MINERALOGY, GEOCHEMISTRY AND UTILIZATION … 49/49-4-355.pdf · Ferruginous clay Gray carbonaceous clay Lignite Sand Residual white clay Precambrian hard rock clay ... ed in thick

Clays and Clay Minerals, Vol. 49, No. 4. 355-369. 2001.

MINERALOGY, GEOCHEMISTRY AND UTILIZATION STUDY OF THE MADAYI KAOLIN DEPOSIT, NORTH KERALA, INDIA

C.S. MANJU l, V. NARAYANAN NAIR 2 AND M. LALITHAMBIKA l

~Regional Research Laboratory (CSIR), Industrial Estate RO., Thiruvananthapuram 695019, Kerala, India 2Department of Geology, University of Kerala, Thiruvananthapuram, Kerala, India

Abst rac t The Madayi clay deposit consists of a thick sequence of residual white kaolinitic clay underlying the sedimentary Warkallai Formation, which includes gray carbonaceous kaolinitic clays, lignite, ferruginous kaolinitic clays, laterite and bauxite with ferricretes. The conditions of clay genesis and the economic significance of the major residual kaolin seam have been investigated. The raw clay and <2 txm fractions were subjected to X-ray diffraction (XRD), chemical analysis, differential thermal analysis (DTA), Fourier transform infrared (FTIR) spectroscopic and scanning electron microscopic (SEM) studies. The firing behavior of the <45 /xm fraction of the major residual clay sequence (L), was investigated systematically to determine the potential industrial use of this kaolin.

Geochemical and morphological studies of different strata indicate the following conditions for clay formation: (1) intense lateritized weathering conditions for kaolinization of the residual white clay from parent quartzo-felspathic mica-gneiss; (2) reducing environment for the gray carbonaceous layers; and (3) oxidizing environment for the uppermost hematite-rich ferruginous clay. Pyrite/marcasite enriched detrital gray carbonaceous clay shows two distinct environments for in situ kaolinite crystallization: (1) within plant fossils influenced by the high organic content and FeS~ leaching; and (2) precipitation from solution.

Incomplete kaolinization of white residual clay is evident from the presence of pyrophyllite, muscovite with lenticular cleavage void and a lower percentage of fines (<2 ixm). The plant fossils from the uppermost portion of residual clay show pyrite mineralization. The Hinckley Index, FTIR and rare earth analysis point towards diverse geochemical environments of deposition and technological evaluation indicates its suitability for application in the ceramics industry.

Key Words--Genet ic Environment, Gray Carbonaceous Clay, Hematite-Rich Clay, Madayi, Morphology, Recrystallization, Residual Kaolinite, Technological Evaluation, Trace Elements.

I N T R O D U C T I O N G E O L O G I C A L S E T T I N G

Kaol ini t ic clays are geochemica l ly and indust r ia l ly ve ry versati le. The major i ty of the k n o w n clay deposits in the southern par t of the Indian Pen insu la are of s ed imen ta ry or res idual origin. These kaol ini t ic clays fo rmed under t ropical wea the r ing condi t ions character ized by al ternate wet and dry seasons wi th h igh hu- midi ty and modera te ly h igh temperatures . These condi t ions wi th per iodic rainfal l p romote rapid chemica l b reak up of the parent material . A sys temat ic s tudy was carr ied out to show the var ia t ions in chemis t ry , s t ructure and m o r p h o l o g y o f kaol in i te col lec ted f rom the deposi t at M a d a y i vil lage, ( 7 5 ~ 1 7 6

12~176 (Figure 1A) in Kannur district, Nor the rn Kerala. The analysis , by var ious techniques , of clays col lec ted f rom dif ferent layers gives a deta i led pic ture of the minera log ica l a s semblages and the nature of the minera l species. A n a t tempt was made to define the process that leads to the fo rmat ion of the genet ica l ly di f ferent var ie t ies of kaol in i te wi th in the same deposi t by charac te r iz ing di f ferent clay layers (Figure 1C). Labora to ry tests were also conduc ted to ident ify poss ib le uses of the ch ina clay seam L (Fig- ure 1C), wh ich is industr ialIy viable , hav ing a rese rve of - -17 mi l l ion tonnes wi th in an area o f 1.2 sq. km.

Geological ly , the area consis ts o f P r ecambr i an crystal l ine rock over la in u n c o n f o r m a b l y by a th ick sequence o f the Warkal la i Fo rma t ion of M i o c e n e age. Th ick deposi ts of res idual and sed imen ta ry clay se- quences , hav ing an overal l th ickness of 45 m are associated wi th these. This clay deposi t is conf ined to the southern par t of the Tert iary sed imen ta ry basin, wh ich ex tends f rom K a n n u r to Kasa rgod .The extent o f the di f ferent types of clays in this loca t ion was identified by the D e p a r t m e n t of M i n i n g and Geology, Ker- ala ( internal report , 1995) (Figure 1B). S ince it was found to be a potent ia l ly v iab le deposi t , de ta i led in- ves t iga t ion was under taken.

