origin and distribution of clay minerals in calcareous

Origin and distribution of clay minerals incalcareous arid and semi-arid soils of Fars

Province southern Iran

F KHORMALI AN D A ABTAHI

Department of Soil Science College of Agriculture Shiraz University Shiraz Iran

(Received 15 December 2002 revised 16 June 2003)

ABSTRACT The clay mineralogy of soils and of the main calcareous sedimentary parent rocks ofsouthern Iran were investigated to determine their origin and factors controlling their distributionpattern in soils The results revealed that the soil-available moisture plays the major role in thedistribution pattern of palygorskite and smectite clay minerals in the arid and semi-arid areas studiedThere is an inverse correlation between palygorskite and smectite with regard to the soil-availablemoisture as expressed by PETordm (ratio of mean annual precipitation to mean annual reference cropevapotranspiration) At PETordm values gt04 palygorskite transforms to smectite Smectite is thought tobe mainly of lsquotransformedrsquo origin It is detected in trace amounts in soils of more arid areas andincreases in soils having greater available moisture The general decrease in illite content with depthis related mainly to its transformation to smectite under favourable moisture conditions of the deeperhorizons Palygorskite is considered to be inherited in plateau soils of the arid regions whereas insaline and alkaline soils and soils with high gypsum it is mainly of authigenic origin The PETordm andgypsum content show a significant correlation with the palygorskite content The occurrence ofkaolinite in some soils is due to its inheritance from the surrounding kaolinite-bearing Cretaceousrocks Illite and chlorite abundance in soils is also largely related to their presence in parent rocksThe rare occurrence of vermiculite in the studied calcareous soils is mainly related to its lowerstability under high pH low Al activity and the presence of large amounts of Si and Mg in soils

KEYWORDS clay mineralogy palygorskite smectite PETordm gypsum arid and semi-arid climate claypedogenesis

Palygorskite smectite chlorite illite kaolinite andvermiculite are the main clay minerals occurring inarid and semi-arid regions Chlorite and illite(micaceous minerals) are commonly believed tobe inherited largely from parent rocks (Wilson1999) However there is some evidence that micain arid soils may form pedogenically as well butonly in special circumstances mainly through Kfixation of pre-existing smectite (Nettleton et al1973 Mahjoory 1975 Niederbudde amp Kussmaul1978 Sanguesa et al 2000) Bronger et al (1998)

found illite and vermiculite to be the two mainpedogenically formed clay minerals in the B or Bthorizons of Holocene soils in Tadjikistan

Environmental conditions are suggested to lead tosmectite formation in Aridisols (Dregne 1976Boettinger amp Southard 1995) Smectite in aridsoils has been noted in Iraq (Al Ravi et al 1969)Iran (Abtahi 1977) and Saudi Arabia (Aba Huseynet al 1980) Nettleton amp Peterson (1983)considered smectite in argillic horizons of theArgid suborder as Pleistocene relics related to amore moist climate and to illuvial deposition to thesoil

Neoformation of smectite was reported byGharaee amp Mahjoory (1984) and by Givi amp

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Clay Minerals (2003)38 511ndash527

2003 The Mineralogical Society

Abtahi (1985) under saline and alkaline conditionswith high concentrations of Si Mg and Al insouthern Iran Abtahi amp Khormali (2001) foundsmectite as a major clay mineral in poorly drainedCalciaquolls of southern Iran The relative abun-dance of smectite increases in the deeper horizonsindicating that its weathering is accelerated in thesurface soil (Ducloux et al 1998)

Minor amounts of smectite were present inOligo-Miocene parent rocks of central Iran(Khademi amp Mermut 1998) but no smectite waspresent in Cretaceous rocks of that region Chloriteand illite were reported as the major clay mineralsof Entisols of alluvial and colluvial fans of thesouthern Iran (Abtahi 1977 Khormali amp Abtahi2001)

Palygorskite in arid soils has been stated tooriginate from two main sources (1) inheritancefrom parent materials and (2) pedogenic formationthrough neoformation or transformation of 21minerals The presence of gypsum (Eswaran ampBarzanji 1974) calcrete or caliche (Roads et al1994 Singer et al 1995 Lopez et al 1996) insaline and alkaline groundwater (Abtahi 1977)would favour the in situ formation of palygorskitefrom the soil solution The occurrence of palygors-kite in the soils and sediments of the Middle Eastgenerally corresponds to the area once covered bythe post-Tethyan intermontane shallow lagoonsduring the Tertiary (Callen 1984) Khademi ampMermut (1998) found palygorskite and sepiolite andappreciable amounts of mica and smectite in theclay fraction of the Oligo-Miocene limestone butonly traces of palygorskite in Cretaceous limestoneThis indicates that the sedimentary environment ofthe study region during the Tertiary was suitable forthe formation of palygorskite and sepiolite

In arid regions of southern Iran palygorskite isthe major mineral in well drained Mollisols whilesmectite is the major mineral in poorly drainedMollisols (Abtahi amp Khormali 2001) Singer(1989) noted that the frequent association ofsmectite and palygorskite is a result of theproximity in their stability fields Paquet amp Millot(1972) showed that palygorskite is not stable andtransforms to smectite when annual precipitation is5300 mm Sanchez amp Galan (1995) concluded thatpalygorskite forms as transformation products ofsmectite and illite Sepiolite is rarely detected insoils by XRD mainly because of its dissolutionunder NaOAC treatment in pH of 45 for theremoval of carbonate (Abtahi 1977 1985) Under

acidic conditions sepiolite is destroyed morerapidly than palygorskite because of its magnesiccomposition and the larger size of its structuralmicrochannels (Myriam 1998) Moreover sepiolitehas a lower stability under the presence of high Alin soil which provides favourable conditions for theformation of palygorskite rather than sepiolite Veryfew occurrences of vermiculite have been reportedin soils of arid and semi-arid regions (Abtahi 1977Khademi amp Mermut 1998 Khormali amp Abtahi2001)

Fars province the study area is one of the mostimportant agricultural regions in Iran Its widerange of climate including xeric aridic and usticsoil-moisture regimes and mesic thermic andhyperthermic soil-temperature regimes provides asuitable area to study the origin and distributionpattern of clay minerals Therefore the mainobjectives of this study were (1) to investigatethe origin of clay minerals in the calcareous aridand semi-arid regions of southern Iran and (2) todetermine the distribution patterns of clay mineralsunder different soil-available moisture regimes

M A T E R I A L S A N D M E T H O D S

Site description

Fars Province the study area is located in thesou the rn pa rt of Iran (50ordm30 rsquo ndash55 ordm38 rsquoE27ordm03rsquo ndash31ordm42rsquoN with a land area of 132 millionha (Fig 1a)

Geology

Fars Province is part of the Zagros orogenic area(Fig 1b) The Zagros mountain ranges with anorthwest to southeast direction are extendedtowards the central parts of the provinceElevation varies from 500 m above sea level inthe southern parts to ~4000 m above sea level inthe northern areas According to Zahedi (1976) theZagros area underwent a relatively moderateorogenic phase (attenuated Laramian phase) nearthe end of the Cretaceous and the beginning of theEocene characterized by folding emergence anderosion The Laramian movements were succeededby a shallow marine transgression A laterregression of the sea eastward resulted in theformation of intermontane lakes in the MiddleTertiary which may have produced an environmentconducive to the formation of fibrous clay minerals

512 F Khormali and A Abtahi

The study area is therefore a part of the Post-Tethyan Sea environment rich in the evaporites(salts and gypsum) in most of the southern andsoutheastern parts

