EXPERT SYSTEM FOR STRUCTURAL CHARACTERIZATION OF PHYLLOSILICATES: II. APPLICATION TO MIXED-LAYER MINERALS

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Clay Minerals (1994) 29, 39-45 EXPERT SYSTEM FOR STRUCTURAL CHARACTERIZATION OF PHYLLOSILICATES: II. APPLICATION TO MIXED-LAYER MINERALS V.A. DRITS AND A.

1 Clay Minerals (1994) 29, EXPERT SYSTEM FOR STRUCTURAL CHARACTERIZATION OF PHYLLOSILICATES: II. APPLICATION TO MIXED-LAYER MINERALS V.A. DRITS AND A. PLAN~ON* Geological Institute, Academy of Sciences, Moscow, Russia, and *Physics Department, Orldans University, Orldans-la-Source, France (Received 22 October 1992; revbed 5 June 1993) ABSTRACT: The expert system described in the first part of this paper has been applied to the identification of mixed-layer phyllosilicates (mica-smectite, mica-vermiculite, chlorite-smectite, chloritevermiculite, chlorite-swelling chlorite, chlorite-mica, chlorite-talc, kaolinite-smectite, talc-smectite), and to the determination of their structural parameters (Reichweite, R, and proportions of constituting layers, Wi). The expert system has been run utilizing the data extracted from (1) experimental XRD patterns for which structural parameters had been evaluated by comparison with calculated patterns, or (2) patterns calculated using pre-selected values of the structural parameters. In all cases examined, the expert system provided correct conclusions concerning the identification of a mixed-layer phyllosilicate and the value of the Reichweite, while the abundances of the component layers were evaluated with a margin of error usually <5%. Microdivided mixed-layer phyllosilicates are widespread in various geological environments. These minerals can be considered as intermediate stages in the phase transition from one periodic layer structure to another. A remarkable feature of mixed-layer minerals is that their fine structural characteristics reflect the change in physical and chemical conditions in the rocks in which they occur. For these reasons, crystallochemical features of mixed-layer minerals (mlm) can be used to study various geological problems. Structural studies of these minerals also help us understand the mechanism of their formation. These are some of the reasons why a large number of publications has been devoted to the structural characterization of mixed-layer phyllosilicates (e.g. Reynolds & Hower, 1970; Drits & Sakharov, 1976; Reynolds, 1980, 1988; Srodofi, 1980, 1984; Moore & Reynolds, 1989). There are currently two methods used for the structural characterization of mixed-layered minerals. The first one is to simulate the experimental X-ray diffraction (XRD) patterns using special computer programs. The other is to use tables or diagrams drawn or deduced from systematic calculations. In Part I of this paper, justification was given for a proposal to determine structural parameters of mixed-layer phyllosilicates through the use of an expert system. The purpose of the present article is to demonstrate the use of the expert system for pure mixedlayer phyllosilicates, specifically excluding associated minerals and periodic phyllosilicates. OPERATIONAL DEFINITIONS OF CLAY MINERALS AND TYPES OF LAYERS APPLIED BY THE EXPERT SYSTEM Table 1, modified from Brown & Brindley (1980), summarizes the different layer types of phyllosilicates and their thicknesses in four states: natural (or air-dry) state, (N); after saturation with ethylene glycol (EG) usually as vapour but sometimes as liquid (Suquet & Pezerat, 1988); after heating to ~ for 1 h, (H); and after heating to 500~i00~ for 1 h, (H550~ Data from these four tests are used for the layer identification. The important structural feature of phyllosilicates is that several different layers may coexist within the same coherent scattering domain (CSD), forming a mixed-layer mineral. The following two-component mixed-layer structures have been found in nature: mica-smectite, micavermiculite, chlorite-smectite, chlorite-vermiculite, chlorite-swelling chlorite, chlorite-mica, chlorite-talc, talc-smectite and kaolinite-smectite The Mineralogical Society

