A comparison of luminescence spectra and structural composition of perthitic feldspars

Pergamon Plh

Radiation Measurements, Vol. 27. No. 2, pp. 137-144, 1997 .~ 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1350-4487(96)00134-5 135o-4487/97 $17.0o + o.oo

A COMPARISON OF LUMINESCENCE SPECTRA A N D STRUCTURAL COMPOSITION OF PERTHITIC FELDSPARS M. L. CLARKE*, H. M. RENDELL*, L. SANCHEZ-MUIqOZt and J. GARCIA-GUINEAt *Geography Laboratory, Arts C, University of Sussex, Falmer, Brighton BN1 9QN, U.K. and tMuseo Nacional Ciencias Naturalem Calle Jose Gutierrez Abascal 2, 28006 Madrid, Spain Abstract--Museum specimensof potassium feldspar, ranging from monoclinic orthoclase through several intermediaries to triclinic microcline, have had their structural and chemical properties characterized by optical microscopy, X-ray diffraction, X-ray fluorescence, ion microprobe and scanning electron microscopy. Thermoluminescence,radioluminescenceand infra-red stimulated luminescencespectra were obtained from these samples showing two emission bands: an ultraviolet band centred around 290 nm and a broad blue band centred around 442 nm. The relative intensities of these two bands change with increasing triclinicity,with the UV band dominating in the microclinesas a result of an increase in sodium exsolution features within the structure of these perthites. TL emission from the intrinsic blue peak also increases in intensity at longer wavelengthswith increasing triclinicity,possibly resulting from an increase in defect sites as a result of the concentration of perthitic intergrowths of sodium lamellae. © 1997Elsevier Science Ltd

I. INTRODUCTION Microclines and orthoclases are the most abundant detrital feldspars found in sedimentary environments (Pettijohn et al., 1972). These minerals are commonly used as dosimeters in luminescence dating, particularly with the recent upsurge in application of IRSL to geological sequences. TL and IRSL spectral studies of potassium-rich feldspars have shown broad emission bands at 290, 335, 400 and 550 nm (Rendell et al., 1995) with the 290 nm emission present in IRSL spectra only after laboratory irradiation (Clarke and Rendell, 1997a, b). This 290 nm UV emission has been linked to the presence of sodium aluminosilicate (NaAISi3Os) either as the dominant phase (e.g. in the sodium feldspar, albite) or as exsolved phases of NaAlSi3Ox within the predominantly potassium-rich (KA1Si3Os) matrix (Rendell and Clarke, 1997) This paper describes a comparison of the luminescence spectra obtained from five potassiumrich perthitic feldspars. Perthites are potassium feldspars (KAISi3Os) which have exsolved phases of sodium feldspar (KAISi3Os) interweaved within the potassium feldspar lattice as a result of the cooling history of the rock matrix. The perthites used in these experiments range from a monoclinic orthoclase to a triclinic microcline with several intermediaries.

2. STRUCTURAL CHARACTERIZATION The samples used in these experiments were perthitic potassium-rich feldspars taken from

granitic pegmatites. The perthite crystals ranged from monoclinic orthoclase (FB35, F4) to triclinic microcline (GcI) with two intermediates (En7 and CR4). The sodium feldspar domains within the perthites are, in all cases, low albite, but a range of domains exists within the potassium feldspar matrix. The structural state of the potassium feldspar has been studied using X-ray diffraction (XRD) and optical microscopy. These techniques have shown that several structural states of different varieties are predominantly observed in the same monocrystal. This is true of both plutonic potassium-rich feldspars and detrital grains from sediments, with the microdomains occurring at a submicroscopic level (FitzGerald and McLaren, 1982). The coexistence of these domains is related to the presence of sodium feldspar lamellae. The Si/AI order is at a maximum next to the N a - K interface resulting in a low microcline structure. The Si/A1 order decreases away from the N a - K interface to that of intermediate microcline and orthoclase. Thus, the orthoclase/low microcline transition is at a minimum in FB35 which is mostly composed of orthoclase and at a maximum in Gcl in which low microcline exists throughout the crystal. Between these extremes, F4, En7 and CR4 have intermediate characteristics with the coexistence of several varieties of potassium feldspar. Sample F4 has an orthoclase XRD pattern which is close to that of intermediate microcline. Sample En7 gives an XRD pattern similar to F4, with intermediate and low microcline structures apparent close to the N a - K interfaces. CR4 has an 137

