Coexistence of halloysite and kaolinite – a study on the genesis of kaolin clays of Campo Alegre Basin , Santa Catarina State , Brazil

Kaolin atCampoAlegreBasin, SantaCatarinaState, Brazilwas formed fromalteration of volcanic acid rocks. Halloysite clays dominate the clay fraction of the matrix of the kaolin body, whereas a poorly crystalline kaolinite is abundant in veins. Some primary blocky structures have high amounts of illite, in one mine, but in general, only low contents of illite-smectite, illite, chlorite-vermiculite, vermiculite and quartz were identified in the clay fraction of the samples. Toward the top of themines, hematite and lepidocrocite appear in horizontal red and ochre colored levels and the amount of kaolinite increases compared to halloysite. The vertical zoning of alteration levels, the changes in mineralogy, the positive correlation between depth and Cation Exchange Capacity of the clays, the preservation of different types of rock textures in the kaolin bodies, the dominant tube morphology of the halloysite clays indicate a supergene genesis for the deposits. Criteria to distinguish between supergene and hypogene kaolin are discussed. Transmission Electron Microscopy of the cross sections of halloysite tubes showed polygonal forms that are ascribed to be transitional between kaolinite and halloysite. It is proposed that some of the kaolinite of these deposits be inherited from the dehydration of halloysite tubes.


INTRODUCTION
Campo Alegre Basin is an important source of clays for the ceramic market of the Mercosul and a place of a welldeveloped ceramic industry; however, these clays have been little studied.Kaolin deposits of Campo Alegre Basin resulted from the alteration of acid rocks of the upper volcanic unit of Campo Alegre Formation (Neoproterozoic).The bedrock is composed of rhyolitic and trachytic lava interspersed with pyroclastic material ei-ther coarse or thinly laminated (Ebert 1971, Daitx andCarvalho 1980).The origin of these deposits is controversial: Biondi et al. (1999) stated they were formed by hydrothermal activity; Oliveira et al. (2000) showed coexistence of halloysite and kaolinite in one mine of the deposit and also showed evidences of a supergene origin.Few data on the mineralogy, chemistry and morphology of the clays are available.

The objectives of this work are:
-To identify and characterize the kaolin clays of this basin (location, macroscopic characteristics, min-eralogy, chemistry, cation exchange capacity, morphology) applying several methods which is useful for mineral exploration.
-Present new data on the crystal morphology of the kaolin clays of this deposit, and in particular to describe transitional forms between halloysite and kaolinite, and implications for the interpretation of geological history.

LOCATION AND GEOLOGICAL SETTING
Kaolin was formed from the alteration of volcanic acid rocks, near São Bento do Sul, Santa Catarina State, Brazil (Fig. 1).These rocks are components of Campo Alegre volcanic-sedimentary basin (Citroni et al. 2001) of almost 400 km 2 .It is a Neoproterozoic sequence (595 ± 16 Ma, Cordani et al. 1999) lying on older Archean granulite terranes.Rhyolite is the most abundant lithotype (Weischel et al. 2000), but the rocks are heterogeneous: trachytes, porphyritic rhyolites, banded rhyolites, tuffs, breccia.The Lower Effusive Sequence is composed of basalt and trachyte.The Intermediary Epiclastic-volcanoclastic Sequence is made of shale and volcanogenic sedimentary and pyroclastic flow.The Upper Volcanic Sequence (80% of the exposed area) of late Proterozoic age consists of trachytes and rhyolites.Pyroclastic flow deposits are common as welded high temperature ignimbrites (Weischel et al. 2000).Old rocks in a plateau up to ±1000 m above sea level, fractures and faults are evidences of a regional uplift (Potter 1997) in the area.Thick (20-40 m) deposits of clays evolve from the surface rocks.Three kaolin mines in Campo Alegre Basin were studied: Oxford (or Floresta) Mine, Incepa (or Cambuí) Mine and Ceramarte Mine (Table I).Ceramarte Mine (26 • 15 21"S-49 • 12 55"W) is located in a down-slope landscape near the centre of Campo Alegre Basin.The mine is 7 to 17 meters deep and exposes differentiated levels over the altered porphyritic rhyolite.At the base the rhyolite alters to a saprolith horizon that maintains the rock texture; the saprock has elongated core boulders and preserves the rhyolite texture and veins of white clays.As the alteration evolves, a network of clayey white segregations becomes denser within a light pink matrix and the rock texture disappears.An oxidate facies with yellow and red spots develops in the clay zone about 3 to 7.5 meters up.The clay zone has anastomosing veins composed of white clays.Black organic soil develops over the white clays.
Incepa Mine (26 • 17 27"S-49 • 16 48"W) is approximately 30 m deep.At the base of the mine there are some relicts of the rock (aphanite) in a light greenish gray clayey matrix cut by fractures, coated with white clays.At some points higher in the profile, the matrix shows a very fine (less than 1 mm) laminated white and pale red structure with some very coarse (more than 10 mm) white granular structures.At the top of the sequence, located in a geomorphic depression, there is a 2 m thick black-colored soil.
Oxford Mine (26 • 19 54"S-49 • 12 50"W) is the third studied occurrence, in the south of Campo Alegre Basin.Kaolin forms from a banded rock with flow structure planes alternating with more or less clayey material; along the planes there are black coatings.Towards the surface, the rock turns into a white to gray clay with rounded or sub-angular coarse blocky structures (up to 50 cm diameter) of green clay.Veins coated with white clay cut all the matrix of the mine walls.Higher in the profile, there is a red level (1 m thick) which is followed by an ochre level (1.5 m thick), which in turn is covered by a white clay soil with black roots.

