Weathering implications of the mineralogy of clay fractions of two Ando soils, Kyushu

Geoderma, 14 (1975) 47--62 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

WEATHERING IMPLICATIONS OF THE MINERALOGY OF CLAY FRACTIONS OF TWO ANDO SOILS, KYUSHU

Y. TOKASHIKI1 and K. WADA

Faculty of Agriculture, Kyushu University, Fukuoka (Japan) (Received May 20, 1974; accepted for publication February 6, 1975)

ABSTRACT Tokashiki, Y. and Wada, K., 1975. Weathering implications of the mineralogy of clay fractions of two Ando soils, Kyushu. Geoderma, 14: 47--62. The mineralogy of the clay fraction was determined by a combination of methods on samples of eight layers from each of two profiles. Among them, five layers were derived from andesitic ash and three layers from dacitic ash. Conclusions that could be drawn from the mineralogy follow: (1) The amounts of layer silicates were much higher in materials from dacitic ash than in those from andesitic ash. (2) Opaline silica was found only in the IA 1 and IIIAI horizons at and near the land surface but not in the intervening II(B) horizon. (3) Allophane and imogolite were absent or nearly absent in the IA~ and the oldest VIIIA~ horizons. All other buried A~ and (B) horizons contained allophane and imogolite. Allophane-like constituents were present in all the horizons except for the VIIIA~ horizon.

INTRODUCTION S h o w e r s o f volcanic ashes h a v e c o v e r e d large areas o f m i d d l e K y u s h u . T h e p r i n c i p a l sources o f t h e s e ashes are Mt. U n z e n , Mt. Aso, Mt. K u j u a n d Mt. Y u f u w h i c h line u p f r o m w e s t t o east. R o c k m a t e r i a l s o f Mt. Aso largely consist o f p y r o x e n e andesite, w h e r e a s t h o s e o f o t h e r v o l c a n o e s largely consist o f h o r n blende, h o r n b l e n d e - b i o t i t e a n d q u a r t z - h o r n b l e n d e andesite. I n t h e Kuju district, s o m e v o l c a n i c activities in t h e R e c e n t h a v e also r e s u l t e d in h e a p i n g u p o f p y r o x e n e a n d o l i v i n e - p y r o x e n e a n d e s i t e ( K u n o , 1 9 5 4 ; M a t s u m o t o e t al., 1962). T a m u r a ( 1 9 6 7 ) r e c o g n i z e d f o u r s u p e r i m p o s e d d e p o s i t s o f v o l c a n i c ash in the K u j u district a n d n a m e d t h e m Kuju-a, Kuju-b, Kirishima-d a n d Kuju-c acc o r d i n g t o t h e p r o b a b l e s o u r c e and t h e e r u p t i o n d a t e . He also n o t e d t h a t t h e s e deposits showed differentiation into A and B horizons, and that humus amount ing t o m o r e t h a n 30% o f t h e soil m a t e r i a l a c c u m u l a t e d in t h e A h o r i z o n s o f 1present address: Faculty of Agriculture, Ryukyu University, Naha (Japan).

48 Kuju-a and Kuju-c deposits. Wada (1969} and Kanno (1971) tabulated unpublished analyses of the soils in the district by Aomine and Wada and Kuwano. The data indicated that these soils have characteristics typical of Ando soils, as defined by Thorp and Smith (1949). Preliminary analyses of clay fractions (Wada, 1969) showed that allophane and related amorphous to poorly crystalline constituents predominate, but there is a considerable variation in the contents of crystalline layer silicates, possibly depending on the nature of parent ash. The first purpose of the present investigation is to provide more detailed data of the clay minerals in Ando soils formed from volcanic ash in the Kuju district. Two profiles were selected to represent ash deposits in the district. Semiquantitative mineralogical analysis covering both amorphous and crystalline constituents was carried out by selective dissolution, differential infrared spectroscopy in combination with X-ray analysis and electron microscopy. The second purpose is to consider weathering of volcanic ash and formation of clay minerals. Emphasis was placed on the effect of the original nature of parent ash on the mineral composition of weathering products and on the mutual interaction between organic matter and amorphous clay constituents. Superimposed deposition of different ashes and soil formation in the profiles provided a unique opportunity to study these problems. SAMPLES AND METHODS

Descriptions of soils and analytical methods Soils were collected from one profile at each of Asahidai (lat. 33°10'6"N, long. 131°15'4"E) and Yoshibu (lat. 33°8'45"7N, long. 131°19'23"6E), Kokonoe-machi, Kusu-gun, Oita-ken. Both profiles were located on the tops of gently rolling hills. The annual rainfall of the area is 2000--2400 mm with a maximum in June and July, and mean summer and winter temperatures are 20--24°C and 0--4°C, respectively. Partial descriptions of the soil profiles and analyses of the samples are given in Tables I and II. The carbon content was determined by Tyurin's method (Kononova, 1961). The pH of the soil suspension was determined at a soil (air-dry) to water (or N KC1) ratio of 10 g to 25 ml. The particle-size distribution was determined after treating 10 g of airdry soil with 6% H202, followed by washing with water, and dispersing the treated soil at pH 4 using sonic waves of 28 kc. The clay, silt and sand were collected by repeated dispersion and sedimentation or by sieving, and their weights were determined. The contents of quartz, cristobalite and feldspar in the silt were estimated by reading the peak heights of the X-ray reflections at 3.34, 4.04 and 3.19--3.20 £, respectively. The observations and analyses listed in Tables I and II led to correlation of the ash deposits with the layers and horizons in the two profiles, as listed in the Tables. In this paper, the layers and horizons were designated according to the conventions described by Taylor and Pohlen (1962). Both root and worm

