Clay Minerals (1986) 21, 401-415
HEXAGONAL
P L A T Y H A L L O Y S I T E IN A N
ALTERED TUFF BED, KOMAKI CITY, AICHI
PREFECTURE, CENTRAL JAPAN
H. NORO
Department of Earth Sciences, Nagoya University, Chikusa-ku, Nagoya, Japan
(Received 10 December 1985; revised 2 April 1986)
A B S T R A C T: XRD analysis and electron microscopy show that hexagonal platy halloysite is
the main component of an altered tuff (Ueno tuff bed) in the Pliocene Seto group, Aichi
Prefecture, Central Japan. In the natural state it shows a single basal peak at 10.1 A, which
collapses to 7:2 A, by dehydration through a segregate-type interstratification. The (02,11)
non-basal band consists of slightly separated peaks which indicates moderate ordering
of the crystal structure. The b-dimension is 8.936-8.939 ,~. The stability of the interlayer
water is intermediate with respect to halloysites of different morphologies. Between 3.5 and
4% Fe203 is present in the deferrated sample and the calculated chemical formula
2+
3+
'
(Fe0.0o3)(All.s54Fe0.146)(Sll.995Alo.o05)Os(OH)4
can not explain the anomalously high CEC of
21.9 mEq/100 g. Because the curvature radius and b-dimension of halloysite increase with
increase in Fe203 content, the platy morphology is ascribed to replacement of AI3+ by Fe 3+ in
the octahedral sheet. Based on the geological and chemical data, the hexagonal platy halloysite is
considered to have been formed from volcanic glass after deposition in a freshwater lake, where
conditions were oxidizing and weakly acidic.
HaUoysite occurs in a variety of morphologies and the relationships between the form of
halloysite and its occurrence, crystal structure, Fe20 3 content and other mineralogical
properties such as dehydration characteristics have been discussed by many authors.
Crystal structure, occurrence and mineralogical properties of long tubular halloysite
have been reported by Honjo et al. (1954), Chukhrov & Zvyagin (1969), Parham (1969),
Nagasawa & Miyazaki (1976), K o h y a m a et al. (1978), Noro et aL (1981), Churchman &
Theng (1984) and Nagasawa & N o r o (1985). Occurrence and mineralogical properties of
spherical and short tubular halloysites have been discussed by Sudo (1953), Sudo &
Yotsumoto (1977), Tazaki (1979) and Churchman & Theng (1984). Other forms of
halloysite have also been reported. Kirkman (1977) described squat cylindrical and disk
halloysite in rhyolitic tephra of New Zealand. Nagasawa & Karube (1975) reported
ribbon-shaped halloysite in altered montmorillonite clay. Tazaki (1979) wrote that various
morphologies of halloysite--spherical, walnut-meat-shaped, acicular, crinkly, platy,
tubular and square-tube--were observed on the surface of altered plagioclase in volcanic
ash,
The occurrence and mineralogical properties of platy or tabular halloysite have also
been reported by many authors. Platy halloysite occurs in soil (Kunze & Bradley, 1964), as
Present address: Information and Analysis Office, Geological Survey of Japan 1-1-3, Higashi, Yatabe,
Tsukuba, Ibaraki, 305, Japan.
t~ 1986 The Mineralogical Society
402
H. Noro
veins in laterite (de Souza Santos et aL, 1966) and granite (Wilke et al., 1978), on the
surface of weathered plagioclase in volcanic ash (Tazaki, 1979), in weathered pyroclastics
(Wada & Mizota, 1982), in hydrothermaUy altered acidic volcanic rocks (Nakagawa &
Shirozu, 1983) and in ferrallitic soil derived from pyroclastics (Quantin et al., 1984).
The present paper describes the occurrence and mineralogical and chemical characteristics of hexagonal platy halloysite from a tuff bed of Pliocene age and also discusses
the relationships between the form of halloysite and its Fe203 content.
OCCURRENCE
Hexagonal platy halloysite was first found in an altered tuff at Ookusa, Komaki City, Aichi
Prefecture, Central Japan. The tuff, termed the Ueno tuff, is a member of the upper Seto
group, which are fresh-water sediments of the Pliocene Tokai lake (Mori, 1971). The Ueno
tuff extends southwards to the Chita peninsula for more than 50 km (Fig. 1) and, where
unaltered, consists mainly of pumice and glass shards.
