Michael ShafferINCO Innovation CentreMemorial UniversitySt. John’s, Newfoundland mshaffer@mun.ca

Advanced Techniques in EPMA Seminar

August 7, 2010University of Oregon

Eugene, Oregon

A brief introduction to theFEI Mineral Liberation Analyzer™:

the technique & results

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

2

BEI: Fe-rich minerals

3

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)

4

Mineral modes

5

Mineral Wt%Hematite 4.57Magnetite 38.54Ti_magnetite 0.09Goethite 0.17Limonite 0.08Ilmenite ndRutile ndCorundum ndQuartz 35.55Aluminosilicate ndMisc_silicates 0.11Siderite 0.06Siderit-Mn 0.11Rhodochrosite ndRhodo-FeMg 0.01Rhodo-MgFe 0.00Siderit-MgMn 7.37Siderit-Mg 0.96Ankerite 0.06Calcit-MgMn ndDolomit-FeMn 11.48Magnesit-FeMn 0.22Dolomite 0.15Calcite 0.08Unknown 0.02

Mineral Wt%Pyrolusite 0.00Bixbyite_lo-Mn ndBixbyite_hi-Mn ndOther_oxides 0.00Olivine 0.00Garnet 0.00Cpx 0.01Opx 0.02Amphibole 0.00Biotite 0.03Feldspar 0.03Muscovite 0.04Serpentine ndChlorite 0.14Mn-rich_clay ndCalcit-REE ndPyrite 0.00Pyrrhotite ndChalcopyrite ndSphalerite ndMisc_sulfides ndApatite 0.08Miscellaneous 0.00Misc_metals 0.01Total 100.0

Mineral Wt%Magnetite 38.54Hematite 4.57Hm_or_Mt 0.00Goethite 0.17Limonite 0.08Other_oxides 0.09Quartz 35.55Misc_silicates 0.38Carbonates 20.50Sulfides 0.00Misc 0.09Unknown 0.02Total 100.0

The particle table

6

4k to 20k particles

Properties of particles

7

DensityWt%Area%Area (microns)Area (pixels)PerimeterMax SpanLength (MBR)Breadth (MBR)Hull AreaHull PerimeterEE Minor AxisHull EE Minor AxisEE Major Axis (P&A)EE Minor Axis (P&A)EE PerimeterEC DiameterAngularityEnclosed Length DeltaForm 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 mineralse.g., Hematite (%)

Magnetite (%)Goethite (%)Limonite (%)Quartz (%)…Misc (%)Unknown (%)

datamining the particle table

8

3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.30.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Si content for particles of density greater than SG

SF+100

SF+200

specific gravity of particles

Si %

Large sections

Spectral discrimination ~ garnet

grain boundaries resolved with BEI

grain boundaries not resolved with BEI

Grain associations

13

Mineral Qtz Biot Plag Ksp Gt_Mg

Qtz - 30 20 7.3 1.3

Biot 35 - 24 7.3 1.7

Plag 32 32 - 8.9 0.9

Ksp 29 25 23 - 0.3

Gt_Mg 14 17 6.7 0.8 -

The grain table

14

More than 52,000 grains

Properties of grains

15

DensityCenter XCenter YWt%Area%Area (microns)Area (pixels)PerimeterMax SpanMax Span AngleWt% (Particle)Area% (Particle)Wt% (Mineral)Area% (Mineral)Particle Max SpanParticle PerimeterLength (MBR)Breadth (MBR)Angle Length (MBR)

Hull AreaHull PerimeterEE Minor AxisHull EE Minor AxisHull EE PerimeterEE Major Axis (P&A)EE Minor Axis (P&A)EC DiameterAspect RatioAngularityEnclosed Length DeltaForm FactorBoundaries with other mineralse.g., Quartz (%)

Orthoclase (%)Garnet (%)Biotite (%)…free surface (%)

datamining the grain table:mineral textures

16

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

plagioclase orientation

angle for MBR

% p

lagi

ocla

se

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

17

Acknowledgements

18

The MUN MLA team:

David GrantAlan Maximchuk

Dylan Goudie

&thank you for your interest!

A typical frame, BSE relative to Ni metal

19

Is it possible with XBSE & MLA spectra?

