Identification of Clays
Preamble
Having prepared the (hopefully) highly orientated clay mounts, the procedure of clay identification involves a series of treatments. The basic ones are normal (air dried), glycolation and heat treatment (350°C). There are other treatments that are used on the basis of what the standard treatments suggest – e.g. heating to higher temperatures (550°C), ion exchange, and treatment with formamide. These will be touched on when specific clays are to be confirmed or identified. Glycolation can either be with ethylene glycol or glycerol but the standard treatment is ethylene glycol atmosphere at 80°C for at least 1 hour (beyond heating up time), however sometimes 4 hours is required as some samples react more slowly than expected.
Basis of Identification
It is hoped that the clay preparation has produced a highly orientated specimen with the (00l) planes (basal spacing) parallel to the sample surface. If this is so, then a strong series of basal reflections - (001), (002), (003), (004), etc. – will be present for each clay. For some clays, some of these reflections are systematically absent or suppressed. One advantage of clay identification is that there are only seven clay classes (depending on whose classification scheme you adopt) - some argue that some classes are not true clays. I’ll settle for illite/mica, kaolinite/kandite, smectite, chlorite, palygorsite (or attapugite), vermiculite and sepiolite. However, there is a series of known {and suspected) mixed layers – the most widely studied is illite-smectite (but also kaolinite-smectite, chlorite-smectite, etc.). The following is directed to the identification of classes and interlayers. It is generally not possible to identify specific minerals within a class. It is usually necessary to prepare powder preparations (of the clay extracts) and use the other hkl lines to differentiate members within a class (or group) - for instance, there are many minerals within the chlorite group.
Illite
Illite is distinguished by the (00l) series 10Å, 5Å, 3.33Å. It is unaffected by glycolation and heat treatment (350°C). It is perhaps the easiest to identify. There may a slight problem with quartz overlap at 3.34Å. The peaks may be narrow (mica like) or broad (illite like) – and where the line should be drawn is open to debate. Iron rich clays such as glauconite and celadonite have severely suppressed 5Å peaks. The only possible confusions are palygorsite at 10.4Å and hydrated halloysite at 10Å but these are distinguished by the absences of the other characteristic illite peaks and the presence of the other palygorsite or halloysite peaks.
Kaolinite/Kandite
It is a big class which ranges from the very ordered (narrow and intense diffraction peaks) to the very disordered (weak and broad diffraction peaks) and beyond to true halloysites. Dehydrated halloysite is about 7.2Å (hydrated halloysite at about 10Å – it is difficult to keep the halloysite hydrated in my air-dried orientated clay preparations and I try usually to ensure that it is dehydrated). Halloysite can be distinguished from disordered kaolinite by the lack of high preferred orientation in the sample preparation (the 4.45Å hkl is stronger than it ought to be) and by a test. Reaction to formamide within 30 minutes is indicative of halloysite (kaolinite will react to formamide but more slowly – a period of hours or days or weeks). Halloysite will expand to 10Å. It is not unusual to have both halloysite and disordered kaolinite present. If the sample fails to react in 30 mins then there is little or no halloysite present and only disordered kaolinite is present. The characteristic lines of kaolinite are 7.1Å and 3.57Å. Possible confusions are chlorite (14Å, 7Å and 3.53Å). It is not unusual for both kaolinite and chlorite to be present in a particular sample. Kaolinite survives heat treatment (350°C) but not at 550°C. Kaolinite is unaffected by glycolation.
Chlorite
Chlorite is a large diverse group – the characteristic lines are 14Å, 7Å, 4.72Å and 3.53Å. There is some variation in line position but not enough to confuse chlorite for kaolinite but enough to make mixtures of kaolinite and chlorite problematic. One could use formamide to expand the kaolinite (this is very slow for ordered kaolinite but the problems of overlap tend to be with the less ordered kaolinites that react within a period of hours). Chlorite is unaffected by formamide so any reaction indicate kaolinite. So called iron chlorites have suppressed 14Å and 4.72Å peaks (hence problems with kaolinite confusion). There is possible confusion with vermiculite. Chlorites generally survive heat treatment (350°C and 550°C), but some chlorites do not and in particular, iron chlorites collapse. It should not be used to differentiate chlorites from kaolinite. Chlorites are unaffected by glycolation.
