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PIXE analysis of pyrite grains
Nuclear Instruments North-Holland
and Methods
in Physics Research
MIMI B
B7S (1993) 420-422
Beam Interactions with Materials&Atoms
PIXE analysis of pyrite grains A.E. Pillay a, C.S. Erasmus b and S. Amos a aDepartment of Chemistry and b Schonland Research Centre, University of the Witwatersrand, P.O. Wits, 2050, South Africa
A unique sample preparation procedure is described for the analysis of inclusions and other trace elements in pyrite grains (Fe&) by PIXE. The grains themselves were about 0.5 mm in “diameter” and a binocular microscope was employed to select and prepare particles for irradiation. Each pyrite particle was decomposed in a small droplet of concentrated HNO, on a 1 km thick kapton foil supported on a hollow plastic backing. The acid was confined to the centre of the foil by a wax ring of 5 mm internal diameter stamped onto it. Finally a thin layer of polystyrene was precipitated over the wax ring to seal the area of interest. A 4 MeV proton beam from the tandem Van de Graaff accelerator at the Schonland Research Centre was used for PIXE analysis of a series of individual pyrite grains prepared by the above technique. Apart from iron and sulphur, low-level impurities in the pyrite such as titanium, chromium and zinc were evaluated quantitatively. The investigation evaluated the potential of PIXE to determine the mass of each grain which was estimated to lie in the region between 0.1 and 1 mg.
1. Introduction PIXE is a well established analytical tool for the measurement of ultralow concentrations of elements in relatively small specimens [l-3]. Due to the low penetrating power of charged particles, however, the method is a surface analysis technique. It is, therefore, a severe limitation when the target material is known to be heterogeneous in composition. Pyrite grains for example may contain small particles of native gold and silver as well as other minute mineral particles and liquid inclusions. Proton and electron microprobe methods [4] and image analyser techniques involving sectional analysis may also prove to be inadequate especially in the case of small single inhomogeneous particles. The main thrust of this work involved the analysis of individual pyrite grains by combining a special sample preparation procedure with the standard PIXE technique. The experimental data were used to obtain the masses of some of the major component elements and inclusions in the grains,
2. Experimental 2.1. Sample preparation Thin homogeneous dry targets were prepared from natural pyrite grains with dimensions between 0.2 and 0.6 mm in length. Each pyrite grain was individually dissolved in concentrated HNO, (droplets) on a kapton foil, 1 Frn in thickness, which was supported on a 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
circular plastic ring that served as backing. In order to prevent the acid from spreading over the entire surface of the kapton, a wax ring of 5 mm internal diameter (i.e. smaller than the beam diameter) was first stamped onto the kapton. A hollow pencil of wax dipped in cyclohexane was used for this purpose, and the cyclohexane was subsequently allowed to evaporate. Once the dissolved sample was confined to the specific area a layer of polystyrene was precipitated over the wax ring to prevent any undissolved material from falling off the kapton. A solution of polystyrene in benzene was used for this. The thickness of the final target was of the order of 1 mg/cm*. Suitable thin calibration standards were available against which the unknown concentrations were measured.
2.2. Irradiation
and measurement
For all irradiations 4 MeV protons from the 6 MV EN tandem Van de Graaff accelerator at the Schonland Research Centre were used. The beam was extracted through a 12.5 urn thick kapton window into the target chamber filled with helium at atmospheric pressure. A six-position rotating sample holder was used to facilitate sample changing. The beam was homogenised by defocussing and collimated to a diameter of 6 mm. Beam currents varied between 1 and 10 nA. The accumulated charge for different targets ranged from 0.5 to 1.5 PC. There was no noticeable thermal degradation of the targets in the helium atmosphere at this beam intensity. X-ray measurements were made with a standard Si(Li) detector and
B.V. Ah rights reserved
421
A.E. Pillay et al. / &rite grains
associated electronics. The sample-to-detector distance was fixed throughout the investigation, and in all cases dead times of less than 10% were maintained.
