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Applied Clay Science 2

Relationships between chemico-mineralogical composition and

color properties in selected natural and calcined Spanish kaolins

E. Gamiza, M. Melgosab, M. Sanchez-Maranonc, J.M. Martın-Garcıad, R. Delgadoa,*

aDepartamento de Edafologıa y Quımica Agrıcola, Facultad de Farmacia, Universidad de Granada, Campus de Cartuja, 18071 Granada, SpainbDepartamento de Optica, Facultad de Ciencias, Universidad de Granada, Spain

cDepartamento de Edafologıa y Quımica Agrıcola, Escuela Politecnica Superior, Universidad de Almerıa, SpaindDepartamento de Geologıa, Facultad de Ciencias Experimentales, Universidad de Jaen, Spain

Received 10 September 2003; received in revised form 15 December 2003; accepted 10 February 2004

Available online 2 July 2004

Abstract

A total of 21 raw, washed and ground Spanish kaolins were studied. Kaolinite, mica, quartz, feldspars and occasionally

anatase and analcime were present. Calcined samples in which the kaolinite had been transformed into metakaolinite (650 8C,3 h), as shown by X-ray diffraction and observed using SEM were also studied. These samples featured particles with

openings and a lateral loss of laminar continuity, generated spaces, joined crystal edges and superficial coatings which

appeared to be fused to the surface of the aggregates.

Whiteness and tint indices (W10, Tw,10), revealed that only nine kaolins could be considered white (limits of CIE

Colorimetry, 1986. 2nd ed. CIE Publication No.15.2. Vienna: Central Bureau of the CIE), though upon calcination, this number

is reduced to three. Calcinating also produced coloring and reddening of the samples. The CIELAB color parameters

significantly correlated with compositional properties such as mineralogy, the chemical analysis of elements, the free oxide

contents (Al2O3, SiO2, Fe2O3) extracted with ammonium oxalate and with citrate–dithionite–carbonate, and trace elements. In

the non-calcined kaolins: Cab* is positively related to SiO2 and negatively to the percentage of kaolinite; L* is negatively related

to K2O; hab is negatively related to K2O and MgO; and W10 is positively related to the percentage of kaolinite and negatively to

the percentages of mica, K2O, MgO, MnO and free Fe2O3. The calcined samples showed correlations of hab and L* (positive)

and of Cab* (negative) with the proportions of both elemental Al and the Al extracted as free oxides. There are nonsignificant

differences between the L* of calcined and non-calcined kaolin samples, meaning that both samples fulfill the requirements of

high lightness for use in the paper industry.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Kaolin; Calcined kaolin; Color; Chemical and mineralogical composition

1. Introduction

0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.clay.2004.02.004

* Corresponding author. Tel.: +34 958 243835.

E-mail address: [email protected] (R. Delgado).

A material is considered to constitute kaolin when

the amount of kaolinite in a rock is greater than 50%

8 (2005) 269–282

E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282270

(Dombrowski, 2000). Kaolins are used in industries

such as pottery, paint, plastics, paper, cements,

pesticides, pharmaceuticals and cosmetics (Bundy,

1993; Konta, 1995; Murray, 2000). The required

quality and characteristics of kaolin depend on its use

(Gomes et al., 1994; Galan et al., 1998). The natural

coloring of kaolins is due both to the presence of

mineral impurities—whether phyllosilicated or not—

and the presence of some chromophore elements in the

crystalline lattice of the kaolinite (Raghavan et al.,

1997), such as Fe in isomorphic substitution with

octahedral Al (Jepson, 1988).

When kaolinite is calcined (Ross and Kerr, 1931;

Grim, 1968), it is transformed into metakaolinite, a

material which also has industrial applications. Meta-

kaolinite features a highly disordered structure with

remnants of the Si–O networks, whereas Al–O–OH

networks dehydroxylate and reorganize (Kaloumenou

et al., 1999; Kakali et al., 2001). Also important is the

change of AlVI to AlIV (Torres et al., 1999). The

calcination of kaolin at 650–700 8C (Bundy, 1993;

Prasad et al., 1991; Murray, 1999) may favorably

modify the properties of resiliency, opacity and

dielectric character, and as the chemical surface

becomes more compatible with organic systems, its

applications increase. The color may change on

calcination owing to mineral and inorganic impurities

and to changes in the oxidation state of the heavy

elements, like Fe, or to the loss of part of the structural

iron from the crystalline structure (Jepson, 1988;

Chandrasekhar, 1996). One requirement for many of

kaolin’s pharmaceutical and industrial applications is

that there is little change in color of the calcined

product (Konta, 1995).

Color is an easily perceived physical characteristic

of materials, and wields great importance for manu-

facturers, particularly in the pharmaceutical sector

(Soriano et al., 2002). In polymineral natural samples

with complex crystallochemistry, the study of color is

more complicated than in minerals of high purity (or

even synthetic ones) where diffuse reflectance spectro-

scopy techniques are employed (Burns, 1993). The

International Commission on Illumination (CIE) rec-

ommends the CIELAB system for estimating color

(CIE, 1986), and indeed this is the most commonly

used method (Kuehni, 1990; Soriano et al., 1998).

