Thermoluminescence emission spectra of calcite and Iceland Spar

Nucl. Tracks. Vol. 10. Nos 4-6, pp. 581-589. 1985

0191-278X/8553.00+0.00 Pergamon Press Ltd.

Printed in Great Britain

T H E R M O L U M I N E S C E N C E EMISSION SPECTRA OF CALCITE AND ICELAND SPAR J. S. DOWN, R. FLOWER,J. A. STRAINand P. D. TOWNSEND MAPS, University of Sussex, Brighton BN1 9QH, U.K. (Received 25 October 1984; in revised form 23 Janua O' 1985)

Abstract--Natural calcite samples have a yellowish hue attributed to Mn impurities whereas the purer CaCO 3crystals of Iceland Spar are colourless. Both minerals give strong thermoluminescence with peaks near 75 and 250°C. This paper reports on detailed measurements of emission spectra recorded during TL. For calcite the 60 and 200°C glow peaks have a broad emission centred at 630 nm which extends from 550 to 750 nm. Under high resolution fine structure is apparent. The 400°C peak in calcite is very different with emission bands less than 30 nm wide centred at 560, 600, 640, 705 and 750 nm. By contrast, the Iceland Spar gives a similar set of broad line features at lower temperatures and lacks a 400°C glow peak. The relative intensities of the lines change with temperature and vary with different samples. It is proposed that Mn is important in all the luminescence sites but in the heavily doped calcite the impurities aggregate into defect complexes at lower temperatures. At high temperatures, or in lightly doped Iceland Spar, the Mn is atomically dispersed so emission characteristic of Mn energy states is feasible. To change the state of the system crystals were quenched or slow cooled from temperatures up to 700°C. Major changes occur.

1. I N T R O D U C T I O N CALCITEis a relatively common mineral and has been the subject of numerous thermoluminescence (TL) studies (e.g. Medlin, 1963, 1968; Townsend, 1968; Levy et aL, 1971; Bapat and Nambi, 1975; Visocekas, 1979; Debenham et al., 1982; Sunta, 1984). Such studies are of value for geological dating. The material is a bright phosphor in the orange region of the spectrum and one may record spatial inhomogeneities or spectra of the various glow peaks. It is generally assumed that the luminescence site includes a manganese ion and the present work gives data from samples of impure calcite (Joplin) and Iceland Spar. The latter are thought to contain less manganese. Irradiation with X-rays, electrons or alpha particles induces different TL characteristics, as do thermal treatments. These differences are not the same in calcite and Iceland Spar.

2. EXPERIMENTAL DETAILS The experimental system records the TL whilst repetitively scanning through the emission spectrum. This TLSP facility was previously described by McKeever et al. (1983), in which data are stored on a

microcomputer and corrected for the wavelength response of the system. Samples were heated from 20 to 500°C at rates ranging from 5 to 40°C min-1 and the monochromator resolution was 6 nm, Higher resolution of 0.4 nm bandwidth did not confirm the presence of finer detail. For a visual appraisal of the data the results have first been displayed as an isometric projection of the variables intensity, wavelength and temperature (L 2, T). Because the spectra are scanned there is a minor distortion of the signal relative to the temperature axis in this simple presentation. Whilst this may be corrected in the computing process the effect is not visible in the isometric plots. The wavelength response correction is essential and the conversion of 'raw' data to those adjusted for the wavelength-dependent response function of the monochromator and photomultiplier tube (EMI 9659QAM) are obvious. A further correction factor is needed if one displays the results in terms of photon energy as the monochromator has a fixed wavelength bandwidth. Since the photomultiplier response and the monochromator decrease in sensitivity from the blue (400 nm) to the red (800 rim), the original calcite spectra change shape on correction with a significant enhancement of the red end of the spectrum. In most 581

582

J . S . D O W N et al.

6.0 5.0 4 0 3.0 d.O

-6O

l.O

]so ]4 o

.0 4.5(

-3o ]20

4 O0

3.5C 3.00 2.50" •

~'~ 2.0C

°*,Oo

~I"

7.25

.75

I .SO

6.25 .00

.75 .SO

m

25

k~.~OO3

FIG. 1. Isometric projection of a calcite crystal TLSP after X-ray irradiation for a heating rate of 40°C min- L

of the following isometric displays the black body component has been removed. The same data may be processed to be shown as contour maps of the wavelength temperature plane or as slices at fixed T (i.e. a spectrum) or fixed 2 (i.e. a simple TL plot). 3. OBSERVATIONS