49

TABLE I Brief descriptions of the Asahidai and Yoshibu profiles Soil and sample number

Layer and horizon

Depth (cm)

Color

Structure

Consistence

Asahidai

T-1-1

IA 1

0--25

2/0

crumb

very friable

T-1-2

IIIAI

25--50

2/0

subangular blocky

friable

T-l-3

VAj

50--70

10YR 2/1

subangular blocky

firm

T-1-4 T-1-5

VIA 1 VI(B)

70--90 90--100

10YR 2/2 10YR 3/2

angular blocky angular blocky

firm very firm

T-1-6 T-l-7 ~

VIIAI VII(B)

100--115 115--130

10YR 2/1 7.5YR 5/8

angular blocky angular blocky

firm friable

T-l-8 T-l-9

VIIIA~ VIII(B)

130--180 +180

5YR 2/1 5YR 5/8

angular blocky angular blocky

firm very firm

10YR 2/1

crumb

very friable

Y o sh ib u

3181

IA~

0--30

3182

II(B)

30--42

10YR 5/6

massive

very firm

3183

IIIAI

42--92

5YR 2/1

subangular blocky

firm

3184

IV(B)

92--111

10YR 4/2

angular blocky

firm

3185 3186

VIA1 VI(B)

111--134 134--151

7.5YR 3/2 7.5YR 4/4

angular blocky massive

firm very firm

3187 31881

VIIA~ VII(B)

151--173 173--183

10YR 2/1 10YR 5/8

angular blocky subangular blocky

firm firm

~"Imogo". channels were observed in the profiles. The analyses listed in Table II indicate, however, th at the effects o f mixing materials b y action of flora and fauna may have red u ced b u t have n o t obliterated the differences bet w een either the layers or the horizons. A comparison b et w een the descriptions b y T a m u r a (1967) and those by the present authors suggests t h a t the Kuju-a ash consists of at least three different ashes: I, III and V (Table I). Ash deposits II and IV were n o t described by him. These deposits are characterized by high contents of coarse sand com bi ned with marked firmness o f soil material (Table II), and their distribution is limited. A " k o r a " (hard pan) soft, t he distribution and properties of which were investigated b y Tsuno and Takada (1969), may correspond t o ash deposit II. Ash deposits VI, VII and VIII pr oba bl y correspond to Kuju-b, Kirishima-d and Kuju-c ashes o f Tamura. A m ong them, deposit VII is characterized by glassy n a t u re o f ash ( " I m o g o " ) .

50

TABLE II

Analyses of Asahidai and Yoshibu soil samples Soil and sample number

Layer and horizon

Particle-size(ram) distribution (%)

Carbon p H (%)

(H~O)

Qz:Cb:Fd contents in silt

(KCI) 2--

0.2--

0.02-- <0.002

0.2

0.02

0.002

Asahidai T-l-1

IA,

14.2

5.2

4.6

5

47

28

21

T-1-2

IIIA 1

14.2

5.2

4.6

4

35

34

27

10:7:6 8:10:5

T-l-3

VA,

6.9

5.4

5.4

4

47

23

26

3:10:5

T-l-4 T-l-5

VIA, VI(B)

7.4 4.2

5.5 5.5

5.4 5.6

2 3

41 55

26 24

30 18

2:8:4 2:7:5

T-l-6 T-1-7

VIIA, VII(B)

8.0 2.4

5.3 5.4

5.1 5.6

2 8

40 49

31 30

28 14

5:4:2 5:3:2

T-1-8 T-1-9

VIIIA, VIII(B)

14.4 1.4

5.2 5.4

4.2 5.4

1 6

14 26

48 35

37 33

27:4:3 26:4:4

3181

IA,

16.7

4.9

4.2

2

42

32

24

10:10:6

3182

II(B)

1.8

5.6

5.6

23

47

20

12

4:7:10

3183

IIIA,

12.0

5.4

4.6

6

30

33

30

7:9:4

3184

IV(B)

2.4

5.9

5.8

27

33

22

18

2:5:4

3185 3186

VIA, VI(B)

5.1 1.1

5.7 5.9

5.4 5.6

3 7

41 57

27 20

28 16

2:5:4 1:7:7

3187 3188

VIIA 1 VII(B)

4.3 1.9

5.6 5.8

5.1 5.6

3

52 .

