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Cathodoluminescence spectroscopy and micro-PIXE: combined high resolution Mn-analyses in dolomites – First results
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 842±845
www.elsevier.nl/locate/nimb
Cathodoluminescence spectroscopy and micro-PIXE: combined high resolution Mn-analyses in dolomites ± First results A. Gillhaus b c
a,*
, D. Habermann b, J. Meijer c, D.K. Richter
a
a Institute of Geology, Ruhr-University Bochum, D-44780 Bochum, Germany Institute of Experimental Physics, TU Bergakademie Freiberg, D-09596 Freiberg, Germany Institute of Experimental Physics III, Ruhr-University Bochum, D-44780 Bochum, Germany
Abstract The combination of high resolution spectroscopy of cathodoluminescence emission (HRS-CL) and lPIXE is used to calibrate the correlation of the Mn-concentration and the Mn2 -activated CL-intensity in well-ordered and stoichiometric natural dolomites CaMg(CO3 )2 . In dolomite from sedimentary, diagenetic and hydrothermal origin, Mn2 is dominantly built in Mg-position documented by data from CL-spectroscopy and ESR-spectroscopy. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.60.Hk; 91.60.Ed; 91.65.Nd Keywords: Cathodoluminescence spectroscopy; lPIXE; Earth sciences; Dolomite
1. Introduction Cathodoluminescence (CL) microscopy is a routine tool for examination of geological materials. Crystal impurities or trace elements like the transition elements (especially Mn2 ) and/or the rare earth elements are mostly referred to as activators aside from crystal lattice defects, whereas Fe2 is a main quencher element of CL in carbonates (e.g. [1]). In carbonate petrology commonly visual CLcolours and -intensities of complex crystal zones
*
Corresponding author. E-mail address: [email protected] (A. Gillhaus).
with dierent conditions of formation are judged in comparison to results of trace element analysis and stable isotope geochemistry in order to get information for instance about porosity evolution in hydrocarbon exploration. Because visual observations are not objective several attempts were made by high resolution spectroscopy of cathodoluminescence emission (HRS-CL) combined with proton induced X-ray emission (PIXE) to analyse the correlation of CLintensity and activator concentration of minerals, for calcite and feldspar, see [2]. Now similar work is focused on the rock-forming mineral dolomite CaMg(CO3 )2 . The rhombohedral crystal lattice of an ideally ordered and stoichiometric dolomite consists of alternating Ca2 and Mg2 cation layers separated
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 9 7 - 0
A. Gillhaus et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 842±845
Fig. 1. A dominant red CL-spectrum of the standard dolomite 5a1 (Mn2 in Mg-position, compare text) with ®tted intensity is subtracted from the mixed CL-spectrum of dolomite sample 1b25 (Mn2 in Mg- and Ca-position) to separate the two broad bands of CL-emission. The residual spectrum is the minor yellow broad band (Mn2 in Ca-position) of sample 1b25.
by carbonate anion layers. In those ideal dolomites Mn2 may substitute for Mg2 and Ca2 resulting in characteristic red k 656 nm and yellow k 575 nm colours of emission, respectively (see Fig. 1). The two broad bands of luminescence emission (red and yellow) may overlap in CLspectra though the red broad band is dominant. Additionally CL-spectra can be rather complex because the cation layers of natural dolomites are rarely perfect causing distortion of the crystal lattice and therefore changing colours of CL to dierent orange. This paper deals with the calibration of Mn2 contents in Mg-position of well-ordered and stoichiometric dolomites with dominant red luminescence.
