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A brief introduction to the FEI Mineral Liberation Analyzer ™ : the technique & results. Michael Shaffer INCO Innovation Centre Memorial University St. John’s, Newfoundland mshaffer@mun.ca. Advanced Techniques in EPMA Seminar August 7, 2010 University of Oregon Eugene, Oregon.
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A brief introduction to the FEI Mineral Liberation Analyzer™: the technique & results Michael ShafferINCO Innovation CentreMemorial UniversitySt. John’s, Newfoundland mshaffer@mun.ca Advanced Techniques in EPMA Seminar August 7, 2010 University of Oregon Eugene, Oregon
MLA:points of interest • Particle analysis • Rocks crushed, sized and representative • Most accurate • E.G, iron ore from Labrador • “Large particle” analysis • e.g., 25x45mm section • Questionably representative • Large grain sizes • textures • E.G, Himalayan garnet shist
Fe-rich minerals of interest& spectral ambiguity • Hematite & magnetite [Fe2O3 versus Fe3O4] • Generally not distinguishable with x-ray spectra • Associations important to client • Titano-magnetite • Distinguishable with x-ray spectra • BSE similar to Hm • Titanium important to client • Goethite or limonite [FeO(OH)•(H2O)n] • Generally with minor Al, Si, Mg, and usually distinguishable with x-ray spectra • BSE darker than Hm (BSE classification would be helpful) • Siderite [FeCO3] • Generally with Ca, Mg, Mn, and usually distinguishable with x-ray spectra • BSE darker than Hm (BSE classification would be helpful)
Mineral modes Mineral Wt% Hematite 4.57 Magnetite 38.54 Ti_magnetite 0.09 Goethite 0.17 Limonite 0.08 Ilmenitend Rutile nd Corundum nd Quartz 35.55 Aluminosilicate nd Misc_silicates 0.11 Siderite 0.06 Siderit-Mn 0.11 Rhodochrosite nd Rhodo-FeMg 0.01 Rhodo-MgFe 0.00 Siderit-MgMn 7.37 Siderit-Mg 0.96 Ankerite 0.06 Calcit-MgMnnd Dolomit-FeMn 11.48 Magnesit-FeMn 0.22 Dolomite 0.15 Calcite 0.08 Unknown 0.02 Mineral Wt% Pyrolusite 0.00 Bixbyite_lo-Mnnd Bixbyite_hi-Mnnd Other_oxides 0.00 Olivine 0.00 Garnet 0.00 Cpx 0.01 Opx 0.02 Amphibole 0.00 Biotite 0.03 Feldspar 0.03 Muscovite 0.04 Serpentine nd Chlorite 0.14 Mn-rich_claynd Calcit-REE nd Pyrite 0.00 Pyrrhotite nd Chalcopyrite nd Sphalerite nd Misc_sulfidesnd Apatite 0.08 Miscellaneous 0.00 Misc_metals 0.01 Total 100.0 Mineral Wt% Magnetite 38.54 Hematite 4.57 Hm_or_Mt 0.00 Goethite 0.17 Limonite 0.08 Other_oxides 0.09 Quartz 35.55 Misc_silicates 0.38 Carbonates 20.50 Sulfides 0.00 Misc 0.09 Unknown 0.02 Total 100.0
The particle table 4k to 20k particles
Properties of particles Density Wt% Area% Area (microns) Area (pixels) Perimeter Max Span Length (MBR) Breadth (MBR) Hull Area Hull Perimeter EE Minor Axis Hull EE Minor Axis EE Major Axis (P&A) EE Minor Axis (P&A) EE Perimeter EC Diameter Angularity Enclosed Length Delta Form Factor All minerals (Wt%) e.g., Hematite (Wt%) Magnetite (Wt%) Goethite (Wt%) Limonite (Wt%) Quartz (Wt%) … Misc (Wt%) Unknown (Wt%) All elements (Wt%) e.g., Al (Wt%) Ca (Wt%) Cr (Wt%) Cu (Wt%) F (Wt%) Fe (Wt%) H (Wt%) K (Wt%) La (Wt%) Mg (Wt%) Mn (Wt%) Na (Wt%) Ni (Wt%) P (Wt%) S (Wt%) Si (Wt%) Ti (Wt%) … Zn (Wt%) Free Boundary, all minerals e.g., Hematite (%) Magnetite (%) Goethite (%) Limonite (%) Quartz (%) … Misc (%) Unknown (%)
The grain table More than 52,000 grains
Properties of grains Density Center X Center Y Wt% Area% Area (microns) Area (pixels) Perimeter Max Span Max Span Angle Wt% (Particle) Area% (Particle) Wt% (Mineral) Area% (Mineral) Particle Max Span Particle Perimeter Length (MBR) Breadth (MBR) Angle Length (MBR) Hull Area Hull Perimeter EE Minor Axis Hull EE Minor Axis Hull EE Perimeter EE Major Axis (P&A) EE Minor Axis (P&A) EC Diameter Aspect Ratio Angularity Enclosed Length Delta Form Factor Boundaries with other minerals e.g., Quartz (%) Orthoclase (%) Garnet (%) Biotite (%) … free surface (%)
Applications at MUN • Mineral modes & associations • Mineral locking & liberation • Mineral searching (e.g., zircon, baddeleyite, monazite) • Includes x-y coordinate export • Precious mineral searching (e.g., Au, PGM) • Includes associations with host minerals • Provenance determinations • Sourcing continental river & till sediments (mineral prospecting) • Sourcing offshore sediments with onshore (oil & gas) • Lateral correlation of offshore sediments (oil & gas) • Some thought toward … • Accurate determination of trace minerals (e.g., apatite, corundum) • Invisible gold with a FEG MLA • Long-count EDX • Auxillary inputs …, e.g., WDX, μXRF
Acknowledgements The MUN MLA team: David Grant Alan Maximchuk Dylan Goudie & thank you for your interest!
