Physicochemical characterization of edible clays and release of trace elements

Applied Clay Science 43 (2009) 135–141

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Physicochemical characterization of edible clays and release of trace elements J.R. Odilon Kikouama a,⁎, K.L. Konan b, A. Katty c, J.P. Bonnet d, L. Baldé a, N. Yagoubi a a

Groupe Matériaux et Santé EA 401, Faculté de Pharmacie, Université Paris Sud-11, 5 rue J.B. Clément, 92296 Châtenay-Malabry Cedex France Laboratoire de Chimie des Matériaux Inorganiques (LCMI), Université de Cocody-Abidjan, UFR SSMT, 22 BP 582 Abidjan 22, Côte d'Ivoire c Laboratoire de Chimie Métallurgique des Terres Rares, CNRS-UPR 209, Bât. F, 2-8, rue Henri Dunant, 94320 Thiais France d Groupe d'Etudes des Matériaux Hétérogènes, Ecole Nationale Supérieure de Céramique Industrielle, 47 à 73 avenue Albert-Thomas 87065 Limoges Cedex France b

a r t i c l e

i n f o

Article history: Received 20 September 2007 Received in revised form 29 July 2008 Accepted 29 July 2008 Available online 9 August 2008 Keywords: Edible clays Mineralogical composition Voltamperometry Release of trace elements

a b s t r a c t The mineralogical composition of seven edible clays from West Africa (Ivory Coast, Guinea and Senegal) was determined by X-ray diffraction, thermal (DTA, TGA and DSC) and chemical (ICP-MS and ICP-OES) analyses and measurement of specific surface areas and density. The major compounds were kaolinite, illite, muscovite, quartz and feldspars. Electrochemical analysis of clay suspensions by voltamperometry (SnO2:F) yielded no results at 6.5 b pH b 7 and pH ≈ 8.3. However, ionic species transfer was revealed at pH ≈ 1.8 by an anodic peak. The analysis by X-ray diffraction of the metal cation layers which were deposited on the blades (SnO2:F), showed the presence of mineral elements having therapeutic implications such as Fe and Zn. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction The use of clays for therapeutic purposes is very ancient and have been described by several authors (Ziegler, 1997; Mahaney et al., 2000; Saathoff et al., 2002; Wilson, 2003; Nchito et al., 2004; Tateo et al., 2006). It is as well internal as external (Carretero, 2002; Woywodt and Kiss, 2002). Soil ingestion or geophagia, also known as pica, is a behavior commonly observed among peoples on all continents (Hunter and De Kleine, 1984; Johns and Duquette, 1991; Reid, 1992; Abrahams and Parsons, 1996; Aufreiter et al., 1997; Grigsby et al., 1999; Mahaney et al., 2000; Tateo et al., 2001; Saathoff et al., 2002; Nchito et al., 2004). Several clay minerals are used as gastrointestinal protectors (kaolinite, palygorskite) because of their high specific area. But the main reasons of the ingestion of argillaceous material, according to Wilson (2003) are: detoxification of noxious or unpalatable compounds present in the diet; alleviation of gastrointestinal upsets such as diarrhea; supplementation of the diet with mineral nutrients; and the alleviation of excess acidity in the digestive tract. In West Africa (Ivory Coast, Guinea, Senegal), the ingestion of clays mainly concerns women during pregnancy. The organoleptic desire (color, taste and odor of clays), abdominal pains with accompanying vomiting, means for relieving nausea and ptyalism (exaggerated secretion of saliva) in pregnant women, and instinctive response to the demands of the body (Kikouama et al., 2002).

⁎ Corresponding author. Tel.: +33 146835443; fax: +33 146835963. E-mail address: [email protected] (J.R.O. Kikouama).