2. Method The samples are homogeneous crystals of diagenetic and hydrothermal dolomites from the island of Hydra, Greece. X-ray diraction pattern (stoichiometry and degree of order) of powder samples were determined with a Philips PW1050/25 with AMR-
843
monochromator using Cu Ka-radiation (excitation: 40 kV, 35 mA) and Quartz standards. Polished thin sections > 70 lm thick of the samples were analysed at standard conditions with the Bochumer HRS-CL-instrumentation mounted on a petrographic microscope [3] to choose only dominant red (Mn2 in Mg-position), homogeneous luminescing, clear crystals (excitation: 0.1 mA, 14 keV; exposure time: 60 s; spot diameter: 30 lm). The spectrum of one hydrothermal sample (sample 5a1) with ± CL-spectroscopically ± least amount of Mn2 in Ca-position (broad band of CL emission at 656 nm, width at half height 84 nm, Fig. 1) is used as a model spectrum (with ®tted intensity) to do the essential separation of red and minor yellow broad bands of CL-emission in mixed CL-spectra by subtraction (see [4,5]; compare Fig. 1). Sample 5a1 (standard) was additionally analysed by electron spin resonance (ESR) to determine the lattice site of Mn in this standard dolomite. The Mn-distribution of the two lattice positions was determined by using the integrated spectrum at the spin transition MS ÿ3=2 ! ÿ5=2 and MI 5=2 ! ÿ5=2: The analysis was done with a Bruker ESP 300e (X-Band; microwave frequence: 9748 GHz; microwave power: 2007 mW; Fig. 3) at the Institute of Experimental Physics of the University of Mining and Technology Freiberg. Absolute Mn concentrations were determined with lPIXE (3 MeV) at the Dynamitron Tandem Laboratorium of the Ruhr-University Bochum (method in [6]). A very low limit of detection (LOD) of 10 ppm Mn in addition to high local resolution (spot diameter: 10 lm) is important because only Mn-poor dolomites (empirically less than 100 ppm Mn) exclusively contain Mn in Mgposition and are additionally stoichiometric and well-ordered. The spectra were analysed by GUPIX-software [7].
844
A. Gillhaus et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 842±845
3. Experimental results The ®rst part of the study deals with the calibration of Mn-activated CL-intensity with absolute Mn-contents in the Mg-position of dolomites. The results prove a linear correlation of red luminescing, well-ordered low Fe2 -dolomites < 800 ppm Fe in the range of 18±94 ppm Mn2 in Mg-position (Fig. 2). At these low Mn and Fe concentrations the self-quenching of Mn2 and quenching of Fe2 has no measurable eect on CL-intensity [8]. The CL-intensity (Ik ) was plotted after subtraction of small amounts of Mn2 in Caposition (CL-spectroscopically compared to ®tted model spectra of standard dolomite 5a1). The plot is in broad agreement with data of low Fe2 -calcites, compare [2]. The slightly higher gradient of the dolomite correlation may be explained by the ESR spectrum of the standard dolomite (Fig. 3) although the lack of local resolution has to be considered in comparison to PIXE and HRS-CL analyses. The integrated ESR-spectrum proves dominant Mn-substitution in the Mg-position of the powder sample. There are additional weak signals of Mn-substitution in the Ca-position of the dolomite (Mn-distribution of Mg-position/Caposition is 8.9:1) which are in part responsible for the higher gradient. Additionally the higher gra-
Fig. 3. Integrated ESR-spectrum of standard dolomite 5a1. The Mn-lines (arrows) indicate the build in of Mn2 dominantly in Mg-position and subordinately in Ca-position.
dient may be caused by a small variation in the transition probability of the d-electrons caused by slightly dierences in symmetry and cation-anion distance of calcite structure and Mg-position of dolomite. The deviations from linear correlation of the dolomite plot result mainly from small inhomogeneities in Mn-distribution. There is an additional statistical error of HRS-CL and lPIXE analyses and also referring to dierent spot size and depth of beam penetration, but although the excited sample volumes of both methods are dierent there is a constant relation of ®ltered CL-data and absolute Mn concentrations.
4. Conclusions
Fig. 2. Correlation of absolute Mn-contents determined by lPIXE and CL-intensity (normed to counts/5 s) of dolomiteswithdominantredluminescence(Mn2 inMg-position). The ®t of the measured data (dots) from dolomites (straight line) shows a linear correlation with dolomite MnMg-position ppm 0:2713 CL-intensity. In comparison, ®tted data from calcite [4] (broken line) have only a little lower gradient with calcite MnCa-position ppm 0:2538 CL-intensity.
