappli- cation of this technique in studying the thermal history of flints or chert, since these minerals are not homogeneous and different samples taken from the same source may be found to contain a dif- ferent spectrum of iron sites. Nevertheless, the presence of a-FeOOH in the spectrum of a sample of Alibates flint would suggest that specimen had not been heated, at least to 275"C, in the past. Simi- lary, if a sample under study can be shown to exhi- bit a marked change in recoil-free fraction for the a-Fe2Or component following heating then this would suggest that the sample had not been heated pre- viously. The complete absence of a-FeOOH in the spectrum of a sample of Alibates flint, together

with an isensitivity of the a-Fe 0 absorption to pre-heating of the sample to 240 to 300°C, would provide tentative evidence that the sample had been heated previously. Clearly it would be unwise to draw any major conclusions solely on the basis of such tentative evidence. Results obtained from MEssbauer spectroscopy may be best used in conjunc- tion with those from other techniques, e.g., ther- moluminescence /l/. It may be of interest to exami- ne other types of flint and chert to determine whether they show a similar behaviour to Alibates

flint.

Finally, it must be noted that it would not be possible to distinguish, using this technique or thermoluminescence, between deliberate heating and accidental heating, for example in a forest fire. Presumably that is a problem for the archaecr logist to solve.

4. Experimental.- The 5 7 ~ e MEssbauer spectra were recorded on single pieces of Alibates flint as des- cribed above, either at room temperature or liquid nitrogen temperature. The source (which was main- tained at room temperature) was a 20 mCi 5 7 ~ o / ~ d source obtained from New England Nuclear. The cons- tant acceleration drive was calibrated against an iron foil absorber. A Kr/C02 proportional detector was used.

Acknow1edvements.- The authors wish to acknowledge a grant in support of this research from the Na- tional Research Council of Canada and support from the Presidents Research Fund, Simon Fraser Univer- sity, to assist in the purchase of the source.

We wish to thank Dr. Erle Nelson (Depart- ment of Archaeology, Simon Fraser University) for

suggesting this project and for kindly supplying samples of the flint.

References

/l/ Melcher, C.L. and Zimmermh, D.H., Science

197

(1977) 1359.

/ 2 / Mandeville, M.D., Plains Anthropologist

2

(1973) 177.

/3/ Weymouth, J. and Mandeville, M.D., Archaeome- try

17

141 Govaert, A., Dauwe, C.

,