Yoshibu

.

31 .

14 .

3:3:2 .

The contents of quartz, cristobalite and feldspar in the silt (Table II) show that the ash deposits were derived from andesitic to dacitic magma. Tamura (1967) also noted differences in mineralogic nature among the Kuju ashes and reported that orthopyroxene, orthopyroxene~linopyroxene and hornblende predominated as heavy minerals in the fine sand of Kuju-a (= I, III and V), b (= VI) and c (= VIII) ashes, respectively. The 14C ages of humus in the ash deposits corresponding to IIIA,, VIIA1 and VIIIAI were reported by Yamada (1968) to be 3500 + 100, 4300 ± 270 and 9060 + 100 years, respectively. Taking the time necessary for humus accumulation into consideration (Wada 1967), it may be inferred that ash deposits III and VIII have been subject to weathering over periods of about 2500 years and 10,000 years, respectively.

51

Analyses of clay minerals The clays collected were kept as flocculated suspensions by adding small amounts of NaC1. The composition and amounts of amorphous components were determined by the following procedure (Wada and Greenland, 1970; Wada and Tokashiki, 1972; Tokashiki and Wada, 1972): First, the clay was successively treated with dithionite-citrate, 2% Na2 CO3 and 0.5N NaOH. Second, each extract was analyzed for Si, A1 and Fe. Third, the infrared spectrum representing the material removed by the dissolution treatment was obtained by placing the clay samples before and after each treatment at the reference and sample side of a spectrophotometer, respectively. Imogolite and crystalline constituents were identified by electron microscopy and X-ray analysis. The contents of layer silicates were roughly estimated by reading the 001 peak heights of the respective minerals and by assuming constant peak height ratios for the 1:1 mixture of the minerals such as I (15 A montmorillonite)/I (10 h illite) = 3, I (17 h montmorillonite)/I (10 h iUite) = 4 and so on (Wada, 1966). Estimates of the contents of quartz, cristobalite and feldspars were made as described earlier. ANALYTICAL R E S U L T S

The results of the analyses of clay constituents are summarized in Table III. The content of the whole soluble fraction was equated with the total weight loss of the sample from successive dithionite-citrate and 0.5N NaOH treatments. The content of each soluble fraction was estimated by allocating this total weight loss to each fraction according to the sum of extracted SiO2, A1203 and Fe203 and assuming that the H20 (+) content of each soluble fraction was the same. The content of each soluble fraction in the clay generally decreased in the order; dithionite-citrate .>\0.5N NaOH > 2% Na2 CO3 soluble fractions (Table III). The proportion of the dithionite-citrate soluble fraction to the whole soluble fraction ranged from 44 to 71%. The highest value was found for the clay from the oldest (VIIIA,) layer. The SiO2/A1203 molar ratio of the dithionite-citrate soiuble fraction was low, ranging from 0.18 to 0.40. The infrared spectra of the dithionite-citrate soluble fractions except for that from the VIIIA, clay showed features of allophane-like constituents and additional alumina-rich constituents (Fig.l), as described by Wada and Greenland (1970) and Wada and Tokashiki (1972). The poorly defined spectrum for the VIIIA, clay suggested that alumina derived from humus complexes may have been dissolved by the treatment. The extracted Fe2 03 content was in the range of 9.5 to 15.7% and did not parallel the extracted A1203 content. The lowest Fe203 contents were found in the clays from the VIIA, and VII(B) horizons, derived from "Imogo" ash. The SiO2/A12 O3 ratio of the 2% Na2 CO3 soluble fraction was higher than that of the respective dithionite-citrate soluble fraction and was in the range

52

,

I

,

[

,

I0 8 X IOOcm -I

Fig.1. Infrared spectra o f dithionite-citrate (full line) and 2% Na2CO s (dotted line) soluble fractions. 1 ffi IA~ (Asahidai); 2 = IA~ (Yoshibu); 3 = II(B), IIA~, IV(B), VA~, VI(B), VIIAI, VII(B), VIII(B); 4 = VIIIA~. The spectrum for the VI(B) clay is shown as an example.

of 0.40 to 0.80 (Table III). The infrared spectra of the 2% Na2CO3 soluble fractions except for that from the VIIIA~ clay indicated that their main constituent was a fairly uniform allophane-like constituent (Fig.l). The presence of opaline silica in the IA~ clays was suggested by their high SIO2/A1203 ratios of 6.2 and 13.0 for the 0.5N NaOH soluble fraction (Table III). As described by Shoji and Masui (1969a, b; 1971), electron microscopy showed that there are thin, circular objects in these clays (Fig.2) which disappear after 0.5N NaOH treatment. The difference infrared spectra showed a strong absorption with a maximum at 1070 cm -1 and with a shoulder at 1200 c m - ' and a weak absorption with a maximum at about 800 c m - ' (Fig.3). A weak shoulder at about 930 cm -1 may be attributed to aUophane, but the amount of aUophane, if any, is much smaller in these clays than in the others. Opaline silica was also present in the IIIA~ clay but in smaller amounts. The main constituents of the 0.5N NaOH soluble fractions in the clays from deposits IV, V, VI and VII were allophane and imogolite (Table III). The values of their SiO2/A12Os ratios were in the range of 1.1 to 1.3. The morphology of allophane and imogolite in these clays and its change after dithionite-citrate and 2% Na2 COs treatments are illustrated in Figs.4a and 4b, respectively. A broad reflection at 14 to 15 h, which appeared on the X-ray patterns of the similarly treated, Mg- or K-saturated and air-dried clays, was attributed to imogolite. However; this broad reflection disappeared upon heating at 100--300°C and development of a reflection at 18--19 A -- an X-ray feature of typical imogolite -- was not evident. The same clays prior to treatment gave X-ray patterns of nearly amorphous materials. Either poor development of imogolite structure or close association of allophane and other constituents with imogolite units in these clays was therefore suggested. In contrast with

