Clay Science 13, 189-197 (2008)

CLAY MINERALOGY OF FERRALITIC SOILS DERIVED

FROM IGNEOUS ROCKS IN VIETNAM

NGUYEN QUANG HAIa AND KAZUHIKO EGASHIRAb

aDepartment of Plant Resources , Graduate School of Bioresource and Bioenvironmental Sciences,Kyushu University, Fukuoka 812-8581, Japan

bDepartment of Plant Resources, Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan

(Received September 2, 2007. Accepted November 30, 2007)

ABSTRACT

Twelve profiles of ferralitic soils derived from different igneous rocks were collected from hilly and

mountainous areas in the Northern, North of Central, Coastal Area of Southern Central and High Plateau Tay

Nguyen regions of Vietnam, and were subjected to clay mineral analysis. The main clay minerals detected

were kaolin (kaolinite and halloysite (0.7nm)), 2:1-type silicate (mainly mica, vermiculite, smectite, the

mica/vermiculite/smectite•`mica/smectite mixed-layer mineral, and chlorite-vermiculite intergrade), and oxide/

hydroxide (gibbsite and goethite) minerals. Based on the abundance of kaolin and oxide/hydroxide minerals

in the clay fraction, twelve profiles were grouped into 3 groups. The mineralogical difference of soils among

the three groups was explained by the difference in rock-forming minerals of igneous rocks rather than by

advancement of ferralitization, different from soils derived from sedimentary and metamorphic rocks in the

previous study (Nguyen and Egashira, 2007). Inherent potentiality of the soils was assessed based on the type

and amount of clay minerals and varied with soils depending mainly on the amount of 2:1-type silicate miner-

als in the clay fraction.

Key words: Ferralitic soils, Igneous rocks, Clay mineralogy, Ferralitization, Inherent potentiality of soil, Viet-

nam

INTRODUCTION

Vietnam is situated within 8•‹10' to 23•‹24'N latitude and

102•‹09' to 109•‹30'E longitude ranges in the Indochina Pen-

insula. This location makes its own tropical to subtropical

monsoons with two distinct seasons: rainy (April to October)

and dry (November to March) seasons. Because of long dis-

tance from north to south, Vietnam has the very complicated

topography and diversified geology. Based on the topography,

geology, climate, and vegetation, the whole country is divided

into 9 agro-ecological regions (Fig. 1). Soils are classified into

13 main soil groups and 31 soil units according to the Viet-

namese soil classification system, which have been correlated

with the FAO/UNESCO soil classification system and USDA

Soil Taxonomy (NISF and DSTPQ, MARD, 2002).

Vietnam has 331,000km2 of the total national land of

which 72% is hills and mountains where ferralitic soils are

widely distributed. Ferralitization is a main process of soil

formation occurring in ferralitic soils. This process comprises

loss of basic cations and silica through weathering of silicate

minerals due to hydrolysis, formation of more stable minerals

such as kaolinite and halloysite, and relative accumulation of

sesquioxides (Driessen et al., 2001).Parent rocks of ferralitic soils in Vietnam are igneous,

sedimentary and metamorphic rocks; the soils derived fromthe former and the latter two occupy more than 7 million haand around 10 million ha, respectively (NISF and DSTPQ,MARD, 2002). The clay mineralogy of ferralitic soils derivedfrom sedimentary and metamorphic rocks was examined inthe previous study (Nguyen and Egashira, 2007), and claymineralogical differences of the soils were explained in refer-ence to the advancement of ferralitization.

In the present study, we focus on the clay mineralogy offerralitic soils derived from igneous rocks. The purposes ofthe study are: (1) to clarify the clay mineralogical compositionof the soils; (2) to characterize the clay mineralogical compo-sition of the soils in relation to ferralitization; and (3) to assessinherent potentiality of the soils based on the type and amountof clay minerals.

