appendix a. mineralogical and chemical data - lyell...

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Appendix A. Mineralogical and chemical data This appendix presents data on the clay mineral, non-clay mineral and chemical composition of a small but varied collection (N-- 105) of clay materials from the British Isles. This collection was assembled and analyzed during the writing of this report, such that the data have not been published previously elsewhere. Of course, there are many sources of mineralogical and chemical data on clay materials. In the British context these include several well-known publications which have attempted to compile and summarize data from many disparate sources (Perrin 1971; Ridgway 1982; Sellwood & Sladen 1981; Shaw 1981). The monograph compiled by Perrin (1971) was published by the Mineral- ogical Society of Great Britain and Ireland, and is yet to be superseded in scope; although it should be noted that, at the time of writing, the Mineralogical Society is in the process of preparing a multi-author book review of the knowledge and progress in understanding of the clay mineralogy of the UK stratigraphic column, made since the publication of Perrin's 1971 monograph (Jeans & Merriman 2006). The publication by Shaw (1981) is an overview of the mineralogy and petrology of the argillaceous rocks of the UK. Shaw's discussion of the non-clay mineralogy of these rocks was severely limited by lack of available data, but trends and patterns in the clay mineral assemblages, based on analyses of clay-sized fractions, throughout the UK stratigraphic column were presented and discussed. The paper by Sellwood & Slade (1981), which appeared in the same thematic volume of the Quarterly Journal of Engineering Geology, focused solely on the Mesozoic and Tertiary argillaceous units of the UK. Although ten years had elapsed between the publication of these reviews and that of Perrin the latter works still drew largely on Perrin's compendium and this remains the most extensive and detailed compilation of clay mineral data available in the UK to date. The British Geological Survey (BGS) did compile a limited UK mineralogical mudrock database in the late 1980s/early 1990's which included data from the NIREX (Nuclear Industry Radio- active Waste Executive) candidate sites for the disposal of low-level radioactive waste, but this is no longer publicly available. The publication by Ridgway (1982) contains a compilation of 85 whole-rock, major element, chemical analyses of clay materials from the UK. These chemical analyses may be directly compared with those in this appendix. Intemationally, Weaver's attempts to compile and ana- lyze the trends in Phanerozoic clay mineral assemblages, based mainly on data from the United States, are also well known, beginning with his 1967 publication on 'The significance of clay minerals in sediments'. More recently he has documented and discussed trends and pattems of clay mineral distribution on a global scale in Chapter 9 of his book 'Clays, muds and shales' (Weaver 1989), an account which is remarkable in scope and coverage. Other publications dealing with the stratigraphic distribution of clay minerals include those of Chatzidimitriadis et al. (1993) and Viczian (2002), which deal with argillaceous rocks in Greece, and Hungary, respectively. No doubt there are others in existence not referenced here. All of these aforementioned publications deal primarily with the clay mineralogy of argillaceous rocks as deter- mined on a clay-sized fraction (usually but not always <2 pm) that is physically separated from the bulk material. As pointed out by Shaw (1981) there are much less published data on the non-clay mineral composition (quartz, feldspars, carbonates and total amounts of clay minerals etc.) of mudrocks, and certainly very few attempts to compile or summarise data in any strati- graphic sense. There have, however, been several attempts to determine the average mineralogical compo- sition of shale, as well as the more easily determined average chemical composition. Literature data on these aspects are discussed more fully in Chapter 3 section 6.4. Additionally, a set of almost 100 whole rock mineralo- gical analyses, mainly from North American shales, can be found in the 'Argillaceous Rock Atlas', compiled by O'Brien & Slatt (1990). A.1. Purpose and scope The primary purpose of obtaining and presenting the new data in this appendix is to supplement previous data with further analyses that, above all-else, are internally consis- tent. Where literature data from diverse and disparate sources are concerned, amongst many other factors, meth- ods and techniques change and vary with time, experi- ence, and purpose. Lack of consistency is, therefore, a perpetual and insoluble problem when data are compiled and compared from different sources. The uncertainties attached to comparisons, and the conclusions that may be drawn from such comparisons, are therefore larger and in many cases unknown. Other problems arising from com- paring data from different sources, often obtained using different analytical approaches have been discussed in Chapter 8. The disadvantage of consistency is that the number of samples that can be compared is inevitably a much by guest on July 8, 2018 http://egsp.lyellcollection.org/ Downloaded from

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Appendix A. Mineralogical and chemical data

This appendix presents data on the clay mineral, non-clay mineral and chemical composition of a small but varied collection (N-- 105) of clay materials from the British Isles. This collection was assembled and analyzed during the writing of this report, such that the data have not been published previously elsewhere.

