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Micromorphology of Halloysite Produced by Weathering of Plagioclase in Volcanic Ash
415
MICROMORPHOLOGY OF HALLOYSITE PRODUCED BY WEATHERING OF PLAGIOCLASE IN VOLCANIC ASH KAZUE TAZAKI Institute for Thermal Spring Research, Okayama University, Misasa, Tottori-ken, 682-02 (JAPAN)
ABSTRACT Halloysite derived from weathered plagioclase in volcanic ash s o i l s in the San-in district, Japan, exhibits various micromorphological forms, namely, of spherical, walnut-meat-shaped, acicular, crinkly, olaty, tubular and square-tube types. Spherical, walnut-meat-shaped and crinkly ones are allophane-halloysite aggregates, whereas the ones of other types are normal halloysite with no allophane. The crinkly type ones consist of rolling spheroidal particles of halloysite and fine allophane granules. The weathering process can be traced morphologically from fresh plagioclase to various types of halloysite. The possible mechanisms of the crystal growth are discussed.
INTRODUCTION Wide variations in morphology in halloysites are well known. Especially, tubular and spheroidal halloysite have wide occurrences in weathered rocks and soils from Hongkong (Parham, 1969), Texas (Kunze and Bradley, 1964, Askenasy et al., 1973)s Japan (Okada, 1973) and New Mexico (Berner and Holdren, 1977). In the present study, the weathering process of plagioclase in volcanic ash soils is investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and with the aid of other conventional methods such as X-ray diffraction and differential thermal analysis. The materials are quantitatively analyzed by electronprobe microanalyzer and electron microscopic microanalyzer (EMMA). Plagioclase samples investigated were collected from the volcanic ash s o i l s and pyroclastic sediments of Mt. Daisen and Mt. Sambe. Mt. Daisen and Mt. Sambe, located on the southwestern coast of the Japan Sea, are Quarternary complex volcanoes, composed of calc-alkali two-pyroxene andesite and biotite-hornblende dacite. Thick pyroclastic flows and volcanic ash soils from
416 these volcanoes widely cover the San-in district. A mechanism of feldspar weathering has been proposed by Berner and Holdren (1977). Acoording to them, the weathering of feldspar is controlled by chemical reaction at the feldspar-solution interface, but not by diffusion, either through aqueous solution or through a protective surface layers. Nevertheless, the present author has revealed an important role of amorphous surface layers for the formation of clays and its related minerals (Tazaki, 1976, 1977, 1978). In the present paper, the weathering process from fresh plagioclase to halloysite of various types will be demonstrated. METHODS AND MATERIALS Halloysite derived from plagioclase was isolated by hand-picking from the volcanic ash soils of Mt. Daisen and Mt. Sambe. Weathered plagioclase was lightly broken to expose the fresh surface and the resultant chips were placed on brass stubs with wet silver cement. The mounted samples were then coated with carbon and gold films with a thickness of several hundred 8 . Morphology of the mineral grains was examined by a JEOL 5A scanning electron microscope. The composition of the near-subsurface of these grains was determined by non-disuersive X-ray spectrometry, using an ORTEC Si(Li) X-ray detector and JEOL EM-ASID-4D scanning image display device. Fresh plagioclase contained 35 to 39 mole per cent An and were identified to be andesine. The difference in d-spacings between (131) and (131) reflections ranged from 1.90 to 1.95 suggesting that the plagioclase was of a high temperature type. Halloysite derived from plagioclase could be easily detected by means of X-ray powder diffraction and electron microscopic observation. The X-ray diffraction peak corresponding to the d-spacing of 10A is characteristic of halloysite and mica-group minerals, and the former is easily distinguishable from the latter by expansion of the 10A spacing to 1 1 A after ethylene glycol treatment. The differential thermal analysis curve of the halloysite showed two endothermic peaks at about 120 and 550°C and a clear exothermic peak at 990°C. RESULTS The SEM studies indicate that dissolution of plagioclase does not initially occur over the entire surface of the mineral uniformly. Instead, the dissolution occurs at sites of chemical erosion on the smooth surface, so that oval- or square shaped etch pits and conical hollows develop on the plagioclase surface (Fig. 1,2 and 3). As erosion proceeds, the whole surface becomes rugged and,further,changes partially into an amorphous state. The reaction product left by the dissolution of plagioclase forms as amorphous thin layer, of which the thickness is less than 0.5pm. Fig. 4 shows the thin layer with irregular-shaped caves which suggest proceeding erosion. Subsurface fresh plagioclase could be seen below the layer (not shown).