Hexagonal platy halloysite was found at all the outcrops of the Ueno tuff examined. At
four outcrops denoted by 'h' in Fig. 1, pure hexagonal platy halloysite clay was found. In
these outcrops the tuff bed is intensely altered to clay and can be subdivided into four
sublayers: white clay, red clay, greenish-yellow clay and pale yellow clay (Fig. 2a).
Hexagonal platy halloysite is the dominant clay mineral of the greenish-ydlow clay. The
white clay transgresses into the red clay as veinlets containing manganiferous concretions.
wI
~z
,~ Ioya
35=N
PACI F I C OCEAN
i
130 k nn
I
FIG. 1. Distribution m a p of Ueno tuff. Surveyed outcrops are denoted by stars with number and
location name. 'h' is added to the location number where pure hexagonal platy halloysite clay
was found.
Hexagonal platy halloysite, Japan
.O 9 . . .
%:10-,..9>&~:.b?
0.r
0..0
~'& 9 : :
".b.'.'.o.'. : :"~"94~;a--gibl:~ite
9" ""
grovel
"0" 'o:
" " o""'~'"
"'O"v"
0
P""
o: .o'.."
~- -/~j w h i t e
~
concretion
1
cloy - ~
pink clay
~ ~ . ~
red clay
"'.o"
....-~-......o...-.... 9
..o: ..o . .. or....9
-o
. . . . .
. . . . . . . .
o- -
1.2m
grovel
/
-
t
00000^0~
o o Oo o~ o\~
O000~OU 0
0 0 ~J 0
0~0~0~
0
00~0~0~0
Ueno tuff
403
0000
0 0
0 000000
0 0 0 0 00~
0 O0000~
0 0 O00r~ 0
unaltered
pum:ce
Ueno tuff
0 0 O00~o0
- 5m
0000 0000
white clay
_ __
_
(a)
creamy
~crearny
!
white clay
slit
white clay
j
__
(b)
FIG. 2. (a) Sketch of the outcrop at Ookusa, Komaki City, Aichi Prefecture (location number 2
in Fig. 1). (b) Sketch of the outcrop at Sakashita, Kasugai City, Aichi Prefecture (location
number 3 in Fig. 1).
The present surface of the red clay is also whitened. The boundary layer between the white
clay and the red clay is pink. The red clay transgresses into the greenish-yellow clay as
veinlets and seams containing manganiferous concretions. A thin layer of the creamy
white clay lies between the greenish-yellow clay and the underlying silt bed. The boundaries
are gradual.
At these outcrops, the Ueno tuff overlies lignite-bearing silt and underlies a gravel bed.
The gravel bed and the overlying soil are coloured red and contain gibbsite concretions.
The outcrops are located close to the high-level terrace plane. This implies that the gravel
bed and the Ueno tuff were exposed to intense weathering during the period of erosion after
deposition of the Seto group was complete (~2 Ma ago).
In other outcrops of the Ueno tuff, clays rich in hexagonal platy halloysite were not
found (in Fig. 1, these outcrops are denoted by locality numbers only). In an outcrop at
Sakashita, Kasugai City (location number 3 in Fig. 1), the Ueno tuff consists of an
unaltered pumice sublayer, white clay (pure roll-shaped halloysite) and creamy white clay
(roll-shaped halloysite with a small amount of hexagonal platy halloysite) (Fig. 2b).
Although the presence of much unaltered pumice shows that alteration was less intense
that that of the above four outcrops, weaker iron staining of clay indicates more rigorous
leaching of iron. An outcrop at Togo-cho shows the same feature. These two outcrops are
located well below the high-level terrace plane, which is why the Ueno tuff at these
outcrops was very little affected by weathering compared with the outcrops mentioned
above.
EXPERIMENTAL
Samples
Channel samples were taken from the Ueno tuff at six outcrops. To avoid possible error
in discriminating between halloysite and kaolinite, the dried surface of the exposure was
404
H. Noro
removed and samples were taken from the inner part. From sampling to X-ray
examination, special care was taken not to dry the samples.