Of course, there are many sources of mineralogical and chemical data on clay materials. In the British context these include several well-known publications which have attempted to compile and summarize data from many disparate sources (Perrin 1971; Ridgway 1982; Sellwood & Sladen 1981; Shaw 1981). The monograph compiled by Perrin (1971) was published by the Mineral- ogical Society of Great Britain and Ireland, and is yet to be superseded in scope; although it should be noted that, at the time of writing, the Mineralogical Society is in the process of preparing a multi-author book review of the knowledge and progress in understanding of the clay mineralogy of the UK stratigraphic column, made since the publication of Perrin's 1971 monograph (Jeans & Merriman 2006).

The publication by Shaw (1981) is an overview of the mineralogy and petrology of the argillaceous rocks of the UK. Shaw's discussion of the non-clay mineralogy of these rocks was severely limited by lack of available data, but trends and patterns in the clay mineral assemblages, based on analyses of clay-sized fractions, throughout the UK stratigraphic column were presented and discussed. The paper by Sellwood & Slade (1981), which appeared in the same thematic volume of the Quarterly Journal of Engineering Geology, focused solely on the Mesozoic and Tertiary argillaceous units of the UK. Although ten years had elapsed between the publication of these reviews and that of Perrin the latter works still drew largely on Perrin's compendium and this remains the most extensive and detailed compilation of clay mineral data available in the UK to date. The British Geological Survey (BGS) did compile a limited UK mineralogical mudrock database in the late 1980s/early 1990's which included data from the NIREX (Nuclear Industry Radio- active Waste Executive) candidate sites for the disposal of low-level radioactive waste, but this is no longer publicly available. The publication by Ridgway (1982) contains a compilation of 85 whole-rock, major element, chemical analyses of clay materials from the UK. These chemical analyses may be directly compared with those in this appendix.

Intemationally, Weaver's attempts to compile and ana- lyze the trends in Phanerozoic clay mineral assemblages, based mainly on data from the United States, are also well

known, beginning with his 1967 publication on 'The significance of clay minerals in sediments'. More recently he has documented and discussed trends and pattems of clay mineral distribution on a global scale in Chapter 9 of his book 'Clays, muds and shales' (Weaver 1989), an account which is remarkable in scope and coverage. Other publications dealing with the stratigraphic distribution of clay minerals include those of Chatzidimitriadis et al. (1993) and Viczian (2002), which deal with argillaceous rocks in Greece, and Hungary, respectively. No doubt there are others in existence not referenced here.

All of these aforementioned publications deal primarily with the clay mineralogy of argillaceous rocks as deter- mined on a clay-sized fraction (usually but not always <2 pm) that is physically separated from the bulk material. As pointed out by Shaw (1981) there are much less published data on the non-clay mineral composition (quartz, feldspars, carbonates and total amounts of clay minerals etc.) of mudrocks, and certainly very few attempts to compile or summarise data in any strati- graphic sense. There have, however, been several attempts to determine the average mineralogical compo- sition of shale, as well as the more easily determined average chemical composition. Literature data on these aspects are discussed more fully in Chapter 3 section 6.4. Additionally, a set of almost 100 whole rock mineralo- gical analyses, mainly from North American shales, can be found in the 'Argillaceous Rock Atlas', compiled by O'Brien & Slatt (1990).

A.1. Purpose and scope

The primary purpose of obtaining and presenting the new data in this appendix is to supplement previous data with further analyses that, above all-else, are internally consis- tent. Where literature data from diverse and disparate sources are concerned, amongst many other factors, meth- ods and techniques change and vary with time, experi- ence, and purpose. Lack of consistency is, therefore, a perpetual and insoluble problem when data are compiled and compared from different sources. The uncertainties attached to comparisons, and the conclusions that may be drawn from such comparisons, are therefore larger and in many cases unknown. Other problems arising from com- paring data from different sources, often obtained using different analytical approaches have been discussed in Chapter 8.