The main objective of this work is to study the

influence of the mineralogical and chemical composi-

tion on the color parameters of different Spanish

kaolins and their corresponding calcined samples. In

order to relate morphological changes to color, some

aspects of the metakaolinization process are inves-

tigated using SEM.

2. Materials and methods

2.1. Materials

We studied 21 kaolin samples from seven deposits in

Spain, provided by different mining companies (Table 1).

All the kaolins were passed through a sieve of 500 Am;

those which were not supplied finely powdered were ground

before sieving. Their corresponding calcined samples (at

650 8C, for 3 h) were also studied.

2.2. Methods

2.2.1. Mineralogical analysisThis was carried out using X-ray diffraction with a

Rigaku Miniflex Ca 2005 and Phillips PW 1730, equipped

with a Ni filter and Cu Ka radiation source. The

crystalline powder method was used with side-filled

sample holders to avoid preferential orientation of the

particles. Quantitative interpretation was based on the

intensity factor method on random powder diagrams

(Schultz, 1964; Gamiz, 1987).

2.2.2. Chemical analysis of elements and trace elementsThe elements were determined by X-ray fluorescence

with a Philips PW1404/10 spectrometer equipped with a

rhodium anode tube and a 3-kW generator. The trace

elements were determined with a Perkin Elmer Sciex-Elan

5000 mass spectrometer with a plasma torch ionization

source and a quadruple ion filter.

2.2.3. Free oxides of iron, aluminum and silicaThe elements obtained from extraction solutions of

ammonium oxalate (McKeague and Day, 1966) and

dithionite–citrate–bicarbonate (Mehra and Jackson, 1960)

were determined using a Perkin Elmer 1100B atomic

absorption spectrophotometer.

2.2.4. Scanning electron microscopyThe morphology of the samples, powdered on adhesive

paper and previously metallized with gold in two different

orientations (Bohor and Hughes, 1971), was studied with a

Hitachi S-510, with a resolution of 70 2 and magnification

between 20 and 150,000. Particle size was measured on the

photographs (dmax).

Table 1

General characteristicsa of the samples

Kaolin Origin Treatment Presentation L.D.b (%) O.M.c (%) CO32� (%)

C1-1 Arguisuelas. Cuenca Washed Pressed 0.64 0.21 1.19



C2-1 Villagarcia de Arosa. Pontevedra Raw Fragments 2.97 0.21 0.90



C3-1 Cervo. Lugo Raw Granular 0.79 0.17 0.79



C4-1 La Guardia. Pontevedra Washed Pressed 0.79 0.12 1.10



C5-1 Oviedo Raw Fragments 0.29 0.16 0.65



C6-A1 Burela. Lugo Washed Pulverized 1.22 0.24 0.99



C6-B1 Burela. Lugo Washed Pulverized 1.28 0.41 0.91



a Adapted from Gamiz et al. (1988a,b).b L.D.: loss on drying (110 8C).c O.M.: organic matter.

E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282 271

2.2.5. ColorThe color measurements of the previously dried kaolins

(110 8C for 24 h) and the samples after calcination (650 8Cfor 3 h) were carried out with a SpectraScanPR-704

spectroradiometer placed in front of a VeriVide portable

cabin with D65 light source. From these measurements, we

calculated the corresponding color parameters in CIELAB

and the indices of whiteness and tint (W10, Tw,10)

recommended by the CIE (1986). Rather than Cartesian

coordinates, polar coordinates (L*, Cab* , hab) were used here.

It should be noted that parameters L*, Cab* and hab are

mathematically derivable from CIELAB and correspond to

the so-called CIELCH.

2.2.6. Statistical analysisThe Kolmogorov–Smirnov test was used to ensure that

the data satisfied the condition of normality. To investigate

the influence of the extraction procedure (citrate or oxalate)

and calcination on the composition of the free oxides (Fe, Al

and Si), two-way analysis of variance (ANOVA) was used:

the dependent variable was either Fe, Al or Si, successively;

and the factors were calcination (vs. uncalcinated kaolin)

and the extraction procedure (citrate vs. oxalate). At the

same time, the possible interaction between the two factors

was explored. To determine whether the calcination had a

significant effect on the color parameters, paired Student’s t-

tests were used (for each kaolin before and after calcination)

(Martinez-Gonzalez et al., 2001). Next, simple and multiple

linear regression equations were calculated using the

stepwise method (only the first two or three variables of

each equation were assigned to approximate the criterion of

one variable for every 10 samples) to predict each color

parameter from the most influential chemical elements. All

statistical analyses were carried out using the Statistical

Package for the Social Sciences, version 11.0 (SPSS, 1997).