Sample preparation
Clay andfine sand. After being dispersed in dilute NaOH solution (pH = 10), clay and
fine sand fractions were collected by a combination of wet sieving and sedimentation.
Excess salt was removed by washing with distilled water and centrifugation. Polished
thin-sections of the fine sand were made for examination under an optical microscope. For
XRD of the clay fraction, oriented aggregates were prepared on glass slides and kept in a
vessel saturated with HEO vapour.
A few grams of the clay fractions were air-dried for chemical analysis and for
measurement of CEC.
A drop of clay suspension was placed on a carbon-coated collodion film supported by a
copper grid and was air-dried for transmission electron microscopy.
'Whole rock' sample, small stubs of the sample were cut and air-dried for scanning
electron microscopy. The surface of the samples were coated with gold using a
sputter-coating apparatus.
~ r a y diffraction.
XRD traces were made of oriented mounts equilibrated at relative humidities of 100, 95,
79, 64, 53, 32, 20 and 0%, and heated at 120~ for 24 h, 200~ for 48 h and 560~ for
3 h. Constant humidity air between 95% r.h. and 20% r.h. was produced by circulation of
air through water saturated with various salts: KNO 3 for 95% r.h., NH4C1 for 79% r.h.,
Mg(CH3COO)2 for 64% r.h., Mg(NO3) 2 for 56% r.h., CaC1 z for 32% r.h. and CH3COOK
for 20% r.h. During XRD examination, air of constant humidity flowed around the
specimen; the humidity was adjusted to the value at which prior drying was done.
Chemical analysis and CEC
The clay fractions were analysed by wet chemical methods after deferration with
dithionite-citrate-bicarbonate (DCB) (Jackson, 1956). SiO2 and HEO were determined
gravimetrically, TiO 1, A120 ~, Fe203, FeO and MnO by colorimetry, MgO and CaO by
the chelate (EDTA) titration method, and NalO and K20 by atomic absorption
spectrometry.
Some samples were also analysed by an electron probe microanalyser (JEOL JXA-733).
About 20 mg of clay sample was pressed into a translucent pellet and, after coating with
carbon, the surface was analysed (Noro et al., 1981). The contribution of unobservable
oxides (H20 etc.) to the matrix effect was estimated by the method of Yusa & Tsuzuki
(1976).
CECs of deferrated samples were measured by MgC1 z and NH4C1 solutions.
Electron microscopy
Transmission electron micrographs of the clay fractions were taken using a JEOL T7
electron microscope operated at an accelerating voltage of 60 kV. Rock samples were
Hexagonal p[aty halloysite, Japan
405
examined with a JEOL JXA-733 electron probe microanalyser in the scanning electron
microscopic mode.
RESULTS
X-ray diffraction
Identification of clay minerals. XRD traces of the clay fractions obtained under 100, 56
and 20% r.h. are shown in Fig. 3. Discrimination between halloysite and kaolinite was
made using the following criteria for samples which had not been dried prior to
examination: (i) a minera! showing a basal reflection at 10 A after equilibration at 100%
r.h. and collapsing to 7 A after dehydration is halloysite; (ii) a mineral showing a basal
reflection at 7 ,s after equilibration at 100% r.h. is kaolinite; (iii) when the specimen
showed two peaks at 10 A and 7 ,/~, it was assumed to contain both halloysite and
kaolinite.
At the outcrops where pure hexagonal platy haUoysite was found, the uppermost white
clay was identified as pure halloysite, the middle red clay as halloysite with a small amount
of kaolinite, the lower greenish-yellow clay as pure halloysite and the lowermost
rhlO0
10
20
rh 56
10
20
rh 20
10
20
28 CuK.~
FIG. 3. X R D traces of clay fractions from the Ueno tuff Ookusa, Komaki City. Samples were
equilibrated at 100, 56 and 20% relative humidity. Arrows indicate peaks due to kaolinite.
H. Noro
406
creamy-white clay as pure halloysite. The existence of kaolinite in the red clay was
confirmed by kaolinite peaks in the (02,11) band region.
At the outcrops where pure hexagonal platy halloysite clay was not found, the white
clay and the creamy-white clay identified as pure halloysite.