MATERIALS AND METHODS
Twenty-six samples from the three mines were studied: 7 from Incepa, 8 from Oxford and 11 from Ceramarte Mine.They were split into sub-samples for various methods of investigation.
Thin sections of impregnated samples were studied using scanning electron microscopy.
The clay (< 0.002 mm) fraction of each sample was removed after settling under gravity according to Stock's Law in an aqueous solution of 33 g of sodium hexametaphosphate and 7 g of anhydrous sodium carbonate in distilled water (1 liter).Two oriented aggregates of each sample were prepared for Ba saturation and Mg saturation.
Cation exchange capacity (CEC) was determined by barium saturation as described by Janik and Keeling (1996).The oriented samples saturated with barium (1M BaCl 2 ) solution were washed several times with deionized water.Then the samples were dried and analyzed  by XRF to determine barium content and hence CEC.The Mg saturated samples for X-ray Diffraction analysis were dried in air, ethylene glycol solvated, formamide treated, and heated up to 110 • , 350 • and 550 • C. Quantitative analyses of the clay separates were done with Siroquant TM software, using X-ray diffraction of random preparations.
Chemical analyses of 11 bulk samples were obtained with X-ray fluorescence in the Geology Department, ANU.
Clays deposited from ethanol suspension onto aluminum stubs, were dried at 40 • C and then gold coated (20ηm) for observation in a Cambridge S360 SEM with Tracor Northern EDXA.
Analysis of particle shape was made by applying NIH Image software to Adobe PhotoShop images of SEM photomicrographs.From each particle NIH image provided area and the length of the major and minor axes of the best fitting ellipse.
For TEM, small fragments of the samples were im- pregnated with Spurr resin.After two weeks the samples were heated up to 60 • C for 24 hours, then cut with a Reichelrt ultramicrotome, mounted on copper grids and coated with carbon.A Philips 430 EM, 300 kV at EMU-ANU and a Philips CM TEM 300 kV, Gatan Camera, at the Research School of Earth Sciences (ANU) were used.