HRS-CL and lPIXE complement each other in making a calibration for quantitative analyses of Mn-activated CL in Ca2 - and Mg2 -position and also in distorted Ca- and Mg-lattice position in dolomites. Additional ESR-analyses contribute to diminish the error of CL-spectroscopically determined Mn-distribution. Although small variations in analyses of geological materials never can be excluded, quantitative HRS-CL provides a very good tool for example to judge the extent of evaporitic conditions during dolomite formation in relation to the partition of Mn2 in the two
A. Gillhaus et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 842±845
cation positions (compare discussion in [4]). The special advantage of the HRS-CL-instrumentation mounted on a petrographic microscope is that several dierent observations are possible. So the additional quantitative analysis after combined calibration provides high geoscienti®c application with low expense.
Acknowledgements This study was ®nancially supported by the Deutsche Forschungsgemeinschaft (Ri 216/16-1) and the Dynamitron Tandem Laboratorium (DTL) of the Ruhr-University Bochum.
845
References [1] D.J. Marshall, Unwin Hyman, Boston, 1988, p. 146. [2] D. Habermann, J. Meijer, R.D. Neuser, D.K. Richter, C. Rolfs, A. Stephan, Nucl. Instr. and Meth. B 150 (1999) 470. [3] R.D. Neuser, F. Bruhn, J. G otze, D. Habermann, D.K. Richter, Zbl. Geol. Pal aont., Teil I H1/2 (1995) 287. [4] D. Habermann, Dissertation, Ruhr-Universitat, Bochum, 1997, p. 152. [5] A. El Ali, V. Barbin, G. Calas, B. Cervelle, K. Ramseyer, J. Bouroulec, Chem. Geol. 104 (1993) 189. [6] J. Meijer, A. Stephan, J. Adamczewski, H.H. Bukow, C. Rolfs, T. Pickart, F. Bruhn, Nucl. Instr. and Meth. B 89 (1994) 229. [7] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. [8] N.G. Hemming, W.J. Meyers, J.C. Grams, J. Sediment. Petrol 59 (1989) 404.
www.elsevier.nl/locate/nimb
Cathodoluminescence spectroscopy and micro-PIXE: combined high resolution Mn-analyses in dolomites ± First results A. Gillhaus b c
a,*
, D. Habermann b, J. Meijer c, D.K. Richter
a
a Institute of Geology, Ruhr-University Bochum, D-44780 Bochum, Germany Institute of Experimental Physics, TU Bergakademie Freiberg, D-09596 Freiberg, Germany Institute of Experimental Physics III, Ruhr-University Bochum, D-44780 Bochum, Germany
Abstract The combination of high resolution spectroscopy of cathodoluminescence emission (HRS-CL) and lPIXE is used to calibrate the correlation of the Mn-concentration and the Mn2 -activated CL-intensity in well-ordered and stoichiometric natural dolomites CaMg(CO3 )2 . In dolomite from sedimentary, diagenetic and hydrothermal origin, Mn2 is dominantly built in Mg-position documented by data from CL-spectroscopy and ESR-spectroscopy. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.60.Hk; 91.60.Ed; 91.65.Nd Keywords: Cathodoluminescence spectroscopy; lPIXE; Earth sciences; Dolomite
1. Introduction Cathodoluminescence (CL) microscopy is a routine tool for examination of geological materials. Crystal impurities or trace elements like the transition elements (especially Mn2 ) and/or the rare earth elements are mostly referred to as activators aside from crystal lattice defects, whereas Fe2 is a main quencher element of CL in carbonates (e.g. [1]). In carbonate petrology commonly visual CLcolours and -intensities of complex crystal zones
*
Corresponding author. E-mail address: [email protected] (A. Gillhaus).