The sed imen ta ry clays o f this reg ion be long to the Warkal la i Fo rma t ion o f M i o c e n e age w h i c h is character ized by a thin layer of sand, a l ternate layers of ca rbonaceous clays and l ignite seams fo l lowed by Fe- r ich var iega ted clay, lateri te and bauxi te wi th ferr icre- tes at the top (Figure 1C). The topmos t clay layer L16 is a s t rong red co lour and is - -0.6 m thick. The carbonaceous clay layers, L15, L13, L11, L9, L7, L5 and L3 al ternate wi th l ignite seams L14, L12, L10, L8, L6, L4 and L2 fo rming an overal l th ickness of --17.5 m. The upper two ca rbonaceous clay seams L15 and L13 show mot t l ed reddish ye l low patches . B lack co lored patches can be seen wi th in the lower L9, L7 and L3

Copyright �9 2001, The Clay Minerals Society 355

356

75 ~

Kasargod

Maniu et aL

7 6 ~

Scale 1 cm:21 km f

Clays and Clay Minerals

12 ~ Madayii ~

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Tertiary

Precambrian

~ ' ;i~ I 7 - t . . . . . . Boundary of Tertiary formation

. . . . . Geological boundary of l ignite s t .......... Geological boundary of China clay I

O Ponds

/Pazhayangadi

/ r iver Scale 1:16000 / /

Figure. 1. (A) Schematic representation of the area under study; (B) geological map of Madayi; (C) schematic cross-section of Madayi deposit.

Vol. 49, No. 4, 2001 Madayi kaolin: geochemistry, mineralogy and utilization 357

8

I

Figure. 1. Continued.

0

5

10

15;

,2o

,.~ 20,

25

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4 5 ,

l i b O

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L16 L15 L14 .~ L13 ~, L12 L l l ,= LIO k9 g~ L8 L7

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L5

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1

Scale

l c m =2.5 rn

Depth from

0--12 12.3

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top (m)

Laterite, bauxite and ferricret~s

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L15

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L l l

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L7

L5

L3

Lib

Lla

D d

.

A

! 1

x I t

Laterite + Bauxite + ferricretes

Ferruginous clay

Gray carbonaceous clay

Lignite

Sand

Residual white clay

Precambrian hard rock

clay seams. Ample fossilized plant remains were found both in the carbonaceous clay and lignite seams. The entire sedimentary deposit is --30 m thick and is underlain by residual white clay. A thin veneer of sand separates the overlying sedimentary and the underlying residual clay.

The residual white clay L (Lla and Llb) which is approximately 10-15 m thick (Figure 1C), has relict structures like foliation and intruded quartz veins in- herent from the parent rock, a quartzo-feldspathic mica-gneiss of Precambrian age. Numerous fossilized plant roots, --0.1-0.35 m from the top of the residual clay L, were identified during field excavation. This shows evidence of plant growth and subsequent weathering prior to Tertiary sedimentation. Field evidence and detailed geological studies of the regions around North Kerala by Rajendran and Narayanas- wamy (1987) indicate the influence of two spells of lateritization, i.e. pre-Warkallai and post-Warkallai-

pre-Quaternary lateritization on the country rock. The pre-Warkallai lateritization resulted in the formation of the thick white residual clay. In addition, weathering after the deposition of the Warkallai Formation resulted in thick deposits of laterites and bauxite which overlie the gray carbonaceous clay seams.

MATERIALS AND METHODS

Representative clay samples were collected from each of the recognizable layers (Figure tC). Sampled materials were crushed in an agate mortar and a fraction of the raw clay was used for mineralogical and chemical analysis. The <2 Ixm clay fraction separated by sedimentation and centrifugation was analyzed for clay minerals and associated impurities by oriented XRD and DTA. Structural characterization of the <2 txm fraction of kaolinite was carried out by FTIR spectroscopy and determination of the Hinckley crystallinity index (Hinckley, 1963), after the removal of

358 Manju et al. Clays and Clay Minerals

Table 1. Semi quantitative mineralogical analysis by XRD (raw clay and <2 ixm fraction).

Sample K Sm M Py I G Q P Ma H A

Raw clay: L la **** L lb **** L3 **** L5 **** L7 **** L9 **** LI1 **** L13 **** L15 **** L16 ****

<2 ~m L la **** L lb **** L3 **** L5 **** L7 **** L9 **** L l l **** L13 **** L15 **** L16 ****

K--kaolinite, Sm--smectite, M--muscovite, Py--pyrophyllite, I--illite, G--gibbsite, Q ~ u a r t z , P--pyrite, Ma--marcasite, H--hematite, A--anatase. The symbols in the table represent: **** major phase, *** minor phase, ** trace phase.

ca rbona te by sod ium acetate, i ron oxides by citrate- b ica rbona te , and organic mat te r by H202 (A1-Khalissi and Worral l , 1982).