417
Fig. 1. Oval shaped etch pits develop on the plagioclase surface.
Fig. 2. Parallel development of square shaped etch pits.
Fig. 3 . Square shaped etch pits formed on the lamellae.
Fig. 4 . The amorphous thin layer with irregular-shaped caves which suggest preceding erosion.
Fig. 5. Spherical grains and etched plagioclase surface.
Fig. 6. Walnut-meat-shaped halloysite occurs on the swollen surface.
418
Fig. 7. Walnut-meat-shaped halloysite occurs on the smooth surface.
Fig. 8. Rounded aggregate of the tubular halloysite looks like a flower of chrysanthemum.
F i g . 9 . The crinkly halloysite forms fringed globules with a honeycomb-like surface.
Fig. 10. The crinkly halloysite consists of spheroidal particles of halloysite and fine granules of allophane.
Fig. 11. The square-tube halloysite has
Fig. 12. The platy halloysite with a fringed edge.
a rectangular cross section.
419 The X-ray diffraction of the thin layer, which was peeled off from the eroded plagioclase by ultrasonic treatment, showed an amorphous pattern with weak peaks of gibbsite Parham (1969) has shown the similar early corrosion pitting and alteration of feldspar along crystal dislocation and/or microjoints, resulting from both natural and artificial weathering. During the subsequent stage, allophane-halloysite aggregates, of spherical, walnut-meat-shaped or crinkly types, grow at the bottom of hollows or pits. spherical type product is conceivably a primitive stage of halloysite.
The
Walnut-meat-
shaped and crinkly type products are weakly crystallized halloysite containing &lophane. In the following, various types of alteration products are described. Spherical t y p e : Spherical grains are 0.02-0.2pm in diameter and are smaller than the other forms. Figure 5 shows formation of spherical grains in the left-hand side and etched plagioclase surface in the right. Spherical halloysite often coexists with a scaly mica mineral.
WaZnut-meat-shaped t y p e :
The walnut-meat-shaped type product consists of irregu-
larly bent flakes and tiny tubes with a diameter less than O.lpm (Fig. 6 and 7). Walnut-meat-shaped halloysite generally occurs on the smooth surface (Fig. I ) , and sometimes on the swollen surface (Fig. 6). Tubular t y p e :
The tubular crystal is morphologically well-known and is character-
istic of halloysite (Sudo and Takahashi, 1956; Kurahayashi and Tsuchiya, 1960; Okada, 1973). The SEM observation of the tubular halloysite was also reported by Borst and Keller (1969) and Eswaran (1972). The tubular halloysite with various lengths was produced from plagioclase in volcanic ash soils.
The size ranges from 0.2 to 2.%m
long and is about 0.lUm wide on the average (Fig. 8).
The rounded aggregate of the
tubular halloysite looks like a flower of chrysanthemum. of feldspars in granites are long and tubular shaped.
Those formed by weathering
Nagasawa and Miyazaki (1975)
reported the long halloysite tubes up to 8pm.
AcicuZar t y p e : Acicular halloysite appears spinous under low magnification ( ~ 3 0 0 0 ) . Under high magnification (x40,OOO) however, it is clear that the needle crystals form brush-like bundles. X-ray diffraction of this halloysite shows a relatively strong 10A peak in spite of its primitive morphology. The cluster of elongated kaolin minerals in Sparta granite shown by Keller (1977) is similar to the present acicular-type halloysite.
Crinkly type:
The crinkly halloysite forms fringed globules 1-5pm in diameter
with a honeycomb-like surface (Fig. 9). in halloysite.
This is the first observation of this form
A morphology similar to this was discovered for smectite (Bohor and
Hughes, 1971; Borst and Keller, 1969) of the A.P.I.
standard samples.
halloysite was also investigated by X-ray powder diffraction and TEM.