2 40 V.A. Drits and A. Planqon A large amount of diversity of these minerals results from variations in layer concentration and by order-disorder in layer distribution. INPUT DATA UTILIZED BY THE EXPERT SYSTEM The standard information used by the expert system is derived from the XRD patterns recorded for three states of the sample: natural (or air-dry), after saturation with ethylene glycol, and after heating to ~ for 1 h. The method of preparation of the air-dry sample has been summarized by Srodofi (1981) while the ethylene glycol intercalation technique has been described by Srodofi (1980). In case of relative humidity (r.h.) <10%, it is necessary to exchange a divalent cation into the interlayer space (Eberl et al., 1986). The samples are prepared for diffraction as oriented powders and recorded using the reflection diffraction technique. X-ray diffraction patterns from oriented powders contain only the 00t reflections, which are sufficient for the determination of the layer types, the abundance of each layer, and the Reichweite (R). The patterns are recorded from 2.5 to 40 ~ 20, using Ni-filtered Cu-K~ radiation, in the conditions described by Srodofi (1980). The standard data used by the expert system are the positions of reflections, expressed in,~. Apart from these standard data, the expert system sometimes needs additional information. This information may include some specific features of the pattern such as the intensity ratio of two reflections (the intensity being measured at peak height), the breadth ratio of two reflections (the breadth being measured at half-maximum), the breadth of a reflection measured in 20 between the points at which the tails of the peak join the X-ray background, etc. The additional information may also require another experiment, e.g. the d value of the 060 reflection which requires the analysis of a randomly oriented sample. Other experiments include heating to 500q500~ for 1 h, intercalation with glycerol, or interlayer cation exchange. Two terms used by the expert system must now be defined: rationality of reflections, and identical XRD patterns. The rationality of reflections is defined by the value of the standard deviation of a set of de(001) values calculated for various experimental 001 reflections as d~(001) = l dex p (001) The coefficient of variation (C.V.) is where /max ]d... (001) - d/(001)/ l=1 C.V. : d... (001) d... (001) -- /max <(001) /=1 /max TABLE l. Thickness, in A, of phyllosilicate layers after various treatments (after Brown & Brindley, 1980, p. 323). Ethylene Mineral Air-dry glycol ~ ~ Kaolinite Disappears Halloysite-7 A Disappears Halloysite-10/k Disappears Serpentine Disappears Talc Mica Chlorite Swelling chlorite Smectite Na, r.h. >10% 12, Smectite Na, r.h. <10% (*) Vermiculite, Mg, Ca Vermiculite, Na (*) from Eberl et al. (1986), (r.h.: relative humidity).

3 By definition (Bailey, 1982), C.V. ~< 0.75% indicates a mineral with a rational series of basal reflections. The expression 'nearly rational series' of reflections used by the expert system will mean that the rationality criterion expressed above is not satisfied, but C.V. is <4%. The effect of the size and distribution of the sizes of the CSD can be strong, displacing the peak positions (e.g. 0.5 A for d- spacings ~-- 30/~). Identical XRD patterns are those for which a defined set of reflections have the same positions (the discrepancies in peak position being <0.1/~) and the same relative intensities (the discrepancies in the intensity ratios not exceeding 10%). ONE OR SEVERAL EXPERT SYSTEMS? The nine mixed-layer phyllosilicates considered in this paper could produce the same number of expert systems but the behaviour of certain mlm is strongly similar under several treatments (e.g. mica-smectite and mica-vermiculite, or chloritemica and chlorite-talc, etc.) and the design of nine expert systems would produce several redundancies. The development of a unique expert system is a second option, but the larger the expert system, the more difficult it is to maintain. A very important feature of an expert system is the simplicity with which it can evolve if additional knowledge appears. The expert system described has three parts: (1) one for mica-smectite and mica-vermiculite; (2) one for chlorite-smectite, chlorite-vermiculite, chlorite-swelling chlorite, chlorite-mica and chlorite-talc; (3) one for kaolinite-smectite and talc-smectite. For a given mixed-layer phyllosilicate, only one part will provide the values for the structural parameters. The two other parts fail to give any result. LITERATURE BASIS OF THE EXPERT SYSTEM The first goal of the expert system is the identification of the mlm. Table 1 allows one to judge which treatments distinguish each type of layer and then distinguish the different twocomponent mira. It could be tedious and redundant to detail this distinction for all families of mlm (Drits & Sakharov, 1976; Brown & Brin- Expert system for characterization of phyllosilicates dley, 1980; Reynolds, 1980, 1988; Srodofi & Eberl, 1984; Moore & Reynolds, 1989; Drits & Tchoubar, 1990). An example is presented here for the set: chlorite-mica + chlorite-talc + chlorite-serpentine. The XRD patterns for N, H and EG samples are identical or very similar, with a reflection situated between 10 and 14/~. For a sample with a rational series of 00l reflections, the conclusion is pure chlorite or chlorite-serpentine. Analysis of the width of basal reflections permits distinction between chlorite and chlorite-serpentine. For an example with a non-rational series of basal reflections, the reflection located between 4.6 and 5.0/~ must be considered. If the width of this reflection is less than that of the /~ reflection and the ,~ reflection, and if it occurs betweeen 4.66 and 4.75 ~, then the mineral is identified as chlorite-talc; if these conditions are not met, the mineral is identified as chlorite-mica. The second goal of the expert system is the determination of the structural parameters, i.e. the Reichweite and the abundance of layers. Depending on the type of mixed-layer mineral, different techniques are used to evaluate these parameters. For example, the structural characterization of mica-smectite minerals is usually based on the Watanabe diagram (Watanabe, 1981) for three reflections with d values between 10 and 14 ~, 8.4 and 9 ~, and 5.0 and 5.5 ~. For Fe-rich minerals, the ~ reflection is absent and Mering's formula (Meting, 1950) is used, for several reflections, in order to obtain the abundance of each layer type. Different examples of mixtures with a predominant component masking the other have been published and are used in the expert system (Srodofi, 1984; Brusewitz, 1986). The intensity ratio of the 001 and 003 reflections from the air-dry and glycolated samples is a sensitive tool for revealing small amounts of smectite accompanying illite (Srodofi, 1984). For other mixed-layer minerals, the main source of literature is the data gathered by Moore & Reynolds (1989). TESTING OF THE EXPERT SYSTEM USING LITERATURE DATA The expert has been tested by using published XRD data. Some of these published results 41