53 other days, no dissolution of allophane and partial dissolution of layer silicates were indicated for the VIIIA1 clay by the difference spectra (Fig.3}. Short threads, possibly imogolite, were seen in this clay but much fewer in number than in the VIII(B) clay. The VIII(B) clay also contained allophane and gibbsite. That the amounts of allophane and imogolite affect the chemical property of a soil is illustrated by a correlation between the pH values (Table II) and the content of allophane--imogolite (Table III) of the respective samples. The difference between the pH in water and in KCI solutions decreases with increasing amounts of allophane--imogolite, reflecting high anion-exchange capacities of those constituents. Table II also shows that the amount of organic matter in the soil material has the opposite effect on pH values measured by the two methods. Considerable amounts of interlayer A1 in 2:1 and 2:2 mineral intergrades present in some clays were also extracted by 0.SN NaOH treatment. This treatment promoted a shift of the 14--15 A reflection of the Mg-saturated clay to 10 A upon K saturation. The species of layer silicates listed in Table III were named after the clays had been treated with 0.5N NaOH. Prior to this treatment, either vermiculite or montmoriUonite existed largely as chloritic intergrades except for montmorillonite in deposits II and VII. The appearance of vermiculite and montmorillonite in the deposits other than II and VII, as shown in Table III, therefore denotes the relative ease with which interlayer A1 was removed by 0.5N NaOH treatment. The amount of residue remaining after 0.5N NaOH treatment was higher in the soil clays from the dacitic ashes I, III and VIII than in those from the andesitic ashes IV, V, VI and VII (Table III), with the exception of the clay from II(B). An approximate estimate of the quantity ratio of layer silicates to silica minerals was obtained by comparing the absorption at 1020 cm -1 with that at 1080 cm -~ in the infrared spectra (Fig.3). The value of this ratio decreases in the following order: VIII > I, VII > I I I > II, IV, V and VI. It may be worthy of note from a genetic viewpoint that a great variety of layer silicates, vermiculite, montmorillonite, chlorite, illite and kaolinite were found in the soils derived from dacitic ashes. Kaolinite was identified by a shift of its 7.1 A reflection to 10.3 A after treatment with hydrazine. The relative contents of quartz, cristobalite and feldspars in the clays paralleled those in the silts (Table II). DISCUSSION

Effects of ash composition Fieldes (1955) postulated from his study of soils derived from rhyolitic or andesitic ash in New Zealand that with increasing age, the clays pass through the sequence, allophane B -. allophane A ~ metahalloysite -~ kaolinite. No particular effect of the difference in ash composition on the clay mineralogy

54 TABLE III Summary of selective dissolution and mineralogical analyses of clay fractions in Asahidai and Yoshibu soil samples Deposit and horizon

Parent ash

2% Na2CO3 .soluble fraction

Dithionite-citrate soluble fraction content ~

(%)2

consti- S i O J tuent 3 AI20 ~

F%O~

content ~

(%)2

(%)2

consti- S i O J tuent 3 Al203

ratio

ratio

Asahidai IA l

dacitic

34

(49) A'

0.48

12.1

12

(17) A'

0.75

IIIAI

dacitic

46

(60) A'

0.20

15.7

13

(18) A'

0.62

VA~

andesitic

41

(47) A'

0.36

11.9

16

(19) A'

0.68

VIA, VI(B)

andesitic andesitic

41 39

(47) A' (44) A'

0.21 0.25

12.1 10.0

16 19

(18) A' (21) A'

0.65 0.60

VIIA1 VII(B)

andesitic andesitic

39 39

(45) A' (44) A'

0.24 0.37

9.5 9.8

17 18

(19) A' (20) A'

0.69 0.64

VIIIA~ VIII(B)

dacitic dacitic

37 33

(71) X (52) A'

0.18 0.33

15.5 12.6

6 9

(11) X (15) A'

0.42 0.60

IA~

dacitic

33

(47) A'

0.26

10.2

9

(13) A'

0.40

II(B)

andesitic

31

(44) A'

0.37

7.8

15

(24) A'

0.80

IIIAI

dacitic

45

(56) A'

