clay materials from the central valley of costa rica and their possible ceramic uses

Applied Clay Science, 1 (1986) 239--254 239 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

C L A Y M A T E R I A L S F R O M T H E C E N T R A L V A L L E Y O F C O S T A R I C A A N D T H E I R P O S S I B L E C E R A M I C U S E S

MARIO BERTOLANI and ANNA G. LOSCHI GHITTONI

Istituto Mineralogia e Petrologia, Universitb, and Centro Ceramico, Modena (Italy) Istituto Mineralogia e Petrologia, Universiti~, Modena (Italy)

(Received January 30, 1985; accepted after revision July 16, 1985)

ABSTRACT

Bertolani, M. and Loschi Ghittoni, A.G., 1986. Clay materials from the Central Valley of Costa Rica and their possible ceramic uses. Appl. Clay Sci., 1 : 239--254.

The clays of the region under investigation are of two principal origins: (1) sedimentary, and (2) residual.

The residual clays derive from hydrothermal or lateritic transformation of volcanic and sedimentary rocks. The marine sedimentary clays appear in the NE part of the region in- vestigated (Peralta area) and, with volcano-sedimentary types, in the area of Col6n. Per- alta clays contain carbonates and montmori l lonite , those of Col6n are illitic.

Lacustrine clays are found in the old basin of Rio Grande, but show total hydrothermal transformation.

The residual hydrothermal clays are very common and abundant as a consequence of the presence, still active, of many warm water springs related to the volcanic structure of the region. These clays are always rich in quartz, are generally kaolinitic, but also contain illite. The residual clays of lateritic transformation are equally rich in quartz, but the prevailing clay mineral is smectite. Amorphous phases are frequently present.

From the industrial point of view kaolinitic and illitic hydrothermal clays are the most interesting as they can be used, with the addit ion of feldspathic volcanic rocks (Costa Rica does not produce feldspar), to make gr~s tiles. The volcano-sedimentary clays can be employed with limestone in the manufacturing of porous tiles.

INTRODUCTION

In A p r i l 1 9 8 3 a r e s e a r c h s t u d y was c o n d u c t e d o n p r i m a r y m a t e r i a l s o f c e r a m i c i n t e r e s t in t h e C e n t r a l V a l l e y o f C o s t a R i c a , w i t h p a r t i c u l a r r e g a r d t o c l a y - b e a r i n g d e p o s i t s . T h e r e s e a r c h was p r o m o t e d a n d f i n a n c e d , as fa r as t h e g r o u n d w o r k was c o n c e r n e d , b y C . E . I . N . S . A . E n g i n e e r R i c a r d o F e r n a n d e z o f C . E . I . N . S . A . a n d P ro f . E n r i q u e Ma lavas s i o f t h e U n i v e r s i t y o f C o s t a R i c a p r o - v i d e d v a l u a b l e h e l p in t h e i d e n t i f i c a t i o n o f p l a c e s a n d d e p o s i t s , a n d in o u t - l i n i n g t h e g e o l o g i c a l b a c k g r o u n d .

0169-1317/86/$03.50 © 1986 Elsevier Science Publishers B.V.

240

GEOGRAPHICAL SETTING

The Central Valley of Costa Rica extends in an E--W direction for 100 km and in a N--S direction from 28 to 57 km. It presents two limits: the Atlan- tic, crossed by the Rio Reventaz6n, and the Pacific, where the Rio Grande runs. The altitudes range from the 240 m of the low valley of the Reven- tazdn and the 200 m of the low valley of the Rio Grande to the 3,432 m of the Cima Torre, near the Irazu Volcano.

In the Central Valley at an altitude of 1,200 m, is San Jose, the capital. The ancient capital of Cartago is about 20 km to the east, at 1,400 m above sea level.

GEOLOGICAL OUTLINE

The territory of Central Valley is predominantly volcanic (see Geological Map at 1:100,000 scale). There are Quaternary and Upper Tertiary vulcanites. In both cases, the lavas range from andesites to basalts (Thorpe et al., 1979, 1981).

Active volcanoes exist in Costa Rica; some of them are on the ridge which borders the north of the Central Valley. The most notable are Irazu {3,364 m) north of Cartago, Poas (2,600 m) northwest of the territory, and Tur- rialba (3,200 m) northwest of the city of the same name (Bullard, 1960).