MATERIALS AND METHODS

SoilsSampling sites of 12 profiles of soils derived from igne-

190 Nguyen Q.H. and K. Egashira

FIG. 1. Soil sampling sites on the map of agro-ecological regions of

Vietnam. I. Northwest region; II. Northern region; III. Northeastregion; IV. Red River Delta region; V. North of Central region;

VI. Coastal Area of Southern Central region; VII. High Plateau

Tay Nguyen region; VIII. East of the South region; IX. West of the

South region.

ous rocks are shown on the map of agro-ecological regionsof Vietnam in Fig. 1, and general information on soil profilesis given in Table 1. All soil samples in the present study weretaken in the project of Surveying and Evaluating Soil Qualityto Establish Soil Reference, Database and Information. Theywere selected from the hilly and mountainous areas in theNorthern, North of Central, Coastal Area of Southern Centraland High Plateau Tay Nguyen regions, where soils developedon igneous rocks are widely distributed.

Out of twelve profiles, six profiles (VN08, VN18, VN19,

VN21, VN38 and VN42) were developed on basalt of basic

igneous rocks, three profiles on dacite (VN39), porphyrite

(VN40) and andesite (VN49) of intermediate igneous rocks,

and three profiles on rhyolite (VN16) and granite (VN50 and

VN54) of acid igneous rocks. Geologic age of the parent

rocks was relatively young for basalt, namely from Middle

subdivision of the Pleistocene epoch (0.78-0.13 Ma) to

Holocene epoch (0.01 Ma-present) or from Middle subdivi-

sion of the Neogene period (16-5.5 Ma) to Early subdivision

of the Pleistocene epoch (1.81-0.78 Ma). Geologic age of

intermediate igneous rocks was placed within 47 Ma (end of

Early subdivision of the Tertiary period) and 1,400 Ma (begin-

ning of Middle period of the Middle Proterozoic era). Rhyo-

lite had geologic age of Middle subdivision of the Tertiary

period (47-25 Ma), while granite had relatively old geologic

age of Early (1,600-1,400 Ma) and Middle (1,400-1,200 Ma)

periods of the Middle Proterozoic era (Phan et al., 1991).

The soils were distributed on various elevation levels rang-

ing from 50 to 830 m above sea level, but normally on flat or

undulating topography with slope mostly less than 8° for soils

derived from basic and intermediate igneous rocks and on

high sloping topography (more than 8-15•‹) for soils derived

from acid igneous rocks. The soils developed on basalt have

been subjected to production of perennial industrial crops

such as rubber, coffee, tea and black pepper. Those devel-

oped on intermediate and acid igneous rocks are covered with

grass or have been subjected to production of perennial crops

(coffee, cashew and mango) and short-term crops (sugarcane

and maize).

Soil samples were taken vertically with profile depth

based on the genetic horizons, air-dried and gently ground to

pass through a 2-mm sieve in Hanoi, Vietnam, followed by

particle-size and clay mineral analyses in Kyushu University,

Japan.

Particle-size analysis

The soil sample was first treated with hot 7% H2O2 to de-

compose organic mater, followed by the treatment with hot

0.3 M sodium-citrate solution and sodium-hydrosulfite pow-

der to digest free iron oxides (Kunze and Dixon, 1986). After

dispersion by the sonic wave treatment and deflocculation by

adjustment of the pH of the suspension to 10 by addition of a

small amount of 1 M NaOH, the soil suspension was stood in

a 1-L sedimentation cylinder for a prescribed time followed

by siphoning-out of the clay fraction. The sample was then

subjected to repetition of sonification-sedimentation-siphon-

ing with intermittent pH adjustment to separate the whole<

2 ƒÊm clay fraction. The whole 2-20 ƒÊm silt fraction was then

separated by repeated sedimentation and siphoning, followed

by separation of the 20-200 and 200-2,000 ƒÊm sand fractions

by wet-sieving. After oven-drying at 105•Ž, each fraction was

weighed to calculate the particle-size distribution of a soil.

