Preparation of powder specimen for quantitative XRD
Reinhard Kleeberg TU Bergakademie Freiberg
Mineralogical Institute [email protected]
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References
Moore, D.M. & Reynolds, R.C.: X-Ray Diffraction and the Identification and Analysis of Clay Minerals. Oxford Univ. Press/CMS 1989, 1997.
Bish, D.L. and Reynolds, R.C., Jr. (1989) Sample preparation for X-ray diffraction. Pp. 73-99 in: Modern Powder Diffraction (D.L. Bish and J.E.
Post, editors). Reviews in Mineralogy 20, Mineralogical Society of America, Washington, D.C.
Środoń, J., Drits, V.A., McCarty, D.K., Hsieh, J.C.C. and Eberl, D.D. (2001) Quantitative X-ray diffraction analysis of clay-bearing rocks from random preparations. Clays and Clay Minerals, 49, 514-528.
Hillier, S. (2003) Quantitative analysis of clay and other minerals in sandstones by X-ray powder diffraction (XRPD). International Association of Sedimentologists Special Publication, 34, 213-251.
Kleeberg, R., Monecke, T., Hillier, S. (2008) Preferred orientation of mineral grains in sample mounts for quantitative XRD measurements: How random are powder samples? Clays and Clay Minerals, 56 (4), 404-415.
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Crystallite statistics 1
Impact of crystallite statistics on the diffraction signal depends on:
•sample volume contributing to diffraction (limited by the primary slit system, sample holder, absorption)
•section of the diffraction rings seen by the detector (limited by Soller slits, receiving slit, detector area)
•number of particles per volume unit (limited by particle size)
-only the particle size may be optimized by sample preparation
-ideal case: “infinite number of crystallites” = continuous cones of diffraction coarse material ideal powder
sections seen by a point detector sections seen by an area detector
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Crystallite statistics 2
Practical consideration: How many particles do contribute to our diffraction signal?
Exemplary calculationat: http://epswww.unm.edu/xrd/xrdclass/07-Errors-Sample-Prep.pdf Quartz in different particle diameter in a conventional Bragg-Brentano diffractometer
Particle diameter 40 µm 10 µm 1 µm Diffracting particles 12 760 38 000
To achieve a standard uncertainty of < 1%, > 52900 particles would be needed!
Conclusions:
-If we want to measure single peak intensities of minerals correctly, we should try to mill any rock samples to < 5 µm.
-This is necessary to get reliable results of Rietveld structure analysis, too.
Problems Milling Filling Examples
Experimental testsee: Klug, H.P. & Alexander, L.E. X-Ray Diffraction Procedures. John Wiley, New York, 1954
Quartz in different particle diameter in a conventional Bragg-Brentano diffractometer Particle size fraction /microns 15-50 5-50 5-15 <5 Standard deviation of I quartz101/% 18.2 10.1 2.1 1.2
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Microabsorption 1
Microabsorption causesloss of intensityinside a particle. If this loss is different between two phases, the beam is interacting with different phase volumina! Thus, the QPA results will be falsified.
High absorption Lower absorption
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The theory of BRINDLEY (1945):
- depends on grain diameter d and linear attenuation coefficient µ
- intensity loss / correction factor can not be determined from the powder data!
- can be ignored if the product µd is equal for all phases
- intensity resp. scale factor can be correctedif µd < ~0.5, and if both values are known for each phase!
Approximated BRINDLEY correction in the BGMN structure file:
GOAL:forsterite=GEWICHT*exp(my*d*3/4)
GEWICHT Rietveld scale factor, mass weighted
my linear attenuation coefficient in 1/µm, provided by BGMN
d estimated particle diameter in µm, to be set by the user in structure or control file
Problems Milling Filling Examples
Microabsorption 2
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When does microabsorption occur in practice?
Example: QPA of sulphide bearing rocks Absorption contrast between quartz and pyrite
coarseness according to BRINDLEY (1945)
phase grain size/µm µd Cu K
µd Co K
quartz 10 0.093
medium
0.146 coarse
pyrite 10 1.012
very coarse
0.516 coarse
Microabsorption 3
Conclusions:
-No problem for fine clay fractions and silicate minerals having similar (low) µ.
-But, if we want to quantify rock samples containing high absorbing materials, we should try to mill the samples to < 5 µm.
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For example, sedimentary rocks:
•large crystals (>mm, e.g. quartz), together with fine grained material (clay)
•hard and soft phases together
•differences in density may occur (pyrite, hematite, rutile, silicates…)
•mechanically or thermally sensitive phases (e.g. clay minerals, zeolites, sulphates)
Problems Milling Filling Examples
What we have
•mean particle size below 5 µm, optimum 1-3 µm
•narrow particle size distribution, no amorphization, no “rocks-in-the-dust”
•no additional disorder ( e.g. by shifting of layers)
•no contamination (e.g. by grinding elements)
•no loss of material (e.g. by dusting or dissolution)
•no phase transformation (e.g. by dissolution-precipitation, hydration-dehydration…)
•no phase separation (e.g. by hardness, density, primary particle size, electrostatic…)
What we want to get
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General principles:
- The maximum grinding energy has to be adapted to the most sensitive phase.
- Some overmuch large grains are better than any destroyed phase(s).
- If a sample is not homogenous or not representative, any further efforts (for phase quantification in general) are unnecessary.
- Working in closed containers to avoid loss of material by dusting, checking for losses by sedimentation, dissolution, electrostatic adhesion...
- Quick working for minimum contact with dissolution agents and air to avoid any phase alteration.