There are also intrusive rocks, predominantly basic, of Miocene--Pliocene age, not very extensive. There are limited dioritic and gabbroic outcrops south of San Jos~; more extensive outcrops of plutonic rocks ranging from granites to gabbros are found on the southwestern edge and in the high valley of the Rio Grande of Orosi. A gabbroic outcrop is found in the low valley of the Rio Reventaz6n. The intrusive rocks are usually profoundly altered.

The territory is rich in hydrothermal manifestations, which followed the volcanic activity. An example is given by the Rio Agua Caliente, a tributary of the Reventaz6n.

Hydrothermalism has acted on the volcanic and sedimentary rocks pro- ducing predominantly kaolinisation products, which often overlap and can be confused with analogous materials produced by diffuse percolation of water and lateritisation, so much as to make the distinction difficult. A great abundance and diversification of hydrothermally derived clay is found in the volcanic and sedimentary areas.

Sedimentary rocks represent less than a fifth of the territory. The oldest sediments are marine limestones of Eocene age, passing on to Oligocene marls. They are present in the lower valley of the Rio Reventaz6n, Peralta zone. At Pecten, there are limestones of Miocene and Oligocene ages, siltites and lutites which form a strip from 2--4 km wide in the central-south of the territory.

Miocene sediments are more extensive, consisting of sandstones, con-

241

glomerates and limestones. The late Miocene sediments are prevalent, generally sandy, with calcareous lenses, which develop in the central-south of the terri tory and in the southwest margin. There are very limited sandstones of the Middle Miocene towards the Pacific, near Turrucares.

The Pliocene deposits, in part clayey, are in the Peralta zone, in the ex- treme northeast of the territory.

The Quaternary is well-represented, with recent alluvial deposits, of fluvial origin and of ten terraced, occurring throughout the territory. Plio--Pleisto- cene lacustrine deposits are very extensive in the western part, corresponding to the ancient basin of Rio Grande and in the Tej~r zone.

SAMPLING AND METHODS OF STUDY

All the clayey outcrops were sampled. Samples of clays of hydrothermal origin, residual clays formed through processes of decalcification and argil- lification, sedimentary marine clays, and lacustrine clays (more or less af- fected by hydrothermal action) were collected. Samples of rocks from which the clayey material had been derived were also gathered and examined.

The sampling programme was extensive and has the aim of constructing a general picture of the type of clays existing in the Central Valley and of their possible industrial use. A more localized sampling was conducted only for those deposits which presented a greater applied interest after testing.

X-ray diffraction analyses were carried ou t on all the clay samples, on the total rock and on the < 1 ~m fraction, complete with different treatments for the identification of the clay minerals (also using the differential thermal analysis for this purpose).

A great deal of the material was subjected to granulometric and chemical analyses, extending the latter to clayey minerals concentrated by sedimentation. Sintering curves were set out for some minerals of greater ceramic interest. The chemical analyses and part of the technological tests were carried out by the "Centro Ceramico di Bologna, Sezione di Modena".

THE CLAYEY OUTCROPS

Residual hydrothermal clays

Tablbn (Samples 1, 39, 40, 41). The kaolinized material crops out on the left side of the Rio Parires valley, along the Cartago--Corralillo road, as a series of pockets, which, altogether extend for 1500 m. The kaolinized rock is a sandstone of the late Miocene in the lower part of the profile, and a basalt in the higher part. The basalt presents a porphyrit ic structure with big plagioclase phenocrysts in the process of transformation set in a hypocrystalline groundmass. The quartz is probably of secondary origin.

Santa Elena. The deposit is divided into two parts: one higher (1,500 m) on the right side of the Rio Santa Elena, between S. Antonio and Santa

242

Elena Arriba (Samples 13, 50, 51); the other lower (1,300 m) on the left side of the same Rio (Samples 14, 52, 53, 54, 56, 57). It is developed between the bridge on the Rio Santa Elena and Frailes for more than 1 km, with a width of 500 m.

Lourdes. (Samples A, 7, 36, 37). The clayey deposits, which occupy an area of 200 ha, begin with the very first elevations emerging from the Carta- go plain to the height of 1,332 m, and persist up to the contour line 1,460 m. The rock which has been subjected to hydrothermal transformation is a fine sandy sediment of the formation called "Molejon" of the Late Miocene. Materials for ceramic use are extracted from a little quarry in the lower part. Other little quarries supply artisan kilns for bricks and the manufacture of pottery.

Hydrothermal clays, but derived from basic volcanic tufts, also exist at the height of 1,500 m, near Finca Trinidad (Sample 38).