3. Results and discussion The interesting feature of the investigation was that the sample preparation procedure facilitated analysis of the complete pyrite grain. The problems that may be associated with surface analysis and hetereogeneous elemental distribution are, therefore, eliminated. The obviation of specimen inhomogeneity thus enables the accurate determination of Fe/S ratios as well as minor and trace element content which may not otherwise be possible. The experimentally determined Fe and S contents of ten pyrite grains analysed are presented in table 1. Also included in this table are the dimensions (lengths and widths) of each grain obtained by means of a calibrated binocular microscope. The iron-to-sulphur ratios in table 1 reveal a low sulphur content in all the samples which would suggest that some sulphur was lost, during the dissolution and decomposition of the pyrite grain, probably as H,S or SO,. The impurities (or inclusions) determined in the pyrites were Ti, Cr and Zn. Cr and Zn were present in most of the samples but Ti occurred in only two. The experimentally determined masses of these impurities are shown in table 2. Table 2 clearly demonstrates the powerful analytical capability of PIXE which comfortably extends down to 81 ng in the case of Zn. Before computing the concentrations of these impurities in the pyrite grains the accuracy of the experimental results were confirmed by comparing them against measured grain sizes. This was achieved by plotting the experimentally determined Fe masses against the surface area of each grain. This plot is shown in fig. 1. Assuming the thickness of the grains did not vary dramatically, a one-to-one correlation was expected which
Table 2 Experimentally determined Ti, Cr and Zn masses ingj Sample number 1 2 3 4 5 6 7 8 9 10
Ti mass
CT mass
Zn mass
_
260 180 300 470 _
230 620 190 _
350 230 490
81 81 _
840 320
930 310
790 670 -
would lend validity to the Fe contents displayed in table 1. The basis of this assumption lies in simple density considerations i.e. the Fe mass of each grain is evidently proportional to its volume; but if the thicknesses of individual grains did not vary widely, the Fe content would be expected to correlate on a one-to-one basis with the surface area. From the best straight line drawn between the data points (in fig. 1) it is evident
Surface
Area
(mm2)
Fig. 1. A plot of Fe mass vs surface area of the pyrite grains. Table 1 Experimentally determined
Fe and S masses of pyrite grains
Sample number
Fe [ctgl
S [pgl
Fe/S ratio
Dimension of grain [mm] X 10
1 2 3 4 5 6 7 8 9 10
209 422 166 854 77 469 104 383 715 788
158 185 129 455 62 227 65 233 430 355
1.32 2.28 1.29 1.89 1.24 2.07 1.60 1.64 1.66 2.22
2.5/2.5 6‘0/2.0 5.2/2.0 5.5/5.0 2.0/2.0 3.0/3.0 2.0/2.0 4.0/4.0 5.0/4.5 4.5/6.0
Table 3 Content of Ti, Cr and Zn relative to Fe (X 10V6) Sample number I 2 3 4 5 6 7 8 9 10
Ti 4759 784
Cr
Zn
1244 426 1807 550
1100 1469 1144 _
746 2211 1279 1175 406
173 779 1300 393
VI. GEOLOGICAL SAMPLES
422
A.E. Pillay et al. / Pyrite grains
that there are no serious deviations, which suggests that the Fe mass values in table 1 are acceptable. The concentrations of Ti, Cr and Zn relative to Fe are presented in table 3. These values varied widely from sample to sample and such variations indicate that heterogeneity would prevail within individual pyrite grains. Hence the analysis of only the surface or a fraction of the grain could lead to serious errors. The proposed sample preparation procedure overcomes this type of unrepresentative sampling and provides valuable information about the magnitude of the inclusions as well as useful major and minor element data.
entire grain by PIXE and ii) accurately determine the mass of each grain. 2) The proposed sample preparation procedure overcomes unrepresentative sampling and provides valuable information about the magnitude of the inclusions as well as useful major and minor element data. 3) Because the method has the potential for complete analysis of the pyrite grain, including its inclusions, it thus strongly rivals microprobe and image analyser techniques.