The X R D analysis was pe r fo rmed us ing Ni-f i l tered C u K a radia t ion (40kV, 20 m A ) at a scan speed of 1 ~ 20/min. The DT A was carr ied out us ing a Seiko 320 T G / D T A ana lyzer at a hea t ing rate of 10~ f rom amb ien t t empera tures to 1100~ wi th a lumina as stan- dard. The F T I R spectra of 1 m g of the pre t rea ted frac- t ion, mixed and pel le t ized wi th 200 m g o f KBr, were recorded over the range o f 4 0 0 0 - 4 0 0 cm ~ us ing a Perk in E l m e r IR Spec t romete r after ove rn igh t d ry ing at 60~

The pH of the raw clay samples was measu red us ing 1:4 solid water ratios. Organ ic ca rbon in these samples was de t e rmined by oxida t ive decompos i t ion us ing K2Cr207 as the oxid iz ing agen t (Gross, 1971). The concen t ra t ion of pyri t ic and non-pyr i t ic i ron was de- t e rmined by the acid d isso lu t ion t echn ique (Schne ider and Schneider , 1990). The raw clay and the < 2 p.m fract ion were ana lyzed for SiOz, A1203, Fe203, TiO2, K20, CaO, Na20, M g O and LOI (Benne t t and Reed, 1971). The t race e l em en t analys is of three selected raw samples (L la , L7 and L16) was carr ied out by ICP- MS. Scann ing e lec t ron mic roscop ic analysis of raw clay and hand-p i cked p lant r emains was pe r fo rmed us- ing a J E O L J S M 5600 LV microscope . Fresh ly b roken natural surfaces of the spec imens were sput ter coa ted wi th gold pr ior to the analysis .

Labora to ry tests were conduc ted us ing the < 4 5 Ixm f rac t ion o f res idual ch ina clay L, an economica l ly vi-

able bel t wi th in the M a d a y i deposit . The phys ica l proper t ies and firing behav io r were tested in order to evaluate thei r u t i l iza t ion in the ce ramic industry. Gra in-s ize analysis of the < 4 5 Ixm f rac t ion was carr ied out by Micromer i t i c s Sed igraph 5100 us ing ca lgon as the dispersant . Viscos i ty m e a s u r e m e n t of 63% w/w slurry was under t aken with a Brookf ie ld v i scomete r us ing m i n i m u m concen t ra t ion of sod ium polysal t as a d ispers ing agent. Crude and ca lc ined samples (350~ 1 h) (Kogel and Hall , 1999) were b l eached increas- ingly wi th 0 .4% sod ium hydrosul f i te for b r igh tness improvement . This was measu red us ing a Color Touch Mode l ISO br igh tness meter. Br igh tness was measu red on dried samples (100~ 1 h) at 457 n m and the yellowness as a d i f ference in b r igh tness be tween 570 and 457 nm. The wate r of plastici ty, g reen M O R , fired M O R (1250~ and l inear shr inkage (dry and fired (1250~ were measu red as specif ied in A S T M stan- dards. Vo lume shr inkage, bulk density, wate r absorp- t ion and apparen t porosi ty ( A S T M ) of the samples were ana lyzed by firing the samples at t empera tures of 600, 800, 1000 and 1250~ (1 h soaking). The fired color o f the s intered samples was also noted.

R E S U L T S A N D D I S C U S S I O N

X R D studies

The minera log ica l analyses of raw clay and the < 2 txm fract ion are s u m m a r i z e d in Table 1. The con- s t i tuent minera l s were expressed as major, m ino r and


trace depending on their concentrations. The residual clays, L la and Llb, consist chiefly of kaolinite and quartz with considerable muscovite, gibbsite and pyrophyllite (Figure 2A). The <2 Ixm clay fraction (Fig- ure 2B) is devoid of pyrophyllite, suggesting its con- version to kaolinite. The same conclusion was reported by Heckroodt and Buhmann (1987) suggesting the formation of pyrophyllite from muscovite during early stages of weathering and its alteration to kaolinite at a later stage. Microcrystalline quartz and fine-grained mica, i.e. illite, constitute minor ingredients.

Most of the gray carbonaceous clays, except L5, exhibit an association of pyrite and marcasite along with kaolinite, quartz and gibbsite. Sedimentary pyrite formation occurs during the reaction of a detrital iron mineral with H2S, which was formed by the reduction of sulfate by bacteria in the presence of organic matter (Bernel; 1984). The small shoulder at 3.52 ,~ in the XRD patterns (Figure 2A) of L3 and L7 indicates anatase. Trace amounts of smectite were identified in L5 (14 A) (Figure 2B). The formation of smectite is fa- vored by a microenvironment with poor drainage and hence minimum leaching conditions with relatively high metal ion concentrations (Keller, 1985). Gibbsite identified by the 4.84 A peak in L3, L5, L7, L9 and L11 indicates intense leaching in a strongly deionized environment under tropical climate (Vazquez, 1981) during desilication, i.e. dissolution of Si from its alu- minosilicate parent material. From the available infor- mation it is not clear whether the gibbsite found in these carbonaceous clay is the product of post or pre- depositional alteration. The association of the above minerals in the same clay resulted from the changes in the microenvironment during the clay deposition or formation. Fine quartz grains (<2 Ixm) were noted in the carbonaceous clays having large amounts of marcasite/pyrite (evident from peak intensity, Figure 2B). Chen et al. (1997) suggest formation of microcrystalline quartz as a result of the rearrangement of the feld- spar and muscovite structures during kaolinization or kaolinite recrystallization.