3. Results and discussion

3.1. Mineralogical composition

In 18 of the 21 kaolins studied (Table 2), kaolinite

was the most abundant mineral (between 40% and

92%), its percentage increasing in the washed

samples. The main mineral impurities were quartz,

dioctahedral mica (muscovite) and feldspars; anatase

was detected in 12 cases and analcime in 3. The XRD

study of kaolins C2-1, C2-2 and C2-3 showed

reflections at 0.45 and 0.72 nm and a band between

1.4 and 1.7 nm, indicating the joint presence of

smectite and kaolinite. A previous study of the clay

Table 2

Mineralogy (XRD) of the kaolins (%)

Kaolin Mica Kaolinite Phyllosilicatesa Quartz Fd-K Plagioclases Anatase Analcime

C1-1 6 80 14 b1 b1

C1-2 4 81 15 b1 b1

C1-3 5 83 12 b1

C2-1 68 12 16 4

C2-2 65 12 20 3

C2-3 69 13 16 2

C3-1 35 56 6 3

C3-2 27 40 19 14

C3-3 37 55 7 1

C4-1 31 59 10 b1

C4-2 36 55 9

C4-3 33 58 9

C5-1 11 83 4 1 1

C5-2 3 92 2 1 2

C5-3 10 80 6 1 3

C6A-1 14 82 3 1 b1

C6A-2 18 76 4 2 b1

C6A-3 19 76 4 1 b1

C6B-1 20 60 18 1 1

C6B-2 23 65 12 b1

C6B-3 21 64 15 b1

Mean 20.2 69.2 67.3 10.0 7.5 3.0 1.0 2.0

S.D. 11.4 13.6 1.7 4.9 7.5 0.8 0.0 0.8

a The percentages are for all the phyllosilicate minerals according to Schultz (1964). In these samples, they are mainly smectites and

kaolinite (Gamiz et al., 1988a).


fraction of these samples with thermal, DMSO, and

EG treatments (Gamiz et al., 1988a) revealed the joint

presence of kaolinite and smectite.

In the samples calcined at 650 8C, an intense and

characteristic band of amorphous materials between

178 and 278 2h was observed, and the reflection at

0.72 nm of kaolinite disappeared, suggesting that all

the kaolinite had been transformed into metakaolin-

ite. Mica, feldspars, quartz and anatase were still

present.

3.2. Chemical composition

As seen in Fig. 1, we detected the oxides that are

components of the ideal formula of kaolinite (SiO2,

Al2O3 and H2O), as well as those elements which may

be present as isomorphic substitutions of the kaolinite

structure (for example Fe3+) or related to various

mineral impurities (e.g. TiO2 from the anatase). With

regard to the ideal composition of the kaolinite (SiO2:

46.54%; Al2O3: 39.50%; and H2O: 13.96%), these

samples had higher SiO2 contents due to the notable

presence of quartz and mica, and thus a lower content

in Al2O3 and H2O (Table 2).

Highly significant regression equations ( pV0.001)were obtained (Table 3), indicating that the percent-

age of Al2O3 [1] is related to kaolinite and, to a

lesser extent, mica; that %SiO2 [5] decreases with in-

creasing phyllosilicates (relative decrease of quartz);

and that %Fe2O3 [2], %K2O [3] and %MgO [4]

depend on mica content, in the form of ions present

in their structure. The loss of ignition (LOI) (%H2O)

[6] is positively related to kaolinite content—of all

the minerals present, it is the one with the highest

LOI.

The percentage of free oxides extracted with

oxalate (ox) and citrate–dithionite–bicarbonate (cdb)

is shown in Table 4. Although they are found in a

remarkably small proportion, they affect the color of

the sample (as will be shown later) since they are

mainly coatings or extracted forms from the edges and

surface of the mineral particles. The oxides extracted

with oxalate were in much lower quantities than those

extracted with citrate. Citrate is a much more energetic

Fig. 1. Oxide contents in kaolins. Mean values (FS.D.).


extractant, dissolving the crystalline and amorphous

forms, whereas oxalate preferentially extracts the

amorphous forms (Schwertmann and Taylor, 1989).

Comparison of the quantities of the three oxides in

each sample, for both extractants, gives the sequence

%Al2O3N%SiO2N%Fe2O3 for most samples. This can

be attributed to their mineralogical composition, rich

in kaolinite, with Al present in the octahedral sheet and

Si in the tetrahedral sheet, and with a very small

particle size entailing high reactivity. A comparison of

kaolins with their corresponding calcined samples

shows, firstly, a considerable increase in Al2O3, which

may double or triple (regardless of extractant), and an

increase in the SiO2 extracted with ammonium oxalate

as a result of calcination. This may be due to the

destructuring of the kaolinite into a more amorphous

material that is more susceptible to extraction. Despite

these numerically confirmed differences, the positive

relationships between the percentage of kaolinite with

Al2O3 cdb and Al2O3 ox in calcined samples are only

significant at 1% and 5%, respectively; for this reason

they are not included in Table 3. There is no correlation

between the percentage of kaolinite and SiO2 extracted

from the calcined samples. The proportions of

extractable Fe2O3 generally decrease on calcination

as well, the only exceptions being the extractions with

oxalate in C4 and C5. This may have to do with the

recrystallization of these iron forms under calcination,

making them less susceptible to extraction.