Climate

Due to the geographical location of the Zagrosmountain range a major part of the rain-producing

FIG 1 (a) Location map and the sites of the different studied pedons (b) geological map (c) soil moisture andtemperature regimes map and (d) average annual Iso-(PETordm) lines for Fars Province

Clay minerals in soils of southern Iran 513

air masses enter the region from the west and thenorthwest with relatively large amounts ofprecipitation for those areas (600 ndash900 mmTable 1) Towards the south and southeast areduction of rainfall is observed Furthermorewinter precipitation in the northwest area is in theform of snowfall but for the other areas it is mostlyin the form of rainfall The mean annualprecipitation for the province ranges from 50 to1000 mm (MPB 1994) (Fig 1cd)

Fars Province comprises xeric ustic and aridicsoil-moisture regimes along with mesic thermic

and hyperthermic soil-temperature regimesaccording to the lsquoSoil Moisture and TemperatureRegime Map of Iranrsquo (Banaei 1998) As shown inFig 1c in mountainous areas of the northwest thexeric moisture regime prevails In the northernnortheastern and southern parts the aridic moistureregime is dominant The ustic moisture regimeprevails in the western and south-central parts

The agroclimatological index (PETordm) was calcu-lated for the Fars Province (Sadeghi et al 2002)and the ISO-(PETordm) lines were plotted on the mapof the province for area identification (Fig 1d) Theindex is an important agricultural and biologicalcriterion for crop growth in arid and semi-aridregions as well as a reliable weathering index basedmainly on climatic factors for soil development andmineral weathering and transformation The indexvaries (Fig 1d) from lt02 in arid areas of the northeast and south to gt06 in mountainous regions ofthe northwest This variation is in accordance withthe soil moisture and temperature regime map ofFars Province

Fieldwork

Thirty intermontane plains were selected for thisstudy (Fig 1a) The total number of 75 soil serieswas selected on the soil survey maps and pits weredug described and classified according to the SoilSurvey Manual (Soil Survey Staff 1993) and Keysto Soil Taxonomy (Soil Survey Staff 1998)respectively Thirty four representative pedonswere considered for further mineralogical study Inaddition eight rock samples from the dominantgeological strata were collected for mineralogicalstudy

Physicochemical analyses

Air-dried soil samples were crushed and passedthrough a 2 mm sieve Particle-size distribution wasdetermined after dissolution of CaCO3 with 2 N HCland organic matter was destroyed with 30 H2O2After repeated washing for removal of saltssamples were dispersed using sodium hexametapho-sphate for determination of sand silt and clayfractions by the pipette method (Day 1965)Alkaline-earth carbonate (lime) was measured byacid neutralization (Salinity Laboratory Staff1954)

Organic carbon was measured by wet oxidationwith chromic acid and back titrated with ferrous

TABLE 1 Climatological data of the studied pedons

Pedon P T ETordm PETordm(mm) (ordmc) (mm)

2 149 1358 1500 013 149 1358 1500 014 149 1358 1500 015 3416 121 1000 03510 3468 123 800 04511 3468 123 800 04512 3468 123 800 04515 698 125 1300 0517 609 124 1000 0621 3282 1589 1300 0324 224 233 3000 0126 224 233 3000 0127 224 233 3000 0128 224 233 3000 0131 251 232 1900 01338 338 148 900 0439 288 18 1100 0340 288 18 1100 0341 288 18 1100 0343 288 18 1100 0344 288 18 1100 0351 257 215 1900 01452 257 215 1900 01460 300 194 1100 0362 224 1585 1350 0264 224 1585 1350 0265 173 167 1500 0166 173 167 1500 0170 856 104 900 gt0771 600 21 1300 0572 600 21 1300 0573 600 21 1300 0574 400 222 1600 0375 400 222 1600 03

from Sadeghi et al (2002) mean annual ETordm mean annual reference crop evapotranspiration


ammonium sulphate according to Nelson (1982)Gypsum (CaSO42H2O) was determined by preci-pitation with acetone (Salinity Laboratory Staff1954) Soil pH was measured with a glass electrodein a saturated paste Electrical conductivity (totalsoluble salts) was determined in the saturationextract (Salinity Laboratory Staff 1954) Cationexchange capacity (CEC) was determined usingsodium acetate (NaOAc) at a pH of 82 (Chapman1965)

Mineralogical analyses

Removal of chemical cementing agents andseparation of clay fractions was carried outaccording to Mehra amp Jackson (1960) Kittrick ampHope (1963) and Jackson (1975) After repeatedwashing for removal of salts (including gypsum)the carbonates were removed using 1 N sodiumacetate buffered at pH 5 The addition of 1 N

sodium acetate was continued until no efferves-cence was observed with 1 N HCl (Jackson 1975)The reaction was performed in a water bath at 80ordmCOrganic matter was oxidized by treating thecarbonate-free soils with 30 H2O2 and digestionin a water bath This treatment also dissolvedMnO2 Iron oxides were removed from the samplesby the dithionate citrate bicarbonate method (Mehraamp Jackson 1960) and iron was determined in thefiltrated solution by atomic absorption spectro-scopy

The iron oxide-free samples were centrifuged at750 rpm for 54 min (RCF = 157 g InternationalIEC Centrifuge) and clay separates were removedAdditionally for removal of fine clay (particleslt02 mm) clay separates were centrifuged at2700 rpm for 40 min according to Kittrick ampHope (1963) X-ray diffraction (XRD) studieswere carried out using a Philips diffractometer onboth fine and coarse clay fractions while equalconcentrations of clay suspensions were used for allsamples in order to allow for a more reliablecomparison between relative peak intensities fordifferent samples

The (00l) reflections were obtained following Mgsaturation ethylene glycol solvation and K satura-tion The K-saturated samples were studied bothafter drying and heating at 330 and 550ordmC for 4 hTo identify kaolinite in the presence of trioctahedralchlorite samples were also treated with 1 N HCl at80ordmC overnight The percentages of the clayminerals were estimated according to Johns et al

(1954) Rock samples were ground and also treatedwith sodium acetate (pH 5) to remove carbonatesand the clay fractions were separated using thecentrifuge technique discussed above A totalnumber of 81 clay samples from soils and parentrocks were studied using XRD

Electron microscopy studies

Small soil aggregates (~1 cm3) were studied byscanning electron microscopy (SEM) Driedsamples were mounted on Al stubs using double-sided tape and carbon paste then coated with Auand examined using a LEO SEM For TEM studiessuspensions of 1500 (g dry clayml water) wereprepared from dried soil and rock clay particles and100 ml of the suspension were dried on 200-meshformvar-coated Cu grids under a heat lamp for 25min and examined using a LEO 906E transmissionelectron microscope (TEM)

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

Physicochemical properties of the soils

The parent material of southern Iran is mainlylimestone and most of the soils are highlycalcareous throughout The electrical conductivities(EC) of the well drained soils are low but inimperfectly drained soils which are mostly saline itreaches a maximum of 240 dSm (pedon 62) Thetotal gypsum content of soils ranges from traceamounts in most pedons to ~30 in the By horizonof pedon 66 Only a few pedons have a high enoughorganic carbon content in their surface layers tomeet the requirements of the mollic epipedon(pedons 51 70 and 72) The pH range in thestudied pedons is 7 ndash85 The CEC of the soilswhich is dependent on the amount and the type ofclay minerals ranges from 3 ndash4 cmole (+) kgndash1 incoarse-textured pedon 52 to a maximum of32 cmole (+) kgndash1 in Bt horizon of pedon 70(Table 2)