Near Ochomogo (Samples 22, 35) some hills at the height of 1,500 m are concerned with quarries of whitish, hard material, probably derived from andesite, which where it is less altered, presents a porphyrit ic structure with plagioclase phenocrysts, green hornblende, biotite and augitic pyroxene in a hypocrystalline groundmass. Its chemical composit ion is the following: SiO: 63.14, TiO2 0.59, A1203 18.08, Fe20~ 4.66, MnO 0.07, CaO 2.75, MgO 1.14, Na20 2.94, K20 3.04, P2Os 0.21, H:O 3.38.

Near the village of Llano Grande north of Cartago, at an altitude of 2,350 m, there is a quarry which exploits a lens of white-powdered material derived from basaltic rock. The rock, of porphyrit ic structure, has plagioclase, ortho- pyroxene and cl inopyroxene phenocrysts. The groundmass, predominantly plagioclase-bearing, is of fluidal structure. The deposit is of small dimensions.

Enpalme (Sample 26). Small deposit at an altitude of 2,400 m, 16 km from Cartago to the south (road to Cerro de la Muerte). It is derived from kaolinisation of basic Tertiary vulcanites.

Turrialba (Sample 45). The deposit is at an altitude of 1,390 m, near Carmen, above Verbena Norte, It is of a light grey, plastic clay, derived from basic pyroclastites.

Salitr~l (Sample 42) and Cerro Minas (Sample 43). They represent the ridge which separates the valley of the Rio Umca from that of the Rio Oro, southwest of Santa Ana. At Salitral there is a small deposit, derived from sandy sediments of the Middle--Late Miocene, rich in flint nodules, used in the local ceramic handicrafts. At Cerro Minas there is a large deposit, derived from basic Tertiary vulcanites.

Residual lateritic clays showing hydrothermal action

Weathering alteration is frequently added to that of hydrothermal alteration. The deposits at Higuito (Sample 16) and Jeric5 (Sample 21) can be used as examples.

243

The first is about 3 km southeast of Higuito at an altitude of 1,370 m and is used for bricks. It could be derived from limestone. The second is east of JericS, at an alti tude of 1,700 m; it is heterogeneous. It derives from siltites and limestones. It was sampled in such a way as to obtain an average composition.

Residual lateritic clays

These are very widespread, because the tropical climate favors lateritic processes. They derive from all the types of rocks present.

They are found at Bermejo (Samples B, 2, 3, 27, 28), exploited to supply the C.E.I.N.S.A. ceramic works. The clays are often sandy, slightly carbon- aceous. They derive from Miocene limestone, sandy and silty sediments.

The clays on the other side of the ridge, towards Tobosi (Samples 4, 5, 6), are of the same type.

Along the Jeric5 road at an altitude of 1,490 m, on the right side of the Quebrada Tablazo, there is an interesting flaky, hazel-grey clay (Sample 15) which passes towards the southeast to kaolinized clay. These are transforma- tions of limestones and siltites more or less marly and sandy of the Coris Formation of the Late Miocene.

On Cerro Enga~o, at 1,300 m above sea level, southeast of Lourdes, the lateritic type clays, which form a deep cover, are derived from volcanic tuff (Samples 11, 12).

Other residual clays are found at Tei~r (Sample 29) a suburb of Cartago, derived from limestones and siltites of the Coris Formation; at Mu~eco, in the hinterland of Lourdes (Sample 34), at Jest~s Maria, along the Turrialba-- Peralta road (Sample 24), where the argillised formation is the calcareous Eocene called "Las Animas".

Clays of marine sedimentation

The clayey sediments of marine origin are very few and show a notable persistance only in the northeast part of the Central Valley, along the Rio Reventaz6n corresponding to Peralta. They begin at Jes~s Maria and end at Bajo Millas (Samples 25, 30, 31, 32, 33). They belong to the Oligocene Format ion called "Senostri", which is represented by grey marly clay layers with differing percentages of carbonates.

Volcano-sedimentary clays

These outcrop near ColSn between the Quebrada Alhizar and the Que- brada Honda (Samples 44, 58, 59, 60, 61, 62, 63). They are red or grey in colour. Their genesis is not at all clear, being devoid of microfossils. They have been used for a short time as additives for ceramic mixtures.