XRD-MINERALOGICAL COMPOSITION
Data were obtained by quantification on the X-ray diffractograms of random preparations of the clays analyzed with the SIROQUANT software and are shown in Table II as well as the name and the character of the samples (rock, regolith, vein, soil, etc).
In the three kaolin bodies, kaolin minerals halloysite and kaolinite are the main constituents.Exceptions are samples Ox1, Ce2m, Ce3b that correspond to saprocks with high percentages of K-feldspar; sample Ox2 has high percentage of illite.Figure 2 shows the overall distribution of the minerals along the three mines obtained from averages of the % mineralogy of Table II.Halloysite is the most abundant clay mineral in the three mines.Only in two samples of veins (Ce2av and Inc5, Table II) the amounts of kaolinite are higher than halloysite.
The XRD diffractograms of oriented, air-dried preparations of representative samples of the three mines are presented in Figure 3.
Toward the top of the mines, iron oxides concentrate in a red (hematite) level followed (in Oxford Mine) by an ochre (lepidocrocite) level.Also some other minor mineral components were detected: gibbsite and anatase (Oxford Mine); cristobalite (Ceramarte Mine).
CHEMISTRY OF CLAYS XRF analyses of 11 bulk samples of the three mines are shown in Table IIIa.Chemical analyses of bulk samples showed that the veins have kaolinitic composition, the samples of the matrix have high amounts of SiO 2 due to quartz and the green clay of the blocky structures from Oxford Mine has higher K 2 O content than the other samples.
Fe 2 O 3 contents vary from 3.15 wt.% in the greenclay sample from Oxford Mine, to 0.185 wt.%, in the veins of Incepa Mine.
TiO 2 values are from 0.51 to 0.01 wt.%.The lower values are from samples of veins in the base of the mines and the higher ones are from samples from the top of the deposits.
The K 2 O content composition may be a good mea-sure of the degree of the alteration of the primary minerals.The lower the K 2 O value the high alteration degree of the sample.There is a positive correlation between CEC and depth in the Ceramarte Mine (Fig. 5 and Table III) as already seen with K 2 O content, but not for the other mines.The average CEC of Oxford Mine clays is the highest (11.2 meq/100g) and may be related to the presence of some Illite-Smectite.The average CEC of Ceramarte clays (10.3 meq/100 g) may be due to the observed very small length of halloysite tubes (mode 0.46µm in the veins of the rock; 0.22µm in the veins of the alterite).Clays from Incepa Mine have the lowest CEC average value (5.6 meq/100 g), possibly related to the highest tube width (mode 0.46µm).

MICROTEXTURES OF THE CLAYS AND ELEMENTAL COMPOSITION
Visible differentiations from the walls of each mine (Table I) were sampled.SEM observations and EDX Analysis of the small fragments and the thin sections were done in order to add new data on the mineralogy and composition of the bulk samples (Table IV).
Ceramarte Mine facies are composed of tubular halloysite and platy xenomorphic crystals of kaolinite (Si=Al), with tubes predominating in the matrix, with few spheres, and platy xenomorphic crystals of kaolinite forming (iron-rich) stacks in the veins (Ce2v).The iron was detected by the EDX Analyses with the SEM (Fig. 6).Exceptions are some hard white veins of halloysite in the rhyolite where platy crystals were not ob- served (Ce1Av).Inherited grains of K-feldspar appear at the base of the mine showing differential alteration as "etch-pit formation" (Fig. 7A), but disappear throughout the top.The spherulitic texture of the rhyolite, due to crystallization of the volcanic glass, is also observed in the samples from the base of the Mine both in thin section and in small fragments and is related to the very fast cooling rate of the high temperature acid flows.The arrow in the thin section photomicrograph (Fig. 7B) points the spherulitic texture of the rhyolite.In a small fragment the porous structure of the spherulite, made of neddles of quartz (Fig. 7C) contains halloysite aggregates.
In the pallid zone of Ceramarte Mine an aggregate of halloysite tubes that apparently merge into kaolin stacks was observed (Fig. 7D).
Incepa Mine facies are composed of tubular and platy xenomorphic crystals of halloysite and kaolinite (Si, Al), some platy xenomorphic crystals of illite (Si > Al K > Fe), and laths of Vermiculite.Some relicts of the rock (aphanite) are indurated with quartz box-work.
A finely laminated white and pale red facies at 4 m depth from the surface is seen by SEM to be caused by layers of halloysite tubes and spheres intercalated with kaolinite plates (Fig. 7E).
Oxford Mine displays the greatest diversity of clay particles.The Gley 8/5BG clays with the blocky structure (16 m depth) showed dominantly platy xenomorphic crystals of illite (Si > Al K > Fe), little illitic material of platy xenomorphic crystals without iron (Si > Al K) and lath-shaped crystals with irregular ends (Si > Al K > Fe) (Fig. 7F).Also small amounts of Mg, Ti and Zr were detected in these clays.The wall of the mine (matrix) is a light greenishgray clay composed of tubular and platy xenomorphic crystals of halloysite and kaolinite (Si, Al), pyramidal crystals of quartz (beta-form), platy xenomorphic crystals of illite (Si > Al K > Fe) and lath-shaped crystals with irregular ends (Si Al).
The white veins in the wall (11 m depth) are composed of platy xenomorphic crystals of kaolinite, tubes of halloysite (Si=Al) and platy xenomorphic crystals of illite (Si > Al K > Fe).These clays also showed very small amounts of Mg, and Ti and Pb.
The red level (2 m depth) is composed of tubes of halloysite (0.3µm width) and platy xenomorphic crystals of halloysite and kaolinite.Other components as hematite and anatase were not observed at this scale of magnification (9.000-14.000x).
Finally the yellowish brown level (1.5 m depth) almost at the top of mine is composed of tubes and platy xenomorphic crystals (Si = Al > Fe K > Ti) of halloysite and kaolinite.The Fe and Ti detected in these particles may be related to very small amounts of lepidocrocite and anatase observed in the XRD analysis.
The white soil (Gley 8/N) at 1m depth is composed of tubes and platy xenomorphic crystals of halloysite and kaolinite and small quantity of laths with regular ends (Si > Al K > Fe or Si = Al Ti > Fe) of vermiculite.Also Ti (anatase) was detected mixed in the clays.