0.19

14.2

14

(18) A'

0.59

IV(B)

andesitic

37

(41) A'

0.30

10.3

15

(16) A'

0.70

Yoshibu

1Figures in parentheses show the contents of the respective soluble fractions as percentages of the whole soluble fractions. 2On the oven-dry basis of total clay. 3Abbreviations: A-Ira = allophane and imogolite; A' = allophane-like, Cb = cristobalite; Ch -chlorite; Fd = feldspars; Gb ffi gibbsite; It = iUite; Kt = kaolinite; L.S. = layer silicates; Mt = montmorillonite; O.S. ffi opaline silica; Qz = quartz; Vt = vermiculite; Vt-Ch = vermiculite-chlorite intergrades; X = unidentified.

o f t h e soils was m e n t i o n e d . K a n n o ( 1 9 6 1 ) s u b d i v i d e d H u m i c A l l o p h a n e soils in J a p a n into f o u r soil genera a c c o r d i n g t o the m o r p h o l o g y o f the soil, m a i n l y t h e c o l o r o f t h e B h o r i z o n , a n d the lithologic c o m p o s i t i o n o f p a r e n t ash. He n o t e d t h a t gibbsite a n d kaolin minerals are p a r t i c u l a r l y p r e s e n t in greater a m o u n t s in softs w h i c h b e l o n g t o t h e light y e l l o w i s h - b r o w n genus (biotiteh o r n b l e n d e andesitic ash). Wright ( 1 9 6 4 ) also p o i n t e d o u t in his review o n A n d o softs o f S o u t h A m e r i c a t h a t even u n d e r s u i t a b l y h u m i d c o n d i t i o n s , n o t all k i n d s o f volcanic ash p r o d u c e A n d o soils, and t h a t t h e great b u l k o f A n d o soils are d e v e l o p e d in volcanic ash o f c o m p o s i t i o n i n t e r m e d i a t e b e t w e e n acid

55 TABLE III (continued)

content' (%)2

constituent ~

Deposit and horizon

0.5N NaOH insoluble fraction

0.5N NaOH soluble fraction SiO2/

content

AJ203 (%)~

constituent 3

other mineral

layer silicate

ratio Asahidai 23

(34) O.S.,A(?)

6.21

31

Vt-Ch > Mr, Vt, It > Ch, K t

Qz > Cb > Fd

IA,

16

(23) O.S.,A-Im

1.83

25

Vt-Ch, V t > It, K t > Mt

Cb > Qz > Fd

IIIA ,

30

(34) A-Ira

1.10

13

Vt-Ch(?)

Cb >> Fd

VA1

31 31

(35) A-Ira (35) A-Ira

1.09 1.19

12 11

Vt-Ch(?) Vt-Ch(?)

Cb >> F d Cb >> Fd

VIA,

31 33

(36) A-Ira (35) A-Ira

1.18 1.25

13 10

Vt-Ch,Mt >> It,Ch,Kt(?) Vt-Ch,Mt >> Ch,Kt(?)

Qz, Cb Qz, Cb

VIIA1 WI(B)

9 20

(18) L.S. (33) A-lm, Gb

1.70 0.85

48 38

Vt, It > Ch > Vt-Ch > Kt, M t Vt-Ch, It > Ch > K t

Qz >> Cb Qz >> Cb

VIIIA , VIII(B)

28

(41) O.S.,A(?)

30

Vt-Ch > Mr, Vt, It > Ch, K t

Qz, Cb > Fd

IAI

25

(33) A-Ira

1.04

29

Mt >> Ch(?)

Cb, Fd > Qz(?)

H(B)

21

(26) O.S.,A-Im

1.33

20

Vt-Ch, Vt > It, K t > Mr(?)

Cb > Qz > Fd

IliA l

39

(43) A-Im

1.27

9

Vt-Ch(?), Mt(?)

Cb, F d

IV(B)

VI(B)

Yoshibu 13.0

and basic. He noted that the relatively inert quartz sand in the parent ash plays an important part in the subsequent development of soil morphology; usually the more quartz sand present, the more readily the soils will respond to leaching and the more rapidly the developing soft will lose its Ando soil characteristics. Few systematic and quantitative studies have been made to examine the effect of ash composition on the formation of clay minerals in softs from volcanic ash. Masui et al. (1966) studied the clay-mineral composition of soils from different ashes in northern Japan. All the ashes were described as andesitic b u t subdivided into three groups of alkalic, intermediate and femic, according to the mineralogical composition. On the basis of the increased 2:1 layer silicate content with weathering, which was measured b y the clay content, it was concluded that the dominating weathering sequence is volcanic ash ~ amorphous materials -* montmorillonite -* intergrade of vermiculite -~ Al-chlorite. Reexamination of these data b y Wada and Aomine (1973) has

56

Fig.2. An electron micrograph o f the dithionite-citrate treated IA~ clay.

4

~ ..//

5 ~

'

I

.

.