Clay mineralogy of soils and rocks

The semiquantitative analysis of clay minerals inthe studied rocks and soils are shown in Table 3Chlorite illite smectite palygorskite and kaoliniteare the major clay minerals of both the parent rocksand soils Studies by XRD and TEM of thesedimentary rocks in the study area revealed that


TABLE 2 Range mean and median of some selected physicochemical properties of the studied pedons

Sand Silt Clay OC Gypsum CCE pH EC CECmdashmdashmdashmdashmdashmdashmdashmdashmdash mdashmdashmdashmdashmdashmdashmdashmdashmdashmdash dS m ndash1 cmol kg ndash1

Range 317 ndash764 7 ndash692 7 ndash54 0 ndash28 0 ndash306 9 ndash74 7 ndash86 02 ndash240 3 ndash321Mean 317 381 303 04 39 43 79 118 146Median 293 40 299 02 02 43 78 10 147

OC organic carbonEC electrical conductivityCCE calcium carbonate equivalentCEC cation exchange capacity

TABLE 3 Relative abundance of clay minerals CEC of the clay fraction and FeDCB of the studied soils andparent rocks

Pedon Horizon Clay Chl+ Ill+ Sme+ Pal+ Kao+ Ver+ Mixed layer CEC of clay FeDCB

size cmol kg ndash1

2 Ap c 30 30 ndash 40 ndash ndash ndash 35 0512 By2 c 20 20 ndash 60 ndash ndash ndash 35 0333 Bk c 20 30 lt10 40 lt10 ndash ndash 365 0834 Bky c 20 25 15 40 ndash ndash ndash 425 0624 By11 c 20 20 15 45 ndash ndash ndash 425 0485 Ap cc 30 35 25 ndash 10 ndash ndash 455 0995 Ap fc 10 15 40 15 10 10 ndash 605 Btk2 cc 30 25 30 ndash 15 ndash ndash 47 0585 Btk2 fc 10 20 35 15 10 10 ndash 57510 Ap cc 25 35 25 ndash 15 ndash ndash 445 10210 Ap fc 20 30 30 10 10 ndash ndash 4510 Btk1 cc 35 30 25 ndash lt10 ndash ndash 45 05710 Btk1 fc 20 20 45 10 lt10 ndash ndash 56511 Bt2 cc 25 35 30 ndash 10 ndash ndash 48 07411 Bt2 fc 15 20 30 20 lt10 10 ndash 5612 Ap c 20 30 30 20 ndash ndash ndash 50 04112 Bss2 c 15 15 40 20 lt10 ndash ndash 525 00815 C c 30 40 15 15 ndash ndash ndash 425 04517 Ap c 25 25 30 ndash lt10 10 ndash 53 07417 Bk c 15 20 35 ndash 20 10 ndash 555 07321 Ap cc 30 50 lt10 ndash lt10 ndash ndash 335 04521 Ap fc 15 20 20 30 lt10 ndash 15 illndashsme 4121 Bt2 cc 30 50 lt10 ndash lt10 ndash ndash 335 0421 Bt2 fc 20 25 20 35 ndash ndash ndash 4524 Cyz2 c 25 20 lt10 45 ndash ndash ndash 36 01226 Ap cc 20 20 20 40 ndash ndash ndash 45 05226 Ap fc 15 10 lt10 65 ndash ndash ndash 3626 Btk cc 20 20 lt10 50 ndash ndash ndash 36 03926 Btk fc lt15 ndash ndash 85 ndash ndash ndash 3527 C c 20 20 ndash 50 lt10 ndash ndash 31 02628 Bk2 c 20 25 ndash 55 ndash ndash ndash 35 03631 Ap c 20 30 ndash 50 ndash ndash ndash 35 02631 By2 c 15 30 ndash 55 ndash ndash ndash 35 02638 Ap cc 40 40 20 ndash ndash ndash ndash 45 04138 Ap fc 20 15 35 20 lt10 ndash ndash 48538 Btk3 cc 40 30 20 ndash lt10 ndash ndash 425 072


38 Btk3 fc 20 15 35 20 lt10 ndash ndash 5039 Bk1 c 30 25 10 30 lt10 ndash ndash 405 0540 Btny cc 20 20 25 35 ndash ndash ndash 475 04340 Btny fc 15 10 15 50 lt10 ndash ndash 4041 Ap cc 20 20 20 20 ndash ndash 20 illndashsme 39 0641 Ap fc 10 15 40 25 ndash 10 ndash 6241 Bty1 cc 20 20 25 35 ndash ndash ndash 475 0541 Bty1 fc 20 10 10 60 ndash ndash ndash 4043 Cy1 c 20 10 10 50 lt15 ndash ndash 375 04344 Ap cc 20 30 10 40 ndash ndash ndash 40 05644 Ap fc 15 15 15 55 ndash ndash ndash 45544 Btk cc 25 25 15 30 lt10 ndash ndash 415 0644 Btk fc 15 15 10 50 ndash ndash 10 illndashsme 3744 By cc 20 15 20 45 ndash ndash ndash 45 03544 By fc 15 10 15 60 ndash ndash ndash 42551 Apg c 25 20 25 30 ndash ndash ndash 475 00551 Bkg2 c 10 10 50 20 lt10 ndash ndash 575 00952 Cz2 c 20 60 20 ndash ndash ndash ndash 35 0260 Ap c 20 20 10 50 ndash ndash ndash 40 0360 Bk1 c 10 25 15 50 ndash ndash ndash 425 03262 Az c 30 40 20 10 ndash ndash ndash 45 06162 By2 c 30 30 ndash 40 ndash ndash ndash 35 03864 Bk2 c 35 30 15 20 ndash ndash ndash 425 05365 C c 25 20 ndash 55 ndash ndash ndash 35 06366 Btk c 20 20 20 40 ndash ndash ndash 45 06366 By c 20 20 10 40 lt10 ndash ndash 375 05570 A cc 10 30 40 ndash 20 ndash ndash 59 09870 A fc 10 15 30 lt10 25 10 ndash 4970 Bt cc 30 15 25 ndash 30 ndash ndash 415 08570 Bt fc 15 15 40 lt10 20 10 ndash 59571 Bk1 c 20 20 20 25 lt10 ndash 10 chlndashsme 41 04272 Ap c 30 30 20 10 10 ndash ndash 43 06472 Btk c 20 20 20 30 10 ndash ndash 39 05473 C1 c 20 20 30 20 lt10 10 ndash 615 06274 Ap c 25 35 ndash 40 ndash ndash ndash 35 03174 By2 c 20 25 ndash 55 ndash ndash ndash 35 02375 Bky c 15 15 lt10 60 lt10 ndash ndash 365 033R1 ndash c 10 15 ndash ndash 75 ndash ndash 17 002R2 ndash c 25 20 25 30 ndash ndash ndash 22 028R3 ndash c 15 15 25 ndash 45 ndash ndash 25 003R4 ndash c ndash 5 ndash ndash 95 ndash ndash 12 024R5 ndash c 20 25 15 40 ndash ndash ndash 27 003R6 ndash c 20 20 20 ndash 40 ndash ndash 26 001R7 ndash c 40 20 20 10 ndash ndash 10 illndashsme 32 001R8 ndash c 15 10 10 65 ndash ndash ndash 25 002

c clay lt2 mm cc coarse clay 2ndash02 mm fc fine clay lt02 mm+ chl chlorite Ill illite Sme smectite Pal palygorskite Kao kaolinite Ver vermiculite illndashsme illitendashsmectitechlndashsme chloritendashsmectite Rock samples