20

0

1

2

3

4

5

6

7

8

0.35 0.45 0.55 0.65 0.75 0.85

eV

Cou

nts

(200

0 sp

ectra

l cou

nts)

HematiteMagnetite

0

5

10

15

20

25

6.10 6.30 6.50 6.70

eV

Cou

nts

(200

0 sp

ectra

l cou

nts)

Hematite

Magnetite

Difference is only

24 counts(2σ ~ 34)

15 counts (2σ ~ 58)

72 wt% Fe versus 70%

28 wt% O versus 30%Sensitive to absorption

Sensitive to charging

The spectral-classification result

21

Red implies mineral grain is either hematiteormagnetite

BSE classification

22

Cumulative or “full”

histogram

Qtz Hm

Other silicates, carbonatesand hydroxides

Mt

“reliable” histogram

BSE-classification results – good & bad

23

MagnetiteHematite“Darks”

MLA BSE mode results – good & badthe smallest size fraction: -200 mesh

24

Before “Merge Overlay”

Mode BSE data

acquisition

Classified data, modes, …

Processed via

gray level segmentation

Mode XBSE data

acquisition

Classified data, modes, …

Processed via

Spectral matching

ORMerge

Overlay

MLA “merge overlay” tool

26

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 27

Reproducibility: mineral modes same samples – 6 months between

28

Samples A, B, C & D

Qtz

Hm Mtno

-ID Qtz

Hm Mtno

-ID Qtz

Hm Mtno

-ID Qtz

Hm Mtno

-ID

0

5

10

15

20

25

30

35

40

45Size +100M mineral modes

2008

2009

Wt%

Reproducibility: mineral modes same samples – 6 months between

29

Samples A, B, C & D

Qtz

Hm Mtno

-ID Qtz

Hm Mtno

-ID Qtz

Hm Mtno

-ID Qtz

Hm Mtno

-ID

0

5

10

15

20

25

30

35

40

45Size +200M mineral modes

2008

2009

Wt%

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

0

5

10

15

20Size +100M mineral associations

2008

2009

Perc

enta

ge o

f gra

in b

ound

arie

s

Reproducibility: mineral associations same samples – 6 months between

30

Samples A, B, C & D

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

Hm w

ith M

tHm

with

Qtz

Mt w

ith H

mMt

with

Qtz

0

5

10

15

20

25

30

35

40Size +35M mineral associations

2008

2009

Perc

enta

ge o

f gra

in b

ound

arie

s

Reproducibility: mineral associations same samples – 6 months between

31

Samples A, B, C & D

Results comparison:MLA v. Rietveld XRD

32

Qtz Mt Hm Qtz Mt Hm Qtz Mt Hm Qtz Mt HmSample ASF+100M

ASF+200M

Sample BSF+100M

BSF+200M

0

5

10

15

20

25

30

35

40

45

50Rietveld 1Rietveld 2MLA

Results comparison:MLA v. Rietveld XRD

Average absolute errors

XRD sampling XRD-v-MLA0

2

4

6

8

10

12

14

16

18

20

Quartz

Magnetite

Hematite

Sources of data processing error

34

Sources of instrumental error:electron beam illumination

35

195 = Hm198 = Mt

192 = Hm195 = Mt

Sources of instrumental error:varying e-beam current

36

195 = Hm198 = Mt

192 = Hm195 = Mt

3rd frame 143rd frame

2 hoursLater …

Remedying BSE problemsNon-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

37

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

38

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 39

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? …

40

SummaryHm–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

41

Consider an independent approach …

42

Exported BEI frames into 3rd-party software

43

The masked & cleaned frames

44

A clean histogram allows for automatic thresholding

45

Independent software resultsfortunate & unfortunate

46

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 47

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 … 48

Results comparison:MLA v. Rietveld XRD

49Rietveld 1 Rietveld 2 MLA Rietveld 1 Rietveld 2 MLA

0

5

10

15

20

25

30

35

40

45

QuartzMagnetiteHematite

Sample 1SFs +100 & +200

sampling error

Results comparison:MLA v. Rietveld XRD

50Rietveld 1 Rietveld 2 MLA Rietveld 1 Rietveld 2 MLA

0

5

10

15

20

25

30

35

40

45

50

QuartzMagnetiteHematite

Sample 2SFs +100 & +200

Merge JKF dialog

51

3rd-party results can sometimesbe a necessary tool

52

MLA BSE mode results – good & badminerals of similar atomic number

53

Results comparison:MLA v. Rietveld XRD

Largest absolute errors

XRD sampling XRD-v-MLA0

5

10

15

20

25

30

35

40

Quartz

Magnetite

Hematite