The disadvantage of consistency is that the number of samples that can be compared is inevitably a much

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450 APPENDIX A

smaller number. In this appendix data are presented for 105 samples. The samples range in age from Cambrian to Quaternary, and it must be stressed that such a small set of samples can not be considered as representative in a sta- tistical sense of any of the individual stratigraphic units from which they were collected. Thus it must be stressed that the data are not presented as definitive analyses of clay materials from particular stratigraphic units. They are simply presented as examples of analyses of particular clay materials. Samples from other localities, even the same locality, in the same stratigraphic unit will inevita- bly be different in detail and in many instances may be wholly different. Nonetheless, certain trends and features in the new data are in general accord with prior knowl- edge, such as that summarized in the compilations of Perrin (1971), Shaw (1981) and Sellwood & Slade (1981). Some comments are therefore made to put these data into context. In addition, some new features are apparent that have not been established previously and these are also worthy of brief note.

As mentioned above, this appendix also contains data on the 'whole-rock' or 'bulk' mineral composition of British argillaceous units. To our knowledge there are no other compilations of such data in existence at this time. To complement the whole rock mineralogical data, whole rock chemical analyses are also presented. These were obtained on a sub-sample of each specimen. For both whole rock mineralogy and chemistry 'average' composi- tions have been calculated for all data and these may be usefully compared with other attempts to compute the 'average' composition of shales, as detailed in Chapter 3, section 6.4. Average values have also been calculated for the various stratigraphic groupings into which the data are divided. In most cases, the samples are too disparate in nature for this to be truly meaningful, but in some cases this is useful for comparative purposes.

The samples were donated by various individuals listed in the acknowledgments. Many come from construction sites, but others were also obtained at outcrop, from min- eral exploitation pits, and previously assembled collec- tions. On the whole, most contributors donated samples believed to be unaffected by weathering, but some sup- plied material noted as weathered. Because of the subjec- tive nature of this judgement, as well as it inconsistent assessment, no remarks are recorded on the weathered versus fresh state of the materials. However, the reader may generally assume that the materials are not weath- ered, unless the presence of some mineral phases clearly suggest otherwise.

A.2. Methods

Mineralogical analyses were made by Dr Stephen Hillier of the Macaulay Institute, Aberdeen. All the analyses have been made using the methods outlined in Hillier (2000) and in more detail in Hillier (2003). The use of one lab, one analyst, and consistent analytical procedure throughout was to ensure internal consistency for com- parison of data from a wide range of clay materials distributed throughout the stratigraphic column.

A.2.1. Clay fraction analysis

For clay mineral analyses, samples were dispersed in de-ionised water, and a < 2 gm size fraction separated by timed sedimentation according to Stokes' Law. The clay was then deposited on a glass slide using the filter peel transfer procedure and examined in a Siemens D5000 diffractometer in the air-dried state, after glycolation (by vapour pressure method overnight) and after heating to 300~ and 550~ for one hour. For each treatment the sample was scanned from 2-45~ in 0.02 ~ steps, count- ing for 1 second per step using Co-Ks radiation, selected by a diffracted beam monochromator. Clay minerals were identified by comparison of the response to the various treatments and their effect on peak positions and intensi- ties as detailed in Hillier (2003). Clay minerals identified were quantified using a normalized (sum 100%) reference intensity ratio approach (Hillier 2003) based on factors obtained from calculated X-ray powder diffraction (XRPD) patterns.

A.2.2. Whole rock mineralogical analysis

Each bulk sample was disaggregated and reduced to a fine powder ( < 2 mm) by hand, and/or using a hammer, depending on hardness. Then a carefully weighed mixture of 2.4 g of each sample and 0.6 g of corundum was wet ground and spray dried to produce a random powder. The 20% corundum addition was used as an internal standard for quantitative phase analysis (QPA). XRPD patterns were recorded from 2-75~ in 0.02 ~ steps, counting for 2 seconds per step, using Co-Ks radiation selected by a diffracted beam monochromator. QPA was performed using an internal standard reference intensity ratio (RIR) method. As detailed in Hillier (2003), a generalized esti- mate of expanded uncertainty using a coverage factor of 2, i.e. 95% confidence, is given by _+X ~35, where X is the concentration in wt%., e.g. 30 wt% + 3.3.