Clay mineral analysis

The<2 ƒÊm clay fraction wholly separated from the fine

soil was used to examine the clay mineralogical composition

by the X-ray diffraction (XRD) method. Duplicate clay sols

containing 50mg clay each were taken in 10-mL glass tubes

and then washed twice with 8mL of an equal mixture of 1

Clay mineralogy of ferralitic soils derived from igneous rocks 191

TABLE 1. General information on soil samples

1) ASL, above sea level.2) QI and QII

, Early (1.81-0.78 million years ago (Ma)) and Middle (0.78-0.13 Ma) subdivisions of the Pleistocene epoch, respectively; QIV, Holoceneepoch (0.01 Ma-present); N2, Middle (16-5.5 Ma) subdivision of the Neogene period; J1 and J2, Early (199-175 Ma) and Middle (175-161Ma) subdivisions of the Jurassic period, respectively; T1 and T2, Early (65-47 Ma) and Middle (47-25 Ma) subdivisions of the Tertiary period,respectively; Y1 and Y2, Early (1,600-1,400 Ma) and Middle (1,400-1,200 Ma) periods of the Middle Proterozoic era, respectively.

3) Estimated based on the book of NISF and DSTPQ (2002); the italic part in the soil name by the Vietnamese system is parent rock.

M NaCl and 1 M NaCH3COO (pH 5.0) by centrifugation to

lower the pH. Of the duplicate sets, one was saturated with K

and the other with Mg by washing 3 times with 8 mL of 1 M

KCl and 0.5 M MgCl2, respectively. Excess salt was removed

by washing once with 8 mL of water and the clay in the tube

was thoroughly suspended with 1 mL of water. An aliquot of

0.4 mL of the clay sol was dropped on to a glass slide (28x

48mm) covering two-thirds of its area, air-dried and X-rayed

(parallel powder mount). The XRD analysis was made with

the air-dried and glycerol-solvated specimens for the Mg-

saturated clay and air-dried and heated (at 300•Ž and 550•Ž

for 2h) specimens for the K-saturated clay. The XRD analysis

was conducted using a Rigaku diffractometer with Ni-filtered

CuKƒ¿ radiation at 40 kV and 20 mA and at a scanning speed

of 2•‹2ƒÆ min-1 with a scanning step of 0.02•‹ and a continuous

scanning mode over a rang of 3 to 30•‹2ƒÆ.

Relative mineral contents in the clay fraction were semi-

quantitatively estimated on the basis of the XRD peak inten-

sities. In the present estimation, the peak height was used as

the peak intensity by assuming the relative proportions of

the minerals of a sample normalized to 100% and the same

proportionality between the peak intensity and the content for

each mineral (Ho et al., 2000; Nguyen and Egashira, 2000; Do

et al., 2002; Nguyen and Egashira, 2005).

RESULTS AND DISCUSSION

Particle-size distribution

The particle-size distribution of soils derived from igne-

ous rocks is given in Table 2. Variability in the clay, silt, and

fine and coarse sand contents was considerably large for all

samples: the clay content varied from 16.8 to 91.6%, the silt

content from 4.8 to 26.4%, the fine sand content from 2.5 to

29.8%, and the coarse sand content from 0.2 to 56.1%. How-

ever, the variation was relatively narrow for soils derived from

the same parent rock. Soils derived from basalt of basic igne-

ous rocks (VN08, VN18, VN19, VN21, VN38 and VN42) had

the high clay (mostly more than 70%) while the low fine sand

(less than 12%, except profile VN19) and coarse sand (less

than 2%, except profile VN38) contents. This was the case for

soils developed on dacite (VN39) and porphyrite (VN40) of

intermediate igneous rocks. One soil developed on andesite

(VN49) had the considerable amount of clay (30.0 to 46.8%)

and the fair amount of fine and coarse sands (around 25%).

Among the soils derived from basalt, the clay content was

affected by the geologic age of basalt. It tended to be higher

for soils from basalt of Middle subdivision of the Pleistocene

epoch to Holocene epoch than for soils from basalt of Middle

subdivision of the Neogene period to Early subdivision of the

Pleistocene epoch. The silt content was not much different for

soils derived from basic and intermediate igneous rocks and

normally less than 12%, but it was higher for profile VN19.