Figures 1 and 2 display the isometric projections I (2, T), viewed from different angles, which illustrate data for samples of calcite and Iceland Spar after X-ray irradiation at 20°C. Some differences are immediately apparent in that the lower temperature ( < 275°C) glow peaks for calcite are broad emission bands, whereas the high temperature calcite signal and all of the Iceland Spar signals are in the form of line structure. In natural calcite the lower temperature, 60°C, glow peak is absent. The Iceland Spar

used here was synthetic so lacked any natural TL. X-ray irradiation only produced Iceland Spar signals up to ~ 350°C. Greater detail is apparent from the contour plots of the data. Figure 3 shows a contour map for X-irradiated calcite taken at a heating rate of 40°C min-L Figure 4 gives a map for electron irradiated Iceland Spar, taken at 40°C min -~ which emphasises some features additional to those shown in Fig. 2. Glow peak temperatures are given in Table 1 for the 20°C min- ~ rate. Their corresponding spectra are listed in the table. Examples of the emission spectra are given in Fig. 5 from which one notes that the line features occur at similar wavelengths for both materials and are coincident with the structure within the broad emission bands of the low temperature peaks. Detectable variations in peak positions occur for different samples. The relative strengths of the component features change with temperature and hence a simple TL

TL EMISSION SPECTRA OF CALCITE A N D I C E L A N D SPAR

583

-70 1o.o -5o ;-,.o :30 :20

Z.O C,O

- i .0

50 .0 4,0 50

5-0

-00

2.0

.50

1.0 0

"¢

2.00

750

.50

oG$ ~O0

7.0C

4

6.5o

;.00 6. O0

'%r, ,Oo, )

5,50 5.5(

5.oo'',t,.~2 ~

4.oo

FIG. 2. Isometric projection of the TLSP from Iceland Spar after X-ray irradiation and heating at 40°C min-). curve taken with a filter would generate TL peak intensities which are a function of the filter. Figure 6 shows examples of T L slices taken at the wavelengths 575, 600 and 635 nm. There is not an obvious pattern to these temperature-dependent changes, but the presence of these variations implies that the luminescence sites are not identical for each glow peak. Electron irradiation from a 9°Sr source produced TLSP data from calcite, which are very similar to those generated by X-rays, except that it enhanced the 200 and 325°C peaks. However, the electron irradiation of Iceland Spar added or strengthened a number of glow peaks, in particular near 75 and 350°C. Also an enhanced emission at 480nm was noted. The changes imply that additional trapping centres were produced as well as variants of the luminescence sites. Alpha particle irradiation from an 24'Am source caused major changes in the TLSP data as briefly indicated in Table 1. No details of dosedependent changes are reported here. 4. DISCUSSION Previous TLSP experiments with calcite (Medlin,

1963, 1968; Townsend, 1968) have detected the broad band emission of the lower temperature glow peaks and also the evidence for structure within the band. Medlin has further argued that in many calcium containing compounds the substitution of Mn impurity ions leads to an efficient luminescence process. In particular, the calculations by Orgel (1955) and later work by Medlin (1963) with rhodochrosite (MnCO3), which is isomorphous with CaCO3, gives a possible configurational co-ordinate diagram for Mn in CaCO3 and from this one can suggest labels for the energy states involved in the luminescence. More recent luminescence studies by Aguilar and Osendi (1982) are in accord with these suggestions. Aguilar and Osendi used both calcite with ~ 1000 p.p.m. Mn and Iceland Spar with ,,, 10 p.p.m. Mn. The present work similarly leads to a series of labels for the line spectra involving the Mn impurity in the CaCO3 lattice. The closest fit to the Medlin configurational co-ordinate diagram suggests the light at 650 nm would correspond to the 4G (Tts) to 6S transition between the first excited and ground state. The 480 nm line, seen after electron irradiation of Iceland

584

J . S . D O W N et al. I

4.7~

, i.,,

1

I

I

i

I

i

i

I G.O3

I

i 6.50

I

i

I

i

4.25-

3.75-

3,25-

2 7.~

o 0

d

2.75

*6 Q

z5

25

.75

.25 , ~,J

i .5,3

] 5 • O0

I

I 5.53

~nm(x

I

l 7

I 03

I 7.50

.Z5

100)