TABLE 3 (contd)


size cmol kg ndash1


kaolinite is the dominant clay mineral in theCretaceous rock samples (R1 R3 R4 and R6) InOligo-Miocene and Pliocene samples palygorskiteis dominant (R2 R5 and R8) However there is nopalygorskite observed in the Cretaceous rocksChlorite and illite are present in all samplesSmectite is less abundant in two of the Cretaceoussamples (R3 and R6) but is present in otheryounger sediments (R2 R5 R7 and R8)

Both fine and coarse clay fractions were analysedby XRD in soils with argillic horizons to study theirdevelopment and process of clay illuviationChlorite illite smectite palygorskite and kaoliniteare the main minerals in the total clay fraction Inthe fine clay fraction palygorskite is dominant inpedons 21 44 26 72 and 66 and smectite inpedons 38 5 10 70 and 11 Almost all the soilsstudied contain illite and chlorite as the major partof the coarse clay fraction Kaolinite occurs insmall amounts in many of the soils studied

Palygorskite is the dominant clay mineral ofpedons 2 3 4 24 27 28 31 40 41 43 44 6062 65 74 and 75 (Table 3) These soils are mainlyfrom more arid areas (Fig 1) Smectite is thedominant clay mineral in soils with vertic properties(pedons 11 and 12) and in poorly drained Aquolls(pedon 51) Illite-smectite is the main interstratifiedclay mineral occurring in small amounts in some ofthe pedons Mixed-layer chlorite-smectite is onlypresent in small amounts in pedon 71 Vermiculitewas only detected in small amounts in the fine clayfraction of pedons 5 11 17 41 70 and 73

Table 3 also presents the CEC of the clayfraction and FeDCB of the studied soils and rocksThe CEC values range from 31 cmolekg in pedon27 to as high as 62 in the fine clay fraction ofpedon 41 The CECs of coarse clays areconsistently less than those of fine clay fractionsThe higher CEC values of the fine clay innorthwestern soils are in accordance with XRDresults showing that they are dominated bysmectite The CEC values are in accordance withthe clay mineralogical data The FeDCB content ofthe soils ranges from lt01 in pedon 51 to 1 inpedon 10 The FeDCB content of the rocks is verylow

Origin and distribution of clay minerals in soils

Illite and chlorite Illite and chlorite are twocommonly observed clay minerals occurring mainlyin areas of steep relief where active mechanical

erosion limits soil formation (Fanning et al 1989Wilson 1999) Their abundance in soils is larglydue to their presence in parent rocks They are thedominant clay minerals of the coarse clay fraction(Table 3) As presented in Table 3 the illite contentof most of the soils of northwestern areas decreaseswith depth Several processes may account for thisdecrease such as aeolian deposition of this mineral(McFadden et al 1986) enhanced physical weath-ering of biotite grains in the near-surface horizonsdue to large daily and seasonal fluctuations intemperature and moisture and to limited lessivagein the aridic climate (Boettinger amp Southard 1995)In the present study simple transformation of illiteto other clay minerals (mainly smectite) may play amajor role in the decrease in illite content withdepth especially in soils of the northwesternregions where the soil-available moisture increasesand provides a relative leaching environment for therelease of K+ from micaceous minerals and mainlyillite Moreover the calcareous environment high inMg and Si mobility might provide favourableconditions for the formation of smectite throughtransformation at the soil surface and its subse-quent lessivage results in greater accumulations ofit in the deeper horizons

As stated earlier there is some evidence thatmica may form pedogenically from K fixation inpre-existing smectites because of the hot and drysoil conditions (Mahjoori 1975) However illite(mica) constitutes the main part of all studiedparent rocks and its presence in soils is mainly ofdetrital origin Chlorite is also inherited in thestudied soils as well as illite but it does not showany regular pattern with increasing depth (Table 3)Chlorite and illite were reported as the major clayminerals of Entisols of alluvial and colluvial fans ofthe southern Iran (Abtahi 1977 Khormali ampAbtahi 2001)

Kaolinite Precipitation of kaolinite from solutionrequires acid conditions with moderate silicaactivity and small amounts of base cations Thisclay mineral forms mainly from feldspars and micaalteration in conditions of low K+ activity and highH+ activity (Curtis 1983 Dixon 1989) Theseconditions occur in warm and humid tropical andsubtropical climates

In the study area kaolinite is mainly detected inthe clay fraction of pedons located near theCretaceous outcrops (Table 3 Fig 2 mainly inpedons 5 10 11 17 70 72 and in trace amountsin some other pedons) Thus it can be concluded


that the presence of this clay mineral in the studiedsoils of arid and semi-arid regions is due to itsinheritance from surrounding parent rocks Asshown in Fig 2 the greater occurrence of kaolinitein pedon 70 is mainly related to the presence ofkaolinite-bearing Cretaceous outcrops and highprecipitation There is no (or only traces of)kaolinite in the southern parts of the study areawhich are formed on or near geological formationswhich are younger than the Cretaceous sediments

Smectite and palygorskite As presented inTable 3 smectite constitutes the major portion ofthe clay minerals in the well drained Alfisols(pedons 5 10 11 21 38 41 44 70 and 72) somepoorly-drained Mollisols (pedon 52) andCalcixerepts with high precipitation (pedon 17) Itis detected in trace amounts in arid soils of thesouthern southeastern southwestern and northernparts Figure 3 shows the distribution of smectite insoils of the study area It can be concluded that in

a b

FIG 2 TEM image of clay in Cretaceous rock (a) and in the Bt horizon of pedon 70 (b) showing hexagonalkaolinite and a few short palygorskite fibres (c) and (d) are XRD patterns of clay in Cretaceous rock (a) and in

the Bt horizon of pedon 70 (b) respectively


well-drained soils with increasing soil-availablemoisture (as expressed in PETordm) smectite shows anincreasing trend

There are three main sources of smectite in soils(1) neoformation from soil solution (2) detritalorigin or inheritance and (3) transformation ofother clay minerals Low-lying topography poordrainage and base-rich parent material favourablechemical conditions characterized by high pH highsilica activity and an abundance of basic cations arethe factors strongly influencing the origin anddistribution of smectite in soils (Borchardt 1989Aoudjit et al 1995) The presence of large amountsof this mineral in poorly drained pedon 52 (asdiscussed by Abtahi amp Khormali 2001) suggests aneoformational origin of this mineral underfavourable soil solution conditions Neoformationof smectite was also reported by Givi amp Abtahi(1985) and Gharaee amp Mahjoory (1984) undersaline and alkaline conditions with high concentra-tions of Si Mg and Al in southern Iran Howeverneoformation can not solely explain the generalincreasing trend of this mineral with increasing soil-available moisture in relatively well-drained soils ofthe northwestern parts (Fig 3)

Increasing soil-available moisture towards thenorthwest and consequently a relatively moreleaching environment for the release of K+ frommicaceous minerals and mainly illite in thecalcareous environment with high Mg++ and highSi mobility might provide favourable conditions forthe formation of smectite through transformationAs discussed earlier illite content decreases withincreasing depth unlike smectite which shows anincreasing trend (Table 3) The presence of inter-stratified illite-smectite in some pedons could be anindication of an intermediate stage of illitetransformation into smectite Transformation ofchlorite into smectite has rarely been reported incontrast to many reports about illite to smectitetransformation which seems rather more likely

