M. L. JACKSO~
                                 D e p a r t m e n t of Soils, U n i v e r s i t y of W i s c o n s i n ,
                                                     Madison, W i s c o n s i n

T h e f r e q u e n c y d i s t r i b u t i o n or r e l a t i v e a b u n d a n c e o f m i n e r a l s in soils varies w i t h t h e
five p r i n c i p a l classes of factors t h a t g o v e r n soil f o r m a t i o n . T h e c h a r a c t e r i s t i c s of t h e
m i n e r a l s of t h e p a r e n t m a t e r i a l , t h e t i m e factor, c l i m a t i c factors, relief factors a n d
biotic factors e a c h c a n be s h o w n to h a v e i m p o r t a n t i n d e p e n d e n t effects on clay m i n e r a l -
ogy of soils u n d e r proper c i r c u m s t a n c e s . T h e soil p a r e n t m a t e r i a l e x e r t s a control o v e r
t h e f r e q u e n c y d i s t r i b u t i o n of m i n e r a l s in soils b y i n t r o d u c t i o n of t h e c l a y m i n e r a l s into
t h e soil directly, b y controlling t h e ecru'so of chemical w e a t h e r i n g i n t h e soil t h r o u g h
t h e relative susceptibility of its m i n e r a l s to w e a t h e r i n g , b y f u r n i s h i n g a b u n d a n t d i v a l e n t
metallic cations, b y i m p e d i m e n t of d r a i n a g e , or b y acceleration of l e a c h i n g w h e n h i g h l y
p e r m e a b l e . T h e t i m e f a c t o r is c o n s p i c u o u s as long t i m e s give a n a d v a n c e d degree of
w e a t h e r i n g e v e n in t e m p e r a t e climates. C l i m a t e is i m p o r t a n t , s i n c e h i g h l y w e a t h e r e d
m a t e r i a l s i n e v i t a b l y occur as a r e s u l t of i n t e n s e leaching in w a r m t r o p i c a l a n d e q u a t o r i a l
climates. R e l i e f is i m p o r t a n t in c o n c e n t r a t i n g leaching w a t e r a n d m e t a l l i c cations, in
affecting o x i d a t i o n or r e d u c t i o n . T h e biotic factor affects m i n e r a l s c o n s p i c u o u s l y w h e r e
a n A 0 horizon develops a n d r e s u l t i n g c h e l u v i a t i o n m o v e s R~Oa o u t of t h e A 2 horizon.
    I n h e r i t e d m i n e r a l s s u c h as illite, q u a r t z , feldspars, f e r r o - m a g n e s i a n m i n e r a l s , car-
b o n a t e s a n d g y p s u m a r e m o s t a b u n d a n t in Clays of l i t t l e - w e a t h e r e d p a r e n t m a t e r i a l s
a n d soils of t h e zonal D e s e r t , B r o w n , C h e s t n u t a n d T u n d r a soils as well as i n t r a z o n a l
M o u n t a i n g r o u p s a n d a z o n a l Regosols a n d Lithosols. S e c o n d a r y l a y e r silicate m i n e r a l s
s u c h as vermiculite, s e c o n d a r y chlorite, m o n t m o r i l l o n i t e , kaolinite a n d halloysite are
most abundant in clays of nloderately weathered parent materials and soils of the
zonal Chernozem, Prairie, Gray-Brown      Podzelic, Podzol, Red-Yellow Podzolic, and
Low l-Iumic Latosol groups as well as intrazonal Planosol, Rendzina, Dark Magnesium
soil, and ~'iesenboden groups. Secondary sesquioxide minerals such as hematite,
goethite, allophane, gibbsita and anatase and residual resistant primary minerals such
as ilmenite and magnetite predominate in the more highly weathered parent materials
and soils of the zonal Ferruglnous l-[umic Latosols, I~ydrol IKumic Latosols, Lat0solie
Brown, and Ando soils and Latorites, as well as the intrazonal Tropical Savannah and
Ground Water Podzol ortstein soils.