The objective of this work is to determine the mineralogical and chemical composition of seven clays and electrochemically analyze their suspensions in solutions simulating oral, stomach and intestinal media. 2. Materials and methods 2.1. Materials Seven edible clays were selected according to semi-quantitative estimates of the mineralogical composition carried out by Kikouama et al., (2007). These raw materials, commonly consumed (crunched, sucked or drunk in suspension) mainly by pregnant women, were collected from three West African areas: Bingerville, Ivory Coast (BNF and RNF, white and red respectively); Conakry, Guinea (CNR, CNJ and CNFG, red, yellow and grey respectively); Dakar, Senegal (DKG and DKFG, grey). All samples were passed through a sieve, yielding powders with particles less than 100 µm in diameter. The reference material diorite (Geostandard DR-N, CRPG-CNRS, France) was used to validate the dissolution method of clays. Fontainebleau Sand (FS, France) was also used as reference material in order to validate the method of evaluation of the amount of quartz contained in the clay samples. 2.2. Simulated dissolution media Three synthetic media (mouth, stomach and intestine) were prepared according to physiological knowledge of gastrointestinal apparatus (Cabral and Small, 1989; Chey, 1991; Hakanson and Sundler, 1991; Thonson, et al., 1994; Tateo et al., 2001; Imbert, 2002).

0169-1317/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.07.031

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Table 1 Particle size distribution of raw materials Diameter of particles (%) Samples

BNF RNF CNR CNJ CNFG DKG DKFG

Ø b 2 µm

2 b Ø b 20 µm

20 b Ø b 80 µm

80 b Ø b 100 µm

Clay

Silt

Thin sand

Rough sand

30.5 19.1 41.7 34.6 44.1 36.8 15.4

53.7 51.9 49.3 57.1 50.4 55.9 72.6

b15 26.9 8.6 8.3 4.7 b 7.3 12

b1 b3 b1 0 b1 0 0

Mouth: 0.02 M NaCl and 0.02 M KCl; volume ratio 1/1; 6.5 b pH b 7; Stomach: 0.02 M NaCl, 0.02 M KCl and 0.12 M HCl; pH ≈ 1.8; Intestine: 0.05 M NaHCO3, 0.11 M NaCl, 0.01 M KCl and 0.12 M HCl; pH ≈ 8.3. A total of 1 g clay were mixed with 100 mL of dissolution media, previously heated to 37 ± 0.5 °C in a plastic container, with magnetic stirrer.

taken again with 10 mL HCl (37%) to avoid interference between elements (Jarvis et al., 1992). The residue obtained was dissolved again with 5 mL HCl (5%). A Rigaku DTA-TGA apparatus was used for thermal analyses with a platinum crucible and technical alumina as reference, under the following conditions: calibration (indium), scanning rate (10 °C/min), air flow, maximum temperature (1100 °C), sample 100 ± 0.3 mg. Differential scanning calorimeter (DSC) was used to evaluate the quantity of quartz contained in the clays. The phase transition α-β of quartz occurs at 573 °C. A mass of 193 mg ± 0.3 of Fontainebleau Sand (FS) was analyzed (repetition n = 5). Setaram DSC 121 was used under the following conditions: nitrogen flow, reference (aluminum crucible of vacuum), calibration with indium (melting temperature 156,6 °C and ΔHf = 28,45 J/g). The thermal program used reached 580 °C at 0.2 °C/min. Specific surface areas were determined (at 77 K) from the adsorptions by applying the Brunauer–Emmet–Teller (BET) equation (Brunauer et al., 1938) and using 16.3 nm2 for the cross-sectional area of nitrogen (Gregg and Sing, 1982). All samples were preliminarily degassed under nitrogen at 200 °C for 2 h with the Micromeritics Flow Sorb II 2300. The densities of the powders were measured with a helium pycnometer (Micromeritics Multivolume Pycnometer, Norcross, USA).

2.3. Methods 2.4. Mineralogical composition The particle-size distribution of the clays was determined by a laser granulometer Malvern Mastersizer 2000, with a measurement range from 0.02 to 2000 µm and of particles of size less than 100 µm. A total analysis of the elements was carried out by inductively coupled plasma mass spectrometry (ICP-MS) by fusion with lithium tetraborate, for silicon. The quantities of all other elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Varian Liberty 200, argon plasma, France). 0.5 g of each sample previously heated to 110 °C for 24 h were dissolved in solution containing 10 mL HF (40%) and 5 mL HClO4 (70%). The mixture was heated to 200 °C; after complete vaporization of the acids, it was

Major crystalline phases were quantified by calculation from following equation: T ðxÞ ¼ ΣMiPiðxÞ T(x): Mi: Pi(x):

ð1Þ

percentage of oxide of chemical element “x”; percentage of mineral “i” in sample containing chemical element “x”; proportion of element “x” in mineral “i” (calculated from the ideal mineral formula).