4 42 V.A. Drits and A. Plangon originate from a comparison of experimental XRD patterns with calculated ones, whereas some others are from calculated patterns only. The results of testings are shown in Tables 2, 3 and 4. The comparison of the results obtained from literature with those from the expert system is given for mica-smectite + mica-vermiculite in Table 2, for chlorite-smectite + chlorite-vermiculite + chlorite-swelling chlorite + chlorite-mica + chlorite-talc in Table 3, and for talc-smectite + kaolinite-smectite in Table 4. The different examples have been chosen to cover a wide range of R and layer abundances, or mixtures of different mixed-layer minerals. The comparison between the structural characteristics obtained from the expert system and those obtained by the authors of the data shows that the expert system runs satisfactorily for the data reported in the literature. It identifies accurately the nature of the mixed-layering and the R value, while the discrepancy between published and evaluated layer abundances is usually <5%. It identifies (Table 2) the occurrence of the mixture of discrete illite with mlm illite-smectite, as well as mixtures of two different mlm illite-smectite. TABLe 2. Structural characteristics obtained from mica-smectite + mica-vermiculite expert system for various samples, compared with literature data. Name Reference Expert system Literature Wsmec. Wsmec. mlm type R or Wv~rm. R Wv~m. Kinnekule B-1 R.H. I-S Two-Medicine I-S Kinnekule A-2 I-S Kalkberg " I-S i0 Sample 1 S. I-S Sample 5 " I-S Sample 12 I-S Sample 19 I-S (**) Sample 32 I + I-S 3 idem Sample 35 " I + I-S 1 idem B31 B. I-S ISO + I-S 1 39 idem 35 Sample 7576 MC.T I-S Sample 7889 I-S Sample 7920 " I-S p. 122 D.K.1 G-S (*) p. 122 D.K.1 I-S p. 101 D.K.2 G-S Colorado sh. R.H. Seg I & S 50 idem 35 p. 265 M.R. M-V R.H. for Reynolds & Hower (1970); S. for Srodofi (1984); B. for Brusewitz (1986); MC.T. for McCarty & Thompson (1991); D.K.I for Drits & Kossowskaya (1990); D.K.2 for Drits & Kossowskaya (1991); M.R. for Moore & Reynolds (1989). I-S for illite-smectite; G-S for glauconite-smectite (Fe-rich mica-smectite); M-V for mica-vermiculite; I + IS 3 for mixture of discrete illite with mlm illite-smectite with R = 3 and Wsmec < 15% ; I + IS I for mixture of discrete illite with mlm illitesmectite with R = 1 and 15 < Wsmec < 50% ; IS 0 + IS I for mixture of two kinds of mlm illite-smectite with different S and Wsmec (the first with R = 0, the second with R/> 1); Seg. I & S for tendency to segregation of illite and smectite layers, R=I. idem: same identification of the components. (*) glauconite-nontronite: the reflection A is missing. (**) R = 1 and R = 3 in Srodofi (1984).