This crinkly The TEM
observation revealed that the crinkly halloysite consists of spheroidal particles of halloysite and fine granules of allophane (Fig. 10). of allophane coagulate into round grains.
Extremely fine granules
These grains morphologically resemble
420
the spherical halloysite described in the foregoing section. Observation under very high magnification (up to X 4 0 0 , O O O ) shows a rolling end of the outer layer from the spheroidal halloysite as has been observed by Sudo and Yotsumoto ( 1 9 7 7 ) . The morphology of partly rolled laths or ribbons may suggest a
'grodhprocess in the spherical halloysite.
Similar TEM morphology has been
shown by Nagasawa and Miyazaki ( 1 9 7 7 ) on halloysite formed by alteration of volcanic glass having shapes of balls and/or scrolls sometimes associated with tubes.
Square-tube and p l a t y t y p e s :
Square-tube and platy crystals coexist occasionally
with crinkly halloysite. The square-tube ranges from 1 to 15vm long and 0.2 to 7vm wide in size and has a rectangular cross section (Fig. 11). The square-tube is probably fragile and easily broken down into platy pieces by grinding.
The
morphology which is similar to the present halloysite can be found in the TEM replica photographs of the tubular halloysite from Wagon Wheel, Colorado, taken by Dixon and Mckee ( 1 9 7 4 ) , although they did not mention anything about the square tube morphology. The platy halloysites may be divided morphologically into the following two groups: The one with a shape of fringed plate of the size about 5pm long and 0.2vm thick on average (Fig. 12), and the other which is characterized by a straight edge with about 0 . a m thickness on average.
EMMA analyses of platy crystals show the presence of Ca and Na as well as Si and Al. Presence of considerable amounts of CaO and Na,O may suggest preservation of plagioclase components in square-tube halloysite. A s noted above, the square
tube is easily broken down into platy chips by grinding for one minute. This is a distinct difference from most of the ha1,loysiteswith other morphologies which do not break down either by grinding for few minutes or by heating at temperature from 200 to 7 0 0 ° C for one hour. DISCUSSION Various morphologies of halloysite can be divided into two groups, viz., primitive halloysite and normal halloysite. The primitive halloysite usually coexists with allophane, whereas the normal halloysite is free from allophane. Spherical, walnut-meat-shaped and crinkly type halloysites contain allophane and therefore they are regarded as primitive, whereas halloysites of other types are normal because of absence of allophane. The primitive type halloysites are characterized by the presence of the allophane-halloysite balls discovered by Sudo and Takahashi (1956). Sequences of development of halloysite from weathered plagioclase is schematically shown in Fig. 13. Among them, it is apparent that, from the morphological point of view, the main sequence starts with the formation of a rugged surface on plagioclase (Ht 2-1) , passing through the spherical type (Ht 2-2), the walnut-meat-shaped type (Ht 2-3) and the short tubular tyDe (Ht 2 - 4 ) ,
to the chrysanthemum-flower like
aggregates (Ht 2-5) which consist of the long tubular crystals, with occasional
421 growth of well-developed halloysite tubes (Ht 2-6). Other subsidiary sequences can be recognized: the one from Ht 2-1 in Fig. 13 to the acicular (Ht 1-1) and further to the brush-like bundles (Ht 1-2), both of which may sometimes grow into the members of the main sequence as indicated in Fig. 13. Another sequence procedes from Ht 2-1 in Fig. 13 to the crinkly type (Ht 3-11, which may change into the walnutmeat-shaped type (Ht 2-3) of the main sequence or into a complex aggregate of various morphologies (Ht 3 - 2 ) composed of the platy, square-tube and crinkly type halloysites. Dissolved silica in the permeating water may contribute to the formation of halloysite within a relatively lower stratigraphic horizon. This may be one of the processes of formation of halloysite in the heavily weathered plagioclase. A variable degree of crystallinity can be observed in weathered plagioclase within the same horizon. For example, tubular halloysite, the typical well-crystallized halloysite, occurs only in the lowest Daisen pyroclastic sediment (Dl). The occurrences of these halloysites, however, do not extend to the nearest outcrop even in the same stratum. In this context, no definite relation is recognized between the halloysite morphology and the stratigraphic succession. Alternate occurrences of primitive and normal halloysites are also found in the vertical SUCCeSSiOn Of pyroclastic sediment (DL). Several factors may be responsible for these fluctuations in the formation of Clay minerals. Among numerous factors, local differences in permeability, pH of permeating water, porosity of the soil, grain size of primary minerals and micro-topography of the localites may be of importance. Thus more detailed investigation is required to clarify the decisive factors out of these possibilities.