An extremely wide variety of minerals occurs in the clay fraction in the major
great soil groups of the world. It is the purpose of the present article to
examine the frequency distribution or relative amounts       or percentages of
minerals that occur in the clay fractions of different great soil groups as
related to the major factors of soil formation. Occasional reference is made to
  the minerals of the silt fraction where they are related to the minerals in the
   clays. The term " layer silicates " is employed for the layer structured group
  of silicates, which makes up the largest portion of the clay fraction of many
  softs of intermediate degree of weathering, but also makes up important
  constituents of silts, sands, and consolidated rocks and thus cannot as
  accurately be termed " clay minerals." " Great soil groups " is used in the
  sense employed in the soil classification field (Thorp and Smith, 1949), falling
  in the categorical listing : soil phase, soil type, soil series, soil family, great
  soil group, soil suborder and soil order. " Factors of soil formation " refers
  specifically to the five principal classes of soil genetic factors : parent material,
  time, climate, relief and living organisms (lV[uckenhirn and others, 1949).
   Since m a n y analytical data on this subject have been published in various
  journals, the main objective centers on clarification of the way the soil genetic
  factors control clay distribution.
     The criteria used in soil classification encompass m a n y characteristics other
  than soft mineralogy, such as the number and thickness of various horizons,
  and the organic content, structure, color, texture (amount of clay, silt and
  sand, as distinct from its mineralogy), acidity, and mottling of each horizon.
  These characteristics are important to soil classification as to use-suitabftity
  and to interpretations of soil genesis. Many of these factors are controlled b y
t h e soft-forming factors more or less independently of the mineral species of
  the softs, and thus significant soil groupings often do not follow soil minerMogy
  in close detail.
     Broad differences in soil mineralogy do, on the other hand, cause differences
  in important soil characteristics, and this gives rise to a rough, broad correla-
  tion between soil groupings and soil mineralogy. The fact t h a t geochemical
  weathering of rocks reflects wide differences in climatic factors leads to " a
  broad g e n e r a l . . , association of chemical weathering processes and products
  with soil formations distributed over the earth " (Jackson and Sherman~
  1953, p. 242).

       SOIL     GLAY MINERALOGY               AS I N F L U E N C E D        BY
                  P A R E N T M A T E I ~ I A L FACTOI%S
   The soft parent material (both mantle rock and hard rock) inttuences soil
clay minerMogy directly and profoundly in three principal ways: (1) it
provides the litho]ogical minerals which young alluvial and little-weathered
soils (Lithosols and l~egosols) inherit in bulk from the mantle rock, and from
which older soils inherit the more resistant minerals accumulated as a residue
as the less stable minerals are removed by the chemical weathering (Jackson
and others, 1948) ; (2) the chemical nature of the mineral suite present in the
rock determines the supply of micas which weather to vermiculite, chlorite
and beidellite, and of divalent and other cations which influence the direction
and extent of chemical weathering of clay minerals ; and (3) the permeability
of the parent material, as controlled by its texture, porosity, and state of
subdivision, greatly influences the rate of leaching and concurren~ accumula-
               ~EQUENCY DISTRIBUTION OF CLAY MXNEI~ALS                       135
tion of specific mineral products of weathering. The importance of parent-
material factors is brought out by the divergence in soil mineralogy in soils
on different parent rocks when the other factors are held constant. Geo-
graphically, the frequency distribution of minerals and soils in county after
county in state after state largely follows the parent rock distribntion
pattern, yet it must be recognized that with enough time, significant func-
tional relationships with climate, vegetation and relief are developed.