M. L. CLARKE et al.

138

Table 1. Chemical composition of the samples Sample

Type

FB35 F4 En7 CR4 Gcl

Orthoclase Orthoclase Ortho/Micro Micro/Ortho Microcline

Source La lsla pegmatite, C/tceres, Spain Fermin pegmatite, Minas Gerias, Brazil Proberil pegmatite, Minas Gerias, Brazil Corrego Rapa, Minas Gerias. Brazil Golconda pegmatite, Minas Gerias, Brazil

intermediate microcline XRD structure close to that of low microcline. The chemical composition of the samples (Table 1) were determined by X-ray fluorescence using six standards including a sodium feldspar (BCS376) and a potassium feldspar (BCS375). All of the samples have similar K_~Oand Na20 compositions. Figure 1 shows the types of planar defect commonly seen in natural alkali feldspars and all of the samples measured show twinning and exsolution features to varying degrees. The photographs of FB35 (Fig. 2) and GcI (Fig. 3) show how the extent of twinning of sodium and potassium phases increases with increasing triclinicity. The presence of sodium phases are clearly shown in Fig. 4, in which samples of FB35, F4 and CR4 have been etched with hydrofluoric acid vapour, Sodium film lamellae are indicated by the arrows in Fig. 4(a,b). Film and vein features are shown in 4(c) with the large area of 4(b) marked by arrow 3 representing a patch of albite.

3. EXPERIMENTAL

Luminescence spectra were obtained using the high sensitivity thermoluminescence spectrometer at Sussex with a spectral range of 200-800 nm and a resolution of 3 nm. Cleaved chips of the feldspar samples were mounted onto aluminium discs using silicone oil. Thermoluminescence (TL)

K,O %

Na,O %

CaO %

13.16 12.21 13.50 13.58 12.99

2.10 2.77 2.13 2.60 2.42

0.00 0.00 0.00 0.00 0.00

measurements were made from 30 to 400:C at a heating rate of 2.5C s ~. Measurements were made of the natural TL and TL after a 50 Gy X-ray dose. This irradiation was carried out within the system using a Philips MCN 101 X-ray tube with a current of 15 mA and a voltage of 25 kV, delivering 10 Gy min- ~ to the sample. The radioluminescence (RL) spectrum was acquired for 100 s during X-ray irradiation at 30°C. IRSL spectra were obtained using a ring of 8 TEMT484 diodes (880A80 nm) which were switched on for 100 s with the sample held at 30°C. The red detector was filtered with two Schott BG38 filters to reject scattered light from the diodes. All spectra were corrected for response of the system.

4. DISCUSSION The TL spectra after 50 Gy for all of the samples are shown in Fig. 5. All of the spectra are dominated by a broad band centred on 442 nm, with the peak intensity at around 100'~C (see Table 2), This broad emission is thought to result from hole centres caused by intrinsic defects within aluminosilicate lattices with the wavelength of the emission dependent upon the type of impurity found in close proximity to the trapped hole (Kirsh and Townsend, 1988; Prescott and Fox, 1993). The other dominant emission band present in the samples is a narrow band centred on 290 nm which is present at all temperatures above

PLANAR DEFECTS IN NATURAL ALKALI FELDSPARS

Twins

Exsolutions

Modulations

Fig. 1. Types of exsolution feature which occur as planar defects in natural alkali feldspars.

COMPARISON OF PERTHITIC FELDSPARS

139

i

j

-

T

':

~ N.,

Fig. 2. Monoclinic FB35--an optical micrograph showing the presence of sodium exsolution features within the bulk potassium feldspar matrix.