244

Clays o f lacustrine sedimentation

T h e s e f o r m p a r t o f t h e g r ea t P l i o - - P l e i s t o c e n e l a c u s t r i n e bas in o f t h e R i o

G r a n d e , w h i c h e x t e n d s f o r 40 k m 2. T h e s a m p l e s w e r e t a k e n a t S. Juan de S. Rambn, a b o v e a n d b e l o w an i g n i m b r i t i c leve l ( S a m p l e s 46, 47, 48, 49) . I t is

v e r y l i ke ly t h a t h y d r o t h e r m a l a c t i o n s h a v e b e e n e x e r c i s e d o n t h e s e sedi-

m e n t s .

GRANULOMETRIC COMPOSITION

T h e g r a n u l o m e t r i c ana lyses ca r r i ed o u t a c c o r d i n g t o W e n t w o r t h ' s c lassif i -

c a t i o n a re s h o w n in T a b l e I. C o a r s e f r a c t i o n s a p p e a r o n l y in c l ays o f h y d r o -

t h e r m a l a n d r e s idua l o r ig in , r e p r e s e n t i n g u n a l t e r e d o r p a r t i a l l y a l t e r e d p a r e n t

r o c k . F o r t h e r e m a i n d e r t h e r e is l i t t l e s u b s t a n t i a l g r a n u l o m e t r i c d i f f e r e n c e

b e t w e e n c lays o f d i f f e r e n t genesis .

MINERALOGICAL COMPOSITION

T h e m i n e r a l o g i c a l c o m p o s i t i o n was d e t e r m i n e d p r e d o m i n a n t l y by X R D

ana lys i s b o t h o n t h e t o t a l r o c k a n d o n t h e < 1 p m f r a c t i o n s e p a r a t e d by

TABLE I

Granulometric analysis (%): I. Residual hydrothermal clays; II. Residual lateritic and mixed clays; III. Sedimentary clays

Samples > 2 mm 2--0.250 mm 250--63 u 63--3.8tt 3.8--1 u < 1 tz

1 6.14 7.01 7.90 48.01 14.48 16.46 7 -- 0.33 6.04 36.69 29.42 27.52

13 -- 0.32 2.65 34.85 31.39 30.79 14 0.98 18.45 11.95 33.71 9.00 25.91 16 -- 0.17 0.21 76.76 9.50 13.36

I 38 -- 0.46 0.93 67.72 12.95 17.94 40 -- 24.36 13.43 44.93 11.57 5.71 43 32.14 9.80 4.78 24.54 7.59 21.15 45 -- 0.17 0.76 23.58 14.70 60.79 50 -- 3.24 8.28 34.89 27.74 25.85 57 -- 21.04 16.07 32.00 13.62 17.27

5 -- -- 0.84 60.02 19.08 20.06 11 -- 0.18 4.41 32.82 16.50 46,09

II 15 -- 0.22 1.96 64.96 12.27 20.59 24 -- 0.74 4.32 25.06 20.89 48.99 34 5.62 2.20 0.30 43.23 23.41 25.24

31 -- 1.64 3.11 36.88 30.21 28.16 33 -- 0.22 0.83 41.96 23.93 33.06

III 46 -- 0.26 5.86 65.50 8.35 20.03 63 -- 7.71 26.91 47.70 9.72 7.96

245

sedimentation. Quantitative mineralogical analysis was carried out on some of the samples using the method developed by the Research Institute for Ceramics Technology (IRTEC) of the National Research Council (C.N.R.), Faenza, Italy. The quantitative data were used for the classification of the samples in "main", "subordinate" and "accessory" components. Differ- ential thermal analyses on the clayey fractions helped to the identification of the mineralogical composition. The results are shown in Tables I I - IV .

It can be seen that the hydrothermal clays, including lacustrine clays, are predominantly characterized by a clay mineral of the kaolinite group, but the illitic minerals are not missing, and are sometimes very abundant. The smectite is always scarce or absent. Sometimes quartz is replaced by cristobalite. In the case of Llano Grande the product of the hydrothermalisa- tion is merely amorphous silica.

Residual lateritic clays have smectite as the characteristic clay mineral. The presence of illite and kaolinite is sporadic; cristobalite never appears. The presence of amorphous phases like allophane, was indicated {Harris, 1971; Tan et al., 1975).