Details of X R D traces of the hexagonal platy halloysite. The clay mineral in the greenish
yellow clay was hexagonal platy halloysite. For detailed XRD study, a sample taken from
Ookusa, Komaki city (location number 2 in Fig. 1) was chosen.
Features of the XRD traces are: (i) the clay is composed of pure halloysite and does not
show any reflections of mixed-layer kaolin-smectite, (ii) the basal reflections (00/) are
broad and symmetrical, (iii) the rate of dehydration is intermediate with respect to
halloysites of different morphological forms, (iv) the (02,11) non-basal band exhibits some
slightly separated peaks, (v) the b-dimension is larger than that of other halloysites. These
features are discussed separately below.
After heating at 560~ for 3 h, no clear peaks remained in the 10-7 A region nor in the
3.5-3.0 A region. This shows that the hexagonal platy halloysite in the present work is
not mixed-layer kaolin-smectite, but pure haUoysite (Schultz et al., 1971; Wiewiora, 1971;
Yoshimura & Kohyama, 1981).
Coherent domain size was estimated from the half-height width of the 10.1 .~ peak using
macroscopic muscovite as a standard. The results are shown in Fig. 4. All the measured
10.1 A peaks of these halloysites are symmetrical under 100% r.h., i.e. they are not
interstratified with any dehydrated layers at this humidity. Thus the major part of the
differences in the half-height width values can be attributed to the differences in the
coherent domain size parallel to the c* axis. The estimates show that the hexagonal platy
halloysite is much thinner than the other halloysites. The relationships between relative
humidity and the probability of existence of dehydrated layers (7 A) in the hexagonal
platy haUoysite, a spherical halloysite and a long tubular halloysite are shown in
Fig. 5. The probability of existence of the dehydrated layer was estimated by comparing
the observed basal peak profile between 10.1 and 7 A with simulated peak profiles
(Churchman et al., 1972). Simulation of the peak profile was performed using the Kakinoki
& Komura (1952) theory of X-ray diffraction from mixed-layer structures. Long tubular
halloysite is very easy to dehydrate whereas spherical halloysite dehydrates most slowly.
Hexagonal platy halloysite is intermediate. The mixed-layer structure which forms during
EIIIIIII3
LT
SP
ST
I
""
-]
PL
,
I
,
I
200
o 300
Size of crystalite A
FiG. 4. X-ray coherentdomainsize of haUoysitesequilibratedat 100% relativehumidity.LT is
long tubular halloysite,SP is spherical and ST short tubular halloysiteand PL is hexagonalplaty
halloysite.
407
Hexagonal platy halloysite, Japan
~.0
O.
o
o
rl
O
~0.5
O.
s
gl
ttl
I
l
20
35
Relative
I
I
56 65
I
I
79
100
humidity%
FIG.5. The relationships between relative humidity and the probability of existence of 10 ,~
phase in the hexagonal halloysite (p), the spherical halloysite in an altered tuff of Naegi,
Gifu Prefecture (s) and the long tubular halloysite in a weathered granite of Naegi, Gifu
Prefecture (1). The values were estimatedby a comparison method based on interstratification
theory.
dehydration of hexagonal platy halloysite belongs to the partial segregation type. This is
the same as for other halloysites (Churchman et al., 1972; Okada & Ossaka, 1983).
In Fig. 6 (02,11) bands of a well-ordered ('well-ordered' compared with other
halloysites) roll-shape halloysite (collected at location 3 in Fig. 1), a hexagonal platy
halloysite and a poorly-ordered spherical halloysite (Naegi, Gifu Prefecture) are shown.
On the XRD trace of the well-ordered roll-shape halloysite, three sharp and intense
peaks can be seen, of which two (marked with arrows) distinctly change position and
intensity on drying. This shows that these two peaks belong to hkl (1 4: 0) reflections.
Based on the lattice parameters by Chukhrov & Zv.yagin (1966) and Kohyama et al.