Fig. 13. The sequences of development of halloysite from weathered plagioclase. Htl-1; acicular, Htl-2; brush-like bundles, Ht2-1; rugged surface, Ht2-2; spherical, Ht2-3; walnut-meat-shaped, Ht2-4; short tubular, Ht2-5; chrysanthemum masses, Ht2-6; long tubular, Ht3-1; crinkly, Ht3-2; crinkly, square-tube and platy.
422
ACKNOWLEDGMENTS The writer would like to express her cordial thanks to Dr. Susumu Shimoda of Thanks
the Tsukuba University for his valuable suggestions, and many instructions.
are also due to Professor Yoshito Matsui of the same Institute for his advice and critical reading of the manuscript. The writer also thanks Dr. Koichi Tazaki of the same Institute for many suggestions and instructions in the course o f the study. REFERENCES Askenasy,P.E.,Dixon,J.B. and Mckee,T.R.,1973. Spheroidal halloysite in a Guatemalan Soil. Soil Sci.Soc.Am.Proc.,37:399-803. Berner,R.A. and Holdren,Jr.,G.R.,1977. Mechanism of feldspar weathering: some observation evidence.Geology,5:369-372. Bohor,B.F. and Hughes,R.E.,1971. Scanning electron microscopy of clays and clay minerals. Clays Clay Miner.,19:49-54. Borst,R.L. and Keller,W.D.,1969. Scanning electron micrographs of API reference clay minerals and other selected samples. Proc. Int. Clay Conf. 1969, 871-901. Dixon,J.B. and McKee,T.R.,1974. Internal and external morphology of tubular and spheroidal halloysite particles. Clays Clay Miner.,22:127-137. Eswaran,H.,1972. Morphology of allophane, imogolite and halloysite. Clay Miner., 9 ~281-285. Keller,W.D.,1977. Scan electron micrographs of kaolins collected from diverse environments of origin. Clays Clay Miner.,25:311-345. Kunze,G.W. and Bradlly,W.F.,1964. Occurrence of a tabular halloysite in a Texas soil. Clays Clay Miner.,12:523-527. Kurahayashi,S. and Tsuchiya,T.,1960. On the clay minerals of the Kanto Loam(3). Geol. SOC. Japan J.,66:586-593. (in Japanese, with English abstract! Nagasawa,K. and Miyazaki,S.,1975. Mineralogical properties of halloysite as related to its genesis. Proc. Int. Clay Conf. 1975, 257-265. Okada,S.,1973. Clay minerals in the Shikotsu pumice fall deposits. Geol. SOC. Japan J.,79:363-375. (in Japanese, with English abstract) Parham,W.E.,1969. Formation of halloysite from feldspar: Low temperature, artifical weathering versus natural weathering. Clays Clay Miner.,17:13-22. Sudo,T. and Takahashi,H.,1956. Shapes of hallpysite particles in Japanese clays. Clays Clay Miner.,4:67-79. Sudo,T. and Yotsumoto,H.,1977. The formation of halloysite tubes from spherulitic halloysite. Clays Clay Miner.,25:155-159. Tazaki,K.,1976. Scanning electron microscopic study of formation of gibbsite from plagioclase. Pap. of the Institute for Themal Spring Research, Okayama Univ., 45:ll-24. Tazaki,K.,1977. Scanning electron microscopic study of clay minerals and non-clay minerals produced from plagioclase in volcanic ashes with regard to weathering process. (Unpublished Doctor thesis, Tokyo University of Education). Tazaki,K.,1978. Micromorphology of plagioclase surface at incipient stage of weathering. Earth Sci. (Chikyij Kagaku), 32:s-12. (in Japanese, with English abstract)
MICROMORPHOLOGY OF HALLOYSITE PRODUCED BY WEATHERING OF PLAGIOCLASE IN VOLCANIC ASH KAZUE TAZAKI Institute for Thermal Spring Research, Okayama University, Misasa, Tottori-ken, 682-02 (JAPAN)
ABSTRACT Halloysite derived from weathered plagioclase in volcanic ash s o i l s in the San-in district, Japan, exhibits various micromorphological forms, namely, of spherical, walnut-meat-shaped, acicular, crinkly, olaty, tubular and square-tube types. Spherical, walnut-meat-shaped and crinkly ones are allophane-halloysite aggregates, whereas the ones of other types are normal halloysite with no allophane. The crinkly type ones consist of rolling spheroidal particles of halloysite and fine allophane granules. The weathering process can be traced morphologically from fresh plagioclase to various types of halloysite. The possible mechanisms of the crystal growth are discussed.