Is it possible with XBSE & MLA spectra? Difference is only 24 counts (2σ ~ 34) 15 counts (2σ ~ 58) 28 wt% O versus 30% Sensitive to absorption 72 wt% Fe versus 70% Sensitive to charging
The spectral-classification result Red implies mineral grain is eitherhematite or magnetite
BSE classification Hm Qtz “reliable” histogram Cumulative or “full” histogram Mt Other silicates, carbonates and hydroxides
BSE-classification results – good & bad Magnetite Hematite “Darks”
MLA BSE mode results – good & badthe smallest size fraction: -200 mesh
Before “Merge Overlay” Processed via gray level segmentation Mode BSE data acquisition Classified data, modes, … Merge Overlay OR Processed via Spectral matching Mode XBSE data acquisition Classified data, modes, …
Results from Merge Overlay • Spectrally classified “Hm-or-Mt” becomes: • Hematite, or • Magnetite, or • “Fe-ox_no-ID” • Which can generally be justified and grouped with limonite or goethite (… although pure siderite is also a possibility) • Smaller size fractions evaluated independently • Hm:Mt modal ratio might be assumed from larger SFs or their trends
Reproducibility: mineral modes same samples – 6 months between Samples A, B, C & D
Reproducibility: mineral modes same samples – 6 months between Samples A, B, C & D
Reproducibility: mineral associations same samples – 6 months between Samples A, B, C & D
Reproducibility: mineral associations same samples – 6 months between Samples A, B, C & D
Results comparison:MLA v. Rietveld XRD Average absolute errors
Sources of instrumental error:electron beam illumination 195 = Hm 198 = Mt 192 = Hm 195 = Mt
Sources of instrumental error:varying e-beam current 3rd frame 143rd frame 195 = Hm 198 = Mt 2 hours Later … 192 = Hm 195 = Mt
Remedying BSE problems • Non-uniform illumination • No remedy if the SEM manufacturer did not anticipate applications in quantitative BSE • Except to use high magnification • Difficult to remedy if the SEM manufacturer did not provide alignment tools for uniformity • FEI Quanta SEMs: • Centering the illumination provided by e-gun tilt • Tetrode & gun alignment should be accurate • Illumination gradients worse for large spot sizes
Remedying BEI problems • Varying beam current • Very common depending on age of filament … • Stability generally monotonic, i.e., not erratic • … allows for breaking the BSE JKF file into 2 to 4 files, thereby creating more reliable histograms that represent time periods during analysis. • Note also that this method is quite dependent on a significant amount of Hm-Mt in the sample, which builds a more accurate reliable histogram
Anticipating problems we haven’t yet encountered, and possible improvements • MUN IIC has not yet applied this method to mineral assemblages other than the minerals discussed here • I.E., a severe complication would arise for significant amounts of titano-magnetite, thereby blurring the distinction of Hm in the reliable histogram • A very helpful improvement, which would allow the same tools to be applied to other applications, would be for the spectra-classified result to mask the minerals of interest to be classified with BSE
MLA Mode BSE conclusions • Hm – Mt BSE discrimination works … • And Hm-Mt associations are possible • … but not specifically with other minerals • and, by itself, cannot discriminate most other minerals because of average atomic number (i.e., BSE ambiguity) • However, it presents a suitable solution for augmenting spectral classification (mode XBSE) • How to augment with spectral classification? …
Summary • Hm–Mt BEI discrimination is possible … • Hm-Mt associations are possible, and with all minerals • Mineral modes and associations can be reproduced with acceptable accuracy • A comparison with quantitative XRD is within errors associated with the difficulty associated with representative down-sampling (XRD sampling independent of MLA sampling) • However, a well-aligned and stable SEM is necessary … • Electron beam illumination must be uniform over 1 – 2mm • Beam current must be stable over the 2 – 3hr analytical time (although data processing can accommodate a monotonic variation) • This technique is more generally applicable, even to more complex mineral assemblages when chemistry (x-ray spectra) aids in masking the minerals of interest
Independent BEI conclusions • Hm – Mt discrimination works … • Associations Hm-Mt are not possible • Minerals of similar atomic number, identified by XBSE, do not affect calculated Hm:Mt • However, results can be biased if: • one mineral does not polish as well, or if • one mineral’s grain size is typically smaller • Not the best solution, but should be in the analyst’s toolbox
The results for the client • Primary modes and associations come from mode XBSE. • Whereas we had been providing Hm:Mt via the independent method … • Because titano-magnetite and pyrite are minimal and correctable, we do not augment XBSE with additional BSE results. • The good news is that Hm-Mt associations are provided but the bad news is that Hm-Mt-Qtz associations are not. • What is needed …
Results comparison:MLA v. Rietveld XRD Sample 1 SFs +100 & +200 sampling error
Results comparison:MLA v. Rietveld XRD Sample 2 SFs +100 & +200