FIG. 3. Contour map of data for TLSP from X-irradiated calcite heated at 40°C rain-~. Spar matches the position of the Tu to 6S transition between the second excited and ground states. Positive identification of other lines is not possible because of the uncertainty in the production of the Medlin configurational co-ordinate diagram, although in broad terms the energy scheme looks sensible in agreement with Aguilar and Osendi. Lower energy transitions at 705 and 750 nm presumably correspond to transitions between higher energy states. A more major problem is whether the energy scheme of Medlin and the various crystal field calcu-

lations for Mn in different lattices (Tanabe an Sugano, 1954; Orgel, 1955) are appropriate--as shall argue that the Mn ions are not atomicall dispersed in the CaCO3. One may consider the well-known problem of th L i F : M g : T i TL dosimeter in which the Mg is atorr ically dispersed at high temperature (400°C), can 1: quenched to produce TL peaks (2, 3) or aggregated t tnmers to form the more stable peaks (4, 5). Furthe: excess Mg does not enhance the signal but instea precipitates out in a Suzuki phase. At first sight this seems a reasonable analogy fc

TL EMISSION SPECTRA OF CALCITE A N D I C E L A N D SPAR

585

.25

3

75

3 25-

2.;'5

O O

2.25.

cl

! l-

I .75-

] .25,

©

.75-

.25, .00

i

4.50

I

)

5.00

.... '

~.~

o.~u

r-oo

?-50

7.7

~nm(x 100)

FIG. 4. Contour map of data for TLSP from electron irradiated Iceland Spar heated at 40°C rain- ].

Mn in CaCO 3. Bapat and Nambi (1975) noted that above 0.3 mol.% the Mn related luminescence decreased with impurity concentration. This is in accord with the suggestion that the heavily doped calcite has Mn clusters or precipitate phases leading to broad emission bands. Whereas at higher temperatures, or in lightly doped material such as Iceland Spar, the Mn solubility is sufficient to produce line spectra from dispersed material. To test this hypothesis we have heated samples at various temperatures up to 700°C and then either slow cooled them or quenched them by dropping them from the furnace into liquid nitrogen. Some of the data are listed in Table 1.

If one proposes that there are at least four regimes in which the manganese exists, namely (i) isolated Mn; (ii) small clusters of Mn (e.g. associated pairs); (iii) larger clusters (e.g. trimers) and (iv) precipitate or new crystal phases, one may then describe all the present data. Although attention is focussed on the Mn ions one cannot exclude the possibility of intrinsic defects, or secondary impurities, being involved in these four divisions. For the present hypothesis the Mn in heavily doped calcite is predominantly in the type (iv) phase and gives broad emission bands. Above 275°C the solubility of Mn in CaCo3 allows relaxation into larger clusters, type (iii), and these control the narrow line emission at higher tem-

Table 1. Examples of TL peaks and emission spectra Calcite Treatment

Tm (°C)

2 (nm)

T~ (°C)

Iceland Spar ~. (nm)

Natural X-ray

400 400 325 200 60

560, 600, 640, 710 560, 600, 640, 710 600, 640 630 (broad) 630 (broad)

Not appropriate ~225 500, (580), 600, 650, 710 ~ 150 480, 600, 650, 710

/~ Irradiation

400 325 200 60

560, 600, 640, 710 560, 600, 640, 700 630 (broad) 630 (broad)

325 225 125 50

600, 650 480, 580, 600, 650 480, 580, 600 580

700°C to 77 K quench and X-ray

200 60

630 (broad) 630 (broad)

225 125

570, 610, 640 580

650°C slow cool and X-ray

325 200 60

600, 640 630 (broad) 630 (broad)

225 ~ 125

560, 600, 640 480, 570, 640

650°C slow cool and ]} irradiation

325 200 60

600, 640 630 (broad) 630 (broad)

225 125

560, 600, 640 480, 570, 600, 640

These are representative data but individual samples show variations in peak content and position. Values are given for a heating rate of 20°C min- ~. /'|

I.i t

e.§

t. i I

r

e.~

~

IL

/

I I¢

Q

' j -"

",

x

t

f /. ~

(a)

0.0

5091

550t

GO~l

6501

WAVELENGTN ( ~ >

7001 I¢itllnd

7501

Spit

ClI{Illl

FIG. 5a. A comparison of spectra taken during TL for calcite and Iceland Spar at 240 C.

TL EMISSION SPECTRA OF CALCITE A N D I C E L A N D SPAR

587

/ i " /i/I

,? ,'i 'i ,' i

~

/i t /

o C

/ t

i i / ~,j

I "

/ t

,/ t!

t

C

/ /"

,

i

/

t

'.

,

~. l.r,

it ./

/

i/ /

N~\, \

:1 /t /

e.~

,! ,/

(b)

a.

/

600'

658'

~AVELENGTH

( e~ )

70e I

750'

5o°c 275".C

3=s'c

...... .

.

.

.

.

.

.