                 Inherited Lithological Minerals in Soil Clays
   Primary illite, quartz and feldspars inherited directly from the mantle
rock parent material occur in the clays of little-weathered soils on late
Pleistocene till in Ontario (Jackson and others, 1948) and China loess deposits
(Hseung and Jackson, 1952). When weathering and leaching are very
limited, ferromagnesi~n minerMs, carbonate, and gypsum of lithologieal
origin occur in Desert, Brown, and Chestnut zonal soil groups (Hseung and
Jackson, 1952). Sierozems, Brown soils, and Chestnut soils of Colorado
surface soil horizons (Schmehl and Jackson, 1957) had 20 to 36 percent illite
in the --2/~ clay fractions, while quartz and feldspars made up 30-35
percent of the coarse clay and fine silt. Other minerals included 4-11 percent
kaolinite (probably lithological). Alpine Turf soils (Retzer, 1956) of the
Colorado I~ocky Mountains developed from basalt, micaeous granite, quart-
zite and shale tended to contain over 50 percent of the ]itho]ogical minerals
quartz, feldspars and illite in the various size fractions of the clay. Chlorite,
kaolinite, vermiculite and montmorillonite made up the remainder. Illite
has been noted in liberal amounts in Tundra and other cold-region soils.
Iliite inherited from shale occurs abundantly in some till-derived soils of the
central United States including some soils of the Prairie, Chernozem, and
Gray-Brown Podzolic zonal groups (Alexander, Itendricks and Nelson,
1939; Bidwell and Page, 1951). Kaolinite inherited from rock (Peterson,
1946) dominates the clay of Gosport soils of Iowa (Gray-Brown Podzolie).

  Chemical Nature of Parent Material as a Control on Soil Clay Minerals
   Soil clay minerals, besides being inherited outright from the parent
material (above), may include secondary minerals which have been influenced
gre~tly by the chemical nature of the parent m~terial as to primary minerals
and cationic content.
   Distribution of vermieulite.--It appears that vermiculite of soils charac-
teristically forms directly from micas, including illite, through cleavage
across the Z-axis concurrent with layer charge reduction (Jackson and others,
1952) as evidenced by the occurrence of large crystals of vermiculite pseudo-
morphic after mica commonly found in clay, silt, sand and gravel sizes,
associated with a decrease in the mica content compared to the parent
material. A soil developed on weathered granite (Mountain soil) of Colorado
has vermiculite (with 15 percent mica still interstratified) in crystals the size
of the parent black mica grains still found in the interior of unweathered
granite fragments, and no appreciable chloritization has occurred (un-
published studies in this labor&tory)." In Ando soils of Japan, vermiculite is
formed only if mica is present in the ash (Aomine and Jackson, unpublished)
while glassy ash forms allophane (below).
  The weathering sequence is represented by equation (1) :

Biotite\ ~4)   ~-~        Vermiculite (8)         ~b) ~    Montmorillonite          (9)
       ~ .                 (Trioctahedral.)                 (Trioctahedral}

                           7 ' chlorite (8).~    (f)