INTRODUCTION Wide variations in morphology in halloysites are well known. Especially, tubular and spheroidal halloysite have wide occurrences in weathered rocks and soils from Hongkong (Parham, 1969), Texas (Kunze and Bradley, 1964, Askenasy et al., 1973)s Japan (Okada, 1973) and New Mexico (Berner and Holdren, 1977). In the present study, the weathering process of plagioclase in volcanic ash soils is investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and with the aid of other conventional methods such as X-ray diffraction and differential thermal analysis. The materials are quantitatively analyzed by electronprobe microanalyzer and electron microscopic microanalyzer (EMMA). Plagioclase samples investigated were collected from the volcanic ash s o i l s and pyroclastic sediments of Mt. Daisen and Mt. Sambe. Mt. Daisen and Mt. Sambe, located on the southwestern coast of the Japan Sea, are Quarternary complex volcanoes, composed of calc-alkali two-pyroxene andesite and biotite-hornblende dacite. Thick pyroclastic flows and volcanic ash soils from
416 these volcanoes widely cover the San-in district. A mechanism of feldspar weathering has been proposed by Berner and Holdren (1977). Acoording to them, the weathering of feldspar is controlled by chemical reaction at the feldspar-solution interface, but not by diffusion, either through aqueous solution or through a protective surface layers. Nevertheless, the present author has revealed an important role of amorphous surface layers for the formation of clays and its related minerals (Tazaki, 1976, 1977, 1978). In the present paper, the weathering process from fresh plagioclase to halloysite of various types will be demonstrated. METHODS AND MATERIALS Halloysite derived from plagioclase was isolated by hand-picking from the volcanic ash soils of Mt. Daisen and Mt. Sambe. Weathered plagioclase was lightly broken to expose the fresh surface and the resultant chips were placed on brass stubs with wet silver cement. The mounted samples were then coated with carbon and gold films with a thickness of several hundred 8 . Morphology of the mineral grains was examined by a JEOL 5A scanning electron microscope. The composition of the near-subsurface of these grains was determined by non-disuersive X-ray spectrometry, using an ORTEC Si(Li) X-ray detector and JEOL EM-ASID-4D scanning image display device. Fresh plagioclase contained 35 to 39 mole per cent An and were identified to be andesine. The difference in d-spacings between (131) and (131) reflections ranged from 1.90 to 1.95 suggesting that the plagioclase was of a high temperature type. Halloysite derived from plagioclase could be easily detected by means of X-ray powder diffraction and electron microscopic observation. The X-ray diffraction peak corresponding to the d-spacing of 10A is characteristic of halloysite and mica-group minerals, and the former is easily distinguishable from the latter by expansion of the 10A spacing to 1 1 A after ethylene glycol treatment. The differential thermal analysis curve of the halloysite showed two endothermic peaks at about 120 and 550°C and a clear exothermic peak at 990°C. RESULTS The SEM studies indicate that dissolution of plagioclase does not initially occur over the entire surface of the mineral uniformly. Instead, the dissolution occurs at sites of chemical erosion on the smooth surface, so that oval- or square shaped etch pits and conical hollows develop on the plagioclase surface (Fig. 1,2 and 3). As erosion proceeds, the whole surface becomes rugged and,further,changes partially into an amorphous state. The reaction product left by the dissolution of plagioclase forms as amorphous thin layer, of which the thickness is less than 0.5pm. Fig. 4 shows the thin layer with irregular-shaped caves which suggest proceeding erosion. Subsurface fresh plagioclase could be seen below the layer (not shown).