(1978), these peaks may be indexed as 021 (4.35 A at 100% r.h. and 4.28 ,~ after
dehydration) and 112 (4.26 ,~ at 100% r.h. and 3.98 ,/~ after dehydration). Although the
parameters of Kohyama et al. well explain the peak positions of the wet-state sample, a
small discrepancy between calculated and observed peak position (0.11 A for the 112
reflection) is seen for the dehydrated state. Chukhrov & Zvyagin's parameters better
explain the peak positions of the dehydrated sample.
The above results suggest that the structure of the well-ordered roll-shape haUoysite is
similar to the two-layer structure of the long tubular haUoysites which have been
investigated by Honjo et al. (1954), Chukhrov & Zvyagin (1966) and Kohyama et al.
(1978).
The XRD trace of the hexagonal platy halloysite shows more broadened peaks than that
of the roll-shape halloysite; however, the 021 and 112 peaks can barely be detected. The
structure of the hexagonal platy halloysite may be similar to that of the roll-shape
halloysite.
Incomplete separation of the peaks in the (02,11) band of these halloysites indicates poor
ordering of the crystal structure. However, a slight peak separation in the (02,11) band of
hexagonal platy halloysite shows that the crystal ordering of the hexagonal platy variety is
poorer than that of the roll-shape one and superior than that of the spherical one.
408
H. Noro
roll shape
hex, platy
spherical
rhlO0
120~
2L,h
\J
J
2O
25
20
25
0
2
20 Cu K.,
FIG. 6. (02, l 1) bands of a well-ordered roll-shaped halloysite, a hexagonal platy halloysite and a
spherical halloysite. Upper:equilibrated at 100% r.h.; lower: heated at 120~ for 24 h. Indices
are based on the data of Chukhrov & Zvyagin (1966) and Kohyama et al. (1978).
The b-dimension values and Fe203 contents after deferration treatment are listed in
Table I. The hexagonal platy halloysite has the largest b-dimension. This increases with
increase in Fe203 content, although a simple linear relation is not observed.
Chemical composition
Chemical compositions of deferrated clay fractions of the red clay (halloysite with a
small amount of kaolinite) and the greenish-yellow clay (hexagonal platy halloysite) are
given in Table 2.
The calculated chemical formula of the hexagonal platy haUoysite is
2+
3+
"
(Feo.oo3) (A11.s54Feo.146)(S11.995Alo.o05)O5(OH)4.
Although precise determination of structural iron is better performed by spectroscopic
methods (Quantin et al., 1984, Nagasawa & Noro, 1985), the present iron analyses were
made by conventional methods after DCB treatment. Thus the Fe203 values in Tables 1
and 2 must be regarded only as estimates of structural iron. However, it is apparent that
among the halloysites investigated platy halloysite contains the largest amount of structural
iron.
The CEC of the hexagonal platy halloysite is 21.9 mEq/100 g, and those of the white
clay (halloysite), the red clay (halloysite with a small amount of kaolinite) and the
roll-shaped well-crystallized halloysite (collected at location 3 in Fig. 1) are 2.60, 14.5 and
1.94 mEq/1Q0 g, respectively. Anomalously high CECs of iron-rich platy halloysite have
been reported by Kunze & Bradley (1964), Wada & Mizota (1982) and Quantin et al.
(1984). Wada & Mizota explained the high CEC of their crumpled lamellar halloysite by
assuming the substitution of ferrous iron for aluminium ions in the octahedral sheet.
Quantin et al. explained the high CEC by the presence of smectitic layers.
Hexagonalplaty halloysite,Japan
409
TABLE 1. b-dimension values and Fe203 contents
after deferration treatment of hal/oysites. The
b-dimension values were estimated from the (060)
peak positions. Quartz from pegmatite and fivenine metallic silicon were used as standard,
Crystal form
b-dimension
Fe203
Hex. platy
Hex. platy
Hex, platy
Long tubular
Long tubular
Long tubular
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical
Spherical
Roll-shape
Roll-shape
Short tubular
Short tubular
8- 939
8.937
8,938
8,929
8-926
8-920
8,914
8.935
8.920
8.914
8.906
8.900
8.938
8.917
8.933
8.930
8,926
8-926
4.24
4.01
3.55
0.79
0-20
0-55
2~28
2.74
2.38
1-11
0.68
0.71
3.01
0.50
2,74
i. 58
2,40
2- 56
TABLE2. Chemical compositions of DCBtreated clay fractions of the red clay (a)
and the greenish yellow clay (b) from the
Ueno tuff at locality number 2 in Fig. 1.