Vermiculite
Vermiculite has spacings at 14.5Å, 7.2Å, 4.80Å and 3.58Å. The ratio of peak intensities depends on ion saturation. If vermiculite is suspected, it is usually best to prepare a Mg-saturated air-dried preparation (which gives strong 14.5Å and weak 7.2Å), glycol with glycerol (not ethylene glycol) for the 14.4Å does not expand (it will slightly expand with ethylene glycol).
Palygorsite
Palygorsite is fibrous and hence does not orientate. It is detected by its characteristic lines at 10.45Å (100), 6.4Å (15), 5.4Å (10), 4.45Å (15) and 3.65Å (10). The ratio of intensities can vary sample to sample. It is not likely to be confused with any other clay.
Sepolite
Sepiolite is fibrous and hence does not orientate. It is detected by its characteristic lines at 12.8Å (100), 7.6Å (5), 5.1Å (10), 4.4Å (35) and 3.77Å. (broad). The ratio of intensities can vary sample to sample. It is not likely to be confused with any other clay.
Smectite
Smectite is a diverse group. In air-dried samples it has a peak in the range 12Å to 15Å which on ethylene glycolation it expands uniformly to 17.2Å (the peak usually sharpens and increases in intensity with glycolation - also an often observed 002 peak occurs at 8.5Å– there is no 002 peak in the air-dried oriented samples). Confirmation (if required) is to K-saturate the air dried preparation and heat to 300C - the first diffraction peak collapses to an illite-like 10Å.
Mixed Layers
The most difficult area is the identification of mixed layers. They tend to be mixed layers with smectite, eg. illite/smectite, kaolinite/smectite, chlorite/smectite but also chlorite/vermiculite, illite/vermiculite and chlorite/kaolinite. There may be others not as yet recognized. It is beyond the scope here to consider all types but there are two main classes – ordered (or regular, R1) and disordered (or random, R0). It is easy enough the think of the disordered types (R0) as some clay layers at irregular interval existing in other layer types. This give rise to broadening and shifting (sometimes it is very slight) of diffraction peaks. An example is a disordered kaolinite/smectite with 20% smectite. There is a slight shift to a higher d-spacing of the basal spacing [d(001)] on ethylene glycolation (with a slight sharpening) and a retreat to about the air-dried value on heating to 375C (with slight broadening). It is subtle and may be missed (one would need also to be confident of the equipment and sample positioning). The relationship of broadening, shift and intensity is dependant on the repeat distances involved – for a particular mixed layer, some pears may be broadened others not, some enhanced in intensity others depressed. The regular mixed layers (R1) have quite different diffraction peaks from the individual clays and the random mixed layers. The ordering leads to superstructures. For instance e-glycolated regular illite/smectite (50/50) [also called rectorite] – has spacings at 27Å (10+17), 13.5Å, etc. Regular chlorite/smectite has d(001)* at 14+17= 31Å. Regular mixed layers are 50/50, however there may be a 50/50 ratio and no ordering (i.e. random mixed layer, R0). Certain mixed layer clays are found only in certain mixing ratios, eg. chlorite/smectite is found 50/50 (regular,R0) and also <10% smectite only (R0), kaolinite/smectite is only found as R0.
A useful reference is X-ray Diffraction and the Identification and Analysis of Clay Minerals. D M Moore, R C Reynolds. 2nd edition 1997 Oxford University Press. ISBN -19-508713-5
Tony Raftery
Faculty of Science GP,
Queensland University of Technology,
GPO Box 2434, Brisbane, Qld 4001. Australia.