Palygorskite is the other possible precursormineral for smectite in arid and semi-arid environ-ments Singer (1989) noted that the frequentassociation of smectite and palygorskite is a resultof the proximity of their stability fields Paquet ampMillot (1972) showed that palygorskite is not stableat precipitation levels above 300 mm and trans-forms to smectite Figure 4 shows that there is areverse correlation between palygorskite and

FIG 3 (a) Distribution of smectite in different climatic regions of Fars Province (b) smectite percentage (as anaverage for the soil profile) vs PETordm R2 = 0497)


smectite in the soils studied Smectite forms thedominant clay mineral of the argillic horizons whenPETordm is gt~04 (Fig 3 pedons 5 10 11 38 and70) However in the argillic horizons of the aridregions (mainly pedons 26 and 66) palygorskite isthe dominant clay mineral

As discussed earlier smectite is also present insedimentary rocks of late Cretaceous and youngerdeposits Thus detrital origin must also beconsidered However this process is not a dominantsource of smectite in soils since there is no uniformdistribution of smectite in soils near the smectite-

bearing outcrops In contrast to kaolinite thiscorrelation is not evident for smectite Thereforetransformation seems to be the dominant factor forthe formation and distribution of smectite in well-drained soils of the arid and semi-arid soils studiedClimate or soil-available moisture has a stronginfluence on the distribution and formation ofsmectite in the different parts of the study areaSmectite in less weathered C horizons of some soilsis most probably of detrital origin (Pedons 15 43and 73 as reported also by Wilson 1993)

In argillic horizons of arid regions (Pedons 2666) most evidence suggests that these minerals arePleistocene relics related to a moister climate and toeluvial deposition in the soil (Nettleton amp Peterson1983) The relative abundance of smectite in deeperhorizons indicates that its weathering is acceleratedin the surface soil Moreover it is dominant in thefine clay portion Therefore it can also beconcluded that smectite is the major illuviatedclay mineral of the smectitic argillic horizons

In contrast to smectite palygorskite shows adecreasing trend with increasing soil-availablemoisture or decreasing aridity Figure 5 shows thedistribution of palygorskite in the studied soils ofthe Fars Province The greatest occurrence of this

FIG 4 Palygorskite () vs smectite () in the studiedpedons (R2 = 0731)

FIG 5 (a) Distribution of palygorskite in different climatic regions of Fars Province (b) palygorskite percentage(as an average for the soil profile) vs PETordm R2 = 0558)


fibrous mineral is in the more arid areas of thesouth southeast and north and it occurs in smallamounts or is even absent in soils of the north-western parts where there is more soil-availablemoisture (PETordm gt04)

Henderson amp Robertson (1958) and Burnett et al(1972) were the first to report traces of palygorskitein sediments and soils from Iran Severalresearchers consider that palygorskite formedpedogenically in the soils of Iran (Abtahi 19771980 Mahjoory 1979 Gharaee amp Mahjoory 1984Khademi amp Mermut 1998 1999)

An environment rich in Si and Mg with high pHbut poor in Al and Fe is favourable for theformation of palygorskite and sepiolite (Singer1989) Palygorskite in arid soils has been stated tohave two main origins (1) inheritance from parentmaterials and (2) pedogenic formation throughneoformation or transformation of 21 minerals Thetransformation of 21 clay minerals mainly illite orsmectite to palygorskite in solutions high in Si andMg and low in Al and K is a possible mechanismreported by many researchers (Suarez 1994) Asstated earlier parent sedimentary rocks of Tertiaryage and younger contain considerable amounts ofpalygorskite which can be inherited by the adjacentsoils Therefore this source of palygorskite can be apossible origin especially in more arid parts of thesouthern Fars province Well preserved longpalygorskite bundles of plateau soils of southernarid regions (eg Pedon 26) formed directly on thelate Tertiary sedimentary rocks (R5) containing alot of palygorskite are probably of inherited origin(Fig 6) This was also indicated by Khademi ampMermut (1998) in plateau soils of central Iran Inpedon 26 palygorskite constitutes the majority ofthe fine clay especially in the argillic horizon Thepresence of palygorskite in alluvial or colluvialsoils (eg pedon 27) could also be related toinheritace from the parent rocks (detrital)

As mentioned ealier gypsum would favour the insitu formation of palygorkite from soil solution Thedistribution of gypsiferous soils in Fars Province isshown in Fig 5 Gypsum like other evaporiteminerals has been precipitated after drying theshallow saline and alkaline lakes present in theTertiary Due to the arid climate this solublemineral is stable in those areas As for palygorskitegypsiferous soils also occur in more arid parts ofthe Fars Province

The relationship of palygorskite with gypsum canbe deduced from the SEM images (Fig 7) this

fibrous mineral is enmeshed with gypsum crystalsand sprouts from their surfaces which suggests theirauthigenic origin consistent with the earlier observa-tions of Khademi amp Mermut (1998) Ingles ampAnadon (1991) Singer et al (1995) and Pletsch etal (1996) It is known that gypsum would increasethe MgCa ratio of the host water which could inturn encourage palygorskite formation (Hassouba ampShaw 1980) The relationship of palygorskitepercentage to gypsum content of the soils and soil-available moisture (PETordm) for the studied pedons ispresented as the following equation

Palygorskite () = 534 +047 Gypsum () ndash 677 PETordm

R2 = 056 n = 34

The percentage of palygorskite in soils issignificantly related to the gypsum content andPETordm of the soils An increase in gypsum ordecrease in PETordm (or increase in aridity) wouldlead to higher levels of palygorskite in soils As forsmectite palygorskite shows an increasing trendwith depth probably due to more favourableconditions in the deeper horizons mainly containinggypsum in arid regions (Table 3)

There is considerable palygorskite in soils of theSarvestan area (Pedons 39 40 41 43 and 44)which were affected by the nearby saline andalkaline lake (Maharlu) These soils contain gypsumin their subsurface horizons The presence of large

FIG 6 TEM image of pedon 26 showing wellpreserved long palygorskite bundles


amounts of well bundled and elongated palygorskitein soils of this area with parent sedimentary rockcontaining little palygorskite (R7) could be relatedto their authigenic formation in the presence ofgypsum (Fig 8a) The presence of shallow salineand alkaline groundwater (pedons 43 62 and 24)could also have favoured the neoformation of

palygorskite from soil solution Under suchconditions palygorskite may also form fromsmectite (Singer 1989)

Towards the northwest palygorskite decreaseswhereas the smectite content of the soils increasesFigure 8b shows a few short palygorskite fibres inpedon 10 indicating their gradual disappearance

a b

FIG 7 (a) SEM image of pedogenic gypsum coated with palygorskite (pedon 74 By3) (b) elemental analysis ofpalygorskite fibres on gypsum crystals (SEM-EDX)

FIG 8 TEM images of fine clay in the By horizon of pedon 44 (a) and the Btk1 horizon of pedon 10 (b)


and possible transformation into smectiteTherefore it can be concluded that the criticalavailable moisture as expressed by PETordm is ~04above which palygorskite is highly unstable andtransforms mainly to smectite Sepiolite is rarelydetected in soils by XRD mainly because of itsdissolution under NaOAC treatment at pH 45 forthe removal of carbonate (Abtahi 1985)

Vermiculite The occurrence and stability ofvermiculite in calcareous and silica-rich soils hasnot been well documented (Boettinger amp Southard1995) It is also reported in trace amounts incalcareous soils of southern or central Iran(Khademi amp Mermut 1998 Khormali amp Abtahi2001) In the present study only rare occurrences ofthis clay mineral were detected mainly in north-western parts (Table 3 pedons 5 11 17 70 and73) where there is more available moisture in thesoils (PETordm gt04) According to the literature micain Aridisols can weather to vermiculite via directsimple transformation (Douglas 1989) Howeverthis cannot explain the rare occurrence of thismineral in the highly calcareous soils of southernandor central Iran

