preparation of powder specimen for quantitative xrd · quantitative x-ray diffraction analysis of...

$: Preparation of powder specimen for quantitative XRD · Quantitative X-ray diffraction analysis of clay-bearing rocks from random preparations. Clays and Clay Minerals, 49, 514-528$
Preparation of powder specimen for

quantitative XRD

Reinhard Kleeberg

TU Bergakademie Freiberg

Mineralogical Institute

[email protected]

2

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.

3

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

Problems Milling Filling Examples

coarse material ideal powder

sections seen by

a point detector

sections seen by

an area detector

4

Crystallite statistics 2

Practical consideration: How many particles do contribute to our diffraction

signal?

Exemplary calculation at: 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.


Experimental test see: 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 quartz 101 /% 18.2 10.1 2.1 1.2

5

Microabsorption 1

Microabsorption causes loss of intensity inside 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


6

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 corrected if 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


Microabsorption 2

7

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)


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

9

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.

10

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


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 %

48Milling time /min 24 12 6

1.8

Milling time /min

1.8Mean 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

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- 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


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…

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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


Making the powder mount for Bragg-Brentano XRD 2Simple tricks may make the difference…

15



Back-loading (special Philips/Panalytical equipment)

- easy to do, but extreme PO

- no chance to use for unstable aggregates like freeze-dried clays

16



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 40

0

100

200

300

2 Theta / ° ( Scan Axis: 2:1 sym. const. area )

Inte

nsity /

cp

s

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 µm and

McCrone milling/ethanol, filtered slurry

halite dissolved

in ethanol

mica + quartz lost by

dusting during dry

sieving



19

Sample contamination by abrasion from grinding elements

Kaolin Spergau reference sample, McCrone ground using

corundum grinding elements and using agate grinding elements

corundum line positions



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. )

Inte

nsity / c

ps

Comparison of 2 Scans

bentonite, fraction < 0.2 µm

005

02,11

06,33

front-loading

side-loading



21

Preferred orientation of grains/aggregates dependent on filling technique



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

002Ms

004

Dc

002

Ms+Dc

110,

020Ms

060Dc

060

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