417
Fig. 1. Oval shaped etch pits develop on the plagioclase surface.
Fig. 2. Parallel development of square shaped etch pits.
Fig. 3 . Square shaped etch pits formed on the lamellae.
Fig. 4 . The amorphous thin layer with irregular-shaped caves which suggest preceding erosion.
Fig. 5. Spherical grains and etched plagioclase surface.
Fig. 6. Walnut-meat-shaped halloysite occurs on the swollen surface.
418
Fig. 7. Walnut-meat-shaped halloysite occurs on the smooth surface.
Fig. 8. Rounded aggregate of the tubular halloysite looks like a flower of chrysanthemum.
F i g . 9 . The crinkly halloysite forms fringed globules with a honeycomb-like surface.
Fig. 10. The crinkly halloysite consists of spheroidal particles of halloysite and fine granules of allophane.
Fig. 11. The square-tube halloysite has
Fig. 12. The platy halloysite with a fringed edge.
a rectangular cross section.
419 The X-ray diffraction of the thin layer, which was peeled off from the eroded plagioclase by ultrasonic treatment, showed an amorphous pattern with weak peaks of gibbsite Parham (1969) has shown the similar early corrosion pitting and alteration of feldspar along crystal dislocation and/or microjoints, resulting from both natural and artificial weathering. During the subsequent stage, allophane-halloysite aggregates, of spherical, walnut-meat-shaped or crinkly types, grow at the bottom of hollows or pits. spherical type product is conceivably a primitive stage of halloysite.
The
Walnut-meat-
shaped and crinkly type products are weakly crystallized halloysite containing &lophane. In the following, various types of alteration products are described. Spherical t y p e : Spherical grains are 0.02-0.2pm in diameter and are smaller than the other forms. Figure 5 shows formation of spherical grains in the left-hand side and etched plagioclase surface in the right. Spherical halloysite often coexists with a scaly mica mineral.
WaZnut-meat-shaped t y p e :
The walnut-meat-shaped type product consists of irregu-
larly bent flakes and tiny tubes with a diameter less than O.lpm (Fig. 6 and 7). Walnut-meat-shaped halloysite generally occurs on the smooth surface (Fig. I ) , and sometimes on the swollen surface (Fig. 6). Tubular t y p e :
The tubular crystal is morphologically well-known and is character-
istic of halloysite (Sudo and Takahashi, 1956; Kurahayashi and Tsuchiya, 1960; Okada, 1973). The SEM observation of the tubular halloysite was also reported by Borst and Keller (1969) and Eswaran (1972). The tubular halloysite with various lengths was produced from plagioclase in volcanic ash soils.
The size ranges from 0.2 to 2.%m
long and is about 0.lUm wide on the average (Fig. 8).
The rounded aggregate of the
tubular halloysite looks like a flower of chrysanthemum. of feldspars in granites are long and tubular shaped.
Those formed by weathering
Nagasawa and Miyazaki (1975)
reported the long halloysite tubes up to 8pm.
AcicuZar t y p e : Acicular halloysite appears spinous under low magnification ( ~ 3 0 0 0 ) . Under high magnification (x40,OOO) however, it is clear that the needle crystals form brush-like bundles. X-ray diffraction of this halloysite shows a relatively strong 10A peak in spite of its primitive morphology. The cluster of elongated kaolin minerals in Sparta granite shown by Keller (1977) is similar to the present acicular-type halloysite.
Crinkly type:
The crinkly halloysite forms fringed globules 1-5pm in diameter
with a honeycomb-like surface (Fig. 9). in halloysite.
This is the first observation of this form
A morphology similar to this was discovered for smectite (Bohor and
Hughes, 1971; Borst and Keller, 1969) of the A.P.I.
standard samples.
halloysite was also investigated by X-ray powder diffraction and TEM.