TABLE II

Mineralogical composit ion of the residual hydrothermal clays

Samples Main phases Subordinate phases Accessory phases

1 Kaolinite, quartz, illite 39 Quartz, feldspar, kaolinite 40 Quartz, kaolinite, feldspar 41 Quartz, kaolinite Illite 13 Cristobalite, kaolinite 14 Quartz, illite 50 Halloysite 51 Halloysite, cristobalite Smectite 52 Illite, quartz 54 Illite, quartz, kaolinite 56 Quartz Feldspar, kaolinite 57 Illite, quartz Feldspar

A Quartz, kaolinite 7 Quartz, kaolinite Feldspar

36 Quartz, kaolinite 37 Quartz Kaolinite, illite 38 Quartz, smeetite Amorphous phases 22 Quartz, illite 35 Kaolinite, quartz, illite 26 Kaolinite Alunite 45 Halloysite 42 Quartz, kaolinite, illite Smectite 43 Quartz, illite

8 Amorphous phases Cristobalite 9 Amorphous phases

Feldspar Smectite Smectite Smectite

Kaolinite

Jarosite

Illite Vermiculite

Halloysite Cristobalite

Cristobalite

Kaolinite, smectite

246

TABLE III

Mineralogical composition of the sedimentary clays: marine (25, 30, 31, 32, 33); lacustrine (46, 47, 48, 49); volcano-sedimentary (44, 58, 59, 60, 61, 62, 63)


25 Smectite, quartz Calcite 30 Smectite, calcite Quartz, feldspar 31 Smectite, calcite, quartz Feldspar 32 Smectite, calcite Quartz 33 Smectite Quartz, calcite 46 Quartz Halloysite, smectite

47 Halloysite Halloysite, quartz 48 Halloysite Quartz, cristobalite 49 Smectite Halloysite, quartz 44 Quartz, illite

58 Quartz, iltite 59 Quartz, illite

Illite, feldspar, hematite

60 Quartz, feldspar, illite Hematite 61 Quartz, feldspar Vermiculite, hematite 62 Quartz Illite, vermiculite,

hematite

Feldspar

Feldspar Kaolinite, feldspar Illite, feldspar, hematite

Feldspar Feldspar, smectite Cristobalite Vermiculite, kaolinite, hematite Vermiculite, kaolinite Kaolinite, vermiculite, chlorite, hematite

Kaolinite, vermiculite Illite

63 Quartz, feldspar, illite Hematite, vermiculite

TABLE IV

Mineralogical composition of the residual lateritic and mixed clays (16 and 21 are mixed)


B Quartz Feldspar Smectite, calcite 2 Quartz Smectite, amorphous

phases 3 Quartz Feldspar Illite, smectite

27 Quartz Feldspar Illite, smectite 28 Quartz Illite, smectite, feldspar

4 Quartz 5 Quartz 6 Quartz

15 Quartz, illite 11 Quartz 12 Quartz 29 Quartz 34 Quartz, illite 24 Smectite, quartz 16 Quartz, amorphous

phases 21 Quartz

Smectite Amorphous phases Smectite, feldspar Cristobalite, Smectite, feldspar

amorphous phases Feldspar, kaolinite Amorphous phases Smectite Amorphous phases Feldspar, illite Smectite Illite

Feldspar Feldspar, illite

Kaolinite Smectite

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248

T h e v u l c a n o - s e d i m e n t a r y c lays c o n t a i n i l l i te a n d v e r m i c u l i t e as p r e d o m i -

n a n t mine ra l s . T h e q u a n t i t y o f f e l d s p a r p r e s e n t is v e r y var iab le .

S e d i m e n t a r y m a r i n e c l ays are m a i n l y m o n t m o r i l l o n i t e - r i c h a n d c o n t a i n

ca lc i t e .

CHEMICAL COMPOSITION

35 s a m p l e s b e l o n g i n g t o 20 d i f f e r e n t d e p o s i t s w e r e a n a l y s e d . T h e re su l t s

are s h o w n in T a b l e s V - - V I I a n d s u b d i v i d e d i n t o v a r i o u s c l ay c a t e g o r i e s ac-

c o r d i n g t o genesis . A m p l e v a r i a t i o n s in t h e p e r c e n t a g e s o f t h e d i f f e r e n t ele-

TABLE VI

Chemical analysis of the residual lateritic clays

Samples : B 28 4 15 11 12 29 34 24 21

SiO~ 64.08 67.07 59.08 66.23 62.47 61.19 61.01 59.98 60.57 58.20 TiO~ 0.62 0.80 0.84 0.93 0.63 0.80 0.88 1.06 0.75 1.07 Al20 ~ 13.17 16.84 17.92 17.96 18.05 19.38 19.94 21.26 13.51 19.52 Fe203 MnO CaO MgO Na20 K~O P~O~ CO2 H20