Table 2 Chemical analysis of major (% mass) and trace (µg/g) elements (ICP-MS, ICP-OES) Major elements (%)

Samples BNF

SiO2 Al2O3 Fe2O3 T MgO CaO Na2O K2O TiO2 P2O5 H2O(100 °C) P.F (1000 °C) Total Trace elements (µg/g) Ba Co Cr Cu Mn Ni Pb Sr V Zn Zr

DRNexp

DRNth

54.85 26.98 2.09 0.14 0.09 0.19 0.85 1.62 0.04 0.50 8.70 98.05

RNF 59.23 24.48 2.64 0.12 0.07 0.13 0.86 1.49 0.06 0.50 10.00 98.58

CNR 47.56 33.11 3.38 0.22 0.07 0.27 1.09 1.63 0.07 0.90 11.00 99.30

CNJ 46.09 33.09 3.35 0.49 0.08 0.14 1.19 1.41 0.07 0.80 13.00 99.72

CNFG 48.17 33.06 2.86 0.27 0.09 0.11 1.04 1.33 0.06 0.80 11.50 99.29

50.49 25.68 1.98 0.89 0.05 0.19 5.07 0.93 0.03 0.70 12.00 98.02

64.61 21.27 1.83 0.93 0.06 0.17 5.70 0.91 0.03 0.06 3.70 99.81

n.d 17.52 9.66 4.28 7.06 2.96 1.68 1.07 0.23 n.d 2.24

n.d 17.52 9.70 4.40 7.05 2.99 1.70 1.09 0.25 n.d 2.26

299.25 12.29 155.18 24.04 2.75 42.88 26.42 50.97 82.75 70.06 270.37

288.76 11.67 139.81 15.05 2.47 37.07 21.02 56.74 117.51 65.20 230.69

419.60 12.59 112.16 11.64 1.63 34.02 27.89 82.68 146.92 75.86 236.36

390.45 18.20 46.35 22.94 20.73 47.25 30.74 26.93 57.70 98.59 161.72

191.97 12.53 63.76 22.59 8.15 42.84 13.79 16.80 78.77 97.57 151.06

568.38 6.76 57.23 14.46 0.44 12.69 12.15 28.05 59.94 43.29 105.89

558.78 6.89 45.58 35.22 0.57 12.42 13.22 37.47 79.43 46.97 108.74

371 40 26 46 0.19 14 56 372 206 158 74

385 35 40 50 0.22 15 55 400 220 145 125

DRNexp: DR-N geostandard (diorite) values obtained after quantification. DRNth: DR-N geostandard (diorite) literature values (Govindaraju, 1982; Govindaraju and Roelandts, 1989). Fe2O3 T: total iron.

DKG

DKFG

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Fig. 1. Diffractograms of raw samples (Kikouama et al., 2007) identification: K: kaolinite; Q: quartz; M (illite, muscovite); V: vermiculite; Sme: smectite; Gib: gibbsite; An: anatase; F feldspar; Alb: albite; Cal: calcite; Gyp: gypsum; Goe: goethite.

2.5. Electrochemical analysis of clay suspensions Voltamperometry was used as a qualitative analytical method for the identification of the cations released, which were settled as a thin layers on tin oxide conducting glass (SnO2) doped with fluorine (F) (SnO2:F). The electrochemical cell was composed of: – a saturated calomel reference electrode (SCE) KCl(sat)/Hg2Cl2(s)/Hg; – a work electrode, connected to a fluorine-doped tin oxide blade (SnO2:F), half of which was dipped in the clay suspension; – a platinum wire counter-electrode. The dissolved oxygen was purged by argon bubbling and the electrolyte solution was homogenized by agitation. Working conditions: scanning rate: 0.1 V/s; electroactivity range: −1.5 to 1.5 V; duration: 3600 s; temperature: 37 ± 0.5 °C. Several metal cations deposited on the glass blade (SnO2:F) were identified by X-ray diffraction (XRD) analysis using CuKα radiation over a 2θ range of 10–70° (PHILIPS, PW 1830 Generator, APD-PW1711 & DIFFRAC-AT). The cations identified were expressed as oxide or hydroxide. 3. Results and discussion 3.1. Physicochemical and mineralogical composition Granulometric distribution showed various groups of particles in the raw materials (Table 1). The values obtained represent an average after 3 repetitions. These clays mainly contained silts, and very little thin sand, except for RNF (26.9%). A relatively high percentage (30 to 45%) of clay particles distinguishes other samples (BNF, CNR, CNJ, CNFG and DKG) of RNF and DKFG (less than 20%). Quantification by ICP-OES required that there be no pollution of samples, no incomplete dissolution of material and no loss of matter. The Geostandard DR-N used at the time of this analysis made it