A.2.3. Whole rock chemical analysis

Whole rock chemical analysis was made by Mrs. Sieglinde Still and Ms. Michele Heibel at the laboratories of Watts Blake and Berne (WBB) Fuchs Laboratory, Ruppach-Goldhausen, Germany using X-ray fluores- cence spectroscopy (XRF). The clays were dried, crushed and mixed thoroughly before being milled to small par- ticle sizes in a vibrating bali mill. The resulting powders were placed in sealed containers prior to approximately 1 g of each being weighed into an alumina crucible. In order to determine values of loss-on-ignition, the cru- cibles were placed in a furnace at 1000~ for 20 minutes, cooled, and re-weighed immediately afterwards. 0.7000 g of each sample was subsequently weighed into a 95 wt% Pt - 5 wt% Au crucible, to which 3.5000 g of flux was added, giving rise to a flux:fired clay ratio of 5:1. The flux used was Spectroflux 100, Johnson Matthey, Batch No. 40499, of chemical composition LizB204. Fusion was carried out in an automatic fusion machine (HD Elektronik, Kleve, Germany), employing a sequence of

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APPENDIX A 451

3 minutes pre-warming, 4 minutes pre-melting and 6 min- utes main melting. Regular agitation of the crucibles was carried out by the machine at 10-second intervals, prior to the melt being poured into pre-heated Pt-Au dishes for casting. The bead-casting cycle was completed by these dishes being cooled in an airstream for 6 minutes. The samples were analysed on a Philips PW2400 Sequential XRF Spectrometer using a multi-element standard-based calibration within Philips SuperQ Software Version 3.0G.

A.3. Notes on data presentation

Data (Table A1) are presented in stratigraphic order but there is not an even distribution of samples and most (about three quarters) are of Mesozoic and Cenozoic age reflecting the distribution of the more significant clay formations in the UK. For each sample 58 numbered col- umns of data are presented. For identification purposes, columns 1 and 2 contain the stratigraphic unit from which the sample was collected and a National Grid Reference of the locality, as supplied by the originator. Additionally, columns 57 and 58 provide the analysis reference codes. The analytical data are divided into four groups. The first group consists of columns 3-23, which contain data on the whole-rock mineralogical composition of the samples. The second group consists of columns 24-35, which con- tain data on the clay mineralogy of the clay size fraction ( < 2 gm). The third group consists of columns 36-45, which contain data on the whole-rock major element chemical composition. And the fourth group consists of columns 46-56, which consists of whole-rock trace element data.

The data are also divided into groups based on strati- graphic age and average values for each data element have been calculated for each of these groupings. As men- tioned previously, this is for illustrative purposes only, since some of the groupings are of quite disparate clay materials, whereas other groups are more focused. An overall average composition of all 105 clay materials is also given at the end of the listing.

A.3.1. Whole rock mineralogy

Thirteen different non-clay minerals were identified and are listed in columns 3-15. Columns 16 and 17, give the total dioctahedral layer silicates (Di-layer silicates), and total trioctahedral layer silicates (Tri-layer silicates). These are general groupings, which added together give a total value of layer silicates in the sample. The term layer silicates is used in preference to clay minerals because the analytical method does not distinguish between clay minerals and non clay-sized phyllosilicates such as large grains of muscovite mica or chlorite. However, for most samples it can be assumed that the values in columns 16 and 17 are to a good approximation, a measure of the total clay mineral content in each sample. Columns 19-23 give a further break down of the layer silicate mineralogy of the whole rock. Columns, 19-21 and 23 are all diocta- hedral clays, such that their sum is equal to column 16.

Column 20 is an estimate of the total amount of expand- able (swells in water) dioctahedral clay. This is calculated as the remaining dioctahedral clay that is unaccounted for after quantifying illite/mica and kaolin separately. For the majority of samples this is an estimate of the mixed-layer illite-smectite content of the whole rock. The only com- mon trioctahedral clay is chlorite so that column 23 is identical to column 17. The main exceptions to this are samples from the Mercia Mudstone, which commonly contain corrensite (trioctahedral chlorite(50)-smectite R1). This mineral is not distinguished from chlorite in the whole rock analysis. Additionally, when present berthie- rine and vermiculite may also contribute to the measured value for total trioctahedral layer silicate. The reader should therefore check to see if these minerals have been identified in the clay fraction in order to judge if the usual equivalence between total trioctahedral clay (column 17) and chlorite (column 23) in the whole rock is a valid assumption.