Consistently significant differences were found

between the quantities of oxides extracted with oxalate

and citrate, and between the uncalcined kaolins and the

Table 3

Multiple regression equations of the different components of the

kaolins ( pV0.001)

Dependent

variable

Equation R2 F Estimated

standard

error

Oxides of elemental analysis with mineralogya

% Al2O3 [1] y=0.87+

0.39(% kaolinite)+

0.19(% mica)

0.825 32.967 1.865

% Fe2O3 [2] y=0.32+0.03(% mica) 0.708 36.355 0.208

% K2O [3] y=�0.02+0.01(% mica) 0.814 65.850 0.537

% MgO [4] y=�0.005+

0.007(% mica)

0.872 102.020 0.033

% SiO2 [5] y=98.67�0.55(% kaolinite)�0.34(% mica)

0.845 38.255 2.199

% LOI [6] y=1.45+

0.13(% kaolinite)

0.831 73.742 0.880

Minor and trace elements with mineralogyb

ppm Ce [7] y=�156.25+

3.86(% kaolinite)

0.623 24.759 43.899

ppm Rb [8] y=�28.04+

7.95(% mica)

0.666 29.892 68.265

ppm La [9] y=�68.02+

1.72(% kaolinite)

0.634 25.945 19.067

ppm Zn [10] y=�3.59+

0.48(% mica)

0.629 25.469 4.502

Minor and trace elements with free oxidesc

ppm Cu [11] y=4.73�17.53(% Al2O3 cdb)+

82.91(% Fe2O3 cdb)

0.703 21.274 2.123

ppm Li [12] y=56.54�496.63(% Al2O3 ox)+

4335.31(% Fe2O3 ox)

0.743 25.995 22.347

ppm Nd [13] y=142.31�7054.18(% Fe2O3 ox)�473.58(% SiO2 cdb)+

1053.11(% SiO2 ox)

0.712 13.990 23.760

ppm Pr [14] y=43.75�2060.57(% Fe2O3 ox)�122.95(% SiO2 cdb)

0.588 12.870 6.733

ppm Rb [15] y=�76.42+

7918.19(% Fe2O3 cdb)+

1565.29(% Al2O3 ox)�21271.48(% Fe2O3 ox)

0.605 8.694 81.467

ppm Sn [16] y=7.33+

421.79(% Fe2O3 cdb)

0.460 16.189 8.172

ppm V [17] y=44.69�245.07(% Al2O3 ox)

0.492 18.371 12.816

ppm Y [18] y=78.44�6961.33(% Fe2O3 ox)

0.633 32.704 19.686

ppm Zn [19] y=0.39+

299.46(% Fe2O3 cdb)�164.12(% SiO2 ox)

0.658 17.281 4.237


calcined samples (ANOVA, Table 5). From a statistical

viewpoint, there is no interaction between calcination

and extraction except in the case of Fe2O3 ( pb0.001).

Fig. 2 gives the mean content (FS.D.) of trace

elements in kaolins. The most abundant (N100 ppm)

are Sr, Rb, Ba, Zr and Ce. The data show a great

disparity in concentrations of each trace element from

one kaolin to the next. Thus, Sr in the samples of

kaolin 5 has quantities greater than 2000 ppm (not

included in the calculation of the mean in Fig. 2) while

values around 5 ppm were observed for kaolin 6B. We

attribute this to differences in the genesis of the

samples, a matter beyond the scope of the present

study. The trace element contents are significantly

associated with the mineralogy, though in very few

cases pV0.001 (Table 3). Ce and La increase with

increasing kaolinite, possibly as ions entering its

structure. The same is true of mica with Rb and Zn.

A greater number of significant correlations with

pV0.001 are found between the trace elements and

the free oxide contents of the kaolins (Table 3),

suggesting they bear a close relationship.

3.3. Color parameters

CIELAB color parameters (Fig. 3) showed that

natural samples, white to the naked eye, were of high

lightness (L*: 84.0–89.81) but with a slight chroma

(Cab* : 2.04–8.45). Only nine kaolins were within CIE

whiteness limits (W10N40 and Tw,10 �3 to 3, CIE,

1986). The negative values of Tw,10 revealed a reddish

trend. Sample C5-2 (W10: 58.88; Tw,10:�0.47) was the

whitest kaolin. The mean values for color parameters

in calcined samples were significantly different (t-test,

pb0.001) from those in natural samples, except L*. On

the average (n=21), calcination caused an increase in

Cab* (44%), and a decrease in hab (14%), W10 (24%)

and Tw,10 (125%), indicating that the samples became

more reddish and chromatic while maintaining their

original lightness. However, the three C5 samples

gained whiteness after calcination, increasing their

lightness. The fact that the values of lightness (L*)

change very little contrasts with the results of

otes to Table 3:a Only carried out with kaolinite, mica and quartz to guarantee

c20 (nz18).b n=18.c

N

n

n=21.