MORPHOLOGY OF THE PARTICLES
Halloysite exhibits tubular, spherical and cone-shaped morphologies throughout the clay fraction (< 2µm) of the three mines.Arrows of Figure 8 points to: Tubes are the most common, and they often show some flat surfaces (Fig. 8A).There are very small (0.14µm) tubes (Fig. 8B), short tubes (0.8µm), and long tubes (3µm).The spherical halloysite appears in all the three mines.The spheres are very small (< 0.5µm) and are interspersed with the other particles (Fig. 8C), except in the Incepa Mine, where they form layers. Cone morphology and partly rolled tubes were also observed (Fig. 8D).
The planar faces of some halloysite tubes (Fig. 9A) were also investigated through observations of the cross-An Acad Bras Cienc (2007) 79 (4) sections of tubes.TEM showed different types of crosssections: i) mixed curved and linear external shape with a circular internal hole and pores between the smoothly curved layers (Fig. 9B), polygonal with a polygonal in-ternal hole (Fig. 9C), ii) partly unrolled (or partly rolled) cross sections (Fig. 9D).Also circular cross-sections with pores between the smoothly curved layers (Fig. 9E) were observed.

MEASUREMENTS OF HALLOYSITE PARTICLES
The clay separates of three samples of Ceramarte Mine were chosen because of their different values of CEC and because of their location in the profile.One sample of Incepa Mine that showed a low CEC value (4.4) was also statistically analyzed in order to obtain quantitative morphological data of the length and width of the halloysite particles.Also a sample from Oxford mine was analyzed.Statistic data are recorded in micrometers in Table V. Sample Ce1v is from white clays in a vein in the saprock, Ce2v is from very pale brown clay in a vein in the saprolite; sample Inc4a,b is from a laminated matrix of Incepa Mine.Also the clay sample Ce9 that is from the upper pallid zone was analyzed.Curves representing the minimum, mean, median, mode and maximum tube length for each sample are in Figure 10A and tube width in Figure 10B.Halloysite and kaolinite coexist in Campo Alegre as well as in several other kaolin deposits (Murray and Keller 1993).Why both minerals occur?Is there any relationship between halloysite and kaolinite with different precursors?Are they related to different microenvironments?Jeong (1998) observed that genetic relation between the two minerals in the weathering environment is rarely established.
Felsic rocks of Campo Alegre Basin show little mineralogical variation: sanidine and quartz phenocrysts are ubiquitous; arfvedsonite and iron oxides may be present; the matrix is a fine-grained quartz feldspar aggregate.As K-feldspar is the major primary alumino-sili-An Acad Bras Cienc (2007) 79 (4) Fig. 9 -Tubes are the most common morphology for halloysite observed using SEM.Planar surface (A) are common.TEM observations of cross-sections of the tubes showed curved shape as well as polygonal shape in the same particle (B).Other cross sections of tubes are polygonal (C).Some unrolled cross sections form plates (D).Also circular cross-sections with pores between the smoothly curved layers (E) were observed.cate of the rocks, it can be assumed it is the kaolin precursor.Under conditions of good drainage in the humid tropics, K-feldspar weathers to give hollow tubes of halloysite (Parham 1969).Also Banfield and Eggleton (1990) observed a mineralogical pathway in K-feldspar weathering, starting in etch pits and cracks with the formation of cell-textured materials of allophane composition, smectite, a second protocrystalline material and spherical halloysite.After dissolution formed networks of cavities, Al and Si precipitated from solution and resulted the growth of tubular halloysite.