50 C. The 290 nm emission is believed to be related to the presence of NaA1Si308 phases either as the .dominant lattice structure e.g. albite, or as exsolved features within a predominantly KA1Si308 lattice (Rendell and Clarke, 1997). The relative intensities of these two bands varies with increasing triclinicity, with the 442 nm band brightest in the orthoctase samples (FB35 and F4) and the 290 nm band showing a greater relative intensity in the microcline (Gcl). This is well illustrated in Fig. 6 whicl~ is a

temperature slice through the TL spectra taken at 100°C. The ratio of the 290:442nm emission increases with increasing triclinicity (see also Table 2) as a result of a greater concentration of perthitic sodium feldspar lamellae present within the potassium feldspar matrix including both twinning and exsolution features in samples CR4 and GcI (see Figs 1 - 4). The TL slices also show a trend of increasing emission at longer wavelengths, defined by a

Fig. 3. Triclinic Gcl--an optical micrograph showing both twinning and forked exsolution features of sodium feldspar within the bulk potassium feldspar matrix.

140

M . L . C L A R K E et al.

a

b

e1

C

I

20 lam

I

Fig. 4. Scanning electron micrographs of chips of (a) FB35; (b) F4; and (c) CR4 after etching with HF vapour. Arrows 1 and 2 in (a) and (b) indicate sodium film lamellae with arrow 3 indicating a patch of albite. Arrow. 1 in (c) indicates sodium vein features.

COMPARISON OF PERTHITIC FELDSPARS

FB35 orthoclase

EN7 ortho >> micro

Fig. 5. 3-D TL spectra after addition of a 50 Gy X-ray dose.

141

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M . L . CLARKE

et al.

Table 2. The wavelength of peak emission intensities and ratios between intensities of the dominant emission bands for the five samples

Sample

UV (nm)

FB35 F4 En7 CR4 Gcl Mean

298 289 287 289 287 290 ± 5

TL at 100 C Blue Ratio (nm) UV/blue 445 441 445 441 437 442 ± 3

0.074 0.182 0.456 0.715 1.554

UV (nm)

RL Blue (nm)

Ratio UV/blue

UV (nm)

IRSL Blue (nm)

Ratio UV/blue

298 289 287 294 287 291 ± 5

445 428 445 445 445 442 ± 8

0.095 0.177 0.434 0.216 1.466 --

294 291 294 294 294 293 ± 1

445 445 445 449 445 446 ± 2

0.119 0.255 0.752 0.456 1.368 --

broadening of the blue peak with increasing triclinicity, although the peak intensity remains fixed at around 442 nm. This increase in intensity of emission at longer wavelengths may be due to an increase in defect sites associated with the perth±tic sodium lamellae and subsequent distortion of the crystal lattice structure. A link between the 570 nm emission and sodium content has been suggested by Prescott and Fox (1993) who observed that the 570 and 275 nm TL emissions are characteristic ofalbites and plagioclases with more than 75 mole % of sodium. The asymmetry of the blue emission at shorter wavelengths may be due to a 335 nm emission which is masked by the dominance of the 290 and 442 nm bands. Figure 7 shows the RL and Fig. 8 the IRSL spectra for all of the samples. The IRSL spectra were acquired after a dose of 400 Gy. The application of this 400 Gy dose, as opposed to the 50 Gy dose used for TL, was because, unlike TL where the stimulation source is located directly underneath the sample, the IR diodes are mounted at a distance of 10 cm above the sample, which decreases the efficiency of the stimulation. The IRSL and RL

spectra show the two emission bands present in TL, with a peak at 290 nm and a broad band centred on 442 nm (RL) and 446 nm (IRSL). As with the TL, comparison of the relative intensities of the 290 nm and ~ 445 nm bands show that as the concentration of exsolved sodium increases, the intensity of the 290 nm emission increases with respect to the intrinsic peak (see also Table 2). This data confirms that the 290 nm band is linked to the presence of a sodium aluminosilicate phase. The RL spectra for CR4 and Gcl (Fig. 7) and the IRSL spectrum for CR4 (Fig. 8) show the presence of a 570 nm defect centre which is probably related to the exsolved sodium phase. The ratios of blue to UV emission for the different samples, shown in Table 2, show an increase from FB35 orthoclase to GcI microcline for the TL and for the RL and IRSL with the exception of sample CR4. The reason for the different behaviour pattern of CR4 with respect to RL and IRSL, when compared to the TL ratio, is not readily apparent and results from the relatively weak 290 nm emission when stimulating the sample with X-rays and IR photons.