E-Mail: [email protected]
Having prepared the (hopefully) highly orientated clay mounts, the procedure of clay identification involves a series of treatments. The basic ones are normal (air dried), glycolation and heat treatment (350°C). There are other treatments that are used on the basis of what the standard treatments suggest – e.g. heating to higher temperatures (550°C), ion exchange, and treatment with formamide. These will be touched on when specific clays are to be confirmed or identified. Glycolation can either be with ethylene glycol or glycerol but the standard treatment is ethylene glycol atmosphere at 80°C for at least 1 hour (beyond heating up time), however sometimes 4 hours is required as some samples react more slowly than expected.
Basis of Identification
It is hoped that the clay preparation has produced a highly orientated specimen with the (00l) planes (basal spacing) parallel to the sample surface. If this is so, then a strong series of basal reflections - (001), (002), (003), (004), etc. – will be present for each clay. For some clays, some of these reflections are systematically absent or suppressed. One advantage of clay identification is that there are only seven clay classes (depending on whose classification scheme you adopt) - some argue that some classes are not true clays. I’ll settle for illite/mica, kaolinite/kandite, smectite, chlorite, palygorsite (or attapugite), vermiculite and sepiolite. However, there is a series of known {and suspected) mixed layers – the most widely studied is illite-smectite (but also kaolinite-smectite, chlorite-smectite, etc.). The following is directed to the identification of classes and interlayers. It is generally not possible to identify specific minerals within a class. It is usually necessary to prepare powder preparations (of the clay extracts) and use the other hkl lines to differentiate members within a class (or group) - for instance, there are many minerals within the chlorite group.
Illite
Illite is distinguished by the (00l) series 10Å, 5Å, 3.33Å. It is unaffected by glycolation and heat treatment (350°C). It is perhaps the easiest to identify. There may a slight problem with quartz overlap at 3.34Å. The peaks may be narrow (mica like) or broad (illite like) – and where the line should be drawn is open to debate. Iron rich clays such as glauconite and celadonite have severely suppressed 5Å peaks. The only possible confusions are palygorsite at 10.4Å and hydrated halloysite at 10Å but these are distinguished by the absences of the other characteristic illite peaks and the presence of the other palygorsite or halloysite peaks.
Kaolinite/Kandite
It is a big class which ranges from the very ordered (narrow and intense diffraction peaks) to the very disordered (weak and broad diffraction peaks) and beyond to true halloysites. Dehydrated halloysite is about 7.2Å (hydrated halloysite at about 10Å – it is difficult to keep the halloysite hydrated in my air-dried orientated clay preparations and I try usually to ensure that it is dehydrated). Halloysite can be distinguished from disordered kaolinite by the lack of high preferred orientation in the sample preparation (the 4.45Å hkl is stronger than it ought to be) and by a test. Reaction to formamide within 30 minutes is indicative of halloysite (kaolinite will react to formamide but more slowly – a period of hours or days or weeks). Halloysite will expand to 10Å. It is not unusual to have both halloysite and disordered kaolinite present. If the sample fails to react in 30 mins then there is little or no halloysite present and only disordered kaolinite is present. The characteristic lines of kaolinite are 7.1Å and 3.57Å. Possible confusions are chlorite (14Å, 7Å and 3.53Å). It is not unusual for both kaolinite and chlorite to be present in a particular sample. Kaolinite survives heat treatment (350°C) but not at 550°C. Kaolinite is unaffected by glycolation.
Chlorite
Chlorite is a large diverse group – the characteristic lines are 14Å, 7Å, 4.72Å and 3.53Å. There is some variation in line position but not enough to confuse chlorite for kaolinite but enough to make mixtures of kaolinite and chlorite problematic. One could use formamide to expand the kaolinite (this is very slow for ordered kaolinite but the problems of overlap tend to be with the less ordered kaolinites that react within a period of hours). Chlorite is unaffected by formamide so any reaction indicate kaolinite. So called iron chlorites have suppressed 14Å and 4.72Å peaks (hence problems with kaolinite confusion). There is possible confusion with vermiculite. Chlorites generally survive heat treatment (350°C and 550°C), but some chlorites do not and in particular, iron chlorites collapse. It should not be used to differentiate chlorites from kaolinite. Chlorites are unaffected by glycolation.