4.54 3.82 7.33 3.50 7.35 9.97 4.25 6.48 7.35 9.15 0.04 0.01 0.03 tr 0.01 0.01 0.01 0.01 0.05 0.10 3.71 0.35 0.75 0.14 0.09 0.05 0.04 0.12 0.94 0.11 1.73 1.42 2.87 1.24 1.38 0.58 2.34 1.06 2.04 0.90 0.82 1.76 0.49 1.01 0.20 0.99 0.11 0.13 0.48 0.54 1.59 1.96 1.53 2.29 0.92 0.67 1.82 2.79 1.27 2.05 0.17 0.08 0.06 0.06 0.03 0.04 0.03 0.07 0.08 0.07 2.81 6.72 5.89 9.10 6.64 8.87 6.32 9.57 7.04 12.96 8.29

TABLE VII

Chemical analysis of the sedimentary clays

Marine and volcano-sedimentary samples: Lacustrine samples: 25 30 44 58 60 63 46 48 49

SiO 2 56.94 51.82 57.54 53.85 63.86 58.77 53.23 49.36 50.01 TiO 2 0.70 0.74 0.75 1.38 1.11 0.91 1.11 0.92 1.16 A120 ~ 12.96 12.76 20.56 19.54 14.84 20.96 24.56 27.78 23.32 Fe203 6.75 5.86 8.70 11.96 9.65 9.34 6.39 7.31 10.58 MnO 0.04 0.03 0.06 0.13 0.04 0.19 0.05 0.03 0.04 CaO 7.13 9.58 0.28 1.13 0.93 0.07 0.73 0.07 0.21 MgO 2.20 1.97 2.02 1.47 1.63 1.93 1.44 0.49 1.33 Na~O 0.78 1.49 0.13 2.36 2.13 2.12 0.68 0.07 0.35 K~O 1.11 0.82 4.79 2.28 1.77 2.21 1.34 0.18 0.32 P20~ 0.08 0.12 0.05 0.17 0.14 0.04 0.04 0.02 0.03 H20 6.65 6.60 5.12 5.73 3.90 3.46 10.43 13.77 12.65 CO s 4.66 8.21

249

ments can be noted. For example A1203 is high in clays of the kanditic type, that is, in many hydrothermal and lacustrine clays. The percentage of K20 is generally allied to illite content and presents high values both in some clays of hydrothermal genesis and in some residual clays, as well as in sedimentary marine clays. CaO is generally low and is also very low in clays of hydrothermal genesis and in residual clays, except in the few cases in which the decalcification is not complete. Fe203 always presents high values, so much so as to prevent a classification of the clays studied among those defined as special. The presence of sulphates is rare, verified only in clays of hydrothermal genesis.

TABLE VIII

Chemical analysis of < 1 um fractions

Samples: 2 13 16 24 31 45 57 38

SiO2 61.11 53.15 63.66 63.25 56.58 42.99 46.64 50.08 TiO2 0.43 0.73 0.69 0.72 0.69 1.70 0.20 0.32 A1203 16.93 27.58 19.47 13.98 13.64 37.11 28.85 30.39 Fe203 4.25 4.47 4.54 5.72 4.80 1.01 6.64 2.51 MnO tr 0.01 0.01 0.01 0.01 0.01 0.15 0.01 CaO 1.06 0.08 0.22 1.08 5.65 0.11 0.09 0.06 MgO 2.27 0.35 1.64 2.33 2.43 0.10 2.84 1.48 Na20 0.48 0.52 0.51 0.37 2.75 0.37 1.03 0.54 K20 1.23 0.04 1.28 1.27 0.79 0.05 5.50 2.81 P205 0.04 0.01 0.01 0.04 0.06 0.05 0.04 tr H20 12.20 13.06 7.97 11.23 7.10 16.50 8.02 11.80 CO2 5.50

In Table VIII the chemical analysis of 8 samples of fractions less than 1 ~m in size is shown. At this size, clay minerals are concentrated and quartz and feldspar are removed. In the fine fraction of the Samples 2, 24 and 31, the diffractometric check indicated that the consti tuent is almost exclusively montmoril lonite with remains of calcite in specimen 31; in specimen 13 it is kaolinite; in specimen 45 it is halloysite at 7 A. In sample 57 illite is pre- dominant along with smectite; in sample 38 it is smectite along with illite.