1.60

1.28

......... ........ .......

.~

0.96

~

0.64

FB35 F4 En7 CR4 GeI

j:,s ";; 0.32

0.00

-~ 200

'

300

'

400

500 Wavelength

600

700

800

(nm)

Fig. 6. A comparison of temperature slices at 100 C normalized to the peak blue emission intensity.

COMPARISON OF PERTHITIC FELDSPARS FB35

3

_~

F4

400

35O

320

28O

24O

210

leo

140

80

7O

200

300

400

500

600

i 700

_f 0 20O

800

300

Wavelength (nm)

500

300

400

240

300

180

.q

zoo

700

800

12o 6O

tO0

0 2OO

400 500 600 Wavelength (rim)

CR4

En7

.~

143

300

400 500 600 Wavelength (nm)

700

800

700

800

0

200

300

400 500 600 Wavelength (rim)

700

S

800

GcI 300

240

iS0 120 60

200

300

400

500

600

Wavelength (nm)

Fig. 7. RL spectra of the samples.

5. CONCLUSIONS Sodium exsolution features in these samples increase with increasing triclinicity from FB35 orthoclase to GcI microcline. The intensity of" the 290 nm emission also increases with increasing triclinicity suggesting that the recombination centre is associated with a sodium phase. A broad band emission, centred on 442 nm, is present in all of

the samples resulting from intrinsic defects in the aluminosilicate lattices of feldspars. In TL, emission at longer wavelengths 442-700 nm increases with increasing triclinicity, although the peak intensity of the blue emission remains constant at 442 nm.

Acknowledgements--This work was funded by Natural Environment Research Council Special Topic Award GST/02/0755. This is LOIS publication number 104.

144

M.L.

et al.

CLARKE

FB35

F4

20

3o

18

25

18 14

2o

12

10

!

8

to

10

6: 4

5

2 0 200

300

400

500

600

700

800

o 2OO

300

Wavelength (nm)

400

500

600

700

fl00

700

800

WavelensI.h(rim) CR4

En7

10

80

70 8 60 5O

6

40 4

30 20

2

10 0

200

300

400

500

600

700

800

Wavelength (nm)

O'

200

300

400

500

600

Wavetength (nm)

GeI 60 50 40 30

,n 20 10

200

300

400

500

600

700

800

Wavelength (nm) Fig. 8. IRSL spectra of the samples after a 400 Gy X-ray dose.

REFERENCES Clarke M. L. and Rendell H. M. (1997a) Stability of the IRSL spectra of alkali feldspars. Physica Status Solidi (b), 199(2), 597 604. Clarke M. L. and Rendell H. M. (1997b) Infra-red stimulated luminescence spectra of alkali feldspars. Radiat. Meas. 27, 221-236. FitzGerald J. D. and McLaren A. C. (1982) The microstructure of microcline from some granitic rocks and pegmatites. Contributions in Mineralogy and Petrology 80, 219-229. Kirsh Y. and Townsend P. D. (1988) Speculations on the blue and red bands in the T L emission spectrum of

albite and microcline. Nuclear Tracks and Radiation Measurements 14, 43~49. Pettijohn, F. J., Potter, P. E. and Siever, R. (1972) Sandand Sandstone. Springer, New York. Prescott J. R. and Fox P. J. (1993) Three-dimensional thermoluminescence spectra of feldspars. Journal of Physics D: Applied Physics 26, 2245-2254. Rendell H. M. and Clarke M. L. (1997) Thermoluminescence, radioluminescence and cathodoluminescence spectra of alkali feldspars. Radiation Measurements, 27, 263-272. Rendell H. M., Townsend P. D. and Wood R. A. (1995) TL and IRSL emission spectra of detrital feldspars. New experimental data. Physica Status Solidi (B) 190, 321-330.