~[

I

14

12 10 X 100cm "1

8

Fig.3. I n f r a r e d spectra o f 0 . 5 N N a O H soluble f r a c t i o n s ( f u l l l i n e ) and residues r e m a i n i r ~

after 0.5N NaOH treatment (dotted line). I = IAI (Asahidai; Yoshibu); 2 = I l i A I (Asahidai); 3 = IIIA~ (Yoshibu); 4 = II(B), IV(B), VAt, VIAl, VI(B); 5 = VIIAI, VII(B); 6 = VIIIA~; 7 = VIII(B). The spectra for the IA~ (Asahidai), VAt and VII(B) clays are shown as examples.

57

Fig.4. Electron micrographs of the VIAj clay. a. Original clay. b. Dithionite-citrate and 2% Na2CO ~ treated clay.

58 TABLE IV Summary of mineralogical analyses of clay fractions derived from superimposed ashes and scoriae at the foot of the Ashitaka volcano (modified from Matsui and Saito, 1971) Layer and horizon

Dominant heavy mineral

Mineral content in clay fraction

IA IAC IIA IIAC III/IVA IVB~ IVB2C ~ VA VC VIAC VIIA VIIC VIIIA VIIIC IXC XC XIA XIIA XIIIA XIIIC XVIC

olivine olivine olivine olivine hypersthene hypersthene hypersthene hypersthene hypersthene hypersthene olivine olivine olivine olivine olivine olivine olivine olivine olivine olivine olivine

+ ++ + + + + + + +++ + + + ++ +++ +++ +++ +++ ++ ++ ++ +++

Allophane Layer silicates ++ + +++ +++ +++ +++ +++++ ++++ ? ++++ ++++ ++++ ++

Gibbsite

--

--+ ? + ++ ++ ++ ? ? ? ?

Quartz 1

+ -+ ++ ++ +++ ++++ ++ -+++ ++ + ++

+

+

+

+ -+ + +++ ++ --

? ? ? ? -? ?

---? + + --

1Quartz content was estimated by reading the 3.35-A peak height in the X-ray patterns given by Matsui and Saito (1971) by the present authors. 2The age of ash IV was estimated to be 14,300 ± 700 year B.P.

s h o w n t h a t t h e r e are c o r r e l a t i o n s b e t w e e n t h e c o n t e n t s o f t h e 2 : 1 l a y e r silicates, t h e p r e s e n c e o f q u a r t z i n t h e c l a y s a n d t h e a n o r t h i t e c o n t e n t o f f e l d spars. T h a t is, t h e m o r e e v i d e n t t h e p r e s e n c e o f q u a r t z i n t h e c l a y a n d t h e lower the a n o r t h i t e c o n t e n t , the higher the 2:1 layer silicate c o n t e n t in the clay. I n t h e p r e s e n t s t u d y , t h e c o n t e n t s o f n o n c r y s t a l l i n e as w e l l as c r y s t a l l i n e c l a y c o m p o n e n t s h a v e b e e n d e t e r m i n e d o n soil c l a y s d e r i v e d f r o m d i f f e r e n t ashes s u p e r i m p o s e d at t h e same place. As s h o w n in T a b l e III a n d Fig.3, the c o n t e n t s of layer silicates were higher a n d the c o n t e n t s of a l l o p h a n e and i m o g o l i t e w e r e l o w e r i n t h e soil c l a y s f r o m d a c i t i c a s h e s t h a n i n t h o s e f r o m a n d e s i t i c ashes, w h e r e t h e ash c o m p o s i t i o n w a s d e s i g n a t e d o n t h e b a s i s o f q u a r t z c o n t e n t i n t h e silt ( T a b l e II). T h i s e f f e c t o f a s h c o m p o s i t i o n o n w e a t h e r i n g p r o d u c t s s e e m s t o o b l i t e r a t e t h e p o s s i b l e e f f e c t s o f o t h e r f a c t o r s s u c h as differences in environmental conditions and duration of time since deposition o f t h e ash.

59 Matsui and Saito (1971) also studied the clay-mineral composition of soils derived from superimposed volcanic ashes and scoriae at the foot of the Ashitaka volcano in central Japan. Their data are qualitative rather than quantitative, particularly for allophane, but may be summarized as shown in Table IV. They used the dominant heavy-mineral species as a marker of ash composition and concluded that environmental factors and weathering time are more important in deciding weathering products than the composition of ash or scoria. However, if the quartz content in the clay is used as a marker of ash composition, the conclusion will be different. Again, there seems a positive correlation between the quartz and layer silicate contents, on the one hand, and a negative correlation between the quartz and allophane contents on the other. Matsui and Saito (1971) also suggested that the weathering follows the sequence; volcanic glass -* crystalline clay minerals with gibbsite -~ allophane. It is evident that caution should be exercised in drawing any sequential relationship in weathering of volcanic ash. Any direct effect of quartz on weathering of volcanic ash is of course not expected. As shown in the Bowen's scheme (Bowen, 1956), generally quartz appears at later stages of fractional crystallization of magma, whereas feldspars continually appear as products with changes in their composition. Cristobalite crystallizes at higher temperatures and therefore at earlier stages than does quartz. Thus, it may be considered that the differing contents of fine-grained quartz actually reflect the different stages in the differentiation of magma and in the composition of the resulting ash. Confirmation of this postulate and the reactions through which the differences in ash composition affect the formation of clay constituents need further studies.