Ittite  (7~ ~         ~Vermiculite    (8)       {a~ : ~"~'Montmorillonite     (9}
Muscovite ~7~          (Dioctahedral)                    (Betdellite)
iil which the reactions are indicated by arrows (and letters) while numbers
in parenthesis represent weathering sequence index numbers (Jackson and
others, 1948, 1952 ; Jackson and Sherman, 1953). Formation of trioctahedral
montmorillonite was observed by MacEwan (1948). Formation of the vermi-
culite 14~_ spacing as a result of Mg treatment of biotite [reaction (a)] was
observed by Barshad (1948). Reaction (a) was described by Walker (1949,
1950) and (a) and (c) by Jackson and others (1952) and Rolfe and Jeffries
(1952). l~eaetions (a) tkrough (d) agree with the Fieldes and Swindale (1954)
data on mica weathering in soils. Vermiculite occurs in Lutosols (Aguilera and
Jackson, 1953 ; Whittig and Jackson, unpublished).
   Dioctahedral vermiculite seems to be much more common in soil clays
than the trioctahedral type. Thus the 060 spacings of the clays of Hiawatha
(Podzol), Iron River (Podzol) and Omega (Brown Podzolie) and other sandy
soils of northern Wisconsin (Brown and Jackson, 1958 ; Whittig and Jackson,
1956) are 1.50A, characteristic of the dioctahedral type. Similar results have
been observed with the Ireland soils Kilcolgan (Regosolie Brown Forest soil
from limestone) and lY[oate (Gray-Brown Podzolie, from limestone till) with
80 percent vermiculite in the clay (Sawhney and Jackson, unpublished data).
   Distribution of secondary chlorite.--Secondary chlorite (equation 1) refers
to chlorite formed from mica during weathering by interlaying with positively
charged aluminum, ferric iron and (or) magnesium hydroxides. The alu-
minum hydroxide interlayer is thought to be most common in secondary
chlorite in acid soil clays.
   Secondary chlorite has the stability advantage of the large stable 2 : 1
layers formed by cleavage of mica crystals. I t was spoken of as " aluminous
chlorite " by Jackson and Sherman (1953, pp. 235-236) and found to follow
mica-vermiculite (hydrous mica) in weathering stability sequence (Jeffries,
Rolfe and Kunze, 1953). Because secondary (aluminous) chlorite forms from
               FI%EQUENCY DISTRIBUTION OF CLAY I~IINEI%ALS                   137
mica immediately following (or even concurrent with) vermiculite under
proper circumstances, it may be assigned the weathering stability (Jackson
and others, 1948, 1952) index number 8, the same as vermiculite (equation 1).
   Secondary chlorite is distinct from ferromagnesian chlorite occurring in
serpentine rocks, which (perhaps owing to its ferrous iron content) appears
to weather much more readily and has been given weathering stability index
4 (Jackson and others, 1948). I f there is a fairly abundant source of aluminum
(and possibly iron) from soil acidity, intergradational vermiculite-chlorite
having a temperature stability much lower than the 550~ characteristic of
ferromagnesian chlorite may form by mica weathering (equation 1). For
example, slightly chloritized vermiculites have been derived from muscovite
schist (Red Yellow Podzolic) soil of Virginia (Rich and ObenshMn, 1955).
And chlorite-vermiculite intergradational materiM occurred in Crosby coarse
clay (Gray-Brown Podzolic soil)of Indiana (K/ages and White, 1957).
Secondary chlorite is associated with abundant montmorillonite in the basalt-
derived Dark Magnesium Clay soils of Hawaii and Ladybrook (Grumosol or
Black Clay) of Queensland, Australia. These are situations in which magnes-
ium (derived from basaltic rock) seepage, or slow drainage, is abundant for
supplying the interlayer cation.
   Distribution of montmorillonite series.--The occurrence of abundant mont-
morillonite in certain soils (reviewed extensively, Jackson and Sherman,
1953) is commonly controlled by the soil parent material. Alluvial deposits
from stagnant or slow-moving water often have montmoriltonite concentrated
by sedimentation processes. Marl-derived Houston clay (Kunze and Templin,
1956) (Rendzina) and basMt-derived Black Cotton, Ladybrook, Aina ItMna
soils (Dark Magnesium clay) and WMpiata soils are rich in montmorillonite
(Sawlmey and Jackson, 1958) because of abundance of divalent cations
supplied by the parent material. The high cation exchange capacity and
water-holding properties of highly montmorillonitic soils makes them to some
extent self-perpetuating, but functional weathering give s rise to mineral
sequences of kaolinite, gibbsite, and free iron oxides (Jackson and others,
1948 ; Ferguson, 1954 ; Hosking, Neilson and Carthew, 1957).
   The moderately weathered Prairie, Gray-Brown Podzolic, and Podzol
soils of central United States, commonly developed on loess or loess-capped
parent material, characteristicMly have a high content of montmorillonite-
series minerals in the clay (Ross and Hendricks, 1945). To a lesser extent,
montmorillonite also occurs in till-derived soils in this region. Some of this
montmorillonite may be directly lithologicM, or unrelated to weathering (as
suggested by Beavers, 1957), but considerable weathering change (equation 1)
of the mica component (mica is abundant in the loess silt) to the expanded
types of mineral is likely (Bray, 1937 ; Jackson and others, 1952 ; Murray
and Leininger, 1956). The greater abundance of montmorillonite in the loess
than till is still influenced by parent materiM (greater amounts of finer mica
particles in loess than present in till). With progressive leaching, the inter-
layers of chlorite are lost and montmorillonite results (reaction f, equation 1),
as shown in the permeability and time functions (below). In the absence of
sufficient interlayering cations, chlorite is by-passed as beidellite (mont.
morillonite series) forms (reaction d) directly. This has also been demon-
strated in the laboratory (White, 1951, 1956).
   Distribution of phosphate.--A majority of soil parent materials have their
phosphate as calcium phosphate. As the free CaCO3 is leached out, that
phosphate becomes fairly soluble and weathers chemically, reacting with
aluminum ions from aluminosilicates and iron ions from iron minerals
(reviewed by Chang and Jackson, 1958). A weathering index or sequence is
found for phosphate of various great groups of soils, measured by the per-
centage of the total inorganic phosphate that is chemically bonded with
calcium, with aluminum, with iron, and (as a further step in the sequence)
with iron and occluded in iron oxides (Chang and Jackson, 1958).