TEMPERATURE GRADIENT KILN

Firing tests, using a temperature gradient kiln were carried out on samples from the clay outcrops of more ceramic interest. The results are shown in Table IX.

Only the clays of Col6n, multimineral materials, rich in iron, have water absorption which, in relation to the sintering process, reach low values (less than 5%) at the temperature of 1,080°C. The illitic--kaolinitic clays of Jeric5 also have low water absorptions at 1,140°C. The kaolinitic, partially

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252

illitic, clays of TablSn sinter at 1,190°C; the kaolinitic--illitic clays of Ocho- mogo and lower S. Elena sinter at 1,200°C and the samples of upper S. Elena, Cerro Minas and Salitr~l, and Lourdes have water absorptions below 5% at temperatures above 1,200°C.

For the ColSn, Jeric5 and TablSn samples shrinkage is limited, i.e., equal to or less than 10%, at the above indicated vitrification temperatures. The Ochomogo and lower S. Elena materials as well as those from Cerro Minas and Salitr~l have shrinkages at the 10% limit. The upper S. Elena sample has considerable shrinkage at the vitrification temperature.

With the exception of the TablSn and lower S. Elena clays, the color of the vitrified products always is red.

These results were predictable from the chemical and mineralogical data for the various samples.

MATERIALS OF GREATER CERAMIC INTEREST

The study of the clayey deposits of the Central Valley of Costa Rica has shown that the clays of residual lateritic type are not suitable for ceramic use, inasmuch as they are essentially smectitic. As is known, the presence of smectite in ceramic mixes, causes very high shrinkage of the piece in both the drying and firing phases. Smectite also imparts high viscosity to the mix which makes this clay unsuitable for use with slip casting and wet grinding techniques. In addition, the residual clays are very rich in quartz. This material is used in ceramic mixes as an " iner t" material or "shortener". At 573°C, however, quartz undergoes a phase transformation from the alpha to the beta phase and a consequent increase in volume. When the percentage of quartz in the mix is very high, this increase in volume results in deformation of the piece and thus high quantities of rejects. Some exceptions occur: for example the deposit on Jeric5 road (Sample 15) has an illitic--kaolinitic composition, favorable for a ceramic mixture, while at the same time it is rich in quartz. This suitability for ceramic mixes for gr~s tiles is also demon- strated by the vitrification tests which show a very low water absorption already at 1,140°C. The shrinkage, which does not exceed 10% can be reduced with the addition of chamotte or " iner t" materials such as andesite or "lapilli" to the batch composition. The same comment could be made for the predominantly illitic Mufieco deposit.

The sedimentary marine clays of the Peralta zone are excluded from ceramic use for the same reason as the residual clays, that is, the predominantly smectitic composition.

This is not an overriding factor for the lacustrine clays. They do in fact contain some smectite, but in quantities subordinate to the halloysite and associated minerals which serve to increase the refractoriness of the product and also condition the swelling potential of the smectite. Their use is con- ditioned, however, by a mixture with other non-smectitic clays or with feldspar in order to obtain gr~s or porcelain-gr~s type products.

253

The hydrothermal clays offer greater possibilities for use in the production of ceramics even though there exists the theoretical possibility of the presence of sulfates, for which the present investigation found evidence only in a few cases. Sulfates, generally in the form of aluntte and jarosite, make the material unsuitable for ceramic use as they cause bloating of the piece during firing as well as damage to the kiln.

The hydrothermal clays are kaolinitic, illitic, kaolinitic--illitic and halloysitic. This allows mixtures to be effected, changing the proport ions needed to temper the refractoriness of the kaolinitic and halloysitic clays with the plasticity of the illitic clays. For example at S. Elena all the clays on the right side of the valley are kaolinitic or halloysitic, those on the left side are illitic or illitic--kaolinitic, with, however, a predominance of the first mineral. Indeed, as can be seen from the data in Table V, the illitic clays from the lower part begin vitrification at 1,140°C, a difference of 50°C with the kaolins and halloysites of the upper part.

At Tabl6n the clays are in general kaolinitic with sporadic occurrences of illite. At Salitr~l the raw material is kaolinitic--illitic. T.he Cerro Minas clays are prevalently kaolinitic. At Ochomogo kaolinitic--iliitic materials prevail (Sample 35), together with predominantly iUitic materials (Sample 22); the important factor is to know the distribution in the outcrop.