possible to validate the dissolution method. No error exceeded 10% (Table 2), except for Cr (35%) and Zr (40%), whose relatively high errors can be explained by the fact that they are better dissolved by attack with lithium metaborate than with fluorhydric acid. The considerable quantities of total iron were obtained, remarkably with Guinea and Ivory Coast materials. Notable amounts of Ti and K were also found. X-ray diffraction confirmed the presence of anatase of low intensity at 25.26° (DKFG), masked by the kaolinite (002) high intensity line for other samples (Fig. 1). The quantities of K reveal the presence of high percentages of micas in DKG and DKFG samples from Senegal. Feldspars were also detected at 27.56° in all samples (Fig. 1). Ca was in trace amounts (Table 2). The mass ratios of SiO2 and Al2O3 (Table 3) were related to the amount of free silica, compared to that generally found in pure kaolinite (SiO2/Al2O3: 1.18). An evaluation of non-combined alumina in kaolinite was also carried out. BNF, RNF and DKFG contained large quantities of free silica (amorphous silica and quartz). However, kaolinite was relatively abundant in CNR, CNJ, CNFG and DKG samples. The mass loss during heating up to 1000 °C (Table 2) was lower than that of pure kaolinite (13.95%), which is in agreement with the hypothesis of the presence of free silica (Hradil and Hostomsk , 2002; Soro, 2003). Non-combined alumina in kaolinite (Table 3) could be associated with the mica phase, feldspars and gibbsite (Fig. 1).

Table 3 Mass ratios of oxides (SiO2/Al2O3) and non-combined Al2O3 in kaolinite Mass ratios and Al2O3 (NCK)

SiO2/Al2O3 Al2O3 (NCK)

Samples BNF

RNF

CNR

CNJ

CNFG

DKG

DKFG

2.03 4.6

2.42 6.1

1.44 5.3

1.39 8.7

1.46 4.5

1.97 1.9

3.04 12.2

Al2O3 (NCK): non-combined Al2O3 in kaolinite.

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Fig. 2. Differential thermal and thermogravimetric analyses of clay materials studied.

J.R.O. Kikouama et al. / Applied Clay Science 43 (2009) 135–141 Table 4 Reproducibility of the temperature of quartz α-β transition and enthalpy

Table 5 Mineralogical composition (% mass)

FS

Transition temperature (°C)

Enthalpy (J/g)

Samples

SFB1 SFB2 SFB3 SFB4 SFB5 Average Standard deviation

573.18 573.53 573.34 573.61 573.16 573.36 0.20

3.68 3.39 3.64 3.55 3.65 3.58 0.12

Kaolinite Mica Feldspar Quartz Others

139

BNF

RNF

CNR

CNJ

CNFG

DKG

DKFG

57 12 b3 19 ≡9

47 10 b2 29 ≡12

70 8 b3 14 5

62 10 b3 15 ≡10

72 10 b3 7 ≡8

49 22⁎ b2 21 ≡6

7 68● b2 19 ≡4

⁎Mica: muscovite-type ● mica: illite-type.

FS: Fontainebleau sand.

The DTA and TGA curves (Fig. 2) were typical of the clay minerals (Bouaziz and Rollet, 1972) and showed common characteristics: two endothermic peaks and one exothermic peak, except CNJ and DKFG. CNJ present an endothermic peak at approximately 300 °C. The thermogram of DKFG also showed two endothermic peaks at 940 °C and 1050 °C. The loss of mass water, occurred at approximately 110 °C. The intense endothermic peaks between 560° and 580 °C (maximum temperatures) corresponded to the elimination of the structural hydroxyls of the octahedral sheet (Toussaint et al., 1963; Ortega et al., 1993). The endothermic peak observed with CNJ at approximately 300 °C corresponded to decomposition of mineral species associated with this material. The relatively high percentage of total iron (3.35%), and the gibbsite detected in CNJ diffractogram, confirmed the presence of Fe and Al oxyhydroxides. For DKFG, the endothermic peaks detected at 950° and 1050 °C were characteristic of the mica (illite and/or muscovite) phase (Sonuparlak et al., 1987). An exothermic peak of average intensity at the maximum temperature between