with dierent conditions of formation are judged in comparison to results of trace element analysis and stable isotope geochemistry in order to get information for instance about porosity evolution in hydrocarbon exploration. Because visual observations are not objective several attempts were made by high resolution spectroscopy of cathodoluminescence emission (HRS-CL) combined with proton induced X-ray emission (PIXE) to analyse the correlation of CLintensity and activator concentration of minerals, for calcite and feldspar, see [2]. Now similar work is focused on the rock-forming mineral dolomite CaMg(CO3 )2 . The rhombohedral crystal lattice of an ideally ordered and stoichiometric dolomite consists of alternating Ca2 and Mg2 cation layers separated
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 9 7 - 0
A. Gillhaus et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 842±845
Fig. 1. A dominant red CL-spectrum of the standard dolomite 5a1 (Mn2 in Mg-position, compare text) with ®tted intensity is subtracted from the mixed CL-spectrum of dolomite sample 1b25 (Mn2 in Mg- and Ca-position) to separate the two broad bands of CL-emission. The residual spectrum is the minor yellow broad band (Mn2 in Ca-position) of sample 1b25.
by carbonate anion layers. In those ideal dolomites Mn2 may substitute for Mg2 and Ca2 resulting in characteristic red k 656 nm and yellow k 575 nm colours of emission, respectively (see Fig. 1). The two broad bands of luminescence emission (red and yellow) may overlap in CLspectra though the red broad band is dominant. Additionally CL-spectra can be rather complex because the cation layers of natural dolomites are rarely perfect causing distortion of the crystal lattice and therefore changing colours of CL to dierent orange. This paper deals with the calibration of Mn2 contents in Mg-position of well-ordered and stoichiometric dolomites with dominant red luminescence.
2. Method The samples are homogeneous crystals of diagenetic and hydrothermal dolomites from the island of Hydra, Greece. X-ray diraction pattern (stoichiometry and degree of order) of powder samples were determined with a Philips PW1050/25 with AMR-
843
monochromator using Cu Ka-radiation (excitation: 40 kV, 35 mA) and Quartz standards. Polished thin sections > 70 lm thick of the samples were analysed at standard conditions with the Bochumer HRS-CL-instrumentation mounted on a petrographic microscope [3] to choose only dominant red (Mn2 in Mg-position), homogeneous luminescing, clear crystals (excitation: 0.1 mA, 14 keV; exposure time: 60 s; spot diameter: 30 lm). The spectrum of one hydrothermal sample (sample 5a1) with ± CL-spectroscopically ± least amount of Mn2 in Ca-position (broad band of CL emission at 656 nm, width at half height 84 nm, Fig. 1) is used as a model spectrum (with ®tted intensity) to do the essential separation of red and minor yellow broad bands of CL-emission in mixed CL-spectra by subtraction (see [4,5]; compare Fig. 1). Sample 5a1 (standard) was additionally analysed by electron spin resonance (ESR) to determine the lattice site of Mn in this standard dolomite. The Mn-distribution of the two lattice positions was determined by using the integrated spectrum at the spin transition MS ÿ3=2 ! ÿ5=2 and MI 5=2 ! ÿ5=2: The analysis was done with a Bruker ESP 300e (X-Band; microwave frequence: 9748 GHz; microwave power: 2007 mW; Fig. 3) at the Institute of Experimental Physics of the University of Mining and Technology Freiberg. Absolute Mn concentrations were determined with lPIXE (3 MeV) at the Dynamitron Tandem Laboratorium of the Ruhr-University Bochum (method in [6]). A very low limit of detection (LOD) of 10 ppm Mn in addition to high local resolution (spot diameter: 10 lm) is important because only Mn-poor dolomites (empirically less than 100 ppm Mn) exclusively contain Mn in Mgposition and are additionally stoichiometric and well-ordered. The spectra were analysed by GUPIX-software [7].
