A Synchrotron Radiation-XPS (SRXPS)

Natural Resources Ressources naturelles 

Canada 

Canada 

A SRXPS and XPS Study of the 

Surface Reactivity of Enargite. 

Allen Pratt 

1 

CANMET MINING AND MINERAL SCIENCES LABORATORIES

A SRXPS and XPS Study of the Surface Reactivity of Enargite 

Allen Pratt, Mining and Mineral Sciences Laboratory, Natural Resources Canada,Ottawa, Canada K1A 0G1 

Abstract 

Enargite is an important ore of copper in many of the giant porphyry deposits that are found in the circum-Pacific. The surface chemistry of fresh and air 

reacted enargite was investigated using SRXPS and XPS. The purpose of this research was to investigate enargite mineral chemistry as it relates to surface 

reactivity. This information is important to the processors of copper ores. 

Enargite is in the orthorhombic crystal system and has the composition Cu 3 

AsS 4 

. Up to 6 wt% Sb can substitute for As. The structure of enargite is a 

derivative of the wurtzite structure, in which 3/4 of Zn is replaced by Cu and 1/4 of Zn is replaced by As. Zn replacement by Cu and As is in an ordered 

manner so as to give discrete MS 4 

groups. All the atoms in enargite are in tetrahedral coordination (Pauling and Weinbaum, Z.Kritst. 88, 48, 1934).This coordination 

can occur whenever the number of available bonding electrons is equal to 4 times the number of atoms in the formula. 

To conduct this study, chemical information on the mineral near surface was obtained using XPS and the mineral surface using SRXPS. In XPS and SRXPS 

low energy photons are directed at a sample, atoms in a matrix are excited and both core and valence band electrons are ejected with characteristic binding 

energies. Binding energies provide qualitative and quantitative surface chemical state information. The basic equation describing XPS and SRXPS is: 

E K 

= h< - E B 

where E K 

is the kinetic energy of the photoelectrons, h< the incident photon energy and E B 

the electron binding energy. For these XPS experiments h< was 

fixed at 1486.6 eV (monochromatic Al K∀ ) and based on E K 

for each element, the XPS sampling depth was several nanometers. For the SXPS experiments 

h< was varied so as to lower E K 

to the optimal value for a SRXPS sampling depth near one nanometer for the elements of interest. 

The Cu2p3/2 and S2p spectra collected from enargite surfaces exposed by fracture in a vacuum using SXPS and XPS are remarkably similar and show no 

obvious features that can be attributed to Cu or S atoms that are resident in unique surface sites. In addition, the Cu2p3/2 and S2p spectra collected from the 

air-reacted enargite surfaces are similar to those collected from the surfaces exposed by fracture in a vacuum. The As3d XPS and SRXPS spectra collected 

from the vacuum exposed surfaces have similar binding energy contributions, the intensities of these contributions, especially near 42 eV, are however, 

substantially different. The appreciable intensity decrease in the 42 eV contribution in the XPS data relative to the SRXPS data indicates that this emission 

originates from surface resident As atoms. These results appear to indicate that enargite surfaces exposed by fracture in a vacuum are made up mainly of As. 

Exposure of the enargite surface to air for two minutes results in the complete disappearance of the 42 eV contribution. The XPS O1s spectra collected for the 

air-reacted surface show contributions from oxide oxygen, oxygen in hydroxide and oxygen in water. The oxide oxygen contribution is sharp and well defined. 

These features suggest that the oxide is found in a specific chemical environment on the surface. The oxide oxygen is most likely associated with the As that 

made up the 42 eV contribution as this is the only atomic component in enargite that has a chemistry specific to the surface . 

Acknowledgments 

Special thanks are extended to my colleagues H.W. Nesbitt, G.M. Bancroft, W. Skinner, I. Uhlig, and A.G. Shaufuss for their support and assistance in this 

study. I also thank H. Höchst, M. Bissen and M. Severson for their assistance with the CMA on the HERMON line and the SRC-Scienta on the PGM line at 

the SRC. Funding was provided by NRCan to A.P. The SRC is supported by the NSF award DMR-95-31099. 

2 


Why Enargite 

Why the Surface Chemistry 

3 


Enargite 

Cu 3 

t 

As t [S 4 ] h 

Cu - S = 2.31Å As - S = 2.21-2.24Å 

(Weunsch, 1974) (Zoltai, 1974) 

4 


Enargite Specimen Description 

Composition: Cu 3 As 0.9 Sb 0.1 S 4 

Source Local: Cero De Pasco, Peru 

Form: Euhedal Crystals 

Structure: Orthorombic, (Mandarino, 1999) 

Characteristics: Essentially monominerallic. 

History: Sample was in the Earth Sci. mineral collection at UWO. 

Condition: Excellent with good homogeneity, 

5 


Methods 

Sample Preparation: Specimens were cut into 1 cm x 1cm x 0.5 cm 

blocks using a high precision diamond saw and ultrasonically 

degreased sequentially in ensolv, acetone and ethanol 

Fresh Surface Preparation: Fresh mineral surfaces were exposed 

by fracture in high vacuum using specialized fracture devices. 

Reaction with Air: The fresh mineral surface was exposed to 

air for 120 seconds. 

Instrumentation 

XPS: Upgraded PHI 5600, Hemispherical Analyzer 

SR-XPS: PHI Cylindrical Mirror Analyzer 

Scienta, Hemispherical Analyzer, sample cooled to liquid He. 