Table 6 Specific surfaces (SBET (m2/g)) and densities (ρ(g/cm3)) Samples

SBET (m2/g) ρ(g/cm3)

BNF

RNF

CNR

CNJ

CNFG

DKG

DKFG

23.77 2.64

26.35 2.64

28.73 2.59

29.29 2.58

22.47 2.63

23.12 2.63

22.56 2.70

970° and 985 °C was observed for all samples except DKFG. This is related to the formation of a spinel phase (Srikrishna et al., 1990; Okada et al., 1986; Gualtieri et al., 1995). The characteristic α-β peak corresponding to the quartz transformation phase at 573 °C was masked by the very intense peaks of the transformation of kaolinite into metakaolinite. The TGA thermograms confirmed the dehydroxylation of the kaolinite between 400° and 650 °C for all samples except DKFG. For this material, dehydroxylation reaction occurs at 750 °C, corroborated

Fig. 3. Thermogram of the quartz α-β transition.

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Fig. 4. Linear voltamperogram of CNFG after 3600 s of deposit time (pH ≈ 1.8). Ip, Ep: anodic peak current and potential, respectively SCE: saturated calomel electrode.

by the mica phase (Castellein, 2000). For CNJ, the exceptional loss between 250° and 350 °C was related to Fe and Al oxyhydroxides, as observed in the DTA curve. For BNF, RNF, CNR, CNG and CNFG, the loss between 400° and 650 °C could only be due to kaolinite. For DKG, between 400° and 650 °C and for DKFG, between 400° and 750 °C, we attribute the loss of mass to a kaolinite-mica mixture. Following Huertas et al., (1997) we estimated the content of kaolinite from the TGA analyses. The mass loss related to the decomposition of the octahedral sheet was considered to indicate the content of kaolinite. CNR (70%), CNJ (62%) and CNFG (72%) contained more kaolinite than DKFG (7%) conceals less kaolinite than BNF (57%), RNF (47%) and DKG (49%). Table 4 and Fig. 3 show the temperature of quartz α-β transition and enthalpies. The relative errors (standard deviation) of enthalpies were relatively slight. This method can thus be applied to the evaluation of the quantity of quartz in clays. Taking into account the previous physicochemical analyses, the quantitative mineralogical composition of the clays studied was estimated (Table 5). Anatase (TiO2) and other components (carbo-

nates) were found in non-determined phase. Concerning the mineralogical composition (Table 5), no sample contained 50% of particles less than 2 µm. Thus, granulometric analysis cannot provide a quantitative evaluation of the main mineralogical phases of the clays. The specific surface areas measured gave values between 22 and 30 m2/g (Table 6) and were compatible with those generally observed (10 to 30 m2/g) for kaolinite-type materials (Guyot, 1969). These values were also related to the relatively high quantities of iron contained in the materials (Arias et al., 1995; Sei et al., 2006). Their densities were also compatible with those generally observed (2.40 to 2.65 g/cm3) for kaolinite-type materials. The samples with abundant kaolinite (CNR, CNJ, CNFG) gave relatively low densities (Table 6). Therapeutic action of the kaolinite is based on its specific surface area and its sorption capacity (Carretero, 2002). The mineralogical composition, can partly explain the ingestion of these materials by pregnant women. Kaolinite is used in oral application as gastrointestinal protector. It adheres to the gastric and intestinal mucous membrane and protects them. Taking into account the mineralogical composition relatively rich in kaolinite, these clays could be beneficial to relieve women ptyalisme, when they are sucked. 3.2. Electrochemical analysis and XRD on metallic deposits The chemical analysis of materials studied has revealed the presence of several ionic species having as well a beneficial implication as hazardous. A qualitative assessment of the release of chemical elements in media of different pH makes it possible to correlate ingestion of these clays with the supplementation of pregnant women. The clay samples in simulated mouth (6.5 b pH b 7) and intestine (pH ≈ 8.3) media yielded no electrochemical deposit on the glass blade (SnO2:F) after 600 s or 3600 s. However, in the stomach medium (pH ≈ 1.8), linear voltamperograms were obtained from the same clay suspensions after 3600 s of deposit time. All curves were similar and characterized by anodic peak and very weak current whose maximum was about 0.01 mA. This peak indicated various cation species in the electroactivity range analyzed. This study presents only CNFG voltamperogram (Fig. 4). Detection of redox couples in the

Fig. 5. Diffractogram of cation layers released in stomach medium (pH ≈ 1.8) (1) N (2) N (3): decreasing intensity.