844
A. Gillhaus et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 842±845
3. Experimental results The ®rst part of the study deals with the calibration of Mn-activated CL-intensity with absolute Mn-contents in the Mg-position of dolomites. The results prove a linear correlation of red luminescing, well-ordered low Fe2 -dolomites < 800 ppm Fe in the range of 18±94 ppm Mn2 in Mg-position (Fig. 2). At these low Mn and Fe concentrations the self-quenching of Mn2 and quenching of Fe2 has no measurable eect on CL-intensity [8]. The CL-intensity (Ik ) was plotted after subtraction of small amounts of Mn2 in Caposition (CL-spectroscopically compared to ®tted model spectra of standard dolomite 5a1). The plot is in broad agreement with data of low Fe2 -calcites, compare [2]. The slightly higher gradient of the dolomite correlation may be explained by the ESR spectrum of the standard dolomite (Fig. 3) although the lack of local resolution has to be considered in comparison to PIXE and HRS-CL analyses. The integrated ESR-spectrum proves dominant Mn-substitution in the Mg-position of the powder sample. There are additional weak signals of Mn-substitution in the Ca-position of the dolomite (Mn-distribution of Mg-position/Caposition is 8.9:1) which are in part responsible for the higher gradient. Additionally the higher gra-
Fig. 3. Integrated ESR-spectrum of standard dolomite 5a1. The Mn-lines (arrows) indicate the build in of Mn2 dominantly in Mg-position and subordinately in Ca-position.
dient may be caused by a small variation in the transition probability of the d-electrons caused by slightly dierences in symmetry and cation-anion distance of calcite structure and Mg-position of dolomite. The deviations from linear correlation of the dolomite plot result mainly from small inhomogeneities in Mn-distribution. There is an additional statistical error of HRS-CL and lPIXE analyses and also referring to dierent spot size and depth of beam penetration, but although the excited sample volumes of both methods are dierent there is a constant relation of ®ltered CL-data and absolute Mn concentrations.
4. Conclusions
Fig. 2. Correlation of absolute Mn-contents determined by lPIXE and CL-intensity (normed to counts/5 s) of dolomiteswithdominantredluminescence(Mn2 inMg-position). The ®t of the measured data (dots) from dolomites (straight line) shows a linear correlation with dolomite MnMg-position ppm 0:2713 CL-intensity. In comparison, ®tted data from calcite [4] (broken line) have only a little lower gradient with calcite MnCa-position ppm 0:2538 CL-intensity.
HRS-CL and lPIXE complement each other in making a calibration for quantitative analyses of Mn-activated CL in Ca2 - and Mg2 -position and also in distorted Ca- and Mg-lattice position in dolomites. Additional ESR-analyses contribute to diminish the error of CL-spectroscopically determined Mn-distribution. Although small variations in analyses of geological materials never can be excluded, quantitative HRS-CL provides a very good tool for example to judge the extent of evaporitic conditions during dolomite formation in relation to the partition of Mn2 in the two
A. Gillhaus et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 842±845
cation positions (compare discussion in [4]). The special advantage of the HRS-CL-instrumentation mounted on a petrographic microscope is that several dierent observations are possible. So the additional quantitative analysis after combined calibration provides high geoscienti®c application with low expense.
Acknowledgements This study was ®nancially supported by the Deutsche Forschungsgemeinschaft (Ri 216/16-1) and the Dynamitron Tandem Laboratorium (DTL) of the Ruhr-University Bochum.
845
References [1] D.J. Marshall, Unwin Hyman, Boston, 1988, p. 146. [2] D. Habermann, J. Meijer, R.D. Neuser, D.K. Richter, C. Rolfs, A. Stephan, Nucl. Instr. and Meth. B 150 (1999) 470. [3] R.D. Neuser, F. Bruhn, J. G otze, D. Habermann, D.K. Richter, Zbl. Geol. Pal aont., Teil I H1/2 (1995) 287. [4] D. Habermann, Dissertation, Ruhr-Universitat, Bochum, 1997, p. 152. [5] A. El Ali, V. Barbin, G. Calas, B. Cervelle, K. Ramseyer, J. Bouroulec, Chem. Geol. 104 (1993) 189. [6] J. Meijer, A. Stephan, J. Adamczewski, H.H. Bukow, C. Rolfs, T. Pickart, F. Bruhn, Nucl. Instr. and Meth. B 89 (1994) 229. [7] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. [8] N.G. Hemming, W.J. Meyers, J.C. Grams, J. Sediment. Petrol 59 (1989) 404.