Mica can transform into smectite in the followingway

mica +Si +Ca +Mg ndashAl ndashKmdashmdashmdashmdashmdashmdashmdashmdashmdashmdash smectite + Al + K

This transformation requires depotassificationdealumination and silication In other wordstemperature and pressure must be low enough todestabilize [Al] in the mineral structure Lowactivity of K+ and Al3+ and high Si(OH)4 activityare other requirements At ~pH gt6 (as is the casefor the studied soils) Al is insoluble and incontrast Si is highly soluble especially at pH of ~8(as in calcareous soils of the Fars Province)Therefore the large amount of Mg present incalcareous materials can substitute for Al in thelattice and form smectite Moreover in the north-western regions there is enough precipitation toallow the process of depotassification At pH lt6the activity of Al increases and vermiculite canform In the case of very low pH and a highlyleaching environment kaolinite may even formfrom vermiculite This explanation is proposed forthe rare occurrence of vermiculite in highlycalcareous soils of the study area Those pedonswith traces of vermiculite in their fine clay holdmore available moisture in contrast to other pedonsThe longer periods of moisture availability forchemical weathering and dampened temperature

fluctuations relative to other areas may havefavoured the stability of vermiculite in those areasas discussed previously for smectite (Boettinger ampSouthard 1995)

Figure 9 presents the major pathways for theformation of clay minerals in arid and semi-aridregions for the studied pedons Vermiculite asdiscussed earlier is considered to be unstableunder the present conditions of the studied areaand its occurrence is of little importance Kaolinitechlorite and illite in soils are considered mainly ofinherited origin from parent rocks Neoformation isthe main mechanism for the occurrence ofpalygorskite and formation of smectite could bemainly related to transformation of illite andorpalygorskite

C O N C L U S I O N S

The presence of some kaolinite in northwesternparts of Fars province is due to its inheritance fromthe surrounding kaolinite-bearing Cretaceous rocksIllite and chlorite are two commonly observed clayminerals and their abundance in the soils is largelydue to their presence in parent rocks Simpletransformation of illite to other clay minerals(mainly smectite) may play a role in its relativedecrease with depth

Smectite constitutes the major portion of the clayminerals in well drained Alfisols somewhat poorlydrained Mollisols and Calcixerepts with highprecipitation It is detected in trace amounts insoils of more arid areas Therefore it can beconcluded that in well drained soils with increasingsoil-available moisture (as expressed in PETordm)smectite increases Increase in soil availablemoisture and consequently a relatively leachingenvironment for the release of K+ from micaceousminerals and mainly illite in the calcareousenvironment with high Mg++ and high Si mobilitymight provide favourable conditions for theformation of smectite through transformation

There is an inverse correlation between paly-gorskite and smectite with regard to the soilavailable moisture Palygorskite is scarce whenPETordm is gt04 This indicates that the criticalavailable moisture as expressed by PETordm is ~04above which palygorskite is highly unstable andweathers mainly to smectite Palygorskite isconsidered to be inherited in plateau soils of thearid regions whereas its occurrence in saline andalkaline soils and soils high in gypsum is mainly of


authigenic origin The rare occurrence of vermicu-lite in the calcareous soils studied is proposed to berelated to its instability under high pH low Alactivity high Si and Mg These conditions arefavourable for smectite formation

ACKNOWLEDGMENTS

The authors thank Shiraz University Research Council(grant No 78-AG-1242-667) Research and TechnologyCouncil of Fars Province and Ministry of ScienceResearch and Technology of the Islamic Republic of Iranfor their financial support We are also grateful to Prof EVan Ranst State University of Ghent Belgium and ProfR Baumhauer Trier University Germany for theirsincere cooperation in XRD and SEM of some samplesand their interpretation

REFERENCES

Aba-Huseyn MM Dixon JB amp Lee SY (1980)Mineralogy of Saudi Arabian soils south westernregion Soil Science Society of America Journal 44643 ndash649

Abtahi A (1977) Effect of a saline and alkaline groundwater on soil genesis in semiarid southern Iran SoilScience Society of America Journal 41 583 ndash588

Abtahi A (1985) Synthesis of sepiolite at roomtemperature from SiO2 and MgCl2 solution ClayMinerals 20 521 ndash523

Abtahi A amp Khormali F (2001) Genesis and morpho-logical characteristics of Mollisols formed in acatena under water Table influence in southernIran Communications in Soil Science and PlantAnalysis 32 1643 ndash1658

Al Ravi AH Jackson ML amp Hole FD (1969)Mineralogy of some arid and semiarid land soils ofIraq Soil Science 107 480 ndash486

Aoudjit M Robert M Elsass F amp Curmi P (1995)Detailed study of smectite genesis in graniticsaprolites by analytical electron microscopy ClayMinerals 30 135 ndash147

Banaei MH (1998) Soil Moisture and TemperatureRegime Map of Iran Soil and Water ResearchInstitute Ministry of Agriculture Tehran Iran

Boettinger JL amp Southard RJ (1995) Phyllosilicatedistribution and origin in Aridisols on a graniticpediment Western Mojave Desert Soil ScienceSociety of America Journal 59 1189 ndash1198

Borchardt G (1989) Smectites Pp 675 ndash727 inMinerals in Soil Environment (JB Dixon amp SBWeed editors) Soil Science Society of AmericaMadison Wisconsin USA

Bronger A Winter R amp Sedov S (1998) Weatheringand clay mineral formation in two Holocene soils

FIG 9 Pathways for the formation of clay minerals in the studied soils Note that kaolinite formation throughtransformation of other minerals is not possible under the current climate of the study area (designated withcrosses) Palygorskite is considered mainly to form as a result of neoformation and smectite forms mainlythrough transformation of illite andor palygorskite The thickness of the arrows indicates the importance of the

pathway The parentheses indicate lesser importance


and in buried paleosols in Tadjikestan Towards aQuaternary paleoclimatic record in Central AsiaCatena 34 19 ndash34

Burnett AD Fookes PG amp Robertson RHS (1972)An engineer ing soil at Kermanshah ZagrosMountains Iran Clay Minerals 9 329 ndash343

Callen RA (1984) Clays of palygorskite-sepiolitegroup depositional environment age and distribu-t ion Pp 1 ndash38 in Palygor sk ite-sepiol iteOccurrences Genesis and Uses (A Singer amp EGalan editors) Developments in Sedimentology 37Elsevier Amsterdam

Chapman HD (1965) Cation exchange capacity Pp891 ndash900 in Methods of Soil Analysis Part 2 (CABlack editor) American Society of AgronomyMadison Wisconsin USA

Curtis CD (1983) Link between aluminium mobilityand destruction of secondary porosity AmericanAssociation of Petroleum Geology Bulletin 67380 ndash384

Day PR (1965) Particle fractionation and particle-sizeanalysis Pp 545 ndash566 in Methods of Soil AnalysisPart 1 (CA Black editor) American Society ofAgronomy Madison Wisconsin USA

Dixon JB (1989) Kaolin and serpentine group mineralsPp 467 ndash525 in Minerals in Soil Environment (JBDixon amp SB Weed editors) Soil Science Society ofAmerica Madison Wisconsin USA

Douglas LA (1989) Vermiculites Pp 635 ndash674 inMinerals in Soil Environment (JB Dixon amp SBWeed editors) Soil Science Society of AmericaMadison Wisconsin USA