The ferruginous LI6 clay shows an assemblage of kaolinite, quartz and hematite. Hematite is concentrated in the fine clay fraction. The presence of hematite indicates an oxidizing environment and the presence of Fe sulfides indicates reducing environment conditions during the formation or deposition of gray carbonaceous and ferruginous clay, respectively.

The Hinckley crystallinity index of kaolinite varies from clay layer to clay layer (Table 2). This variability may be attributed to differences in the geological environment such as intensity of weathering or the extent of transportation of the clay during formation or deposition (Brindley, 1986). The crystallinity index varies from 0.44 (L5) to 0.86 (Lla).

Chemica l analyses

The chemical analysis and pH values of the samples are listed in Table 2. The SiOz and AlzO3 contents of L la and L l b were consistent with mineralogical ob- servation. The relatively lower K~O concentration in L l b compared to L la from the same seam at a differ- ence in depth of 10 m indicates its preferential leaching from muscovite.

The major oxide in gray carbonaceous clay, Sit2, is present in lower concentrations than in the residual clays above. This might be due to the winnowing ac- tion during transportation and sedimentation. Lack of a particular trend in the Ti t2 distribution with depth, indicates its immobile nature during weathering. The MgO concentrated in the <2 txm fine fraction is attributed to smectite. High Fe203 accounts for the presence of FeS2 minerals, marcasite and pyrite (Table 2). Hematite contributes to the major source of iron in ferruginous clay and the maximum iron content of this deposit is found in this clay fraction (22.20%) (Ta- ble 2).

The pH of the clays ranges from 2.64 to 5.48. The high acidity implies the effect of ongoing weathering. The acidity correlates with the FeS2 and organic carbon content. The pH is lowest in L3 (2.64) where organic carbon (7.23%) and FeS2 (9.96%) are relatively high. The FeS 2 is highly concentrated in L9 (15,37%), L7 (8.47%) and L3 (9.96%) where the carbon content and Fe203 are also high.

The distribution of trace elements in Lla , L7 and L16 (Figure 3) shows some variation. Vanadium (V 4+) is more concentrated in the hematite-rich clay in the presence of relatively high Ti t2 and FezO3 (Table 2). Vanadium (V 4+) with an ionic radius of 0.61 A re- places Ti4+(0.64 A) (Rankama and Sahama, 1952) and also precipitates in ferric hydroxides (Goldschmidt, 1958). Zirconium, almost entirely contained in the mineral zircon, is found to be concentrated more in the hematite-rich sedimentary clay (180 ppm), fol- lowed by the gray carbonaceous clay (118 ppm), suggesting concentration during the course of weathering, transportation and deposition. Manganese shows sim- ilar ranges in concentration. The Cu can be correlated with Zn; where an increase in the concentration of Cu occurs, the Zn content also increases. The total trace element concentrations for Lla, L7 and LI6 are 498.71, 837.61 and 881.69 ppm, respectively.

DTA studies

The DTA patterns (<2 ixm) (Figure 4) show features related to the mineralogical assemblage and the kaolinite structure. The dehydroxylation and exothermic peaks of kaolinite range from 522 to 542~ and 972 to 1000~ respectively. Higher exothermic temperatures for kaolinite were noted in L l a (1000.9~ and L l b (1003~ and lower peak temperatures were


A K K

. , . II

o = j . aA I Q

Q Ma

K'~, P Q M a Q

Ma

Q Q

65 60 55 50 45 40 35 30 020 (CuK(x)

K

; Q G i 1K

LK Gi

M

K : M

25 20 15 10 5

L16

L15

L13

L l l

L9

L7

L5

L3

Llb

Lla

Figure 2. (A) XRD pattern of raw clay samples: Lla and L l b ~ t h e lowermost residual clay layer; L3, L5, L7, L9, L l l , L 13, L I 5-- the carbonaceous gray clay with decreasing order of depth from the top; and L l~-ferruginous clay. K--Kaolinite, Py--Pyrophyllite, Gi--Gibbsite, Q--Quartz, P--Pyrite, Ma Marcasite, M~Muscovite, H--Hematite, A~Anatase. (B) XRD pattern of oriented <2 txm clay fraction: Lla and L l t~- the lowermost residual clay layer; L3, L5, L7, L9, L11, L13, L15-- the carbonaceous gray clay with decreasing order of depth from the top; and L16--ferruginous clay. K~Kaolinite, G i - - Gibbsite, Sm--smectite, Q--Quartz, P--Pyrite, Ma--Marcasite, I--illite, H--Hematite, A--Anatase.


B K K

H ~.~ ~ L.,- -.'~L16

_ ~ _ = . ~ ~ ~ ~ ~ . , / ~ L13

~K Q K

K K K Q . IK.K Q

:

65 60 55 50 45 40 35 30 25 20 15 020 (Cu,K(~)

Figure. 2. Continued.