(a)
(b)
SiO2
TiO:
AI203
Fe203
FeO
HzO+
41.84
nd
37,04
1-58
0.07
19.73
43.49
nd
34.38
4-24
0-07
17.83
Total
100.26
100.01
In the present investigation, the calculated SiO2/(A1203 + Fe203) molar ratio of 1.99
can not explain the high C E C of the hexagonal platy halloysite. Since X-ray examination
eliminated the possibility of mixed-layer kaolin-smectite in the greenish-yellow clay, its high
C E C m a y possibly be attributed to surface properties.
Morphology
Transmission electron micrographs o f the white clay (pure roll-shape halloysite), the red
clay (halloysite with a small amount of kaolinite), the greenish-yellow clay (pure hexagonal
410
H. Noro
#
~9
0
Q
"6
,x=
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zL
t~
"6
t~
"6
"6
e~
E
.o
e.
e~
[.
ez
Hexagonal platy halloysite, Japan
411
U
E
O
0
o_
i-
,..<
.JD
t~
i.
o
t-
06
412
H. Noro
platy haUoysite) and the creamy white clay (roll-shape halloysite) are shown in Fig. 7.
Halloysite in the white clay (Fig. 7a) and the creamy-white clay (Fig. 7d) is composed of
long elfipsoidal or roll-shaped particles and short tubular particles. Particles in the red clay
(Fig. 7b) resemble those in the white clay but irregular shaped platy crystals are also seen;
these may be kaolinite. The greenish-yellow clay (Fig. 7c) is composed of very thin
pseudo-hexagonal platelets. The platelets have curled edges and are <0.3/~m in diameter.
X-ray examination excludes the possibility that these hexagonal plates are kaolinite,
mixed-layer kaolin-smectite or gibbsite. The calculated SiO2/(AI203 + Fe203) molar ratio
of 1.99 of the greenish-yellow clay also excludes the possibility that these plates are
amorphous silica flakes such as those reported by Kirkman (1977). It can be concluded
that these hexagonal plates are also halloysite.
Scanning electron micrographs are shown in Fig. 8. In the photographs of the white clay
(Fig. 8a) and the red clay (Fig. 8b), particles show the same form as in the TEM
photographs. The greenish-yellow clay (Fig. 8c,d) is characterized by 'rose flower'-like
aggregates. Monocrystals of the hexagonal platy halloysite may correspond to the petals of
the 'flower'. The hexagonal platy halloysite may not be a completely flat plate but a portion
of a very large sphere.
Sand fraction
The sand fractions of the tuff were examined under an optical microscope and found to
comprise rock fragments, quartz, feldspar, glass, altered biotite, orthopyroxene, amphibole,
garnet and opaques.
Altered biotite. In the greenish-yellow clay, biotites were not vermicular and still
pleochroic. Biotites in the red clay were vermicular and weakly pleochroic; the partings
being filled with hematite. Biotites in the white clay were vermicular and completely
replaced by kaolinite.
Glass. Glass fragments in the greenish-yellow clay were transparent and the edges sharp.
Those in the red clay were brown and dirty and had rounded edges. The white clay
contained a small amount of rounded glass fragments.
Opaques. Opaques in the greenish-yellow clay were goethite and manganese micronodules. In the red clay, hematite and manganese micronodules occurred. The white clay
did not contain any opaque minerals.
These observations show that during the formation of the red clay and the greenishyellow clay, iron leaching was not rigorous and the akeration environment was oxidizing.
They also indicate that the alteration temperature of the red clay was higher than that of
the greenish-yellow clay (Schwertmann et al., 1979).
DISCUSSION
Platy morphology
The most characteristic property of the hexagonal platy halloysite, other than the crystal
form, is its large Fe203 content. Kunze & Bradley (1964) and de Souza Santos et al.