Table 4

Free oxides in kaolins and in calcined samples (values in ppm)

Kaolin Al2O3

ox K.

Al2O3

cdb K.

Fe2O3

ox K.

Fe2O3

cdb K.

SiO2

ox K.

SiO2

cdb K.

Al2O3

ox C.S.

Al2O3

cdb C.S.

Fe2O3

ox C.S.

Fe2O3

cdb C.S.

SiO2

ox C.S.

SiO2

cdb C.S.

C1-1 85 534 54 366 315 684 1642 8461 50 192 670 4679

C1-2 126 661 63 338 207 881 1889 4431 49 96 834 1944

C1-3 173 616 57 211 291 759 1503 2172 34 51 665 663

C2-1 1275 2620 33 77 266 1356 838 1013 31 58 736 407

C2-2 1294 2504 37 236 341 1011 771 938 34 46 978 714

C2-3 1346 2595 30 130 306 957 860 1662 29 96 639 1083

C3-1 865 1524 135 663 132 571 1204 1759 55 83 620 910

C3-2 1267 4127 97 609 437 2078 1134 1791 45 53 686 653

C3-3 876 1932 85 481 174 910 1177 1771 51 78 658 806

C4-1 758 1534 102 355 89 971 2347 6300 134 287 611 2073

C4-2 830 1607 86 390 174 1260 2261 5155 148 206 550 1368

C4-3 756 1876 95 363 148 1180 2125 4383 145 193 599 1177

C5-1 401 691 10 46 146 1697 3855 6214 19 46 512 346

C5-2 961 3725 10 107 374 1699 4619 9044 31 42 487 651

C5-3 926 2504 9 17 378 1295 5679 11,016 22 28 440 618

C6A1 1793 2671 106 196 92 1274 2320 6367 79 178 599 2099

C6A2 1722 2479 104 237 21 1257 2291 5808 64 112 639 1525

C6A3 1762 2425 94 247 115 860 2221 5191 86 116 766 1347

C6B1 1204 2355 51 163 38 1297 2091 4254 33 92 660 1622

C6B2 1166 2372 64 176 108 1347 2008 4661 38 65 736 1318

C6B3 1209 2344 43 124 45 2221 1981 4337 31 39 642 1321

Mean 990 2081 65 263 200 1217 2134 4606 57 103 654 1301

S.D. 501 950 36 174 124 429 1235 2753 39 69 118 934

ox: extraction with ammonium oxalate.

cdb: extraction with citrate–dithionite–bicarbonate.

K.: kaolins.

C.S.: calcined samples.


Chandrasekhar (1996), who attributed a decrease in

lightness to the oxidation of the iron present during

metakaolinization. Finally, the samples exhibit high

lightness (L*N85.5) in all cases and satisfy the strictest

requirements for use in the paper industry, as clay

coating, for which values must be at least 83.5 (de

Mesquita et al., 1996).

Table 5

Proportion of the free oxides (%) extracted with oxalate or citrate–dithion

Free

oxide

Kaolins Calcined samples

ox (meanFS.D.) cdb (meanFS.D.) ox (meanFS.D.)

Al2O3 0.099F0.050 0.208F0.095 0.213F0.124

Fe2O3 0.007F0.004 0.026F0.017 0.006F0.004

SiO2 0.020F0.012 0.122F0.43 0.065F0.012

* pb0.05.

*** pb0.001.

3.4. SEM study

With SEM, the powdered kaolins appear to be

composed mainly of roughly spherical particle aggre-

gates with an internal fabric tending towards concen-

tric that is possibly acquired during grinding (Fig. 4a).

Accompanying the aggregates is a matrix of smaller

ite–bicarbonate in kaolins, before and after calcinations

ANOVA ( F)

cdb (meanFS.D.) Effect of

calcination

Effect of

extraction

Interaction

calcination�extraction

0.461F0.275 27.56*** 25.98*** 3.91

0.010F0.007 15.65*** 32.72*** 12.93***

0.130F0.093 5.59* 53.59*** 2.65

Fig. 2. Trace element contents (ppm) in kaolins. Mean values (FS.D.). Sr*=in the mean value (FS.D.) data for samples C5-1, C5-2 and C5-3,

are not included.


generally isolated particles. The average size of the

aggregates is around 86 Am, with a range of 4–304 Am.

A distribution by size (Fig. 5A), however, shows that

particles smaller than 30 Am are the most frequent.

Some bstacksQ of kaolinite over 50 Am (Fig. 4b)

survived grinding. They would have originally been

Fig. 3. Color parameters in kaolins and calcined samples. Mean

values (FS.D.).


vermiform or accordeon-shaped; relic vermiform

aggregates after comminution have been described

by Psyrillos et al. (1999). Folded mica crystals

sometimes appear, due to the elasticity of the mica

layers (Fig. 4c), after comminution.