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electroactivity range used here was not feasible. Indeed, the voltamperograms did not indicate the presence of a peak in relation to the explored redox couples. However, the metal cation layers were analyzed by XRD. That made it possible to identify the ions (mineral elements) associated with the anodic peaks. The XRD results showed two groups of samples: CNFG and CNR with high intensity peaks, and BNF, RNF, CNJ, DKG and DKFG with low intensity peaks to over 30° (Fig. 5). The first two samples are rich in kaolinite (70% on average). They also contain remarkable quantities of Al, Fe, Ti and Zn, whose presence was noted in Fig. 5. These elements were released mostly in an acidic medium as has already been mentioned by Tateo et al. (2001). Among these elements, Fe and Zn are known to have therapeutic implications (Tuvemo and Gebre-Medhin, 1983–1985; Richardson, 2002; Baqui et al., 2004). 4. Conclusion The clays contained mainly kaolinite, micas (illite and muscovite), quartz and feldspars. CNR, CNJ and CNFG samples from Guinea were rich in kaolinite (60 to 73%). The clays released the mineral elements as Al, Fe, Ti and Zn in an acidic medium (pH ≈ 1.8). Acknowledgements We are grateful to Ms. G. Magnusson ([email protected]) for her proofreading of the manuscript. References Abrahams, P.W., Parsons, J.A.,1996. Geophagy in the tropics: a literature review. Geographical Journal 162, 63–72. Arias, M., Barral, M.T., Diaz-Fierros, F., 1995. Effect of iron and aluminium oxides on the colloidal and surface properties of kaolin. Clays and Clay Minerals 43, 406–416. Aufreiter, S., Hancock, R.G.V., Mahaney, W.C., Stambolic-Robb, A., Sanmugadas, K., 1997. Geochemistry and mineralogy of soils eaten by humans. International Journal of Food Sciences and Nutrition 48, 293–305. Baqui, A.H., Black, R.E., El Arifeen, S., Yunus, M., Zaman, K., Begum, N., Roess, A.A., Santosham, M., 2004. Zinc therapy for diarrhoea increased the use of oral rehydration therapy and reduced the use of antibiotics in Bangladeshi children. Journal of Health Population and Nutrition 22 (4), 440–442. Bouaziz, R., Rollet, A.-P., 1972. L'analyse thermique; Tome 1: les changements de phase. Editions Gauthier-Villars, Paris. Brunauer, S., Emmett, P.H., Teller, E., 1938. Journal of the American Chemical Society 60, 309. Cabral, D.J., Small, D.M., 1989. Physical chemistry of bile. In: Shultz, S.G. (Ed.), Handbook of Physiology: The Ga7strointestinal System III Section. Waverly Press, Baltimore. Carretero, M.I., 2002. Clay minerals and their beneficial effects upon human health: a review. Applied Clay Science 21, 155–163. Castellein, O., 2000. Influence de la vitesse du traitement thermique sur le comportement d'un kaolin: Application au frittage rapide. Doctorat de l'Université de Limoges, France. Chey, W.J., 1991. Regulation of pancreatic endocrine secretion. International Journal of Pancreatology 9, 7–20. Govindaraju, K., 1982. Geostandards Newsletter 6, 91–159. Govindaraju, K., Roelandts, I., 1989. Geostandards Newsletter 13, 5–67. Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity, (second ed.). Academic Press, London. Grigsby, R.K., Thyer, B.A., Waller, R.J., Johnston Jr., G.A., 1999. Chalk eating in middle Georgia: a culture-bound syndrome of pica? South Med. J. 92, 190–192. Gualtieri, A., Belloto, M., Artioli, G., Clark, S.M., 1995. Kinetic study of the kaolinite– mullite reaction sequence. Part II: mullite formation. Physics and Chemistry of Minerals 22, 215–222. Guyot, J., 1969. Mesure des surfaces spécifiques des argiles par adsorption. Annales Agronomiques 20, 33–359.

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