An almost pure halloysite, good for refractories is that occurring at Turrialba. The kaolin at Enpalme is also almost pure, but, apart from the small size of the deposit, its use is impaired by the presence of ferrous oxides, but undoubted ly it is interesting for mixing.

The material at Llano Grande, essentially composed of amorphous silica, can be used as an additive instead of quartz in the refractory industry.

All the hydrothermal clays are more suitable for gr~s than for porous products. The high percentage of iron which is reduced to about 1% only at Turrialba and Enpalme, impedes the preparation of light colored gr~s mixtures. Therefore they are to be considered as common clays. However, the absolute lack of carbonates allow the preparation of good mixtures to which feldspar could be added, excluding the quartz--feldspar sands, inasmuch as the quartz is already present in the clay. Feldspar is lacking in Costa Rica, therefore it would be necessary to import it, perhaps even from nearby countries, although a possible substi tute exists in the form of the effusively feldspathic rocks. The andesites of Ochomogo and of Frailes, with sufficient plagioclase and devoid of quartz are possible contenders.

It is possible to use the hydrothermal materials of the Central Valley of Costa Rica for the product ion of ceramic floor- and wall-tiles using technology presently employed by the Italian ceramic tile industry for the production of fast single fired gr~s. They consist of the addition to the mixture of about 20% of volcanic "lapilli" or similar raw materials, to favor the emis- sion of gases during the firing and the entrance of oxygen (Bertolani and Loschi Ghittoni, 1980; Fabbri and Fiori, 1984). There would be few difficulties in Costa Rica in finding "lapilli", for example near craters of the Irazu and Poas volcanoes.

254

One material which has in practice proved suitable for the preparation of ceramic gr~s for the production of tiles is the volcanic sedimentary clay of Col6n, predominantly illitic, feldspathic, and rich in iron oxides. This is shown, in addition to the vitrification tests which indicate a good equilib- rium between water absorption and shrinkage already at 1,080°C, by the present industrial use for the production of porous, glazed tiles, where the material from Col6n is mixed with limestone and kaolinitic clay from Lourdes.

To obtain industrial porous ceramic material both in double- and single- firing, the many existing clays of Peralta cannot be counted upon as their bentonitic nature would create great difficulties in the drying and firing phases.

Apart from the clays of Col6n, it would therefore be necessary for the production of porous material to turn to the hydrothermal clays previously mentioned or to the few suitable residual deposits, mixing kaolinitic clays with illitic clays and adding limestone.

On the whole, the clays of the Central Valley of Costa Rica should be suitable for supplying a ceramic industry oriented towards the production of ceramic floor- and wall-tiles and in particular, single-fired gr~s tiles.

REFERENCES

Bergoeing, J.P., Malavassi, E. and Jimenez, R., 1980. Sintesis geologica del Valle Central de Costa Rica. Inst. Geogr. Nat.

Bertolani, M. and Loschi Ghittoni, A.G., 1980. Alcune osservazioni sulla formazione del cuore nero helle piastreUe da gr~s rosso ottenuto con cottura rapida. Ceramica Inf., 15: 519--521.

Bullard, F.M., 1960. Active volcanoes of Central America. XX ° Congr. Geol. Int., Mexico, 1960, Sec. 1, Vulcanologia del Cenozoico, pp. 351--371.

Fabbri, B. and Fiori, C., 1984. La cinerite un additivo naturale per prevenire il cuore nero. Ceramica Inf., 19: 135--144.

Harris, S.A., 1971. Podsol development on volcanic ash deposits in the Talamanca range, Costa Rica. Paleopedology, Syrup. Age Parent Materials and Soils, Amsterdam, 1970, pp. 191--209.

Tan, K.H., Perkins, H.F. and Mc Creery, R.A., 1975. Amorphous and cristalline clays in volcanic ash soils o f Indonesia and Costa Rica. Soil Sci., 119: 431--440.

Thorpe, R., Francis, P.W. and Moorbatb, S., 1979. Strontium isotope evidence for petro- genesis o f Central American andesites. Nature, 277 : 44--45.

Thorpe, R., Locke, C.A., Brown, G.C., Francis, P.W. and Randal, M., 1981. Magma chamber below Po~s Volcano, Costa Rica. J. Geol. Soc., 138: 367--373.

clay materials from the central valley of costa rica and their possible ceramic uses

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