For soils derived from acid igneous rocks, one soil devel-

oped on rhyolite (VN16) had the fair amount of clay (24.5 to

29.6%) and the considerable amount of fine and coarse sands

(around 30%). For soils derived from granite (VN50 and

VN54), the clay content (16.8 to 32.8%) was usually lower

than that of soils derived from basic and intermediate igneous

rocks while the coarse sand content (30.6 to 56.1%) was high-

er than that of them. Between the two soils derived from gran-

ite, the clay content tended to be higher for soil from granite

of Middle period of the Middle Proterozoic era than for soil

from granite of Early period of the Middle Proterozoic era.

According to the IUSS system, soils derived from basic and

intermediate igneous rocks, excluding some layers of profile

VN49, had a very fine texture of heavy clay, and soils derived

from acid igneous rocks had a coarser texture of light clay or

sandy clay loam. The texture hardly varied with depth except

soils from andesite and rhyolite having the intermediate level

of the clay content.

Mineral identification

The XRD patterns of the<2 ƒÊm clay fraction from the

surface horizon of profiles VN08, VN18, VN39, VN16, VN50

192 Nguyen Q.H. and K. Egashira

TABLE 2. Particle-size distribution and texture of soils derived from igneous rocks

Subordinate distinctions within master horizons and layers:

p-plowings-illuvial accumulation of sesquioxides and organic matter

t-accumulation of silicate clay

w-development of color or structure in a horizon but with little or no apparent illuvial accumulation of materials.

Refer to Table 1 for explanation of symbols of geologic age of parent rocks.

Clay mineralogy of ferralitic soils derived from igneous rocks 193

VN08-1 VN18-1

VN39-1 VN16-1

VN50-1 VN54-1

FIG.2. The XRD patterns of the<2 ƒÊm clay fraction from the surface horizon of profiles VN08, VN18, VN39, VN16, VN50 and VN54. Spacing is

in nm. Treatments: a, Mg-saturation and glycerol-solvation; b, Mg-saturation and air-drying; c, K-saturation and air-drying; d, K-saturation and

heating at 300•Ž; e, K-saturation and heating at 550•Ž.

and VN54 are illustrated in Fig. 2. Mica was identified by the

presence of the 0.99-1.00 nm peak along with its higher-orderreflections at 0.497-0.500 and 0.334 nm. The presence of ka-olinite was ascertained by the relatively sharp peaks at 0.717-0.719 and 0.357 nm while the presence of halloysite (0.7 nm)

was by the relatively broad peaks at 0.720-0.729 and 0.357

nm; the basal spacing for halloysite (0.7 nm) was 0.72 to 0.75

nm by Brindley (1961) and 0.72 to 0.76 nm by Jackson (1964).

The peaks reflected by both kaolinite and halloysite (0.7 nm)

all disappeared by heating at 550•Ž in the K-saturated speci-

194 Nguyen Q.H. and K. Egashira

men. Vermiculite was identified by the decrease in the inten-

sity of the 1.42-1.44 nm peak with the corresponding increase

in the intensity of the 0.99-1.00 nm peak by K-saturation and

air-drying. Smectite was noticed by the broad peak around 1.80

nm in the Mg-saturated and glycerol-solvated specimen, and

talc and serpentine minerals were detected by the peaks at 0.93

and 0.73 nm, respectively, throughout the treatments.

The mica/vermiculite/smectite•`mica/smectite mixed-layer

mineral (abbreviated as Mx thereafter) was noticed on the

XRD pattern by the poorly defined diffraction effect between

1.0 and 2.0 nm in the Mg-saturated and glycerol-solvated

specimen and by the increase in the intensity of the 1.00 nm

peak after K-saturation and air-drying (Egashira, 1988). The

irregular mica/vermiculite mixed-layer mineral was noticed by

the 1.20 nm peak in the Mg-saturated and glycerol-solvated

specimen which shifted to 1.00 nm by K-saturation. Chlorite-

vermiculite intergrade was detected by the 1.42 nm reflection

in the Mg-saturated specimen, which shifted toward 1.00 nm

by heating at 300 and 550•Ž in the K-saturated specimen. In

the present samples this mineral showed the poorly defined

diffraction pattern between 1.42 and 1.00 nm in the treatment

of K-saturation and air-drying, suggesting the wide variation

in the interlayer property ranging between the two end mem-

bers of chlorite and vermiculite.