                  Soil Parent Material Permeability Factor
   High permeability of the soil parent material to water has a profound
influence on the frequency distribution of the soil clay minerals formed by
chemical weathering. High permeability leads to rapid leaching of the metallic
cations and other substances dissolved by weathering reactions, leaving the
product formed with lowered contents of monovalent and divalent cations
and even of silica. Four analogous situations will be considered, leading,
respectively, to beidellite, kaolinite, allophane and gibbsite.
   Highly sandy soils of northern Wiseonsin.--The Hiawatha (Podzol) and
Ahmeek (Brown Forest group) sandy soils of northern Wisconsin show greatly
different clay minerals in different horizons as a function of soil depth. The
high permeability of the coarse sandy parent material allows the mica of A
horizons to be leached of potassium and rapidly transformed through
vermiculite and chlorite to beidellite (equation 1). Some of the fine fractions
of these soils are more extensively leached and are (Whittig and Jackson,
1955) nearly amorphous relics of 2 : 1 layer silicates, and possibly could be
characterized as allophane-like. The clay of Plainfield sand of central Wis-
consin (Gray-Brown Podzolic group), a somewhat less sandy soil, is high in
chlorite presumably derived by chloritization of the glauconitic mica charac-
teristic of the Cambrian sandstone parent rock. These soils illustrate excep-
tionally fast weathering in consequence of coarseness of parent material,
giving high permeability and leaching rate, low water-holding capacity, and
little clay to be weathered, all contributing to accelerated weathering.
   Kaolinite in red earths.--High permeability of the granite parent material
of soils of the Peidmont Plateau of southeastern United States has been
interpreted (t~oss and Hendricks, 1945) as the cause of kaolinite occurring
in abundance in the Cecil and related soils (Kelley and others, 1939 ; Coleman,
Jackson and Mehlich, 1950). Less permeable rocks gave montmorillonite and
chlorite. Porous rocks of Hawaii form deeper soils (more rapid weathering)
than hard rocks (Dr G. D. Sherman, private communication). Kaolinite and
associated iron oxides maintain permeability, and quartz maintains the silica
supply, and thus the system has a kind of self-perpetuation. But an increase
               FREQUENCY DISTRIBUTION OF CLiY MINERALS                       139
of rainfall decomposes the kaolinite in Hawaii to give allophane and free
oxides (Tanada, 1951 ; Tamura, Jackson and Sherman, 1953).
   Allophane in Ando soils.--Volcanic ash of Japan, when finely divided and
open and porous in drainage character, quickly forms allophane. Ashes of
only a few decades age have considerable a]lophane, pseudomorphic after
the original ash particles, and also as thread-like precipitates (Aomine and
Jackson, in manuscript). Silica leaches rapidly from freshly deposited ash
(S. Aomine, private communication). Older buried layers, the age of which is
established as several thousand years by several means of dating, contain
halloysite formed by resftication of the allophane, by silica leached fl'om fresh
ash overburden. In older layers that received insufficient silica (no over-
burden), the allophane forms crystalline gibbsite and halloysite.
   Gibbsite in old Appalachian mountain soils and tropics.--High gibbsite
concentrations in soil clays have been found in the old mountain softs of the
Appalachian and Piedmont areas (Alexander, I-Iendricks and Faust, 1942;
Coleman, Jackson and Mehlieh, 1950). These soils have an abundance of other
minerals, including lithologica] illite, vermiculite, etc. (Coleman, Jackson and
Mehlich, 1950). Rotten (porous) trachite of Hawaii consists of nearly pure
crystalline gibbsite (Tamura and Jackson, unpublished). Gibbsite commonly
is abundant under tropical weathering (as reviewed, Jackson and Sherman,
1953, p. 284). Precipitation of gibbsite (like that of allophane in the previous
section) is favored by high rock permeability which allows the dissolution
and rapid removal of ions released by weathering. Gibbsite (also like allo-
phane) takes on silica in the presence of silica in solution (Alexander,
Hendricks and Faust, 1942; Goldman and Tracey, 1946). When silica is
abundant relative to leaching rate, kaolinite and halloysite are abundant
relative to gibbsite. Conversely, the greater the rate of leaching, the greater
the thickness of the gibbsite deposit. Hence, gibbsite occurrence in soils
reflects a higher weathering (leaching) index number (11) than kaolinite
index number (10) (Jackson and others, 1948).