L3

~ . , Llb

I

k~L.,~ Lla -

10 5

noted in the gray carbonaceous clays. The exothermic peak typical of kaolinite, showing the formation of a new phase, mullite, is not prominent in L3, L7 and L9 where there is a higher concentration of pyrite. This mineral may act as flux, which affects the thermal property. An additional peak at 450~ was also noticed in the above clays, which is characteristic of pyrite (Searle and Grimshaw, 1960). Mineralogical and chemical analysis also supports this evidence. The de-

hydroxylation temperature at 277-290~ is that of gibbsite.

117 studies

The structural analysis of kaolinite by XRD and DTA was further complemented by detailed FTIR studies. The spectra (Figure 5) give valuable infor- mation about the degree of crystallinity and regu- larity of structure of the clay minerals (Russell,


Table 2. Chemical analysis of clay samples from different layers (wt. %).

L l a L i b L 3 L 5 L 7 L 9 L l l L I 3 L 1 5 L I 6

SiO2 65.03 63.9 34.92 51.07 46.49 35.02 52.60 43.24 37.67 43.47 45.26 46.02 36.82 38.98 34.06 37.59 38.31 40.38 9.76 28.21

AI203 21.61 23.91 26.23 26.92 25.45 28.72 27.08 33.69 37.30 29.77 38.02 36.17 29.20 36.85 34.06 37.01 35.89 36.02 38.06 35.22

Fe203 1.43 1.27 10.76 1.97 6.64 9.81 1.35 2.36 2.89 11.90 0.38 0.75 6.82 1.26 5.34 2.87 1.42 1.34 2.68 22.20

TiO2 0.02 1.30 1.00 0.98 1.44 1.59 1.35 1.84 3.35 1.85 0.06 0.32 0.10 0.81 2.27 2.86 3.62 2.54 1.05 0.16

Na,O 0.16 0.12 0.26 0.18 0.29 0.22 0.21 0.17 0.18 0.37 0.45 0.33 0.45 2.04 0.67 0.53 0.67 0.58 0.98 0.36

KzO 2.51 1.90 1.05 0.60 2.24 2.24 3.73 1.50 1.20 0.09 0.65 0.39 0.37 1.83 0.55 0.51 0.49 0.58 0.54 0.58

MgO 0.01 0.33 0.04 0.07 0.01 0.03 0.01 0.05 0.27 0.21 0.06 0.11 0.13 0.52 0.02 0.15 0.06 0.33 0.40 0.49

CaO 0.10 0.13 0.13 0.28 0.39 0.41 0.18 0.13 0.14 0.38 0.11 0.13 0.41 0.55 0.42 0.28 0.28 0.13 0.28 0.70

LOI 9.59 9.38 24.49 18.20 17.33 21.62 13.04 17.13 17.13 12.81 14.41 14.69 25.56 17.04 18.36 17.93 16.02 15.43 16.75 12.08

C # 0.21 0.45 7.23 1.08 3.31 5.84 0.99 0.64 0.73 0.42 FeS2 # 0.18 0.31 9.96 1.15 8.47 15.37 1.34 0.95 1.36 - - pH * 3.99 3.49 2.64 3.14 2.75 2.67 3.29 2.77 2.77 5.48 HI* 0.86 0.69 0.72 0.44 0.70 0.70 0.51 0.64 0.50 0.67

Upper readings relate to raw clay, lower readings to the <2 ~xm fraction. #--raw clay, * <2 txm fraction, HI--Hinckley Index, - - below detection limit.

1987). The IR spec t ra s h o w the p r o m i n e n t peaks o f kaol ini te : 3 6 2 0 - 3 7 0 0 cm t for - O H s t re tch ing , 1 0 0 0 - 1 120 c m -1 fo r - S i - O s t r e t c h i n g , 9 1 0 - 940 c m -] for - O H b e n d i n g and 4 0 0 - 5 5 0 cm ~ for - S i - O b e n d i n g v i b r a t i o n s (Fa rmer , 1979) .The d i f f e r e n c e s in band in tens i t i es as ev iden t f rom the peak he igh t d i f f e r ences , sugges t a var ie ty of de fec t s in kaol in i te s t ructure . The doub le t b e t w e e n 3694 and 3619 cm -~ for L l a , L3, L7, L l l , L13 and L15 is ind ica t ive o f w e l l - o r d e r e d kaol in i te s t ructure (Farm- er, 1979; Russe l l , 1987). A deg ree o f o rder for the c lays above is a lso c lear f r o m the IR band in tens i t ies

200

160

~Q. 120

8O

0 - - - , , i , p . i . I . T I , a . l . i . a , a . a . | . l .

Sc V CrMnCoNi CuZnSr Y Zr Oa LaPb

Trace e lements

Figure 3. Trace element patterns of three selected raw clay samples, Lla, L7 and L16.

at 791 and 751 cm-~. The re la t ive in tens i ty o f these bands is the same for o rde red kaol in i te and the band at 791 cm -1 b e c o m e s weak wi th inc reas ing disorder . The p r e s e n c e of the doub le t in the gray ca rbonaceous clays, w h i c h have a l ower H ink ley index (Ta- ble 2), m i g h t be due to the mix tu re o f o rde red and d i so rde red kaol ini te . The fea tures above cou ld be exp l a ined by the o rde r /d i so rde r index p r o p o s e d by Plan~on et al. (1989), w h i c h meas u re s the re la t ive p ropo r t i on o f h i g h - d e f e c t c o m p o n e n t s . The be t te r o rde red kaol in i tes , as ev iden t f rom FTIR, were f o r m e d as a resul t o f the r ec rys t a l l i za t ion of low- o rder detr i ta l ones . This e v i d e n c e o f rec rys ta l l i za - t ion is r e in fo rced by the S E M studies .