6 


XPS - X-ray Photoelectron Spectroscopy 

or 

ESCA - Electron Spectroscopy for 

Chemical Analysis 

XPS is a micro-beam technique which provides chemical state 

information on the major and minor elements in the first 10 to 

50Å of a surface. XPS analysis are conducted at ultra high 

vacuum (>10 -9 Torr). 

Information Acquired: 

- Compositional information, all elements He through U are 

detectable. 

-Oxidation State 

-Bonding information 

7 


Energy Analyzer 

A schematic diagram of the PHI Model 5600 MultiTechnique system. 

8 


Counts 

7000 

6000 

5000 

4000 

3000 

XPS S 2p 

h< = 1486.6 eV 

Enargite Surface Exposed by 

Fracturing in UHV 

161.8 eV 

FWHM = 0.8 eV 

2000 

1000 

0 

168 

166 

164 

162 

160 

158 

Binding Energy (eV) 

9 


Counts 

24000 

18000 

12000 

XPS Cu 2p3/2 

h< = 1486.6 eV 

Enargite Surface Exposed 

by Fracturing in UHV 

931.9 eV 

FWHM = 1.1 eV 

6000 

0 

938 

937 

936 

935 

934 

933 

932 

931 

930 

929 

928 


10 


Counts 

8000 

6000 

4000 

XPS As 3d 

h< = 1486.6 eV 


by Fracturing in UHV 

43.7 eV 

FWHM = 0.7 eV 

43.2 eV 

FWHM = 0.7 eV 

42.3 eV 

FWHM = 0.7 eV 

2000 

0 

48 

47 

46 

45 

44 

43 

42 

41 

40 


11 


Synchrotron Radiation X-ray Photoelectron Spectroscopy 

SR-XPS 

Principle features relative to XPS 

1) Enhance surface sensitivity 

2) Non destructive depth profiling 

E B = hν - E K 

E B = electron binding energy 

hν = incident photon energy 

E K = kinetic energy of photoelectron 

12 


E K 

= hv - 162.0 eV (E B 

for S2p) 

E K = kinetic energy 

hv = incident photon energy 

E B = electron binding energy 

E K = 43.0 eV 

hv = 205.0 eV 

hv = 1486.6 eV 

E K = 1324.6 eV 

Surface 

>5Å 

Depth 

≈50Å 

13 


* 

* 

14 


10 100 1000 

15 


HERMON Beamline SRC Port 033 

16 


6000 

SRXPS Cu 2p3/2 

h< = 1025 eV 

CMA on the HERMON 

931.9 eV 

FWHM = 2.1 eV 

Counts 

4000 

2000 

0 

938 

937 

936 

935 

934 

933 

932 

931 

930 

929 

928 


17 


3000 

2000 

SRXPS S 2p 

h< = 195 eV 

SRC-Scienta on the PGM 

161.75 eV 

FWHM = 0.8 eV 

Counts 

1000 

0 

168 

166 

164 

162 

160 

158 


18 


Counts 

12000 

9000 

6000 

3000 

SRXPS As 3d 

h< = 100 eV 

SRC-Scienta on the PGM 

43.5 eV 

FWHM = 0.5 eV 

As in Matrix 

43.2 eV 

FWHM = 0.5 eV 

As in Matrix 

42.1 eV 

FWHM = 0.7 eV 

As at Surface 

0 

48 

47 

46 

45 

44 

43 

42 

41 

40 


19 


Air Reaction Experiment 

XPS Results 

20 


Counts 

24000 

18000 

12000 

XPS Cu 2p3/2 

h< = 1486.6 eV 

Enargite Surface 

Exposed To Air 

For 2 Minutes 

931.9 eV 

FWHM = 1.1 eV 

6000 

0 

938 

937 

936 

935 

934 

933 

932 

931 

930 

929 

928 


21 


Counts 

7000 

6000 

5000 

4000 

3000 

XPS S 2p 

h< = 1486.6 eV 


To Air For 2 Minutes 

161.7 eV 

FWHM = 0.8 eV 

2000 

1000 

0 

168 

166 

164 

162 

160 

158 


22 


8000 

6000 

XPS As 3d 

h< = 1486.6 eV 

Enargite Surface 

Exposed To Air 

For 2 Minutes 

43.2 eV 

FWHM = 0.7 eV 

As in Matrix 

Counts 

4000 

2000 

43.7 eV 

FWHM = 0.7 eV 

As in Matrix 

0 

48 

47 

46 

45 

44 

43 

42 

41 

40 


23 


Counts 

2000 

1000 

XPS O 1s 

h< = 1486.6 eV 


To Air For 2 Minutes 

533.7 eV - H 2 

O 

FWHM = 2.0 eV 

532.1 eV - OH 

FWHM = 1.8 eV 

529.5 eV - O 

FWHM = 1.0 eV 

0 

537 

535 

533 

531 

529 

527 


24 


Results and Conclusions 

(1) Cu2p3/2 and S2p Spectra in all the experiments are remarkably 

similar and show no obvious features that can be attributed to 

atoms in surface sites. 

(2) As3d spectra show marked differences. SRXPS spectra show 

structure that is attributed to atoms is surface sites. 

(3) Exposure to air appears to show that As is the most 

reactive enargite component. 

25

A Synchrotron Radiation-XPS (SRXPS)

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