The ungrouped or only slightly grouped particles in

kaolins have an average size of approximately 5 Am—

within the range 0.4–61 Am—although the size

distribution (Fig. 5B) reveals that the most frequent

are smaller than 2 Am. Most of these particles are flat

fragments of mica and flat kaolinite crystals with edges

corrugated by grinding. The somewhat larger isolated

particles that also appear are mostly tectosilicates

(quartz and feldspars) of which considerable percen-

tages are present in the samples (Table 2).

The calcination process increased the size of the

aggregates, which had a mean size of 116 Am (Fig.

5A). The generation of a bulky product by calcination

of the kaolin at 650–700 8C and dehydroxylation and

the escape of water vapour has been previously

reported by Prasad et al. (1991). This did not occur

in the case of our ungrouped particles, however, with a

mean size of 5.5 Am: virtually no differences were

observed in their distribution with regard to the natural

particles (Fig. 5B).

A detailed observation of the calcined samples is

difficult because the destruction of the kaolinite and its

amorphization entail a certain loss of ultramicrorelief

and the generation ofmore static electricity, leading to a

poorer SEM image. Nevertheless, it was clear to see

that the metakaolinite aggregates had been discontin-

uously coated by material that appeared to be bmeltedQonto them (Fig. 4d). The same was observed on the

particles in plane 00l, with adhered planar crystals that

appeared to be bsolderedQ (Fig. 4e); the joining of the

edges of particles after calcination has been described

by Berube (1978). The detailed image of the meta-

kaolinite particles in hk0 planes (Fig. 4f) shows the

opening of layers, the generation of spaces with a loss

of lateral continuity of the structure, the apparent

joining of layers, and what we have called bmeltedQ orbsolderedQ material as a superficial coating. The

generation of spaces would explain the increase in size

of the aggregates. Metakaolinization must be respon-

sible for these features, as the destruction of the gibbsite

sheet would create spaces for the escape of OH�—

hence, the loss of lateral continuity of the layers—and

form an aluminic material which could function as a

kind of cement amongst the remains of the tetrahedral

sheet, joining layers atop the aggregates. It can also be

presumed that the changes detected in the ultramicro-

fabric of the powder affect the color of the metakao-

linite, though this effect has not been demonstrated.

3.5. Relationships between color and composition

The most significant correlations with color

parameters (Tables 6 and 7) were those with chemical

composition and free forms while the least significant

were those with mineralogical composition. The

regression equations of whiteness (W10) and tint

(Tw,10) were only calculated for the kaolins whose

indices were within the limits indicated by CIE

(1986). Chroma (Cab* ) [1] was negatively correlated

with the main mineral component, kaolinite. White-

ness (W10) was positively correlated with the kaolinite

[2] and negatively with the proportions of mica [3],

another component always present in the samples.

This mica (muscovite) is a more colored and darker

Fig. 4. SEM images. Kaolin 3-1 and corresponding calcined sample. (a) Kaolin. Field of particle aggregates, together with others slightly

aggregated or unaggregated. The arrow indicates the somewhat concentric internal fabric of the aggregates, as a result of grinding. (b) Kaolin.

Some kaolinite bstacksQ, originally vermiform, that have survived grinding. (c) Kaolin. Mica particle with folded morphology due to grinding.

(d) Calcined sample. Metakaolinite aggregate coated with particles (shown by arrow) which appear to be bsolderedQ to it. (e) Calcined sample.

Metakaolinite particle (shown by arrow), 00l plane; the particles appear to be bsolderedQ to the surface. (f ) Detailed image of kaolinite particle,

hk0 planes, showing opening of laminae, generation of spaces, loss of lateral continuity of the laminae and generation of a discontinuous

material on the surface, bsolderingQ the laminae.


phase than the kaolinite, as its structure accepts a

higher proportion of isomorphic substitutions of

chromophore elements.

The correlations between the color parameters and

the elemental chemical composition of the kaolins

(Table 6) support some of the mineralogical relation-

ships already described. For instance, the increase in

whiteness (W10) with a decreasing percentage of

Fe2O3, MgO, K2O and MnO of the samples can be

explained by the lower percentage of mica. Likewise,

L* decreases with greater K2O content, and hue is less

intense with greater K2O and MgO, reflecting a clear

relationship with mica. For the same reason, Cab* in-

creases in tandem with proportions of MgO and SiO2

(mica, a 2:1mineral, has more SiO2 than kaolinite, 1:1).

Interestingly, the Cab* also decreases along with the

percentage of TiO2 (C*=6.84�2.63% TiO2, R2=0.322,

pb0.01); though a number of authors (Bundy and

Ishley, 1991; Prasad et al., 1991; de Mesquita et al.,

1996; Raghavan et al., 1997) report a negative effect of

TiO2 on lightness (L*) and whiteness (W10).