Goethite, gibbsite, quartz, and feldspars are minerals

other than layer silicate minerals and were identified by the

peaks at 0.418, 0.484, 0.425 and 0.334, and around 0.32 nm,

respectively. Irrespective of the deferration treatment in the

particle-size analysis, goethite peak was detected in most of

soils derived from basic and intermediate igneous rocks. The

deferration treatment was prioritized in order to get sufficient

dispersion of soil particles. Goethite is relatively stable among

soil iron oxide/hydroxide minerals and hence it was not totally

removed after the pretreatment. If we suppose that goethite

was removed with an equal amount from soils, the magni-

tude relation in the goethite content among the soils does not

change before and after the deferration treatment. The goethite

content in the clay fraction of ferralitic soils in Vietnam is rel-

atively low and normally less than 10% even in a soil without

pretreatment for removal of free iron oxide/hydroxide miner-

als (Ho et al., 2000), and we considered that removal of free

iron oxide/hydroxide minerals by the deferration treatment, if

any, does not much affect the mineralogical estimation of fer-

ralitic soils.

Clay mineralogical characterization of soils

The semi-quantitative estimates of the minerals in the clay

fraction are shown in Table 3. Six soils (VN08, VN18, VN19,

VN21, VN38 and VN42) derived form basalt were character-

ized by the fairly to very high amount of kaolin minerals (54

to 94%), followed by the small to considerable amount of

gibbsite (4 to 36%) and the small amount of goethite (less than

10%). In addition, the small to fair amount of mica (6 to 14%)

in profile VN19 and the small amount of chlorite-vermiculite

intergrade (less than 7%) in the other profiles were included.

The kaolin minerals content decreased from around 90% in

profiles VN08, VN19 and VN21, to around 77% in profiles

VN18 and VN 42, and finally to around 55% in profile VN38.

The gibbsite content increased in the same direction, from 0-

5% to 10-17% and finally to 33-36%. Some basaltic rocks arerich in aluminum, leading to the high gibbsite content in theirweathering products and hence to the relatively low content ofkaolin minerals. Dominance of kaolin minerals and gibbsiteobserved for soils derived form basalt in Vietnam was also thecase of soils derived from basic igneous rocks in Thailand, asreported by Yoothong et al. (1997).

The clay mineralogical composition of soils derived frombasalt was not affected by the geologic age of basalt. How-ever, profile VN19 contained mica with leaving other five pro-files having chlorite-vermiculite intergrade. In addition, both

goethite and gibbsite were not detected in profile VN19. In thesoil classification, VN19 was classified as Violet brown soilsor Nitisols and other five profiles were as Reddish or Yellow-ish brown soils or Ferralsols by the Vietnamese system or bythe FAO/UNESCO system. Normally Violet brown soils aredistributed at foothill or intrude into the area of Reddish andYellowish brown soils in a narrow belt (NISF and DSTPQ,MARD, 2002), and profile VN19 was taken at the foothill.Although the detail is unknown, the present example suggeststhat petrological characteristics of basalt or site-environmentalfeatures in profile VN19 may have some effects on the claymineralogical differences between profile VN19 and other five

profiles.Among three soils derived from intermediate igneous

rocks, soils from dacite (VN39) and porphyrite (VN40) hadthe comparable clay mineralogical composition with soilsfrom basalt. One soil from andesite (VN49) also had the highhalloysite (0.7 nm) content (58 to 84%) in the clay fraction.However, the clay mineralogical composition of this soil wassomewhat different from the above soils, because the fair toconsiderable amounts of 2:1-type silicate minerals (mica, ver-miculite and Mx) were detected instead of oxide/hydroxideminerals (goethite and gibbsite).

One soil derived from rhyolite (VN16) was unusually char-acterized by the clay mineralogical composition of the highcontent of chlorite-vermiculite intergrade (62 to 66%) with thesmall to fair amounts of halloysite (0.7 nm) (7 to 8%), talc (8to 9%), serpentine minerals (12 to 16%) and feldspars (6%).The presence of talc and serpentine minerals in the clay frac-tion strongly suggests that the rhyolite, located in the northernend of the country with bordering with China (Fig. 1), wassubjected to sepentinization, a kind of metamorphism. A largeamount of chlorite-vermiculite intergrade is probably a weath-ering product of chlorite which is a product of serpentiniza-tion.