References 4. Conclusion 1) It has been shown that a special sample preparation procedure (for thin targets) - which entails dissolving individual pyrite grains on kapton foil - is useful because it is thus possible to: i) analyse the
111T.B. Johansson, R. Akselsson and S.A.E. Johansson, Nucl. Instr. and Meth. 84 (1970) 141. 121 S.A.E. Johansson and T.B. Johansson, Nucl. Instr. and Meth. 137 (1976) 473. [3] F. Folkmann, C. Gaarde, J. Hum and K. Kemp, Nucl. Instr. and Meth. 116 (1984) 487. [4] J.R. Bird, Nucl. Instr. and Meth. B45 (1990) 516.
and Methods
in Physics Research
MIMI B
B7S (1993) 420-422
Beam Interactions with Materials&Atoms
PIXE analysis of pyrite grains A.E. Pillay a, C.S. Erasmus b and S. Amos a aDepartment of Chemistry and b Schonland Research Centre, University of the Witwatersrand, P.O. Wits, 2050, South Africa
A unique sample preparation procedure is described for the analysis of inclusions and other trace elements in pyrite grains (Fe&) by PIXE. The grains themselves were about 0.5 mm in “diameter” and a binocular microscope was employed to select and prepare particles for irradiation. Each pyrite particle was decomposed in a small droplet of concentrated HNO, on a 1 km thick kapton foil supported on a hollow plastic backing. The acid was confined to the centre of the foil by a wax ring of 5 mm internal diameter stamped onto it. Finally a thin layer of polystyrene was precipitated over the wax ring to seal the area of interest. A 4 MeV proton beam from the tandem Van de Graaff accelerator at the Schonland Research Centre was used for PIXE analysis of a series of individual pyrite grains prepared by the above technique. Apart from iron and sulphur, low-level impurities in the pyrite such as titanium, chromium and zinc were evaluated quantitatively. The investigation evaluated the potential of PIXE to determine the mass of each grain which was estimated to lie in the region between 0.1 and 1 mg.
1. Introduction PIXE is a well established analytical tool for the measurement of ultralow concentrations of elements in relatively small specimens [l-3]. Due to the low penetrating power of charged particles, however, the method is a surface analysis technique. It is, therefore, a severe limitation when the target material is known to be heterogeneous in composition. Pyrite grains for example may contain small particles of native gold and silver as well as other minute mineral particles and liquid inclusions. Proton and electron microprobe methods [4] and image analyser techniques involving sectional analysis may also prove to be inadequate especially in the case of small single inhomogeneous particles. The main thrust of this work involved the analysis of individual pyrite grains by combining a special sample preparation procedure with the standard PIXE technique. The experimental data were used to obtain the masses of some of the major component elements and inclusions in the grains,
2. Experimental 2.1. Sample preparation Thin homogeneous dry targets were prepared from natural pyrite grains with dimensions between 0.2 and 0.6 mm in length. Each pyrite grain was individually dissolved in concentrated HNO, (droplets) on a kapton foil, 1 Frn in thickness, which was supported on a 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
circular plastic ring that served as backing. In order to prevent the acid from spreading over the entire surface of the kapton, a wax ring of 5 mm internal diameter (i.e. smaller than the beam diameter) was first stamped onto the kapton. A hollow pencil of wax dipped in cyclohexane was used for this purpose, and the cyclohexane was subsequently allowed to evaporate. Once the dissolved sample was confined to the specific area a layer of polystyrene was precipitated over the wax ring to prevent any undissolved material from falling off the kapton. A solution of polystyrene in benzene was used for this. The thickness of the final target was of the order of 1 mg/cm*. Suitable thin calibration standards were available against which the unknown concentrations were measured.
2.2. Irradiation
and measurement
For all irradiations 4 MeV protons from the 6 MV EN tandem Van de Graaff accelerator at the Schonland Research Centre were used. The beam was extracted through a 12.5 urn thick kapton window into the target chamber filled with helium at atmospheric pressure. A six-position rotating sample holder was used to facilitate sample changing. The beam was homogenised by defocussing and collimated to a diameter of 6 mm. Beam currents varied between 1 and 10 nA. The accumulated charge for different targets ranged from 0.5 to 1.5 PC. There was no noticeable thermal degradation of the targets in the helium atmosphere at this beam intensity. X-ray measurements were made with a standard Si(Li) detector and
B.V. Ah rights reserved
421
A.E. Pillay et al. / &rite grains
associated electronics. The sample-to-detector distance was fixed throughout the investigation, and in all cases dead times of less than 10% were maintained.