(1966) reported that their platy halloysites were rich in Fe203 (8-25 and 1-42%,
respectively). Thin-plate halloysite from clay veins in a granite of the Bayerischer Wald
contained 2.9% Fe203, and only a third of the iron was dithionite-soluble (Wilke et aL,
Hexagonal platy halloysite, Japan
413
1978). Crumpled lamellar halloysite described by Wada & Mizota (1982) contained 12.8%
iron oxides after DCB treatment. They concluded that the crumpled lamellar form of
halloysite could be attributed to the presence of Fe 3+ in octahedral sites. Nakagawa &
Shirozu (1983) reported that in the Omura hydrothermal halloysite deposit, the Fe20 a
content of halloysites increased with the amount of platy particles in the samples. Quantin
et aL (1984) reported that thin, irregular and sometimes crumpled lamellar halloysite-like
clay mineral contained ~7% Fe203.
To demonstrate the semiquantitative relationship between the crystal form of halloysite
and the Fe203 content, in Fig. 9 the curvature values (--the reciprocal of radius) are plotted
against the Fe203 contents after deferration. Here, 'radius' represents the average value of
the shortest radius for spherical, tubular and roll-shape halloysites. The shortest radius of
about 50 particles in each sample were measured randomly. The 'radius' of platy
halloysites is regarded as infinite, i.e. the curvature is zero. Fig. 9 shows that the curvature
of the halloysite crystal decreases with an increase of Fe203 content. So far as the data of
this study show, the halloysite crystal approaches the platy form above 3.5-4% Fe20 3.
The ionic radius of Fe a+ in six-fold coordination is 64.5 pm, and that of A13+ is 53 pm.
Thus the size of the octahedra in halloysite increases by ~6% when A1a+ is replaced by
Fe 3+. The expansion of the octahedra decreases the mismatch between the octahedral sheet
and the tetrahedral sheet, and consequently the crystal may take the platy form above
3.5-4% Fe203.
The b-dimension also increases with the Fe203 content and reaches that of kaolinites
(8.935-8.940 ,~) above 3.5-4% Fe203. This tendency is concordant with that of the
crystal form.
i
40
i
~3
.,..;
~2
080
0
1
o:4 o12
o.';
Crystal r
o.o'8 0.06
1/).ira
FIG. 9. Fe203 content of halloysitesplotted against curvature. Solid circles: hexagonal platy
halloysites; open circles: spherical and short tubular halloysites:circles with vertical line: long
tubular halloysites.
Genesis
From details of occurrence and mineralogical observations, the following genesis
of various halloysite clays in the tuff bed is deduced. At first, greenish-yellow clay
consisting of hexagonal platy halloysite was formed widely over the area. This is an
alteration product of pumice soon after its deposition in the Tokai lake in Pliocene times.
After deposition of the Seto group was complete, this area was subjected to intense
414
H. Noro
weathering. Under the near-surface oxidizing conditions, the upper part of the pumice bed
changed into red clay, which partly or entirely replaced the greertish-yellow clay. Under
subsurface conditions, on the other hand, the greenish-yellow clay changed into
creamy-white clay containing a small a m o u n t of platy halloysite. White clay above the red
clay or creamy-white clay formed by recent weathering.
Thus the hexagonal platy halloysite is considered to be a product of lacustrine diagenesis
of pumice.
CONCLUSION
The hexagonal platy form of the halloysite which was examined in this study can be
attributed to its high iron content. Ionic substitution of F e for A1 in octahedral sites
increases the curvature radius o f halloysite crystals,
The b-dimension o f halloysite increases with increase in F e 2 0 3 content but the
relationship between b-dimension and F e 2 0 3 content is not a simple linear one.
The anomalously high C EC (21.9 m E q / 1 0 0 g) o f the hexagonal platy halloysite can not
be explained by its charge deficit due to isomorphous substitution.
Geological d a t a indicate that the hexagonal platy halloysite was formed b y fresh-water
alteration o f tuff which occurred in the Pliocene L a k e Tokai.
ACKNOWLEDGMENT
I am greatly indebted to Professor Keinosuke Nagasawa, Shizuoka University, for his helpful suggestions and
critical readings of the manuscript. I also express my gratitude to Professor Kanenori Suwa and Dr Kazuhiro
Suzuki, Nagoya University for encouragement. I thank anonymous referees for many helpful suggestions and
improvements of English usage.
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