Two soils (VN50 and VN54) derived from granite had thedifferent clay mineralogical compositions. Profile VN54, de-veloped on granite of Early period of the Middle Proterozoicera, had the high content of kaolinite (around 70%), followedby the fair amounts of mica (11 to 16%) and gibbsite (10 to11%) and the small amount of the mica/vermiculite mixed-layer mineral (5 to 8%). Whereas, profile VN50, developedon granite of Middle period of the Middle Proterozoic era,was composed of a series of 2:1-type silicate minerals suchas mica (8 to 10%), vermiculite (11 to 22%), smectite (14 to49%) and Mx (2 to 26%) in addition to the fair amount ofhalloysite (0.7 nm) (20 to 26%).

The clay mineralogical composition of profile VN50 is

Clay mineralogy of ferralitic soils derived from igneous rocks 195

TABLE 3. Approximate relative mineral contents (%) in the clay fraction of soils derived from igneous rocks

Abbreviations: Mc, mica; Vt, vermiculite; St, smectite; Mx, mica/vermiculite/smectite•`mica/smectite mixed-layer mineral; Mc/Vt, mica/vermiculite

mixed-layer mineral; Ch-Vt, chlorite-vermiculite intergrade; Tc, talc; Kt, kaolinite; Ht, halloysite (0.7 nm); Sp, serpentine minerals; Gt, goethite;

Gb, gibbsite; Qr, quartz; Fd, feldspars.

Refer to Table 1 for explanation of symbols of geologic age of parent rocks.

196 Nguyen Q.H. and K. Egashira

rather unusual as a soil derived from granite, because the con-tent of kaolin minerals is too low and smectite was detectedin a fair to considerable amount. Since mica is a rock-formingmineral of granite, mica existing in soil is highly probablyinherited from the rock. Vermiculite and Mx are weathering

products of mica. As a result, one explanation for the claymineralogical composition of profile VN50 is formation ofsmectite instead of kaolin minerals from rock-forming miner-als under a base-rich condition or marine environment. An-other idea is that granite of profile VN50 is highly dominatedwith mica and that smectite as well as vermiculite and Mx isa weathering products of mica. Mica was selectively trans-formed into vermiculite and Mx in the first layer while intosmectite in the second through fourth layers.

Clay mineralogy of soils in relation to parent rocks and

ferralitizationThe clay mineralogy of soils derived from igneous rocks

is mainly controlled by neoformation of kaolin minerals asweathering of rock-forming minerals under leaching conditionand by transformation of mica into 2:1 expandable minerals.Aluminum and iron oxide/hydroxide minerals are produced asbyproducts in weathering of rock-forming minerals. Forma-tion of kaolin minerals and oxide/hydroxide minerals followsthe mineralogical change by ferralitization (Driessen et al.,2001). Wang et al. (1996) found the increase in the kaolinitecontent corresponding with the advancement of weathering insoils of Fujian Province, China.

In the previous paper (Nguyen and Egashira, 2007), claymineralogical composition of ferralitic soils derived fromsedimentary and metamorphic rocks were examined, and soilswere grouped into three groups by the clay mineralogicalcomposition. The clay mineralogical differences among thethree groups were explained by the extent in the advancementof ferralitization.

Following the previous study (Nguyen and Egashira,2007), twelve profiles were grouped into three groups basedon the abundance of kaolinite, halloysite (0.7 nm), goethiteand gibbsite in the clay fraction. Two profiles (VN16 andVN50) having the low total content (less than 30%) of theabove four minerals were put into group 1, two profiles (VN49and VN54) having around 85% of the four minerals andaccompanied with a fair to considerable amount of 2:1-typesilicate minerals were into group 2, and remaining profiles

(VN08, VN18, VN19, VN21, VN38, VN39, VN40 and VN42)having more than 85% of the four minerals were into group 3.