FIG. 5b. A comparison of spectra taken during TL for three calcite glow peaks.

peratures, and in particular there was evidence for a peak in the region of 375°C. Traces of smaller clusters could exist, e.g. type (ii), and these are detectable at the 325°C glow peak and are characterised by line spectra. For Iceland Spar the low Mn concentration precludes the type (iv) defects so all emission is in the form of lines and since these only exist below 325°C it also suggests the larger type (iii) clusters are non-existent in the X-ray irradiated samples. Electron irradiation generates new variants of the simple defects and so one records a TL peak at 350°C, stronger emission at the lower temperatures and signals in the line at 480 nm. It may be that association of an

intrinsic defect with the Mn favours the transition from the second excited electronic state, as mentioned previously. The heat treatments in Iceland Spar scarcely altered the TL spectra, presumably because the low Mn concentration is already atomically dispersed or in the type (ii) form. However, for calcite quenching from a high temperature excludes the aggregation of types (ii) and (iii) whereas slow cooling allows the aggregation of the type (ii) small clusters. It should also be noted that the comparison with L i F : M g : T i is instructive in that the TL peak from small clusters is at a lower temperature than that for the trimers, as we are proposing here for the equivalent types (ii) and

588

J . S . D O W N et al.

5~01 / fx

t / /

\

/ t

/

j

/

I /

/

3o~

~2~++

!'!, l!,

/r \ ,~-

/ 7 ," '/

+

'

'

'"

"

+o,

\

. "'

+

,+, rErIOERaTURE

rC~

.++,

575nm

. . . . .

(lOOnm 635nrn

FIG. 6. TL curves taken from the TLSP data at three wavelengths for calcite for a heating rate of 20+C min- t.

(iii) of M n in CaCO3. The difference to note is that for LiF the clusters influence the charge trapping centres and the T L peak temperatures whereas for M n in CaCO3 the association of impurities is influencing the spectra of the luminescence site.

thermal treatments would not have been suspected from simple T L records using a broad band optical filter. Acknowledgements--We are grateful to SERC for financial support, to Professors E. J. Zeller and J. Thomas for samples, and to Dr P. J. Chandler for discussions.

6. C O N C L U S I O N The luminescence observed during T L of CaCO3 is directly linked to the M n impurities and the present data are interpreted to be evidence for at least four types of site in which M n is associated in small and large clusters plus precipitate phases. The use of a wavelength dispersive T L system has proved invaluable in the detection of changes in the emission spectra observed for each grow peak. Variations in spectra as a consequence of radiation or

REFERENCES Aguilar (3. M. and Osendi M. [. ([982) Fluorescence of Mn2÷ in CaCO3. J. Lumin. 27, 365-375. Bapat V. N. and Namhi K. S. V. (1975) TL of CaCO~:Dy and CaCO3:Mn. Proc. Nat. Syrup. TL Appl., Bhabha ARC, Bombay, pp. 606-614. Debenham N. C., Driver H. S. T. and Walton A. J. (1982) Anomalies of TL in young calcites. PACT 6, 555-562. Levy P. W., Mattern P. L. and Lengweiler K. (1971) Three dimensiona] thermoluminescent analysis of minerals. Mod. Geol. 2, 295 297.

TL E M I S S I O N S P E C T R A O F C A L C I T E A N D I C E L A N D S P A R McKeever S. W. S., Ahmed K., Chandler P. J., Strain J. A.. Rendell H. M. and Townsend P. D. (1983) Analysis of the emission spectra of meteorites during thermoluminescence measurements. P A C T 9, 187-204. Medlin W. L. (1963) Emission centres in thermoluminescent calcite, dolomite, magnesite, aragonite and anhydrite. J. opt. Soc. Am. 53, 1276-1285. Mexilin W. L. (1968) The nature of traps and emission centres in thermoluminescent rock materials. In Thermoluminescence of Geological Materials (Edited by McDougall D. J), Ch. 4. Academic Press, New York. Orgel L. E. (1955) Spectra of transition metal complexes. J. chem. Phys. 23, 1004-1014.

589

Sunta C. M. (1984) A review of thermoluminescence of calcium fluoride, calcium sulphate and calcium carbonate. Radiat Protect Dosim. 8, 25--44. Tanabe Y. and Sugano S. (1954) On the absorption spectra of complex ions. J. phys. Soc. Japan 9, 753-766. Townsend P. D. (1968) Measurement of charge trapping centres in geological specimens. In Thermoluminescence of Geological Materials (Edited by McDougall D. J.), Ch. 2,4. Academic Press, New York. Visocekas R. (1979) Miscellaneous aspects of artificial TL of calcite: emission spectra, athermal detrapping and anomalous fading. P A C T 3, 258-265.

~oo