The investigation of chemical decomposition process of stalactites by electron spin resonance (ESR) technique

In imestigating the ESR properties of stalactites (in calcite form)_ we observed some anomalous resonance behavior of the hIn?+ lines in the process of heating_ This obsenntion hrtsled us to m3ke 3 critical study of the pammagnetic resonance lines, especi3lIy the ones due to mrtngzlceseimpurity ions, in strtlactite. The impze szunpks used in this study were tnken from 3 polycrystaiiine stahctite obtrtined from DiIara C3ve in the southern part of Turkey_ In order to f3ciiitate 3 compzrrisonwith our nmples 3 commercially prepared CaCO; obtained from Hopkin and Witliams Company w3s used as reference sample. This will be called the “pure sample”

from now on.

A Varian model E-12 type ESR spectrometer oper-

ating at X-band (S_S-9.6 GHz) was used to obtain the ESR spectra of the sampIes_ The ESR spectrum of the stalactite consists mainly

of a group of six fines of &In’*. In addition, 3 singIe line c”A” in fig_ ia) is also observed at the center of this qoup and it has been reported [I] to be due to the radiation induced defects (CO, and CO:-) stsbilized by impurities_ The sampfataken from the stahctite were subjected to heat treatment at different temperatures from 200°C to 9CO”C for 30 min_ The “six-doubIe line” * Some of the rf3nIts of this work tuw2 bern t&en from a joint work xkhichwill appeu in the Proceedings of “Specialisr Seminar onTherrnolumin-nce Drtt+ O_xf-or&3-8 July.

1978. I54

form of the JInZt ESR spectrum changes with increasing temperature of the he3t treatment snd becomes the “six-singie line” form 3f 750°C_ The average separ3tion of these single lines is 86 G. At this temperature the s3mpie becomes fluffy and kery white in color_ The observations

are very similnr to the characteristics

C30 [?I_ It is concluded from this f3ct that the stalactite decomposes when it ~3s heated at 750°C for 30 min and gave C30 and CO, 3ccording to the reaction of manganese-doped

c3c03

= c30 •t co,

_

(1)

In order to better understand the decomposition process two groups of sampies were heated at 700°C and 750°C respecthely for the same time intervals, and the ESR spectrum of the latter is shown in fig. lb. As seen in this figure, only the six-doubk

line form of

the Mn’* ESR spectrum is observed for the sampIes heated at 750°C for Iess than 15 min_ Increasing the heating time beyond 1.5min produced a six-singe link form of the hIn2+ spectrum in addition to the sixdouble iine form_ As the heating time ~3s increased further, the intensity of the double-line form of the Mn’* spectrum decreased and the intensity of the single-line form increased_ When the heating time nxched 60 min, the intensity of the double-line form became too small to be distinguished from the noise and only the six-single line form was observed in the spectrum (fig_ lb).

Voiumc

67, number

1

CHI:VICAL

h4n++hyperfme

lmes

40F

(0)

35 Mm.

1 November

PHI SICS LlTi-II3IS

corresponding v3htes of the mzrnganese-doped CaO [2] _ Since the total area under the single-line form is proportional to the number of CaO molecules and th3t under the double-line form is proportional to the number of CaC03 molecules, the graph of the percentage are3 versus heating time for the samples at 750°C is pIotted and the results are seen in fig_ ?a_ Thus the concIusion can be drawn that heat treatment at 750°C for 15 min causes the production of CaO from the decomposition of CaCO;. The decompositton process continues until the heating time reaches 60 min_ After that, the decomposition ceases and only the singleline form of the Mnzt spectrum similar to that obtained in MnZt doped CaO [2] is observed_ The same considerations have been given to the samples heated at 700°C and the results showed that the 60-min heat treatment is needed to start the decomposition of CaCO, at this temperature_ From these observations it is concluded that at 750°C the decomposition starts

20

40

60

ea

100 HEATING

Fig I_ (~3) ESR spectrum of natur.11std.xrite. (b) The change in JIn’+ ESR lines b> hearing AI 750°C for different rime interv& (for con.wxience. only lmes II and III arc shown).

The double-line form of the hin2+ spectrum is due to the characteristics of the calcite structure of CaCO; which has rhombohedral symmetry (space group R%), The structure gives rise to two non-identical calcium sites [3-61. The local symmetry of each of the sites is 3 due to a distorted oxygen octahedron of the nearest neighbors. On the other hand the hyperfine splittings and the g-value of the single-line form of the M&+ spectrum give very good agreement with the

1979

120 TINE

I50

w

(Hln)

COO

Fig. 2. (3) Decomposition of stalactite usin~‘ESR technique_ (b) Decomposition of stalactite by the method of gravimetry.

155

v*>:ume67.

1nIu;kr

< ill

I

uI<-.\L

1’11\‘S1<‘s 1.1--l-11 11s

01

I Sowmbsr

1979

The stime esperiments \\ere also performed for the pure samples which undoubtedly cont3in manganese impurities_ The results obtdned L’iln be seen in figs. 33 and jb_ The slight difference between figs. 23 and 33 might be esplnined by the possible effects of the p3ranugnetic impurities other than manganese which may be present in the sample_ The results are summdrired in table I_ It can be ccnciuded that the decomposition process can also be

HEATING

TINE

mii)

Fii 3_(a) Dcromposirionof pure CKO, US- ESR rtxhnique. (b) Decompositionof pure CzKOx by the method of grznimerry__

earlier than at 700°C_ The decomposition rate seems to be in equilibrium with the recombination rate of (30 3nd CO-, for the samples heated at 750°C for

rime periods greater than 60 minThe decomposition process was also investigated by the method of gravimetry- The purpose in conducting this esperiment was to establish a ratio between CaO and C&O5 as the result of heating time at 750°C_ The resuIts can be seen in fig- 2b. It is interesting to note that both fig_7-b and fig- la have the szme characteristics_ That is, the rate of change of CaO and that of CaCO, in fig. 2b are in very good agreement with those obtained by the ESR technique as seen in fig_21..

156

studied successfully by the ESR teclmique which gives almost the sztme results as those obtained by the method of gr3vimetry_ The authors wouId like to 3cknowIedge Mr. S_ Kiiltkr for his h+Ip in obtaining the results for pure siimplcn

References

[I] J. Cass, R.S. Kent, S_k Ymxhall and S.A. Zagx, J. NtgRcwn

14 (19744)17’0_

[ 71 Y. Gre& and G. Vign.md.Cornet. Rend. -&-ad_ Sci_ Wrist 8270 ( 1970) 1X35_ [3 1 F-K Hunds. 31. Sachs and W-D_ Hershberger, Phys, Rev_ 93 (1954) 373. [41 S-A Mershall and AR Reinberg. Phys_ Rev_ 132 (196;) 131 [ 5 1 S. %nkowin and IV_ LOW. Phys. Rev. B2 (1970) 2S_ 161 RX Gotdmg and 1V.C. Temunr, Yoi. Phys. 39 (19X) 167_