S E M studies

Scann ing e lec t ron m i c r o g r a p h s of clay f r o m L l a , L3, L5, L15, L16 and the foss i l i zed plant r ema ins c lear ly ind ica te the g e o c h e m i c a l p r o c e s s e s and en- v i r o n m e n t for the genes i s o f three d i s t inc t c lays in this par t icu lar depos i t .

Residual white clay. Vermicu la r and book- l ike mor- p h o l o g y is o b s e r v e d for the res idual L l a clay (Fig- ure 6A). A l m o s t all kaol in i te gra ins exh ib i t a face- to - face pat tern o f a r r a n g e m e n t o f pla ty c rys ta l s (Fig- ure 6A). The res idual kaol in i te s h o w s c rys ta l s wi th angular edges (Figures 6A and 6B), w h i c h sugges t in situ fo rmat ion . The i r regular angular kaol in i te edge is charac te r i s t i c o f ac t ive ly g r o w i n g c rys ta l s (Keller , 1985). Kao l in i t e gra ins were found at the edge of m i c a c e o u s s tacks or books sugges t ing the


Figure 4.

U

E O X

.2

e-

"O e-

\

Lll

3 9 1 ~ " " ~ L9

L7

L5

~ ~ , , 522 L3

532 L | J | II

0 287 575 862 1150 Temperature (~

DTA curves of <2 Ixm fraction showing varying endothermic-exothermic temperature.

alteration of muscovite to kaolinite (Figure 6B). This particular morphology implies the initiation of kaolinization from the edges of the muscovite stacks and weak cleavage planes. The muscovite stacks show a typical fanning out texture, formed by the release of K during hydration or hydrolysis (Stoch and Sikora, 1976; Chen e t al. , 1997) forming numerous lenticular voids along the cleavage planes. The formation of these lenticular voids during

weathering is attributed to the volume decrease during the collapse of the muscovite structure (Robert- son and Eggleton, 1991). These lenticular voids indicate an incomplete stage for kaolinization since they disappear during the completion of weathering of the primary phyllosilicates (Robertson and Eg- gleton, 1991; Chen e t al . , 1997). Compact tabular muscovite grains (Figure 6C) without any indication of alteration could also be identified. The K-deft-


t -

. m

E t - .

I - -

-- , . . ,

!

3800

L16

L15

L13

L l l

L9

L7

L5

~ L3

0 r

E m e -

Llb

~ Lla

I I It | J J i . . i . I , , , i l i

3700 3600 1500 1000 500

W a v e n u m b e r (cm -~) W a v e n u m b e r (cm -1)

L16

L15

L13

L l l

L9

L7

L5

L3

Llb

L la

Figure 5. FTIR spectra of <2 Ixm clay sample (3900-3700 cm -~) (1500-400 cm ~) with weak doublet between 3694 and 3619 cm -~ and relative intensities of 791 cm ~ and 751 cm -t reflecting the crystalline order of kaolinite.

c i e n t m u s c o v i t e c r y s t a l s c h a r a c t e r i s t i c a l l y had a m o r e i r r e g u l a r shape t h a n the t yp ica l p r i s m a t i c la ths o f f l e s h m u s c o v i t e .

Foss i l ized p lant roots f rom uppe rmos t res idual clay show minera l iza t ion . In s i tu pyri te crys ta ls wi th the

m o r p h o l o g y of oc tahedron, pyr i tohedron , dodecahe- dron and oc tahedron-cube combina t i on (Figure 6D) were o b s e r v e d to grow f rom the s idewal l of p lant fossils. These par t icular c rys ta l morpho log ie s are s imi lar to that of pyri te fo rming f rom an undersa tu ra ted so-

Vol. 49, No. 4, 200l Madayi kaolin: geochemistry, mineralogy and utilization 365

Figure 6. Scanning electron micrographs illustrating residual clay: (A) kaolinite books (arrow), with irregular edges showing in situ crystallization and friated appearance, indicative of mica alteration; (B) fanning out of muscovite (m), along with kaolinite (k) at the edges, with numerous lenticular voids (v) between the flakes due to K leaching; (C) muscovite grain (m) without alteration, embedded in the kaolinite platelets; (D) mineralization within hand-picked plant remains from uppermost portion Llb residual clay: in situ crystallization of Fe sulfide crystals with o (octahedral), d (dodecahedral), p (pyritohedral), c (cube-octahedral) combination. The grains show etched pits on their surfaces.

lution (White et al., 1991). Numerous etched pits were also found on the surface of the grain (Figure 6D), formed as a result o f ongoing weathering, during the percolation of ion-rich solutions. The FeS2 mineralization in the residual white clay might have resulted f rom the influence o f the leached acid-rich pore-water f rom the over lying organic and FeS2-enriched gray carbonaceous clay layers during oxidation.