The origin of the kaolinite needs other interpretations.In Campo Alegre kaolin deposits, kaolinite occurs as (iron-rich) stacks filling veins either in the rocks or in the saprolite, or as tight aggregates in the saprolite.At Incepa and Oxford Mines also illite and illite-smectite (inherited from pyroclastic rocks) are mixed in the matrix and could be remaining of a hydrothermal process.Banfield and Eggleton (1990) showed that kaolinite develops from an illitic material -(Si > Al K > Fe) by crystallizing epitactically.The weathering releases K and the crystals acquire irregular shape and small quantities of iron.But there is no muscovite or illitic material at Ceramarte Mine to explain kaolinite origin.In this case, the veins, where iron-rich kaolinite occurs, are microenvironments of highly weathered material inherited from the top of the weathering profile.Kaolinite appears in stacks with small halloysite tubes between the kaolinite plates.In an experimental transformation, Singh and MacKinnon (1996) caused hydrated Georgia kaolinite plates to roll, forming tubes with both smoothly curved layers and planar faces, as is commonly observed in natural halloysites.Kaolinite rolling in the solid state was observed in natural samples by Robertson and Eggleton (1991) ascribed to transitional forms between kaolinite and halloysite.The reverse case, unrolling of hal- loysite tubes, is also possible.Singh (1996) explained how the 1:1 structure switches from halloysite to kaolinite: as a result of prolonged dehydration, hydrogen bonding between layers gradually became stronger, and the structure shifts from rolling mechanism to tetrahedral rotation, causing tubes to produce laths by unrolling.Wouatong et al. (1996) observed this morphological transformation (unrolled spherical-shaped halloysite to tubular and finally unrolled again producing platy kaolinite) in a granite saprolite.Present study observed the following sequence of morphological changes in halloysite particles (Fig. 11) in a weathering profile: Tubes -Tubes with planar faces and internal cylindrical holes -Tubes with planar faces and internal hole with planar walls-unrolled tubes-plates.The correlation of the forms with depth is not linear.All the forms occur along the weathering profile, but the transitional forms between tubes and plates were observed most frequently in the samples near the surface.
The end members of the solid-state halloysite to kaolinite transformation at Campo Alegre basin are low Hinckley Index and very broad basal peaks kaolinites, similar to the end-members kaolinites described by Jeong (1998).All observations agree that halloysite is a metastable mineral at Campo Alegre basin, and transform to stable kaolinite with aging (weathering).

CONCLUSIONS
This study resulted in the following conclusions: -The kaolin from Campo Alegre Basin is composed of halloysite (10 Å and 7 Å) and disordered kaolinite.Halloysite content is higher than kaolinite, except in some veins; low abundant minerals are quartz, K-feldspar, illite, illite-smectite, vermiculite, anatase, hematite and lepidocrocite.
-K-feldspar weathering is the starting point of the halloysite formation as hollow tube particles with -Kaolinite is interpreted to have two likely origins: the weathering of illitic material from pyroclastic rocks (Incepa and Oxford Mine) and the dehydration of halloysite at or near the upper part of the weathering profiles.The observed sequence of morphological changes in halloysite to kaolinite particles is -tubes -tubes with planar faces and internal cylindrical holes -tubes with planar faces and internal hole with planar walls -unrolled tubes -plates.