A.3.2. Clay fraction mineralogy

The main clay minerals identified in the samples are illite (mica), mixed-layer illite-smectite, kaolin, and chlorite. Other clay minerals are much rarer. Precise identification of the mixed-layer illite-smectite identified has not been attempted, but based on the overall character of the XRPD patterns and peak positions an estimate of its expand- ability has been made as listed in column 35. Expand- ability may be equated with the percentage of smectite layers in the mixed-layer clay. Some of the diffraction data also suggest that it may be a gross oversimplification to identify some of the expandable clay minerals as mixed-layer illite-smectite and that other layer types, such as vermiculite may be present. Nonetheless, for the pur- poses of this appendix if the clay mineral collapsed to 10A after heating it is labeled mixed-layer illite-smectite. In addition, to an approximate measure of expandability, column 34 indicates whether or not a peak at 17A was observed, following treatment of the sample with ethyl- ene glycol. When the clay mineral has been identified as illite-smectite, the presence of this peak is a reliable indi- cator of a so-called randomly interstratified variety of mixed-layer illite-smectite. Frequently, this clay mineral is incorrectly identified as pure smectite, which also has a peak at 17A. It was also noted that some chlorites where partially susceptible to heating at 300~ This is an unusually low temperature for chlorite to be labile, and this behavior could indicate the presence of berthierine. This is indicated by a # symbol in the berthierine column (column 33).

A.3.3. Whole rock chemical data

The whole rock chemical data require little explanation. There are only three significant points to note. Firstly, the data are presented normalized to a 100 wt% minus Loss On Ignition (LOI) basis. This was deemed the best way to present the data for direct comparison with the whole rock

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458 APPENDIX A

mineralogical data. Secondly, some trace elements were below the limit of detection in some samples. A blank cell in the table indicates such data. Thirdly, organic matter and sulphur are other chemical components that will be present in some samples in significant amounts but these were not analysed.

A.4. Comments on the mineralogical data

observed. This is a well-known feature (Perrin 1971), indeed some types of Carboniferous mudrocks may be almost pure kaolin and have been commercially exploited as a result. Compared to older rocks, concentrations of expandable mixed-layer illite-smectite are also generally elevated, but highly variable. In the bulk analyses the fre- quent occurrence of the iron carbonate siderite is a notable feature.

A.4.4. Permo-Trias

The following comments on the mineralogical composi- tions of the clay materials analyzed for this appendix are made simply in relation to existing data, mainly from Perrin (1971). The aim is to attempt to put the new data into context with existing data and to remark upon fea- tures which are not known to have been noted previously, or that require emphasis. In keeping with Perrin (1971) and in geological tradition this starts with the oldest clay materials.

A.4.1. Lower Palaeozoic

A total of eight Lower Palaeozoic samples were analyzed, which may be compared with 46 reported by Perrin (1971). In the clay fraction, these samples consist of assemblages of 'well crystallized' illite and chlorite, with very little if any trace of expandable mixed-layer illite- smectite, and only rare occurrences of kaolin. Perrin (1971) noted the exception of bentonites to this general trend since these may contain larger amounts of more highly expandable mixed-layer illite-smectite and this is borne out by the present data. Additionally, the present data indicated the occurrence of the sodium mica called paragonite in two samples, and berthierine which was positively identified in one sample and suspected in several. In the whole rock none of the analyzed samples contained any significant amount of any carbonates, but the generality of this observation is unknown. It is also notable that the Lower Palaeozoic samples are the only set of samples where, on average, plagioclase is more abundant than K-feldspar.

A.4.2. Devonian

Devonian samples show a similar pattern to the Lower Palaeozoic samples, in that illite and chlorite are domi- nant, kaolin rare and mixed-layer illite-smectite moderate in abundance but generally of low expandability. One sample was unusual in consisting of an assemblage of extremely well crystallized, illite and chlorite together with minor but pure smectite. In the bulk analyses carbon- ates are not uncommon, with both calcite and dolomite present, and some samples have unusually high contents of K-feldspar.

A.4.3. Carboniferous

The most notable feature of the Carboniferous samples is that relatively high concentrations of kaolinite are

All the materials analyzed are Mercia Mudstone, formerly Keuper Marl. Although sample coverage of the Permo- Trias is clearly unbalanced in a geological sense, eco- nomically and from an engineering perspective the Mercia Mudstone is the most important clay material of this age. The most notable feature of this group of samples is the common presence of abundant chlorite and corrensite. Other notable features of the clay mineralogy include the complete absence of kaolinite and the low concentrations of mixed-layer illite-smectite, which is generally detectable but of very low expandability. Perrin (1971) also noted the rare (absent?) occurrence of kaolin in Mercia Mudstone. In the bulk analyses substantial amounts of dolomite are very common and the abundance of potassium feldspar is frequently high.