Dregne HE (1976) Soils of Arid Regions ElsevierNew York

Ducloux J Guero Y amp Fallavier P (1998) Clay particledifferentiation in Alluvial soils of SouthwesternNiger (West Africa) Soil Science Society ofAmerica Journal 62 212 ndash222

Eswaran H amp Barzanji AF (1974) Evidence for theneoformation of attapulgite in some soils of IraqTransactions of the 10th International Congress ofSoil Science Moscow Russia 7 154 ndash161

Fanning DS Keramidas VZ amp El-Desoky MA(1989) Micas Pp 551 ndash634 in Minerals in SoilEnvironment (JB Dixon amp SB Weed editors) SoilScience Society of America Madison WisconsinUSA

Gharaee HA amp Mahjoory RA (1984) Characteristicsand geomorphic relationships of some representativeAridisols in southern Iran Soil Science Society ofAmerica Journal 48 115 ndash119

Givi J amp Abtahi A (1985) Soil genesis as affected bytopography and depth of saline and alkaline ground-water under semiarid conditions in southern IranIran Agricultural Research 4 11 ndash27

Hassouba H amp Shaw HF (1980) The occurrence ofpalygorskite in Quaternary sediments of the coastal

plain of northwest Egypt Clay Minerals 15 77 ndash83Henderson SG amp Robertson RHS (1958) A

Mineralogical Reconnaissance in Western IranResource Use Ltd Glasgow UK

Ingles M amp Anadon P (1991) Relationship of clayminerals to depositional environment in the non-marine Eocene Pontils Group SE Ebro Basin(Spain) Journal of Sedimentary Petrology 61926 ndash939

Jackson ML (1975) Soil Chemical Analysis AdvancedCourse University of Wisconsin College ofAgricult ure Department of Soils Madison Wisconsin USA

Johns WD Grim RE amp Bradley F (1954)Quantitative estimation of clay minerals by diffrac-tion methods Journal of Sedimentary Petrology 24242 ndash251

Khademi H amp Mermut AR (1998) Source ofpalygorskite in gypsiferous Aridisols and associatedsediments from central Iran Clay Minerals 33561 ndash575

Khademi H amp Mermut AR (1999) Submicroscopy andstable isotope geochemistry of carbonates andassociat ed palygorsk ite in Iranian Aridisol sEuropean Journal of Soil Science 50 207 ndash216

Khormali F amp Abtahi A (2001) Soil genesis andmineralogy of three selected regions of FarsBushehr and Khuzestan Provinces of Iran formedunder highly calcareous condit ions IranAgricultural Research 20 67 ndash82

Kittrick JA amp Hope EW (1963) A procedure for theparticle size separation of soils for X-ray diffractionanalysis Soil Science 96 312 ndash325

Lopez-Galindo A Ben Aboud A Fenoll Hach-Ali P ampCasas Ruiz J (1996) Mineralogical and geochemicalcharacterization of palygorskite from Gabasa (NESpain) Evidence of a detrital precursor ClayMinerals 31 33 ndash44

Mahjoory RA (1975) Clay mineralogy physical andchemical properties of some soils in arid regions ofIran Soil Science Society of America Proceedings 39 1157 ndash1164

McFadden LD Wells SG amp Dohrenwend JC (1986)Influences of Quaternary climatic changes on theprocesses of soil development on desert loessdeposits of the Cima volcanic field CaliforniaCatena 13 361 ndash389

Mehra OP amp Jackson ML (1960) Iron oxide removalfrom soils and clays by a dithionite citrate systemwith sodium bicarbonate Clays and Clay Minerals7 317 ndash327

MPB (Ministry of Programming and Budgeting) (1994)Economic and Social Status of Fars Province Publication Center for Informatic and DevelopmentStudies (in Farsi)

Myriam M (1998) Structural and textural modificationsof palygorskite and sepiolite under acid treatment


Clays and Clay Minerals 46 225 ndash231Nelson RE (1982) Carbonate and gypsum Pp

181 ndash199 in Methods of Soil Analysis part 2 (ALPage editor) American Society of AgronomyMadison Wisconsin USA

Nettleton WD amp Peterson FF (1983) Aridisols Pp165 ndash216 in Pedogenesis and Soil Taxonomy Part2 The Soil Orders (LP Wilding NE Smeck ampGF Hall editors) Elsevier Science PublishersBV Amsterdam

Nettleton WD Nelson RE amp Flach KW (1973)Formation of mica in surface horizons of drylandsoils Soil Science Society of America Proceedings 37 473 ndash478

Niederbudde EA amp Kussmaul H (1978) Tonmineraleigenschaften und Umwandlungen in Parabraunerde-P ro f i l p ae r en un te r Acke r und Wald inSuddeutschland Geoderma 20 239 ndash255

Paquet H amp Millot G (1972) Geochemical evolution ofclay minerals in the weathered products and soils ofMedi terranean climates Pp 199 ndash202 inProceedings of the International Clay Conference Madrid Spain

Pletsch T Daoudi L Chamley H Deconinck JF ampCharroud M (1996) Palaeogeographic controls onpalygorskite occurrence in Mid-Cretaceous sedi-ments of Morocco and adjacent basins ClayMinerals 31 403 ndash416

Roads M Luque FJ Mas R amp Garzon MG (1994)Calcretes palycretes and silcretes in the Paleogenedetrital sediments of the Duero and Tajo BasinsCentral Spain Clay Minerals 29 273 ndash285

Sadeghi AR Kamgar-Haghighi AA Sepaskhah ARKhalili D amp Zand-Parsa Sh (2002) Regionalclassification for dryland agriculture in southernIran Journal of Arid Environments 50 333 ndash341

Salinity Laboratory Staff (1954) Diagnosis andImprovement of Saline and Alkali Soils USDAHandbook No 60 Washington DC

Sanchez HS amp Galan E (1995) An approach to thegenesis of palygorskite II A neogene-Quaternarycontinental basin using principle factor analysisClay Minerals 30 225 ndash238

Sanguesa FJ Arostegui J amp Suarez-Ruiz I (2000)Distribution and origin of clay minerals in the LowerCretaceous of the Alava Block (Basque-CantabrianBasin Spain) Clay Minerals 35 393 ndash410

Singer A (1989) Palygorskite and sepiolite groupminerals Pp 829 ndash872 in Minerals in SoilEnvironment (JB Dixon amp SB Weed editors)Soil Science Society of America Madison Wisconsin USA

Singer A Kirsten W amp Buhmaan C (1995) Fibrousclay minerals in the soils of Namaqualand SouthAfrica Characteristics and formation Geoderma66 43 ndash70

Soil Survey Staff (1993) Soil Survey Manual HandbookNo 18 USDA Washington DC

Soil Survey Staff (1998) Keys to Soil Taxonomy USDANRCS Washington DC

Suarez M (1994) Evidence of a precursor in theneoformation of palygorskite new data by analyticalelectron microscopy Clay Minerals 29 255 ndash264

Wilson MJ (1993) Pedologic factors influencing thedistribution and properties of soil smectite Trends inAgricultural Science 1 199 ndash216

Wilson MJ (1999) The origin and formation of clayminerals in soils past present and future perspec-tives Clay Minerals 34 7 ndash24

Zahedi M (1976) Explanatory text of the Esfahanquadrangle map 1250000 Geological Survey ofIran


Abtahi (1985) under saline and alkaline conditionswith high concentrations of Si Mg and Al insouthern Iran Abtahi amp Khormali (2001) foundsmectite as a major clay mineral in poorly drainedCalciaquolls of southern Iran The relative abun-dance of smectite increases in the deeper horizonsindicating that its weathering is accelerated in thesurface soil (Ducloux et al 1998)