This crinkly The TEM
observation revealed that the crinkly halloysite consists of spheroidal particles of halloysite and fine granules of allophane (Fig. 10). of allophane coagulate into round grains.
Extremely fine granules
These grains morphologically resemble
420
the spherical halloysite described in the foregoing section. Observation under very high magnification (up to X 4 0 0 , O O O ) shows a rolling end of the outer layer from the spheroidal halloysite as has been observed by Sudo and Yotsumoto ( 1 9 7 7 ) . The morphology of partly rolled laths or ribbons may suggest a
'grodhprocess in the spherical halloysite.
Similar TEM morphology has been
shown by Nagasawa and Miyazaki ( 1 9 7 7 ) on halloysite formed by alteration of volcanic glass having shapes of balls and/or scrolls sometimes associated with tubes.
Square-tube and p l a t y t y p e s :
Square-tube and platy crystals coexist occasionally
with crinkly halloysite. The square-tube ranges from 1 to 15vm long and 0.2 to 7vm wide in size and has a rectangular cross section (Fig. 11). The square-tube is probably fragile and easily broken down into platy pieces by grinding.
The
morphology which is similar to the present halloysite can be found in the TEM replica photographs of the tubular halloysite from Wagon Wheel, Colorado, taken by Dixon and Mckee ( 1 9 7 4 ) , although they did not mention anything about the square tube morphology. The platy halloysites may be divided morphologically into the following two groups: The one with a shape of fringed plate of the size about 5pm long and 0.2vm thick on average (Fig. 12), and the other which is characterized by a straight edge with about 0 . a m thickness on average.
EMMA analyses of platy crystals show the presence of Ca and Na as well as Si and Al. Presence of considerable amounts of CaO and Na,O may suggest preservation of plagioclase components in square-tube halloysite. A s noted above, the square
tube is easily broken down into platy chips by grinding for one minute. This is a distinct difference from most of the ha1,loysiteswith other morphologies which do not break down either by grinding for few minutes or by heating at temperature from 200 to 7 0 0 ° C for one hour. DISCUSSION Various morphologies of halloysite can be divided into two groups, viz., primitive halloysite and normal halloysite. The primitive halloysite usually coexists with allophane, whereas the normal halloysite is free from allophane. Spherical, walnut-meat-shaped and crinkly type halloysites contain allophane and therefore they are regarded as primitive, whereas halloysites of other types are normal because of absence of allophane. The primitive type halloysites are characterized by the presence of the allophane-halloysite balls discovered by Sudo and Takahashi (1956). Sequences of development of halloysite from weathered plagioclase is schematically shown in Fig. 13. Among them, it is apparent that, from the morphological point of view, the main sequence starts with the formation of a rugged surface on plagioclase (Ht 2-1) , passing through the spherical type (Ht 2-2), the walnut-meat-shaped type (Ht 2-3) and the short tubular tyDe (Ht 2 - 4 ) ,
to the chrysanthemum-flower like
aggregates (Ht 2-5) which consist of the long tubular crystals, with occasional
421 growth of well-developed halloysite tubes (Ht 2-6). Other subsidiary sequences can be recognized: the one from Ht 2-1 in Fig. 13 to the acicular (Ht 1-1) and further to the brush-like bundles (Ht 1-2), both of which may sometimes grow into the members of the main sequence as indicated in Fig. 13. Another sequence procedes from Ht 2-1 in Fig. 13 to the crinkly type (Ht 3-11, which may change into the walnutmeat-shaped type (Ht 2-3) of the main sequence or into a complex aggregate of various morphologies (Ht 3 - 2 ) composed of the platy, square-tube and crinkly type halloysites. Dissolved silica in the permeating water may contribute to the formation of halloysite within a relatively lower stratigraphic horizon. This may be one of the processes of formation of halloysite in the heavily weathered plagioclase. A variable degree of crystallinity can be observed in weathered plagioclase within the same horizon. For example, tubular halloysite, the typical well-crystallized halloysite, occurs only in the lowest Daisen pyroclastic sediment (Dl). The occurrences of these halloysites, however, do not extend to the nearest outcrop even in the same stratum. In this context, no definite relation is recognized between the halloysite morphology and the stratigraphic succession. Alternate occurrences of primitive and normal halloysites are also found in the vertical SUCCeSSiOn Of pyroclastic sediment (DL). Several factors may be responsible for these fluctuations in the formation of Clay minerals. Among numerous factors, local differences in permeability, pH of permeating water, porosity of the soil, grain size of primary minerals and micro-topography of the localites may be of importance. Thus more detailed investigation is required to clarify the decisive factors out of these possibilities.