                             TIME     FACTORS
   Time is a necessary component of all of the other soft-forming factors
affecting soil clay mineralogy. But time is not unique in this overlapping ;
for example, climatic factors require parent material to operate on. Thus to
assess the time factor, one must observe its effect with the other factors held
more or less constant. The present day trend in soft genesis interpretation
has been to recognize changes in soils and soil mineralogy as a function of
time, with de-emphasis on the earlier steady state (" equilibrium ") concept,
because older landscapes inevitably have minerals of greater stability index
than younger landscapes (as reviewed by Jackson and Sherman, 1953, pp. 257
and 281-290). For example, older limestone-derived softs of central Pennsyl-
vania are rich in kaolinite (Alexander, IIendricks and Nelson, 1939) ; nearby
the younger Hagerstown soils are rich in illite and beidellite (Jackson and
others, 1954). The great soft group may be Gray-Brown Podzolic in both

cases. As another example, clays weathered a million years from limestone
were montmorillonite throughout the profile, and less weathering (equation
1) was noted in Illinois till (140,000 years old) and still less in Wisconsin till
(18,000 years old) (Murray and Leininger, 1956). The clay mineral differences
in both examples clearly reflect the effect of different weathering time.

                           CLIMATIC        FACTORS
    Climatic factors are traditionally the prime " active       "    factors of soil
formation. The clay mineralogy of soil, in time, responds sensitively to
 climatic factors, as reviewed elsewhere (Jenny, 1941, pp. 118-191 ; Jackson
 and Sherman, 1953). Thus for example, rainfall and temperature are climatic
intensity factors that interact with permeability of parent rock (a capacity
factor) reviewed above. Long times of course invariably introduce possible
 changes in climatic intensity factors. Yet broad geographic ranges in present
 climate tend to give corresponding mineral sequences in different hemis-
pheres (I-Iseung and Jackson, 1952 ; Jackson and others, 1948 ; Fie!des and
Swindale, 1954), progressing from primary minerals in cool dry climates
through layer silicates, to abundant AI(OH)a, Fe2Oa and TiO 2 in warm moist
climates. Authigenic kaolinite and halloysite make up 80 percent of clay in
soils from granite and basalt in western Australia (Hosking, Neilson and
Carthew, 1957). High temperatures of warm temperate and l~ropical climates
bring out the red and brown colors indicative of the accumulation of free
iron oxides, hematite and goethite, sometimes in large amounts (15-60
percent). Ten to 15 percent hematite occurs in the Red-Yellow Podzolic soils
of Southeastern United States, in soils such as the Cecil (Coleman and
Jackson, 1946). Rainfall need be only a moderate 40 in. (not unlike
temperate regions)to give 20-30 percent free iron oxide in the Humic
Latosols of Hawaii under tropical temperatures (Tamura, Jackson and
Sherman, 1953). Accumulation of much iron and titanium oxide is favored
by a w e t - d r y seasonal distribution of rainfall, as in the Ferruginous Humic
Latosol and the Latosolic Brown soil of Hawaii which have 35-50 percent
hematite, goethite and magnetite, and 20 or more percent of anatase and
ilmenite (Tamura, Jackson and Sherman, 1955). Gibbsite or kaolinite occurs
in the C horizons, in amounts up to 30-50 percent.