- Samples should not be exposed any enhanced temperature.
Golden rules for milling
- Note (or keep in mind) what treatments and changes your sample was exposed between sampling and XRD measurement.
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Hand grinding in agate mortar - easy, but strenuous
- allows a stepwise sieving of < 20 µm, very mild
- resulting powder is mostly to coarse for high absorbing materials - danger of loss by dusting
Problems Milling Filling Examples
Grinding methods useful for clay-bearing rocks 1
Foto: Detlev Müller Foto: Detlev Müller
11 McCrone micronising mill
- wet grinding (in water or alcohol) produces optimum grain distribution - crushing the starting material < 0.4 mm is necessary
- danger of contamination by grinding elements (corundum, quartz, ZrO2) - dissolution and/or alteration by the grinding liquid (water, ethanol, hexane…) - filtering or drying of the slurry and additional homogenisation is necessary
Grinding methods useful for clay-bearing rocks 2
Particle diameter/µm Volume %
48 Milling time /min 24 12 6
1.8 Milling time /min
1.8 Mean diameter /µm 2.6 5.8 11.4
0 1 2 3 4 5 6
0. 04 0. 1 0. 4 1 2 4 10 20 40 100 400
Particle size distribution of McCrone milled quartz,
from Hillier (2003) Foto: Detlev Müller
12 - is necessary for admixing of any standards as well as to overcome the (unavoidable) separation processes during grinding, sieving, and sedimentation - should be applied with minimum energy
- should be done immediately before filling the sample Useful methods:
- admixing of standards by grinding together(to get similar grain size and intimate mixing)
- larger amounts of easily flowing powders by overhead-shaking
- small amounts by manual stirring
- homogenisation and destroying of aggregates by swirling the powder with small steel balls („Ardenne vibrating mill“ or Fritsch mini-mill “pulverisette 23”) - can also used for destroying of aggregates
Problems Milling Filling Examples
Homogenisation
13 are an old matter of debate, and sometimes treated like religion…
Possible pre-treatment of powder before filling:
- No pre-treatment (filling the milled and homogenised powder “as is”) -Forming irregular shaped aggregates
-by freeze-drying -by spray-freeze-drying
-by admixing glass powder, plastics or organics, e.g. cork powder -Forming spherical aggregates
-by spray-drying with binder (cellulose-acetate, polystyrene, polyvinyl-alcohol) -by spray-drying without binder, application of heat
-by mechanical aggregation in a shaking/sieving procedure (“sieving in”)
Making the powder mount for Bragg-Brentano XRD 1
Aspects for decision:time consumption, material consumption, stability and density of the samples, loss of intensity by dilution, sample transparency, alteration of minerals, reproducibility, labour protection…
14 Front-loading (standard holders, very popular)
- powder pressed using a glass slide: very easy, but extreme PO
- powder roughened by emery paper: not so much PO, but badly reproducible and depending of the properties of the powder
- surface roughened by a razor blade: as above, plus high roughness
- sprinkling/sieving some powder on the surface: good randomness, but errors in sample height, high roughness, sometimes phase separation
- possible to do with any kind of aggregates, perfect randomness possible
Problems Milling Filling Examples
Making the powder mount for Bragg-Brentano XRD 2
Simple tricks may make the difference…
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Making the powder mount for Bragg-Brentano XRD 3
Back-loading (special Philips/Panalytical equipment) - easy to do, but extreme PO
- no chance to use for unstable aggregates like freeze-dried clays
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Problems Milling Filling Examples
Making the powder mount for Bragg-Brentano XRD 4
Side-loading (with special side-opened holders, but also for standard holders) - with normal powder: relatively easy, reproducible, acceptable PO - with irregular aggregates: perfect randomness possible
- general problems are density, homogeneity, and stability, e.g. for rotated samples
17 Influence of overmuch large grains on the powder pattern
topaz bearing granite, samples ball-milled < 63 µm and hand-ground < 20 µm
25 30 35 4 0
0 100 200 300
2 Theta / ° ( Sc an Axis : 2:1 s ym. cons t. area )
Intensity / cps
Comparison of 2 Scans
kfsp 002/040 preferred orientation!
plag 002/040
topaz 230
„rocks in the dust“
< 63 µm
< 20 µm
Errors in sample preparation 1
18 Phase separation and loss of material during powder preparation
Mineral mixtures, ground by hand/dry sieving < 20 µmand McCrone milling/ethanol, filtered slurry
halite dissolved in ethanol mica + quartz lost by
dusting during dry sieving
Problems Milling Filling Examples
Errors in sample preparation 2
19 Sample contamination by abrasion from grinding elements
Kaolin Spergau reference sample, McCrone groundusing corundum grinding elements and using agate grinding elements
corundum line positions
Errors in sample preparation 3
20 Preferred orientation of grains/aggregates dependent on filling technique
20 30 40 50 60
0 100 200 300 400
2 Theta / ° ( Scan Axis: 2:1 sym. )
Intensity / cps
Comparison of 2 Scans
bentonite, fraction < 0.2 µm
005 02,11
06,33 front-loading side-loading
Problems Milling Filling Examples
Errors in sample preparation 4
21 Preferred orientation of grains/aggregates dependent on filling technique
Errors in sample preparation 5
Mixture quartz/dickite/muscovite 40:30:30, McCrone milled, different filling techniques (from Kleeberg et al., 2008)
Powder front-loaded Powder side-loaded Spray-dried front-loaded Ms
002 Ms
004 Dc 002
Ms+Dc 110,
020 Ms
060 Dc 060