Fig. 13. The sequences of development of halloysite from weathered plagioclase. Htl-1; acicular, Htl-2; brush-like bundles, Ht2-1; rugged surface, Ht2-2; spherical, Ht2-3; walnut-meat-shaped, Ht2-4; short tubular, Ht2-5; chrysanthemum masses, Ht2-6; long tubular, Ht3-1; crinkly, Ht3-2; crinkly, square-tube and platy.
422
ACKNOWLEDGMENTS The writer would like to express her cordial thanks to Dr. Susumu Shimoda of Thanks
the Tsukuba University for his valuable suggestions, and many instructions.
are also due to Professor Yoshito Matsui of the same Institute for his advice and critical reading of the manuscript. The writer also thanks Dr. Koichi Tazaki of the same Institute for many suggestions and instructions in the course o f the study. REFERENCES Askenasy,P.E.,Dixon,J.B. and Mckee,T.R.,1973. Spheroidal halloysite in a Guatemalan Soil. Soil Sci.Soc.Am.Proc.,37:399-803. Berner,R.A. and Holdren,Jr.,G.R.,1977. Mechanism of feldspar weathering: some observation evidence.Geology,5:369-372. Bohor,B.F. and Hughes,R.E.,1971. Scanning electron microscopy of clays and clay minerals. Clays Clay Miner.,19:49-54. Borst,R.L. and Keller,W.D.,1969. Scanning electron micrographs of API reference clay minerals and other selected samples. Proc. Int. Clay Conf. 1969, 871-901. Dixon,J.B. and McKee,T.R.,1974. Internal and external morphology of tubular and spheroidal halloysite particles. Clays Clay Miner.,22:127-137. Eswaran,H.,1972. Morphology of allophane, imogolite and halloysite. Clay Miner., 9 ~281-285. Keller,W.D.,1977. Scan electron micrographs of kaolins collected from diverse environments of origin. Clays Clay Miner.,25:311-345. Kunze,G.W. and Bradlly,W.F.,1964. Occurrence of a tabular halloysite in a Texas soil. Clays Clay Miner.,12:523-527. Kurahayashi,S. and Tsuchiya,T.,1960. On the clay minerals of the Kanto Loam(3). Geol. SOC. Japan J.,66:586-593. (in Japanese, with English abstract! Nagasawa,K. and Miyazaki,S.,1975. Mineralogical properties of halloysite as related to its genesis. Proc. Int. Clay Conf. 1975, 257-265. Okada,S.,1973. Clay minerals in the Shikotsu pumice fall deposits. Geol. SOC. Japan J.,79:363-375. (in Japanese, with English abstract) Parham,W.E.,1969. Formation of halloysite from feldspar: Low temperature, artifical weathering versus natural weathering. Clays Clay Miner.,17:13-22. Sudo,T. and Takahashi,H.,1956. Shapes of hallpysite particles in Japanese clays. Clays Clay Miner.,4:67-79. Sudo,T. and Yotsumoto,H.,1977. The formation of halloysite tubes from spherulitic halloysite. Clays Clay Miner.,25:155-159. Tazaki,K.,1976. Scanning electron microscopic study of formation of gibbsite from plagioclase. Pap. of the Institute for Themal Spring Research, Okayama Univ., 45:ll-24. Tazaki,K.,1977. Scanning electron microscopic study of clay minerals and non-clay minerals produced from plagioclase in volcanic ashes with regard to weathering process. (Unpublished Doctor thesis, Tokyo University of Education). Tazaki,K.,1978. Micromorphology of plagioclase surface at incipient stage of weathering. Earth Sci. (Chikyij Kagaku), 32:s-12. (in Japanese, with English abstract)