Interactions between organic matter and amorphous clay constituents Two important features of Ando soils are the presence of allophane, imogolite or both in the clays and the accumulation of humus in considerable amounts in the A~ horizons, as illustrated in some layers of the profiles (Tables I, II and III). Recognition of these two features points to the possibility that allophane and imogolite play an important role in the accumulation of humus. As reviewed by Wada and Harward (1974), there have been evidence and observations that seem to support this idea. Table III also shows that all clays from the A~ horizons except for the present
60 stabilizes organic matter against biotic degradation and leaching is aluminum, though this does not exclude the possibility that aUophane, imogolite and aUophane-like constituents contribute to the stability. Oba and his collaborators (1967, 1971) pointed out that a " n o n allophanic", amorphous constituent rather than allophane is important to accumulation of humus in soils from volcanic ash. This "nonaUophanic" constituent was extracted by dithionite-citrate treatment and characterized by an SiO2/A12 03 ratio lower than 0.5. Kato (1970a) also emphasized the importance of dithionitecitrate soluble sesquioxides in the accumulation of humus in soils from volcanic ash and in " K u r o b o k u " soils. Kato (1970b) gave the name " K u r o b o k u " to the soils which were very similar to Ando soils in morphology but not in mineralogy. On the basis of the low contents of volcanic glass in the sand and of allophane in the clay, he considered that volcanic ash is not the major parent material of these soils. That even the absence of allophane does not exclude the possibility of volcanic ash origin of the soil is illustrated in the IAI and VIIIA~ horizons of the profiles in the present study. The effects of humus accumulation in the soil on the formation and transformation of clay minerals have received less attention. An accelerative effect on clay formation m a y be inferred by comparison o f the particle-size distribution between the corresponding A, and (B) horizons (Table II). There have also been observations which suggest that the clay-mineral composition in the AI horizon is different from that in the (B) horizon. Shoji and Masui (1971) studied the occurrence of opaline silica in Ando soils in Hokkaido, Kanto and Kyushu. T h e y found that opaline silica particles are abundant in the A horizons o f young soils as compared with the B or C horizons of the old soils. Shoji and Masui (1972) also found that allophane Was present in the fine clays of B or C horizons but not in the A horizons developed in volcanic ashes more than several hundred years old in Hokkaido. In the profiles studied here opaline silica was present in the IA~ and IIIA~ but not in the intervening II(B) and allophane was absent in the IAI horizon (Table III). In the oldest ash VIII, about 10,000 years old, allophane, imogolite and allophane-like constituents were nearly absent in the AI horizon but were present in considerable amounts in the (B) horizon. No such clear-cut difference was found in ashes VI and VII where the a m o u n t of humus accumulated in the A~ horizon was relatively small. These observations suggest that the accumulation of humus m a y favor t h e formation of opaline silica, when the supply of silica is as plentiful as it is in early stage of weathering of volcanic ash, and m a y retard the formation of allophane and imogolite, particularly when the soil is developing at the land surface. In 1955, Fieldes noted that aUophane B in which he thought amorphous silica is discrete is predominating in surface layers of young soils derived from volcanic ash and that reduction of humus accumulation is associated with increasing amounts of allophane A in which alumina and silica are randomly combined. He interpreted his observation as indicating that in the presence of colloidal humic material an appreciable a m o u n t of alumina m a y be bound in a

61 stable organic c o m p l e x limiting t h e p o s s i b i l i t y o f c o p r e c i p i t a t i o n o f a l u m i n a a n d silica, so t h a t t h e silica in t h e clay m a y b e c o m e discrete. T h e o b s e r v a t i o n s d e s c r i b e d a b o v e l e n d s u p p o r t this i n t e r p r e t a t i o n . F u r t h e r a c c u m u l a t i o n o f r e l e v a n t d a t a is, h o w e v e r , n e c e s s a r y t o describe a n d e x p l a i n f u l l y t h e m e c h a n isms o f t h e r e s p e c t i v e r e a c t i o n s .