No highly significant correlations ( pV0.001) werefound between the color parameters of kaolins and free

oxides. However, Cab* correlates with the percentage of

Fe2O3 cdb (Cab* =4.47+50.42% Fe2O3 cdb, R2=0.364,

pb0.01), W10 with Fe2O3 cdb (W10=56.48�389.20%

Fig. 5. Frequency distribution of aggregate and particle sizes (dmax) measured with SEM in kaolins and calcined samples. (A) Aggregates; (B)

isolated or slightly aggregated particles.


Fe2O3 cdb, R2=0.740, pb0.01) and W10 with the

percentage of Fe2O3 ox (W10=56.82�1291.19%

Fe2O3 ox, R2=0.774, pb0.01). In other words, since

these are forms of Fe2O3 which are coatings or

extracted from the surface and edges of the mineral

particles, their percentage increase logically affects the

color of the sample, increasing chroma and decreasing

whiteness. Prasad et al. (1991) described a decrease in

whiteness with the presence of iron oxides. However,

the results of our study cannot be used to support the

affirmations of Bundy and Ishley (1991) and de

Mesquita et al. (1996) regarding the decrease in

lightness (L*) of the kaolin with the presence of these

oxides.

The correlations among the calcined samples

(Table 7) provide interesting findings. No correlations

with W10 and Tw,10 could be seen since only three

samples fulfilled the CIE (1986) conditions for

whiteness. The correlations of hab and L* (positive)

and of Cab* (negative) with the proportions of Al, in

many of its forms (whether elemental or extracted,

both with citrate and oxalate) are noteworthy: the

samples are less colored and lighter when they contain

more Al2O3, a white pigment.

Table 6

Simple regression equations between color parameters and the

mineralogical and chemical composition of the kaolins ( pV0.001)

Dependent

variable


standard

error

Color parameters with mineralogy

Cab* [1] y=11.223�0.08(% kaolinite)

0.520 17.320 1.107

W10y [2] y=22.67+

0.36(% kaolinite)

0.690 15.561 3.396

W10y [3] y=56.47�

0.43(% mica)

0.791 22.729 2.624

Color parameters with chemical composition

L* [4] y=88.33�0.56(% K2O)

0.474 17.110 1.388

Cab* [5] y=�3.17+

0.16(% SiO2)

0.440 14.923 1.118

Cab* [6] y=5.16+

2.96(% MgO)

0.215 5.192 1.324

hab [7] y=88.89�2.34(% K2O)

0.547 22.954 5.003

hab [8] y=88.22�25.19(% MgO)

0.628 32.101 4.533

W10y [9] y=58.15�

12.02(% Fe2O3)

0.919 79.621 1.733

W10y [10] y=55.86�

62.45(% MgO)

0.876 49.486 2.146

W10y [11] y=59.39�

6.96(% K2O)

0.861 43.518 2.270

W10y [12] y=57.05�

1462.73(% MnO)

0.789 26.170 2.801

Only carried out with kaolinite, mica and quartz to guarantee nc20

(nz18).y Only in those kaolins fulfilling the requirements for whiteness

(W10N40 and Tw,10 �3 to 3; CIE, 1986).

Table 7

Simple regression equations between different color parameters and

the chemical composition and free oxide content of calcined

samples ( pV0.001)

Dependent

variable


standard

error

Cab* [1] y=21.30�0.42(% Al2O3)

0.612 29.989 1.800

Cab* [2] y=11.94�7.75(% Al2O3 cdb)

0.573 25.483 1.889

Cab* [3] y=12.72�20.35(% Al2O3 ox)

0.796 74.154 1.305

Cab* [4] y=4.45+

4.97(% Fe2O3)

0.435 14.630 2.172

Cab* [5] y=15.90�0.76(% LOI)

0.574 25.562 1.887

Cab* [6] y=�12.64+

0.38(% SiO2)

0.646 34.613 1.721

hab [7] y=58.26+ 0.476 17.228 8.579


The multiple regression equations between the

color parameters and the trace elements (Table 8) gave

values of R2N0.85 and pb0.001. The predictive role of

the color of these components is clear.
28.92(% Al2O3 cdb)
hab [8] y=55.10+

77.20(% Al2O3 ox)

0.682 40.792 6.677

L* [9] y=74.71+

0.38(% Al2O3)

0.552 23.389 1.859

L* [10] y=83.45+

6.61(% Al2O3 cdb)

0.452 15.656 2.056

L* [11] y=79.21+

0.74(% LOI)

0.584 26.643 1.791

L* [12] y=105.01�0.34(% SiO2)

0.543 22.571 1.877

W10 and Tw,10 are not included since only three samples remained

white according to CIE (1986) after calcinations.