5 Expert system for characterization of phyllosilicates 43 TABLE 3. Structural characteristics obtained from chlorite-other (other: smectite, vermiculite, mica, talc, swelling chlorite) expert system for various samples, compared with data from the literature. Name Reference Expert system Literature type R Wchlor. R Wchlo~. p. 604 R. Ch-S 0 " R. Ch-S 1 p. 618 R. Ch-S 0 " R. Ch-S 0 p. 619 R. Ch-S 0 p. 617 R. Ch-M Ch-M Ch-Sp identified p.267 M.R. Ch-Sp identified p.258 M.R. Ch-V 1 p.615a R. Ch-T (**) small amount 0 90 of talc R. for Reynolds (1988); M.R. for Moore & Reynolds (1989). Ch-S for chlorite-smectire; Ch-M for chlorite-mica; Ch-Sp for chlorite-serpentine; Ch-V for chloritevermiculite; Ch-T for chlorite-talc. identified: type of mineral identified, without determination of the structural parameters. (**) bibliographic data correspond to a heated sample whereas the expert system requires data for the sample saturated with K. TABLE 4. Structural characteristics obtained from kaolinite-smectite + talc-smectite by the expert system for various samples, compared with data from the literature. Name Reference Expert system Literature type R W~mect. R Wsmect. p. 260 M.R. K-S sample 1 W. K-S (*) p.353 D.T. K-S-I idem B5 R.W. K-S p. 357 D.T. T-S D.T. for Drits & Tchoubar (1990); W. For Watanabe et al. (1992); R.W. for Robinson & Wright (1987); M.R. for Moore & Reynolds (1989). K-S for mlm kaolinite-smectite; T-S for talc-smectite; K-S-I for a three component system kaolinite-smectite-illite. idem: same identification of the components. (*) described as mlm halloysite-smectite. DISCUSSION The expert system in its present form has certain limits which could give incorrect results in certain cases. First of all, only two-component systems have been considered, except for one example (Table 4, for kaolinite-smectite-illite). Three component mixed-layer minerals have been described in the literature (Drits & Sakharov, 1976; Moore & Reynolds, 1989; Drits & Tchoubar, 1990) but their occurrence in nature seems limited. In any case, their identification is difficult, requiring calculations. The expert system has some limits which are characteristic of

44 approaches based on the use of tables and graphs. Variation in layer thicknesses is one of the main sources of errors in the estimation of the mixedlayer component content.