A.4.5. Jurassic

Samples from the Jurassic are very varied. However, as far as the clay mineralogy is concerned it is notable that the appearance of a peak at about 17,~ in diffraction pat- terns of glycolated samples becomes common in Jurassic samples. This peak is due to the presence of random mixed-layer illite-smectite. Previously, especially in the older literature, this peak has often been taken to indicate the presence of pure smectite. Although pure smectite may be encountered, the results of the current analyses indicate that this is rarely the case. Thus many of the clays minerals identified as smectites in some of the older lit- erature are probably more accurately identified as random mixed-layer illite-smectites. Data in this appendix shows when a 17A peak was observed, and also gives an estimate of the expandability (% smectite layers) of the mixed-layer clay. Small amounts of chlorite were detected in all the Liassic samples. Previously Perrin (1971) noted that the occurrence of chlorite tended to become sporadic above the Lias. The new data presented here, generally concur with this observation. Three samples all from the same locality in the Blisworth clay contained appreciable amounts of mixed-layer kaolinite- smectite. This is a clay mineral that has not previously been reported in the published literature from any geological clay formation in the UK. As far as the bulk analyses are concerned, traces of dolomite are notable in many Liassic samples, siderite also is a common carbon- ate detected in minor amounts throughout the Jurassic set, but the main carbonate is calcite, which can be abundant.

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APPENDIX A 439

Jurassic samples are also conspicuous for the consistent, but not ubiquitous presence, of minor amounts of pyrite.

A.4.6. Cretaceous

Most of the Cretaceous samples examined for this report are from the Gault Formation. Compared to other samples, on average, the Gault contains more mixed-layer illite-smectite in the clay fractions than any other strati- graphic grouping analyzed for this report. Some samples also contained traces of a clay mineral behaving like vermiculite, and this feature was also noted in Perrin (1971). Analysis of the whole rock indicates that some Gault samples are also exceedingly clay-rich _> 70%, although like some Jurassic samples substantial amounts of calcite may 'dilute' the silicate component.

A.4.7. Palaeocene

All Paleocene samples are from the Reading Formation. The most distinctive feature here was the recognition of several samples that contain mixed-layer kaolinite- smectite. To our knowledge this clay mineral has not 3een reported previously from this formation. Kaolinite- smectite is, however, well known in the early Eocene Argites Plastiques formation from the Paris Basin, France, which is of a very similar age, for example see Amouric & Olives (1998).

A.4.8. Eocene/Oligocene

The Eocene samples analyzed are all London Clay, but in addition a set of Devon Ball Clays of Eocene/Oligocene age were also analyzed. The London Clay samples are generally rich in mixed-layer illite-smectite, but also may contain realtively large amounts of kaolinite. Chlorite is also a consistent component. Some London Clay samples contained minor amounts of carbonates, but in common with the other Tertiary samples analyzed carbonates do not appear to figure as much as they do in Mesozoic samples. Another notable feature is the potassium feld- spar content, which appears to be higher than average. As expected the Ball Clays were all dominated by kaolin- ite, but most also contain substantial amounts of illite and illitic mixed-layer illite-smectite. In contrast to the London Clay samples, chlorite was not found in any of the Ball clays.

A.4.9. Quaternary

The Quaternary samples are an extremely varied set and have extremely varied compositions. Undoubtedly, this

is a reflection of their diverse origins as glacial, fluvi- glacial, alluvial and aeolian materials. One notable feature is the generally elevated feldspar contents, with respect to both potassium and plagioclase feldspar. Cal- cite is also a major constituent of some samples. Both of the Brick Earth samples have high quartz contents.

Acknowledgements. G. West, P. Hobbs, R. Merriman, S. Kemp, C. and M. Adams, K. Privett, G. Witte, M. Czerewko, J. Cripps, R. Price, A. Merrit, M. Crilly, P. Sherwood, P. Homer, M. Lawler, P. Halsam-Brunt, G. M. Reeves and D. Jordan are gratefully acknowledged for provision of samples.

References

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