REFERENCES Bowen, N.L., 1956. The Evolution of the Igneous Rocks. Dover Publications, New York, N.Y., pp.54--62 Fieldes, M., 1955. Clay mineralogy of New Zealand mils. Part II: Allophane and related mineral colloids. N.Z. J, Sci. Technol., 37B: 336--350 Kanno, I., 1961. Genesis and classification of main genetic soil types in Japan. I. Introduction and Humic Allophane soils. Bull. Kyushu Agric. Exper. Station, 7:1--185 (in Japanese) Kanno, I., 1971. Soil data for Kyushu tour. In: I. Kanno (Editor), Soils and Agriculture of Kyushu. Organizing Committee, Kyushu Meeting, The Society of the Science of Soil and Manure, Japan, Fukuoka, pp.156--186 (in Japanese) Kato, Y., 1970a. A model for amorphous matters of humic soils in Japan -- a preliminary report. Pedologist, 14:16--21 (in Japanese) Kato, Y., 1970b. A consideration on distribution, profile characteristics and parent materials of "Kuroboku" soils in Tokai district. J. Sci. Soil Manure, Jap., 41: 89--94 (in Japanese) Kononova, M.M., 1961. Soil Organic Matter. Pergamon Press, Oxford, pp.342--344 Kuno, H., 1954. Volcano and Volcanic Rock. Iwanami-shoten, Tokyo, pp.85--87 (in Japanese) Masui, J., Shoji, S. and Uchiyama, N., 1966. Clay mineral properties of volcanic ash soils in the northeastern part of Japan. Tohoku J. Agric. Res., 17 : 17--36 Matsui, T. and Saito, I~, 1971. Clay mineralogy of the recent and buried soils derived from the Pleistocene tephras ("Loams") at the foot of the Ashitaka volcano, central Japan. Quaternary Res., 10:69--79 (in Japanese) Matsumoto, T., Noda, M. and Miyahisa, M., 1962. The Geology of Kyushu District. Asal~ura-shoten, Tokyo, pp. 180--197 (in Japanese) Oba, Y. and Hayeshi, M., 1971. Amorphous inorganic constituents in volcanic-ash soil clays. Abstr. Pap. Soc. Sci. Soil Manure Jap., 17:43 (in Japanese) Oba, Y. and Okano, K., 1967. Aluminum extracted from volcanic ash soils by deferration treatments. Abstr. Pap. Soc. Sci. Soil Manure Jap., 14(II): 23 (in Japanese) Shoji, S. and Masui, J., 1969a. Amorphous clay minerals of recent volcanic ash soils in Hokkaido (I). Soil Sci. Plant Nutr. Tokyo, 15:161--168 Shoji, S. and Masui, J., 1969b. Amorphous clay minerals of recent volcanic ash soils in Hokkaido (II). Soil Sci. Plant Nutr. Tokyo, 15:191--201 Shoji, S. and Masui, J., 1971. Opaline silica of recent volcanic ash soils in Japan. J. Soil Sci., 22:101--108 Shoji, S. and Mesui, J., 1972. Amorphous clay minerals of recent volcanic ash soils (HI). J. Sci. Soil Manure, Jap., 43:187--193 (in Japanese) Tamura, S., 1967. Studies on the distribution and characteristics of volcanogenous soils in Kyushu (Part 1). J. Sci. Soil Manure, Jap., 38:443--448 (in Japanese) Taylor, N.H. and Pohlen, I.J., 1962. Soil Survey Method. Soil Bureau, Taita Experimental Station, Lower Hutt, pp.69--75 Thorp, J. and Smith, G.D., 1949. Higher categories of soil classification: order, suborder, and great soil groups. Soil Sci., 67:117--126

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Tokashiki, Y. and Wada, K., 1972. Determination of silicon, aluminum and iron dissolved by successive and selective dissolution treatments of volcanic ash soil clays. Clay Sci., 4: 105--114 Tsuno, R. and Takada, K., 1969. Distribution and characteristics of "kora" (hard pan) soil in Oita-ken. Kyushu-Nog~yo, 26:173 (in Japanese) Wada, K., 1966. Qualitative and quantitative determinations of clay minerals. J. Sci. Soil Manure, Jap., 37 : 9--17 (in Japanese) Wada, K., 1967. Accumulation and 14C age of organic matter in volcanic ash soils. Pedologist, 11:46--58 (in Japanese) Wada, K., 1969. Beppu to Kumamoto. In: H. Minato and N. Imai (Editors), Clay Minerals, Clay Deposits, Zeolite Deposit, Volcanic Ash Soils and Volcanoes. 1969 Int. Clay Conf., Tokyo, pp 43--50 Wada, K. and Aomine, S., 1973. Soil development on volcanic materials during the Quaternary. Soil Sci., 1 1 6 : 1 7 0 - 1 7 7 Wada, K. and Greenland, D.J., 1970. Selective dissolution and differential infrared spectroscopy for characterization of "amorphous" constituents in soil clays. Clay Miner., 8: 241--253 Wada, K. and Harward, M.E., 1974. Amorphous clay constituents of soils. Adv. Agron., 26: 211--260 Wada, K. and Tokashiki, Y., 1972. Selective dissolution and difference infrared spectroscopy in quantitative mineralogical analysis of volcanic-ash soil clays. Geoderma, 7 : 199--213 Wright, A.C.S., 1964. The "Andosols" or "Humic Allophane" soils of South America. FAO World Soil Resour. Rep., 14:9--22 Yamada, Y., 1968. Relation between 14C age and color of humic acid solution from some volcanic ash soils in Japan. J. Sci. Soil Manure, Jap., 39:447--451 (in Japanese)