Sedimentary gray carbonaceous clay. This clay shows two types o f kaolinite formation in addition to its detrital character. Clay fraction L5 shows a swirl texture (Figure 7A) with face-to-face arrangement of grains suggesting their detrital origin (Keller, 1978). In the sedimentary clay L I 5 , finer clay grains are agglomerated during their deposit ion by the cement ing effect of organic matter (Figure 7B), thereby giving a flower- like appearance.

The lowermost gray carbonaceous clay (L3) has two types of grains (Figure 7C), suggesting the diversi ty

of the envi ronment for their formation. The smaller kaolinite grains are angular with a typical face- to-face arrangement, surrounding larger hexagonal grains having smooth edges. These bimodal features indicate that the finer grains are a recrystal l ized product and the coarser grains are o f detrital origin. This texture in kaolinite occurs due to in situ formation of this particular mineral in a water-laden envi ronment f rom silicate parent material (Keller, 1978).

The second type of in situ kaolinite within gray carbonaceous clay is observed as mineral izat ion within the fossi l ized plant remains collected f rom L15 strata. Stacks of kaolinite platelets were found grown into the cubic etched pits formed by the dissolution of the cubic pyrite (Figure 7D), suggesting kaolinite crystallization after the removal of FeS2 during oxidation. Ox- idation of organic matter along with FeS2 leads to the formation of acid-rich pore-fluid which will disinte- grate and dissolve the kaolinite and reprecipitate this


Figure 7. Scanning electron micrographs of sedimentary clays: (A) swirl texture with face-to-face arrangement of platy grains indicating the detrital origin of gray carbonaceous clay; (B) agglomeration of kaolinite by organic matter giving a flower-like appearance; (C) kaolinite of bimodal origin, face-to-face arrangement of fine platelets of kaolinite (arrow) surrounding larger ones, product of in situ crystallization from a water-laden environment; (D) plant remains from uppermost gray carbonaceous clay L15: kaolinite crystals, product of crystallization from Si -and-Al-rich solution and found to be intergrown in the etched pits of cubic Fe sulfide.

A1-Si-r ich ionic solution within the microenvironment of plant fossils, thereby developing authigenic kaolinite. Simultaneously, dissolution of FeS 2 under oxidizing conditions, allows the percolation of acid-rich water through the available pore-structure and cavities within the clay. The iron released during dissolution precipitates as Fe oxyhydroxides. Microscopic exam- ination of reddish yel low clay encircl ing the plant fossils in mott led L15 clay shows some minerals which are amorphous and with cornflake-like texture. This may be smectite, probably formed as a result of low leaching envi ronment due to the precipitation of amorphous Fe oxyhydroxides (Figure 8A) during FeS2 dissolution. The typical morphology of an Fe oxyhydroxide grain indicating in s i tu crystal l ization is shown in Figure 8B.

H e m a t i t e - r i c h clay. The microscopic examinat ion of this clay shows the agglomerat ion of finer particles, by the cement ing effect of hematite or amorphous Fe oxy-

hydroxides (Figure 9). In this genetic environment, the in s i tu kaolinization or the features of kaolinite re- crystall ization are absent. This is also evident from the FTIR patterns, where kaolinite shows a lower crystallinity.

This evidence indicates that the environmental con- d i t ions - -ox id iz ing and r e d u c i n g - - h a v e influenced the entire clay deposit. The oxidized weathering condit ion which led to the formation of hemati te-r ich clay, en- hanced the dissolution of the FeS2 present in the gray carbonaceous clay layers, creating a very acidic environment in which dissolution and reprecipitation of microcrystal l ine quartz and authigenic kaolinite oc- curred. In addition, the percolation of these SO4 2 ions rendered the deposit more acidic resulting in the subsequent crystall ization of FeS2 in the microenvironment within the plant fossils in the residual white clay. The presence of over lying sedimentary deposits of the Warkallai Formation preserves the underlying residual


Figure 8. Scanning electron micrographs showing smectite and secondary iron oxides: (A) smectite precipitation (s) with cornflake-like texture and amorphous Fe hydroxides (a); (B) crystal of Fe oxyhydroxide showing in situ crystallization.

white clay deposit from intense post-Warkallai-pre- Quaternary lateritization.

Indus tr ia l appl icat ion

Commercial evaluation of the representative sample collected from the residual white clay (L) is given in Table 3. This residual clay deposit shows a recovery of 46% (Table 3) for the <45 ~m fraction. In this, the <2 p~m fraction comprises 48%. Calcination of kaolinite prior to bleaching causes an enhancement in brightness of --7.91 units compared to the crude clay. Correspondingly, the yellowness value also decreases by 1.57 units. Such an improvement indicates that the clay contains goethitic iron impurity (Kogel and Hall, 1999) which converts to hematite during calcination and is leached more efficiently. The pinkish-white

fired color of the clay, even after bleaching, shows the presence of color-imparting impurities. Table 3 pre- sents the properties of the fired samples as fired shrinkage, water adsorption, apparent porosity and bulk density. Increasing the sintering temperature to 800~ resulted in an increase in all the properties measured. Above this temperature, samples sintered at 1250~ exhibit increase in fired shrinkage (24.26%) and bulk density (0.81), with a corresponding reduction of apparent porosity (17.3%) and water adsorption (33.26%). This also results in a drastic increase in MOR of about --235 kg/cm 2.

