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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
C1-2 Arguisuelas. Cuenca Washed Pressed 0.69 0.01 1.41
C1-3 Arguisuelas. Cuenca Washed Pressed 0.68 0.21 1.53
C2-1 Villagarcia de Arosa. Pontevedra Raw Fragments 2.97 0.21 0.90
C2-2 Villagarcia de Arosa. Pontevedra Raw Fragments 2.94 0.13 0.90
C2-3 Villagarcia de Arosa. Pontevedra Raw Fragments 2.94 0.10 0.86
C3-1 Cervo. Lugo Raw Granular 0.79 0.17 0.79
C3-2 Cervo. Lugo Raw Granular 0.65 0.33 1.08
C3-3 Cervo. Lugo Raw Granular 0.72 0.07 0.91
C4-1 La Guardia. Pontevedra Washed Pressed 0.79 0.12 1.10
C4-2 La Guardia. Pontevedra Washed Pressed 0.78 0.05 0.55
C4-3 La Guardia. Pontevedra Washed Pressed 0.80 0.12 0.80
C5-1 Oviedo Raw Fragments 0.29 0.16 0.65
C5-2 Oviedo Raw Fragments 0.38 0.17 0.77
C5-3 Oviedo Raw Fragments 0.37 0.17 0.61
C6-A1 Burela. Lugo Washed Pulverized 1.22 0.24 0.99
C6-A2 Burela. Lugo Washed Pulverized 1.27 0.29 1.11
C6-A3 Burela. Lugo Washed Pulverized 1.30 0.24 0.95
C6-B1 Burela. Lugo Washed Pulverized 1.28 0.41 0.91
C6-B2 Burela. Lugo Washed Pulverized 1.29 0.40 0.75
C6-B3 Burela. Lugo Washed Pulverized 1.27 0.30 0.95
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).
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282272
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.).
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282 273
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
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282274
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.
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282 275
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.
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282276
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.).
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282 277
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.
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282278
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.
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282 279
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
Equation R2 F Estimated
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
Equation R2 F Estimated
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
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282280
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
Equation R2 F Estimated
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).
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282 281
(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
E. Gamiz et al. / Applied Clay Science 28 (2005) 269–282282
anonymous referee for their critical comments on the
manuscript and valuable suggestions.
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