Even if soils derived from igneous rocks are fundamentallysubjected to ferralization, the above grouping of soils wasexplained by the difference in rock-forming minerals amongigneous rocks rather than by the extent in the advancementof ferralitization. Soils of the group 1 are considered to be aspecial case, as discussed in the preceding section. Mica andits weathering products were included in soils of the group 2.They were derived from andesite and granite containing micaas a rock-forming mineral. Soils of the group 3 were derivedfrom basalt, dacite and porphyrite of basic and intermediateigneous rocks and were apparently in the advanced stage offerralitization. Those rocks often lack mica as rock-formingmineral, and mica was not detected in soils of group 3 except

profile VN19.In the previous study (Nguyen and Egashira, 2007), change

of halloysite (0.7 nm) to kaolinite due to advancement offerralitization was widely observed for soils derived fromsedimentary and metamorphic rocks. However, it was rarelyobserved for soils derived from igneous rocks of the presentstudy. This would be attributed to the susceptibility of rocksto ferralitization. In soils derived from rocks less susceptibleto ferralitization, like granite of acid igneous rock, it takeslonger time to reach the advanced stage of ferralitization andthe time is enough for change of halloysite (0.7 nm) to kaolin-ite. Whereas, ferralitization is considered to take place much

quicker in soils derived from rocks of more susceptible to fer-ralitization, such as basalt of basic igneous rock. As a result,ferralitization had already progressed to the advanced stagebut halloysite (0.7 nm) had not been changed to kaolinite dueto time limitation.

Assessment to inherent potentiality of soilsInherent potentiality is defined by one of the authors as

the natural fertility of a soil controlled by clay mineralogyand has been roughly assessed by the clay content (amount inclay minerals) and the clay mineralogical composition (typein clay minerals). In general, the inherent potentiality of fer-ralitic soils derived from igneous rocks was normally low,since most of them contained high kaolin minerals whichare inactive in retention of nutrients and water. Variation inthe inherent potentiality among the soils could be attributedto the amount of 2:1-type silicate minerals in addition to theclay content. Inherent potentiality was highest for soils of the

group 1 (VN16 and VN50), followed by soils of the group 2(VN49 and VN54), and lowest for soils of the group 3 (VN08,VN18, VN19, VN21, VN38, VN39, VN40 and VN42).

CONCLUSIONS

Ferralitic soils derived from igneous rocks were character-ized by the considerably large variability in the particle-sizedistribution due to chemical and mineralogical differencesin the parent rock. Soils derived from basic and intermediateigneous rocks normally had the high clay content with thefine texture of heavy clay, whereas those derived from acidigneous rocks had the lower clay while higher fine and coarsesands contents with the coarser texture of light clay or sandyclay loam.

Because mineralogical change occurring in soils derivedfrom igneous rocks under leaching condition follows themineralogical change by ferralitization, the clay mineralogi-cal composition of the soils was controlled by the differencein rock-forming minerals of igneous rocks rather than by ad-vancement of ferralitization. The advanced stage of ferraliti-zation is generally attained more easily by soils derived frombasic and intermediate igneous rocks than by soils from acidigneous rocks. Inherent potentiality of the soils was assessedbased on the type and amount of clay minerals and variedwith soils depending mainly on the amount of 2:1-type silicateminerals in the clay fraction.

Clay mineralogy of ferralitic soils derived from igneous rocks 197

ACKNOWLEDGEMENTS

We would like to express our sincere thanks to managers of the proj-ect of Surveying and Evaluating Soil Quality to Establish Soil Referencefor their permission of using soil samples and related documents fromthe Soil Reference and Information Center of Vietnam, National Institutefor Soils and Fertilizers, Ministry of Agriculture and Rural Develop-ment, Hanoi, Vietnam. Profound appreciation and sincere thanks are alsoextended to Dr. Ho Quang Duc, Mr. Nguyen Van Ty, and other staffs ofthe Department of Soil Genesis and Classification Research, NationalInstitute for Soils and Fertilizers, Ministry of Agriculture and Rural De-velopment, Hanoi, Vietnam, who directly managed and conducted theabove project.

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