Minor amounts of smectite were present inOligo-Miocene parent rocks of central Iran(Khademi amp Mermut 1998) but no smectite waspresent in Cretaceous rocks of that region Chloriteand illite were reported as the major clay mineralsof Entisols of alluvial and colluvial fans of thesouthern Iran (Abtahi 1977 Khormali amp Abtahi2001)

Palygorskite in arid soils has been stated tooriginate from two main sources (1) inheritancefrom parent materials and (2) pedogenic formationthrough neoformation or transformation of 21minerals The presence of gypsum (Eswaran ampBarzanji 1974) calcrete or caliche (Roads et al1994 Singer et al 1995 Lopez et al 1996) insaline and alkaline groundwater (Abtahi 1977)would favour the in situ formation of palygorskitefrom the soil solution The occurrence of palygors-kite in the soils and sediments of the Middle Eastgenerally corresponds to the area once covered bythe post-Tethyan intermontane shallow lagoonsduring the Tertiary (Callen 1984) Khademi ampMermut (1998) found palygorskite and sepiolite andappreciable amounts of mica and smectite in theclay fraction of the Oligo-Miocene limestone butonly traces of palygorskite in Cretaceous limestoneThis indicates that the sedimentary environment ofthe study region during the Tertiary was suitable forthe formation of palygorskite and sepiolite

In arid regions of southern Iran palygorskite isthe major mineral in well drained Mollisols whilesmectite is the major mineral in poorly drainedMollisols (Abtahi amp Khormali 2001) Singer(1989) noted that the frequent association ofsmectite and palygorskite is a result of theproximity in their stability fields Paquet amp Millot(1972) showed that palygorskite is not stable andtransforms to smectite when annual precipitation is5300 mm Sanchez amp Galan (1995) concluded thatpalygorskite forms as transformation products ofsmectite and illite Sepiolite is rarely detected insoils by XRD mainly because of its dissolutionunder NaOAC treatment in pH of 45 for theremoval of carbonate (Abtahi 1977 1985) Under

acidic conditions sepiolite is destroyed morerapidly than palygorskite because of its magnesiccomposition and the larger size of its structuralmicrochannels (Myriam 1998) Moreover sepiolitehas a lower stability under the presence of high Alin soil which provides favourable conditions for theformation of palygorskite rather than sepiolite Veryfew occurrences of vermiculite have been reportedin soils of arid and semi-arid regions (Abtahi 1977Khademi amp Mermut 1998 Khormali amp Abtahi2001)

Fars province the study area is one of the mostimportant agricultural regions in Iran Its widerange of climate including xeric aridic and usticsoil-moisture regimes and mesic thermic andhyperthermic soil-temperature regimes provides asuitable area to study the origin and distributionpattern of clay minerals Therefore the mainobjectives of this study were (1) to investigatethe origin of clay minerals in the calcareous aridand semi-arid regions of southern Iran and (2) todetermine the distribution patterns of clay mineralsunder different soil-available moisture regimes

M A T E R I A L S A N D M E T H O D S

Site description

Fars Province the study area is located in thesou the rn pa rt of Iran (50ordm30 rsquo ndash55 ordm38 rsquoE27ordm03rsquo ndash31ordm42rsquoN with a land area of 132 millionha (Fig 1a)

Geology

Fars Province is part of the Zagros orogenic area(Fig 1b) The Zagros mountain ranges with anorthwest to southeast direction are extendedtowards the central parts of the provinceElevation varies from 500 m above sea level inthe southern parts to ~4000 m above sea level inthe northern areas According to Zahedi (1976) theZagros area underwent a relatively moderateorogenic phase (attenuated Laramian phase) nearthe end of the Cretaceous and the beginning of theEocene characterized by folding emergence anderosion The Laramian movements were succeededby a shallow marine transgression A laterregression of the sea eastward resulted in theformation of intermontane lakes in the MiddleTertiary which may have produced an environmentconducive to the formation of fibrous clay minerals



Climate








Fieldwork







2 149 1358 1500 013 149 1358 1500 014 149 1358 1500 015 3416 121 1000 03510 3468 123 800 04511 3468 123 800 04512 3468 123 800 04515 698 125 1300 0517 609 124 1000 0621 3282 1589 1300 0324 224 233 3000 0126 224 233 3000 0127 224 233 3000 0128 224 233 3000 0131 251 232 1900 01338 338 148 900 0439 288 18 1100 0340 288 18 1100 0341 288 18 1100 0343 288 18 1100 0344 288 18 1100 0351 257 215 1900 01452 257 215 1900 01460 300 194 1100 0362 224 1585 1350 0264 224 1585 1350 0265 173 167 1500 0166 173 167 1500 0170 856 104 900 gt0771 600 21 1300 0572 600 21 1300 0573 600 21 1300 0574 400 222 1600 0375 400 222 1600 03
























size cmol kg ndash1





TABLE 3 (contd)


size cmol kg ndash1















a b

























R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES



































































Climate








Fieldwork







2 149 1358 1500 013 149 1358 1500 014 149 1358 1500 015 3416 121 1000 03510 3468 123 800 04511 3468 123 800 04512 3468 123 800 04515 698 125 1300 0517 609 124 1000 0621 3282 1589 1300 0324 224 233 3000 0126 224 233 3000 0127 224 233 3000 0128 224 233 3000 0131 251 232 1900 01338 338 148 900 0439 288 18 1100 0340 288 18 1100 0341 288 18 1100 0343 288 18 1100 0344 288 18 1100 0351 257 215 1900 01452 257 215 1900 01460 300 194 1100 0362 224 1585 1350 0264 224 1585 1350 0265 173 167 1500 0166 173 167 1500 0170 856 104 900 gt0771 600 21 1300 0572 600 21 1300 0573 600 21 1300 0574 400 222 1600 0375 400 222 1600 03
























size cmol kg ndash1





TABLE 3 (contd)


size cmol kg ndash1















a b

























R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES






































































Fieldwork







2 149 1358 1500 013 149 1358 1500 014 149 1358 1500 015 3416 121 1000 03510 3468 123 800 04511 3468 123 800 04512 3468 123 800 04515 698 125 1300 0517 609 124 1000 0621 3282 1589 1300 0324 224 233 3000 0126 224 233 3000 0127 224 233 3000 0128 224 233 3000 0131 251 232 1900 01338 338 148 900 0439 288 18 1100 0340 288 18 1100 0341 288 18 1100 0343 288 18 1100 0344 288 18 1100 0351 257 215 1900 01452 257 215 1900 01460 300 194 1100 0362 224 1585 1350 0264 224 1585 1350 0265 173 167 1500 0166 173 167 1500 0170 856 104 900 gt0771 600 21 1300 0572 600 21 1300 0573 600 21 1300 0574 400 222 1600 0375 400 222 1600 03
























size cmol kg ndash1





TABLE 3 (contd)


size cmol kg ndash1















a b

























R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES























































































size cmol kg ndash1





TABLE 3 (contd)


size cmol kg ndash1















a b

























R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES




































































TABLE 3 (contd)


size cmol kg ndash1















a b

























R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES















































































a b

























R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES























































































R2 = 056 n = 34








a b

















ACKNOWLEDGMENTS


REFERENCES





































































a b

















ACKNOWLEDGMENTS


REFERENCES















































































ACKNOWLEDGMENTS


REFERENCES


































































origin and distribution of clay minerals in calcareous

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