Clay Analysis By XRD Part 1
Sample Preparation
The protocol for clay preparation varies from lab to lab, from sample suite to sample suite (and sometimes from sample to sample). The goal remains constant, to characterise the clays present - but there can be more than one consideration. Firstly - what is clay?
I'll define it as members of a group of hydrous aluminium silicate minerals with particles that are either < 2 µm or < 5 µm in nominal spherical diameter (depending upon which grain-size classification used). The actual size may be +100 µm in one dimension and very much less than 1um in another. These minerals fall into 6 groups (depending on whose classification scheme one chooses):
Disaggregation
Disaggregation causes the sample to separate into it constituent particles - to break the sample apart along the particle boundaries and to minimize the fracturing of the particles. If the samples is friable then this is straight forward, if the sample is a consolidate, it may be difficult or impractical. A gentle push down with a mortar (in a pestle, along with some water) can work wonders particularly if the sample is left to soak.
Dispersion
is the process of releasing the clays from surface of the particles or from between the particles and to deagglomerate any massive clay. In the mortar and pestle, gently rolling the particles (avoiding crushing them) is effective. Gentle ultrasonic treatment (high power setting but low time) can work wonders - ultrasonically disperse, pour off suspension, add more water, disperse, pour off then do it again. This sort of process can lead to large suspension volumes. Dispersion agents such as Calgon can be used.
Flocculate
Flocculation reduces the suspension volumes. Flocculate/settle or flocculate/centrifuge. Flocculation is the process of causing the clay to come together (the opposite of dispersion). Calcium ions are the most effective at causing flocculation. Flocculation invariably leads to the use of a centrifuge to recover the floc and to wash it to remove the excess soluble salt. In some samples the clays will not disperse as they are naturally floced (due to excess calcium, say in gypsum) and need to be defloced using things like Calgon (at 0.1 wt % to 0.4 wt %).
Centrifuge
Centrifugation is a way to wash a floc or wash a suspension (to remove soluble salts). There are standard tables (and for clays, you are in the regime of ultra-centrifuge). There is likely to be the loss of the very finest clays (say < 100 nm).
Washing/dissolution
Sometimes there is the need to remove a component/s prior to further work - eg. removal of carbonates by weak acids, dissolution of iron oxides/hydroxides by citrate-bicarbonate-dithionite treatment, dissolution of oil in sandstones, etc. These treatments are always fraught - eg. unwanted reactions, loss of desired phases, etc. They need to be trialled before use on particular samples.
Having arrived at the point of having a clay suspension, the next step is the production of an orientated clay film. The ideal is to have the ( 00l ) planes exclusively orientated parallel to the sample surface. This is rarely if ever achieved. A high degree of preferred orientation allows the basal spacings to be used for the diagnostic tests that are required for clay analysis. Non-clay fines tend to disrupt the degree of preferred orientation.
There is a bit of art, in that a particular technique may need much practice to achieve a good result. Popular techniques include vacuum assisted deposition onto a ceramic frit, vacuum assisted deposition onto a metal filter then transfer while wet to another substrate and lastly (but not finally) my preferred method which is to transfer of the suspension to a silicon wafer 511 plate and to dry slowly. These techniques (and so much more) are outlined in:
"X-Ray Diffraction and the Identification and Analysis of Clay Minerals" by D. M. Moore and R. C. Reynolds Jr.
An advantage of the ceramic frit is that it easily allows for saturation by K+ or Mg++ ions.
Tony Raftery
Faculty of Science GP,
Queensland University of Technology,
GPO Box 2434, Brisbane, Qld 4001. Australia
The protocol for clay preparation varies from lab to lab, from sample suite to sample suite (and sometimes from sample to sample). The goal remains constant, to characterise the clays present - but there can be more than one consideration. Firstly - what is clay?
I'll define it as members of a group of hydrous aluminium silicate minerals with particles that are either < 2 µm or < 5 µm in nominal spherical diameter (depending upon which grain-size classification used). The actual size may be +100 µm in one dimension and very much less than 1um in another. These minerals fall into 6 groups (depending on whose classification scheme one chooses):
- kandite (kaolinite, halloysite, berthierine, nacrite, dickite and others),
- smectite (montmorillonite, nontronite and others),
- illite (illite, glauconite, celadonite and others),
- chlorite (clinochlore, greenalite and others),
- sepolite (sepolite, palygorsite), and
- vermiculite - plus a few interlayers (illite/smectite, illite/chlorite, kaolinite/smectite, etc. that may be regular or random).
Disaggregation
Disaggregation causes the sample to separate into it constituent particles - to break the sample apart along the particle boundaries and to minimize the fracturing of the particles. If the samples is friable then this is straight forward, if the sample is a consolidate, it may be difficult or impractical. A gentle push down with a mortar (in a pestle, along with some water) can work wonders particularly if the sample is left to soak.
Dispersion
is the process of releasing the clays from surface of the particles or from between the particles and to deagglomerate any massive clay. In the mortar and pestle, gently rolling the particles (avoiding crushing them) is effective. Gentle ultrasonic treatment (high power setting but low time) can work wonders - ultrasonically disperse, pour off suspension, add more water, disperse, pour off then do it again. This sort of process can lead to large suspension volumes. Dispersion agents such as Calgon can be used.
Flocculate
Flocculation reduces the suspension volumes. Flocculate/settle or flocculate/centrifuge. Flocculation is the process of causing the clay to come together (the opposite of dispersion). Calcium ions are the most effective at causing flocculation. Flocculation invariably leads to the use of a centrifuge to recover the floc and to wash it to remove the excess soluble salt. In some samples the clays will not disperse as they are naturally floced (due to excess calcium, say in gypsum) and need to be defloced using things like Calgon (at 0.1 wt % to 0.4 wt %).
Centrifuge
Centrifugation is a way to wash a floc or wash a suspension (to remove soluble salts). There are standard tables (and for clays, you are in the regime of ultra-centrifuge). There is likely to be the loss of the very finest clays (say < 100 nm).
Washing/dissolution
Sometimes there is the need to remove a component/s prior to further work - eg. removal of carbonates by weak acids, dissolution of iron oxides/hydroxides by citrate-bicarbonate-dithionite treatment, dissolution of oil in sandstones, etc. These treatments are always fraught - eg. unwanted reactions, loss of desired phases, etc. They need to be trialled before use on particular samples.
Having arrived at the point of having a clay suspension, the next step is the production of an orientated clay film. The ideal is to have the ( 00l ) planes exclusively orientated parallel to the sample surface. This is rarely if ever achieved. A high degree of preferred orientation allows the basal spacings to be used for the diagnostic tests that are required for clay analysis. Non-clay fines tend to disrupt the degree of preferred orientation.
There is a bit of art, in that a particular technique may need much practice to achieve a good result. Popular techniques include vacuum assisted deposition onto a ceramic frit, vacuum assisted deposition onto a metal filter then transfer while wet to another substrate and lastly (but not finally) my preferred method which is to transfer of the suspension to a silicon wafer 511 plate and to dry slowly. These techniques (and so much more) are outlined in:
"X-Ray Diffraction and the Identification and Analysis of Clay Minerals" by D. M. Moore and R. C. Reynolds Jr.
An advantage of the ceramic frit is that it easily allows for saturation by K+ or Mg++ ions.
Tony Raftery
Faculty of Science GP,
Queensland University of Technology,
GPO Box 2434, Brisbane, Qld 4001. Australia