4. Final discussion

The study of 21 Spanish rough, washed and

ground kaolins leads us to conclude that kaolinite is

the most abundant mineral phase of the kaolins,

together with dioctahedral mica, quartz, feldspars,

and sometimes, anatase and analcime. This mixture

of minerals diverts the ideal chemical composition

towards higher SiO2 content and lower Al2O3 and

H2O. Free forms of aluminum, silica and iron

(extracted with citrate–bicarbonate–dithionite and

oxalate) were also present; due to the composition

of the kaolins, rich in kaolinite, the sequence of the

percentages was Al2O3NSiO2NFe2O3. Trace elements

were also detected, the most abundant being Sr, Rb,

Ba, Zr and Ce. These trace elements are significantly

associated both to the minerals present and to the

free forms. The color of all the kaolins, while

apparently white or almost white, could be consid-

ered white according to CIE limits in only nine cases

(W10N40 and Tw,10 from �3 to 3) (CIE, 1986).

However, due to their high lightness (L*N85.5), all

21 samples satisfy the requirements for the paper

industry (L*N83.5) (de Mesquita et al., 1996). The

SEM study of the kaolins revealed aggregates of

crystals with a roughly concentric ultramicrofabric

Table 8

Multiple regression equations between color parameters (in kaolins

and calcined samples) and trace elements ( pV0.001)

Dependent

variable


standard

error

Kaolins

Cab* [1] y=5.54+

0.01(ppm Rb)�0.005(ppm Ba)�1.35(ppm Mo)

0.906 54.849 0.483

hab [2] y=65.89+

0.37(ppm Ga)+

0.39(ppm Sn)�0.02(ppm Rb)

0.909 56.509 2.373

L* [3] y=90.40�0.01(ppm Ba)�0.08(ppm Sn)

0.742 25.835 0.999

Tw,10y [4] y=�4.23+

1.12(ppm Mo)+

0.04(ppm Cr)

0.930 39.624 0.255

W10y [5] y=53.50�

0.09(ppm Rb)+

1.68(ppm Mo)

0.989 258.891 0.705

Calcined samples

Cab* [6] y=3.64�0.0004(ppm Sr)+

0.02(ppm Rb)+

0.36(ppm Ni)

0.916 61.766 0.885

hab [7] y=66.09+

16.74(ppm Mo)+

0.003(ppm Sr)�0.09(ppm Li)

0.846 31.186 4.910

L* [8] y=92.81�0.03(ppm Rb)�0.03(ppm Li)�0.06(ppm Cr)

0.928 72.622 0.789

y Only in those kaolins fulfilling the requirements for whiteness

(CIE, 1986).


(sometimes stacks or relict vermiform aggregates) and

slightly or ungrouped planar particles as a result of

comminution.

Calcination of the samples (650 8C, 3 h) led to the

disappearance of the kaolinite reflections (band of

amorphous material between 178 and 278 2h) from the

X-ray diagram and to a significant increase (shown by

ANOVA) in the contents of the free phases of Al2O3

and SiO2, as a consequence of the amorphization of

the kaolinite. Some morphological effects of meta-

kaolinization, described as the destruction of the

gibbsite sheet by dehydroxylation (Kakali et al.,

2001; Kaloumenou et al., 1999), were observed with

SEM. These include the opening of layers and the

generation of spaces, the loss of lateral continuity,

fusion of the edges of the crystals and a superficial

coating that appears to be melted onto the surface of

the aggregates.

The quantitative measurement of the relationship

between compositional properties and color param-

eters, the main objective of this study, is demon-

strated using simple and multiple linear regression

equations. The greater content of kaolinite, a white

mineral, is vital for explaining the whiteness of the

kaolins (W10), while higher proportions of mica

decrease whiteness. Correlations are also established

between color parameters and the percentages of

some of the oxides present in the chemical

composition (K2O, SiO2, Fe2O3, MgO and MnO

affect L*, Cab* , hab and W10). Both whiteness and

chroma are correlated with the content of free iron

(%Fe2O3): negatively in the case of whiteness and

positively for chroma.

As a result of calcination, the kaolin reddens

(bhab), acquires more color (increase in Cab* ) and

maintains its lightness (L*); only three samples can

be considered white according to CIE (1986). The

most noteworthy correlation between composition

and color parameters in the calcined samples is seen

with Al2O3, in nearly all its forms (elemental and

free). Thus, for example, the calcined sample is

lighter and has a lower chroma when the contents of

Al2O3 and free oxides extracted with citrate–dithion-

ite–bicarbonate and ammonium oxalate increase.

Metakaolinization, with the consequent destruction

of the gibbsite sheet and the generation of an

amorphized mineral rich in Al partially covering

the particle surface, as shown by SEM, must be

responsible for the close relationship between color

parameters and differing percentages of the various

forms of Al2O3. Finally, highly significant correla-

tions are found between color parameters and trace

element contents.

Acknowledgements

This study was supported by the Spanish Ministry

of Science and Technology, project no. BT Espana

2000–1152. We thank Professor E. Murad and an


anonymous referee for their critical comments on the

manuscript and valuable suggestions.

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