Vermiculite
Vermiculite has spacings at 14.5Å, 7.2Å, 4.80Å and 3.58Å. The ratio of peak intensities depends on ion saturation. If vermiculite is suspected, it is usually best to prepare a Mg-saturated air-dried preparation (which gives strong 14.5Å and weak 7.2Å), glycol with glycerol (not ethylene glycol) for the 14.4Å does not expand (it will slightly expand with ethylene glycol).
Palygorsite
Palygorsite is fibrous and hence does not orientate. It is detected by its characteristic lines at 10.45Å (100), 6.4Å (15), 5.4Å (10), 4.45Å (15) and 3.65Å (10). The ratio of intensities can vary sample to sample. It is not likely to be confused with any other clay.
Sepolite
Sepiolite is fibrous and hence does not orientate. It is detected by its characteristic lines at 12.8Å (100), 7.6Å (5), 5.1Å (10), 4.4Å (35) and 3.77Å. (broad). The ratio of intensities can vary sample to sample. It is not likely to be confused with any other clay.
Smectite
Smectite is a diverse group. In air-dried samples it has a peak in the range 12Å to 15Å which on ethylene glycolation it expands uniformly to 17.2Å (the peak usually sharpens and increases in intensity with glycolation - also an often observed 002 peak occurs at 8.5Å– there is no 002 peak in the air-dried oriented samples). Confirmation (if required) is to K-saturate the air dried preparation and heat to 300C - the first diffraction peak collapses to an illite-like 10Å.
Mixed Layers
The most difficult area is the identification of mixed layers. They tend to be mixed layers with smectite, eg. illite/smectite, kaolinite/smectite, chlorite/smectite but also chlorite/vermiculite, illite/vermiculite and chlorite/kaolinite. There may be others not as yet recognized. It is beyond the scope here to consider all types but there are two main classes – ordered (or regular, R1) and disordered (or random, R0). It is easy enough the think of the disordered types (R0) as some clay layers at irregular interval existing in other layer types. This give rise to broadening and shifting (sometimes it is very slight) of diffraction peaks. An example is a disordered kaolinite/smectite with 20% smectite. There is a slight shift to a higher d-spacing of the basal spacing [d(001)] on ethylene glycolation (with a slight sharpening) and a retreat to about the air-dried value on heating to 375C (with slight broadening). It is subtle and may be missed (one would need also to be confident of the equipment and sample positioning). The relationship of broadening, shift and intensity is dependant on the repeat distances involved – for a particular mixed layer, some pears may be broadened others not, some enhanced in intensity others depressed. The regular mixed layers (R1) have quite different diffraction peaks from the individual clays and the random mixed layers. The ordering leads to superstructures. For instance e-glycolated regular illite/smectite (50/50) [also called rectorite] – has spacings at 27Å (10+17), 13.5Å, etc. Regular chlorite/smectite has d(001)* at 14+17= 31Å. Regular mixed layers are 50/50, however there may be a 50/50 ratio and no ordering (i.e. random mixed layer, R0). Certain mixed layer clays are found only in certain mixing ratios, eg. chlorite/smectite is found 50/50 (regular,R0) and also <10% smectite only (R0), kaolinite/smectite is only found as R0.
A useful reference is X-ray Diffraction and the Identification and Analysis of Clay Minerals. D M Moore, R C Reynolds. 2nd edition 1997 Oxford University Press. ISBN -19-508713-5
Tony Raftery
Faculty of Science GP,
Queensland University of Technology,
GPO Box 2434, Brisbane, Qld 4001. Australia.
E-Mail: [email protected]