3. Results and discussion The interesting feature of the investigation was that the sample preparation procedure facilitated analysis of the complete pyrite grain. The problems that may be associated with surface analysis and hetereogeneous elemental distribution are, therefore, eliminated. The obviation of specimen inhomogeneity thus enables the accurate determination of Fe/S ratios as well as minor and trace element content which may not otherwise be possible. The experimentally determined Fe and S contents of ten pyrite grains analysed are presented in table 1. Also included in this table are the dimensions (lengths and widths) of each grain obtained by means of a calibrated binocular microscope. The iron-to-sulphur ratios in table 1 reveal a low sulphur content in all the samples which would suggest that some sulphur was lost, during the dissolution and decomposition of the pyrite grain, probably as H,S or SO,. The impurities (or inclusions) determined in the pyrites were Ti, Cr and Zn. Cr and Zn were present in most of the samples but Ti occurred in only two. The experimentally determined masses of these impurities are shown in table 2. Table 2 clearly demonstrates the powerful analytical capability of PIXE which comfortably extends down to 81 ng in the case of Zn. Before computing the concentrations of these impurities in the pyrite grains the accuracy of the experimental results were confirmed by comparing them against measured grain sizes. This was achieved by plotting the experimentally determined Fe masses against the surface area of each grain. This plot is shown in fig. 1. Assuming the thickness of the grains did not vary dramatically, a one-to-one correlation was expected which
Table 2 Experimentally determined Ti, Cr and Zn masses ingj Sample number 1 2 3 4 5 6 7 8 9 10
Ti mass
CT mass
Zn mass
_
260 180 300 470 _
230 620 190 _
350 230 490
81 81 _
840 320
930 310
790 670 -
would lend validity to the Fe contents displayed in table 1. The basis of this assumption lies in simple density considerations i.e. the Fe mass of each grain is evidently proportional to its volume; but if the thicknesses of individual grains did not vary widely, the Fe content would be expected to correlate on a one-to-one basis with the surface area. From the best straight line drawn between the data points (in fig. 1) it is evident
Surface
Area
(mm2)
Fig. 1. A plot of Fe mass vs surface area of the pyrite grains. Table 1 Experimentally determined
Fe and S masses of pyrite grains
Sample number
Fe [ctgl
S [pgl
Fe/S ratio
Dimension of grain [mm] X 10
1 2 3 4 5 6 7 8 9 10
209 422 166 854 77 469 104 383 715 788
158 185 129 455 62 227 65 233 430 355
1.32 2.28 1.29 1.89 1.24 2.07 1.60 1.64 1.66 2.22
2.5/2.5 6‘0/2.0 5.2/2.0 5.5/5.0 2.0/2.0 3.0/3.0 2.0/2.0 4.0/4.0 5.0/4.5 4.5/6.0
Table 3 Content of Ti, Cr and Zn relative to Fe (X 10V6) Sample number I 2 3 4 5 6 7 8 9 10
Ti 4759 784
Cr
Zn
1244 426 1807 550
1100 1469 1144 _
746 2211 1279 1175 406
173 779 1300 393
VI. GEOLOGICAL SAMPLES
422
A.E. Pillay et al. / Pyrite grains
that there are no serious deviations, which suggests that the Fe mass values in table 1 are acceptable. The concentrations of Ti, Cr and Zn relative to Fe are presented in table 3. These values varied widely from sample to sample and such variations indicate that heterogeneity would prevail within individual pyrite grains. Hence the analysis of only the surface or a fraction of the grain could lead to serious errors. The proposed sample preparation procedure overcomes this type of unrepresentative sampling and provides valuable information about the magnitude of the inclusions as well as useful major and minor element data.
entire grain by PIXE and ii) accurately determine the mass of each grain. 2) The proposed sample preparation procedure overcomes unrepresentative sampling and provides valuable information about the magnitude of the inclusions as well as useful major and minor element data. 3) Because the method has the potential for complete analysis of the pyrite grain, including its inclusions, it thus strongly rivals microprobe and image analyser techniques.
References 4. Conclusion 1) It has been shown that a special sample preparation procedure (for thin targets) - which entails dissolving individual pyrite grains on kapton foil - is useful because it is thus possible to: i) analyse the
111T.B. Johansson, R. Akselsson and S.A.E. Johansson, Nucl. Instr. and Meth. 84 (1970) 141. 121 S.A.E. Johansson and T.B. Johansson, Nucl. Instr. and Meth. 137 (1976) 473. [3] F. Folkmann, C. Gaarde, J. Hum and K. Kemp, Nucl. Instr. and Meth. 116 (1984) 487. [4] J.R. Bird, Nucl. Instr. and Meth. B45 (1990) 516.