                            RELIEF       FACTORS
  Relief or topography affects soil clay mineralogy through concentrating
water (causing hydrolysis and leaching), seepage (bringing in cations in
solution), and b y affecting oxidation or reduction (through water and air
supply). Leaching is more severe when the waters are acid or neutrM than
when high in metallic cations. Leaching is greatly affected by the permeability
of the parent material, making relief interact with the parent materiM
factors (discussed above).
                 FREQUENCY DISTRIBUTIONOF CLAY MINEICALS.                         141
   Abundance of divalent cations, in seepage or runoff water has been
reported by many workers to favor montmorillonite formation, as in the Dark
IV[agnesinm Clays of Hawaii, and in Wiesenboden and Planosol soils (reviewed
by Jackson and Sherman, 1953). Iron is accumulated by seepage on to
Ferruginous Humic Latosol areas (Tamura, Jackson and Sherman, 1953,
1955). Poor drainage and its associated oxidation or reduction concentrated
70 percent goethite in Ortstein concretions of a Ground Water Podzol of
Wisconsin (Whittig and Jackson, unpublished), and 85 percent hematite in
concretions of Tropical Savannah Podzol of Haiti (Tamura and Jackson,
  Relief also acts by supplying alluvial soil parent material, as noted from
montmorillonite sediments (above), but once a soil material is moved, clarity
of nomenclature requires its mineralogy to be a parent material factor rather
than a relief factor.
                           BIOTIC FACTORS
   Plant residues return large amounts of metallic cations and silica to soils
and the literatnre suggests that these substances may contribute greatly to
clay formation. A most conspicuous biotic effect on soil mineralogy is under
mor-forming type of vegetation. For example, Kauri trees of New Zealand
give a thick A~ horizon consisting of more than 90 percent free SiO 2 (quartz
and cristobalite) (Swindale and Jackson, 1956), while nearby soils under grass
and other type of trees show little podzolization. The silica is residual from
the parent material, while the other minerals have weathered ont. Layer
silicates and gibbsite are retold in still deeper horizons. The organic A 0
horizon supplies organic compounds to leaching waters which are able to
chelate Fe and A1, and this accelerates the dissolution of primary and
secondary silicates and oxides and their subsequent eluviation, the combined
process being termed cheluviation (Swindale and Jackson, 1956). Similar
but less extreme effects operate in forming zonal Podzol and Gray Wooded
A 2 horizons of North America, so that illite, vermiculite, chlorite and
montmorillonite and other layer silicate clays are left after cheluviation of
the iron oxides.
   This contribution from the Department of Soils, University of Wisconsin,
an invited paper for the Conference session oll Genesis of clays, was supported
in part by the University I~eseareh Committee through a grant of funds
from the Wisconsin Alumni Research Foundation, for equipment, materials,
and graduate student assistantships which made possible many of the
mineralogical analyses referred to.
Aguilera, N. H. and Jackson, M. L. (1953) Iron oxide removal from soils and clays :
  Soil Sci. Soc. Amer. Prec., v. 17, pp. 359-364 ; v. 18, pp. 223, 350.
Alexander, L. T., I-Iondricks, S. B. and Faust, G. T. (1942) Occurrence of gibbsite in
  some soil-fbrming materials : Soil Sci. See. Ame*'. P~'oc., v. 6, pp. 52-57.
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                      FREQUENCY          DISTI~I]~UTION        OF CLAY       MINERALS                        143
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