6 44 approaches based on the use of tables and graphs. Variation in layer thicknesses is one of the main sources of errors in the estimation of the mixedlayer component content. The expert system reduces this type of error if the composition of interlayers is known (exchangeable cations). Another way could be the use of the two-peak technique such as that described by Srodofi (1980), and correcting the result (Srodori, 1981) for the case of a mixture producing peak interference. Certain problems may also occur in distinguishing between some non-swelling mlm and a mineral with a strict periodicity along the c* axis but with very small coherent scattering domains (Drits & Sakharov, 1976). In both cases there is no rationality of the 00l reflections. The expert system may also fail when it is applied to a mixedlayer structure with unusual structural characteristics (new type of layers, a new type of distribution, etc.). As with the expert system for kaolinites (Planton & Zacharie, 1989), which provided a good appreciation of structural parameters for stacking defects, the expert system for characterization of mixed-layer phyllosilicates offers a promising technique for the study of mlm. Future development of the expert system for disordered phyllosilicates will follow two main avenues. The first will be the analysis of mixtures of mixed-layer phyllosilicates with associated minerals as well as periodic phyllosilicates belonging to one of the main groups of phyllosilicates: micas, smectites, 7/~ kaolin or serpentine minerals, chlorites, talc-pyrophyllites, or palygorskitesepiolites. It will allow identification and structural characterization of mixed-layer minerals in an analysed sample. It will also include a computing tool for calculation of an XRD pattern for the confirmation of the structural parameters evaluated by the expert system. The second direction of development will be devoted to the determination of polytypes, unit-cell parameters, nature of stacking faults, and other structural features of phyllosilicates. DISTRIBUTION OF THE EXPERT SYSTEM The expert system runs on personal computers, compatible with MS-DOS version 3.0 or later. Copies are available from A. Plan~on. V.A. Drits and A. Plan(on REFERENCES BAILEY S.W. (1982) Nomenclature for regular interstratifications. Am. Miner. 67, BROWN G. & BRINDLEY G.W. (1980) X-ray diffraction procedures for clay mineral identification. Pp in: Crystal Structures of Clay Minerals and their X-Ray Identification (G.W. Brindley & G. Brown, editors), Mineralogical Society, London. BRtJSEWITZ A.M. (1986) Chemical and physical properties of Paleozoic potassium bentonites from Kinekalle, Sweden. Clays Clay Miner. 34, DRn'S V.A. & KOSSOWSKAYA A.G. (1990) Clay Minerals. Smectites, Mixed-layer Formation. Nauka editor, Moscow (in Russian). DRffS V.A. & KOSSOWSKAYA A.G. (1991) Clay Minerals. Micas, Chlorites. Nauka editor, Moscow (in Russian). DRITS V.A. & SAKHAROV B.A. (1976) X-ray Structural Studies of Mixed-layer Minerals. Nauka editor, Moscow (in Russian). DRITS V.A. & TCHOUBAR C. (1990) X-ray Diffraction by Disordered LameUar Structures. Springer Verlag, Berlin. EBERL D.D., ~RODOI~ J. & NORTHROP H.R. (1986) Potassium fixation in smectite by wetting and drying. ACS Symposium Series 323, McCARXV D.K. & THOMPSON G.R. (199t) Burial diagenesis in two Montana Tertiary basins. Clays Clay Miner., 39, MERING J. (1950) Des rgflections des rayons X par les mingraux argileux interstratifigs. Trans. 4th Int. Congr. Soil Sci., Amsterdam 1, MOORE D.M. & REYNOLDS R.C. JR. (1989) Pp in: X-ray Diffraction and the Identification and Analysis of Clay Minerals. (Oxford University Press editor) Oxford. PLAN~ON A. & ZACHARIE C. (1989) An expert system for the structural characterization of kaolinites. Clay Miner. 22, REYNOLDS R.C. JR. & HOWER J. (1970) The nature of interlayering in mixed-layer illite-montmorillonites. Clays Clay Miner. 18, REYNOLDS R.C. JR. (1980) Interstratified clay minerals. Pp in: Crystal Structures of Clay Minerals and their X-ray Identification (G.W. Brindley & G. Brown, editors). Mineralogical Society, London. REYNOLDS R.C. JR. (1988) Mixed-layer chlorite minerals. Pp in: Hydrous Phyllosilicates (Exclusive of Micas). (S.W. Bailey editor). Mineralogical Society of America. ROaINSON D. & WRIGHT V.P. (1987) Ordered illite,smectite and ka01inite-smectite: pedogenic minerals in a Lower Carboniferous paleosol sequence, South Wales. Clay Miner. 22, ~aodolq J. (1980) Precise identification of illite-smeetite interstratification by X-ray powder diffraction. Clays Clay Miner. 28, ~RODOr~ J. (1981) X-ray identification of randomly interstratified illite-smectite in mixture with discrete illite. Clay Miner. 16,

$~l~ooon J. (1984) X-ray powder diffraction identification of illitic materials. Clays Clay Miner. 32, 337-349. ~RODOfi J. & EBERL D.D. (1984) Illite. Pp. 495-544 in: Micas (S.W. Bailey, editor).$

7 ~l~ooon J. (1984) X-ray powder diffraction identification of illitic materials. Clays Clay Miner. 32, ~RODOfi J. & EBERL D.D. (1984) Illite. Pp in: Micas (S.W. Bailey, editor). Mineralogical Society of America, Washington, DC. SUQUET H. & PEZEP, AT H. (1988) Comments on the classification of trioctahedral 2:1 phyllosilicates, Clays Clay Miner. 36, Expert system for characterization of phyuosilicates WATANABE T. (1981) Identification of illitie-montmorillonite interstratification by X-ray powder diffraction. J. Mineral. Soc. Japan Spec. Issue, WATANABE T., SAWADA Y., RUSSELL J.D., MCHARDY W.J. & WILSON M.J. (1992) The conversion of montmorillonite to interstratified halloysite-smectite by weathering in the Omi acid clay deposit, Japan. Clay Miner. 27,

DETERMINATION OF ILLITE-SMECTITE STRUCTURES USING MULTISPECIMEN X-RAY DIFFRACTION PROFILE FITTING

Clays and Clay Minerals, Vol. 47, No. 5, 555-566, 1999. DETERMINATION OF ILLITE-SMECTITE STRUCTURES USING MULTISPECIMEN X-RAY DIFFRACTION PROFILE FITTING BORIS A. SAKHAROV, HOLGER LINDGREEN, 1 ALFRED SALYN,