ACKNOWLEDGMENTS
The authors thank Electronic Microscopy Unit of the Australian National University, Dr David Lewellyn help with the TEM, Robin Westcott and Alain Meunier.

Fig. 1 -
Fig. 1 -Location and geology of Campo Alegre volcanic-sedimentary basin.The Lower Effusive Sequence is composed of basalt and trachyte.The Intermediary Epiclastic-volcanoclastic Sequence is made of shale and volcanogenic sedimentary and pyroclastic flow.The Upper Volcanic consists of trachytes and rhyolites.Three kaolin mines were studied: Oxford (or Floresta) Mine, Incepa (or Cambuí) Mine and Ceramarte Mine.

Fig. 2 -
Fig. 2 -The overall distribution of the minerals along the three mines.The lines represent the average of mineral % in each mine.Av = average, Inc = Incepa, Ce = Ceramarte, Ox = Oxford.

Fig. 4 -
Fig. 4 -K 2 O content used as a measure of the degree of the alteration of the primary minerals in the mines.

Fig. 5 -
Fig. 5 -(A) correlation between CEC of the clays and depth in Ceramarte Mine and (B) the variation of CEC with depth in the three Mines.

Fig. 6 -
Fig. 6 -Iron-rich platy crystals of kaolinite forming stacks in the veins (CE2v).The iron was detected by the EDX Analyses with the SEM.

Fig. 7 -
Fig. 7 -Microtextures of the clays: (A) inherited grains of K-feldspar at the base of Ceramarte mine showing "etch-pit formations"; (B) the arrow in the thin section photomicrograph points the spherulitic texture of the rhyolite; (C) in a small fragment, the porous structure of the spherulite, made of neddles of quartz, contains halloysite aggregates; (D) in the pallid zone of Ceramarte Mine it was observed that an aggregate of halloysite tubes apparently merge into kaolin stacks; (E) the stratification at Incepa saprolite is due to layers of halloysite tubes and spheres intercalated with kaolinite plates; (F) blocky structures in Oxford mine showed platy xenomorphic crystals of illite and also lath-shaped crystals with irregular ends.Photomicrographs D, E and F have graphic scale = 1µm.
Figure 10A shows that particles of sample CE2v have the lowest statistical values of the four samples, except the value of max length.Particles of sample Inc4a,b have the highest statistical values of the four samples, except the value of mode length.
Figure 10B also shows that particles of sample CE2v have the lowest statistical values of the four samples, except the value of max width.Particles of sample Inc4a,b have the highest value of maximum width.COEXISTENCE OF HALLOYSITE AND KAOLINITE

An
Fig. 11 -Observed morphological changes in the halloysite particles: Tubes (A), tubes with planar faces with internal cylindrical holes (B), tubes with internal and external planar faces (C), unrolling tubes (D, E, F).The sequence of changes in morphology (A→B→C→D→E) due to dehydration of halloysite, evolves into platy kaolin particles.Small numbers correspond to position in meters of depth in the profile.

TABLE II The kaolin samples from Campo Alegre Basin.
*samples that were not quantifiable with siroquant, x represents Table IIIb shows the K 2 O content of the clay fraction from samples from Ceramarte Mine, Incepa Mine and Oxford Mine.It indicates that Ceramarte Mine has a high degree of alteration but not the highest because at Incepa and Oxford the presence if illite, which is more resistent than K-feldspar (of Ceramarte), may invalidate this measure of comparison.Figure4shows the relation between K 2 O content and the position of the samples of Table IIb in the profiles.Samples of Ceramarte Mine are heterogeneous; the veins have low values and the matrix has decreas-

TABLE III -a XRF analysis (wt. %) of major elements of 11 bulk samples from kaolin clays of Campo Alegre Basin.
follow this trend, because it still has some illite.The av-

TABLE III -b XRF analysis (wt. %) of K 2 O clay fractions of samples from kaolin clays of Campo Alegre Basin.
erage K 2 O content of the clay fraction from Ceramarte Mine is 0.97 wt.%; of Incepa Mine is 1.24 wt.%, and of Oxford Mine is 1.34 wt.%.