CONCLUSIONS

Our investigation indicates that the mineralogy, chemistry and morphology of the associated minerals

Figure 9. Agglomerated kaolinite platelets with amorphous iron oxide coating in hematite-rich clay.

368 Manju

Table 3. Physical properties of china clay (<45 fxm sample).

Clay recovery 46% Raw color Buff

Particle size distribution analysis: <2 Ixm 48% <5 txm 72%

<10 ~m 89% <20 txm 99.5% >20 Ixm 0.5%

Brightness/yellowness of raw clay 58.35/10.6% Brightness/yellowness of calcined clay 62.32ll 1.02% Brightness/yellowness after bleaching 59.48110.9% Brightness/yellowness after calcination and

bleaching 66.26/9.35% Viscosity 621 cps Water of plasticity 40.2% Dry linear shrinkage 3.9% Green MOR 5.59 kg/cm z

Fired properties Fired color (1250~ pinkish white Fired linear shrinkage (1250~ 16.08% Fired MOR (1250~ 235 kg/cm 2

800~ 1000~ 1250~ 600~

Fired volume shrinkage (%) 1.17 2.30 10.00 24.46 Water adsorption (%) 44.35 46.69 46.27 33.26 Apparent porosity (%) 22.40 23.35 23.13 17.3 Bulk density (g/cm 3) 0.66 0.65 0.69 0.81

in this deposi t are c losely re la ted to the fo rmat ion en- v i ronment . The var ia t ion in kaol ini te charac te r in each layer as ev iden t f rom XRD, DTA, IR, chemica l analysis and S E M is related to the pet rogenet ic , strati- graphic and pos t -depos i t iona l condi t ions . It is found that kaol in i te wi th in the same deposi t has i tse l f been fo rmed under three genet ic env i ronmen ta l condi t ions : (1) ox id iz ing condi t ions for the detrital hemat i t e - r i ch uppe rmos t fe r ruginous clay fo l low on L16; (2) reducing e n v i r o n m e n t for the depos i t ion of FeS2-enr iched gray ca rbonaceous kaol in i te seams L3 to L15; and (3) intense lateritized weathering conditions for the forma- t ion o f the economica l ly v iab le res idual whi te c lay L.

The s t ructural and morpho log ica l s tudies indicate the recrys ta l l iza t ion of kaol in i te in the gray carbonaceous clays. Lower L3 clay showed textures charac- terist ic of detri tal and also in situ crys ta l l iza t ion under w a t e r - l a d e n cond i t i ons . In situ k a o l i n i z a t i o n in to e tched FeS2 pits f rom A1-Si-enr iched ionic solut ion occurs wi th in the p lant r emains of the (L15) ca rbonaceous clay in an o rgan ic -enr i ched env i ronmen t . Min - era l izat ion in p lant r emains in L i b indica tes that this m i c r o e n v i r o n m e n t does not favor the in situ kaol in i te c rys ta l l iza t ion in the absence of an ample amoun t of organic chela tors as in gray ca rbonaceous clay. The minera log ica l a s semblages such as pyrophyl l i te , mus- covi te wi th its typical f ann ing-ou t m o r p h o l o g y wi th lent icular voids, and l eached pits in pyri te crys ta ls found wi th in the plant r emains f rom L l b indicate that

et aL Clays and Clay Minerals

kaol in iza t ion of this deposi t is not yet complete . This is fur ther p roven by the fact that the concen t ra t ion of trace e lements is lowes t for the res idual clay L l a , wh ich has unde rgone re la t ively less weather ing. The sed imen ta ry clay deposi ts o f the Warkal la i fo rmat ion play a ma jo r role in protec t ing the under ly ing res idual whi te clay f rom the ongo ing lateri t izat ion.

Technologica l eva lua t ion o f this clay, wh ich has a r ecove ry rate of 46%, shows that this large deposi t could be exploi ted commerc ia l ly for use in the ce ram- ics industry. A compar i son o f the specif icat ions of this clay wi th A S T M indicates tha t this clay is sui table for whi te ware and sani tary ware appl icat ions.

A C K N O W L E D G M E N T S

This research work is funded by the Council of Scientific and Industrial Research, India. The authors thank Robert J. Pruett and Jessica Elzea Kogel for their helpful comments during review of the manuscript. We also thank the Director, Regional Research Laboratory, Thiruvananthapuram, for pro- viding the necessary facilities throughout this work.

R E F E R E N C E S

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Planqon, A., Giese, R.E, Snyder, R., Drits, V.A. and Bookin, A.S. (1989) Stacking faults in kaolin-group minerals: defect structures of kaolinite. Clays and Clay Minerals, 23, 249- 260.

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E-mail of corresponding author: [email protected] (Received 7 April 1999; revised 8 January 2001; Ms. 330;

A.E. Jessica Elzea Kogel)

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