1
GES 166/266, Soil Chemistry
Lecture Supplement 1
Soil Solids
I
SOIL MINERALOGY
I-1, Mineral Stability
Minerals will both degrade (dissolution) and form (precipitation) due to metastable conditions. We can break the minerals into two formation categories: PRIMARY
& SECONDARY. Primary minerals are formed at high temperatures and pressures
through what we may term ‘geologic’ processes. Consequently, they are unstable under
atmospheric conditions. Secondary minerals form under atmospheric conditions (such as
those found in soils) but may be unstable if conditions change from those of their
formation. Primary minerals tend to release their ionic constituents; this can lead to the
"neo" formation (i.e., precipitation) of secondary minerals. Secondary minerals can also
be formed from the solid-state transition of a primary mineral, e.g., mica->vermiculite.
The formation conditions of primary minerals leads to their stability, and is
summarized by the Bowen Reaction Series shown below. This series starts with the
minerals formed at the highest temperature with subsequent minerals formed at
progressively lower temperatures. The stability of the minerals in soils increases with
lower temperatures of primary mineral formed.
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I-2, Ion Bonding: A quick review
Ions or molecules aggregate in response to electrostatic and chemical attractions. The
interaction strength depends on the type of bond, while the coordination numbers depend
on the ratios of ion or molecular sizes.
Types of Bonds:
Ionic: bonds that occur between ions of opposite charge without the sharing of
electrons.
- occur between elements on opposite ends of the periodic table
- non-directional bonds
- relatively strong bonds giving high melting points
- very common for soil materials, e.g., NaCl, CaCO3, etc..
Covalent: the sharing of electron pairs between the combining atoms
- occur between atoms of similar electronegativity, e.g., H2O, CH4
- strongest bond
- directional
- bonding within molecules is usually covalent
H-bonding: a weak electrostatic bond between H+ and highly electronegative
atoms, e.g., F-, O2-.
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- although weak individually, they can be very strong when
multiplied
- water forms H-bonds
van der Waals: weak attractive forces between atoms. They can be significant
when present in large numbers but generally result in very weak planes of
attraction
Bonding is rarely exclusively of one kind, e.g., in crystals most bonds are a
composite of both covalent and ionic character; the degree of each character does vary
significantly, however.
The Bond Valence () (electrostatic bond strength) is a convenient way to denote
the charge balanced by each bond. It is the formal charge observed between the central
cation and each coordinating atom.
= (cation valence) / cation coordination number = Zcation / CNcation
examples: Al(OH)6 :  = 3 / 6 = 1/2
OH
-1/2
OH
-1/2
-1/2
OH
OH
-1/2
Al -1/2
SiO4:  = 4/4 = 1
O
1
1 Si 1
O
1
O
O
I-3, Ion Coordination
OH
-1/2
OH
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Atoms bond to stabilize their electronic configuration or to provide charge
shielding. We will call the cation the central ion, and it will be coordinated by anions that
we will term ligands. The coordination of a cation by a ligand is dependent on the ratio
of their sizes--it is strictly a geometric factor. The following table provides an estimate of
usual coordination observed in nature for size ratios.
Table of Radius Ratios and expected coordination.
Radius Ratio
(rcation / ranion)
Coordination
Type
0.15 - 0.225
0.225-0.414
0.414-0.732
0.732-1.000
1
Coordination
Number
trigonal
tetrahedron
octahedron
cube
dodecahedron
(closest packed)
3
4
6
8
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Examples:
High temp: Al-O: RAl = 0.51 , RO = 1.40 ; RAl/RO = 0.364 =>tetrahedron
Low temp: Al-O: RAl = 0.60 ; RAl/RO = 0.429 => octahedron
Si-O: RSi = 0.42 ; RSi/RO = 0.3 => tetrahedron
I-4, Building Blocks of Soil Minerals
Oxygen is the most abundant element in the earth's crust and is the dominant
element in the solids of soils. Si is the second most abundant and Al the third; they along
with O make up most of the minerals we will consider. We also need to appreciate Mg
since it makes up a class of minerals we will discuss.
Basic building block on the molecular scale are:
SiO4
Al(OH)6
Mg(OH)6
OH
OH
OH
O
OH
Mg
OH
OH
OH
OH
Si
O
O
O
OH
OH
Al
OH
OH
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These units can be linking and stacked in various arrangements to give the
common minerals we observe in soils. This can be thought of as either 1) the build-up of
molecular linkages or 2) oxygen sheets filled with cations that are needed for electrical
neutrality.
On the following page, both octahedral and tetrahedral sites can be seen by the
arrangement of oxygen sheets. There are two types of 'voids' present in these oxygen
sheets, which are designated by 'b' and 'c'. Due to charge constraints, only one type of
void will be filled in the phyllosilicate minerals. It does not matter if it is the 'b' or 'c' type
voids that fill, but only that the mineral has one type consistently (exculsively) filled.
Now, based on electrical neutrality you should be able to determine the octahedral or
tetrahedral cation occupancy.
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The octahedral vacancies will be filled to maintain charge neutrality. We can
determine their filling by first using the bond valence theory. For trivalent ions such as
Al3+ in octahedral coordination,  = 1/2. Therefore, we know that each anion will have
1/2 of a charge satisfied by each bond to the trivalent cation. If the anions (ligands) are
hydroxyls, as in gibbsite [Al(OH)3], then each OH- must be bound to 2 Al3+ for charge
neutrality. We can write a formula for the needed coordination of the ligand based on v:
[The number of cations coordinating each anion] = [ (anion charge) /  ]
So, for a divalent ions, such as Mg2+, in octahedral coordination by OH- [Mg(OH)2]:
v = 1/3, and 3 (1 / 1/3) Mg2+ must surround each OH-.
In fact, these are the observed coordination environments in minerals. In the diagrams
below the position of divalent and trivalent cations is shown to give the observed
coordination.
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Note that every octahedral site of the same type must be filled to obtain charge neutrality
with divalent cations. For trivalent cations this is not the case; here, every third
octahedral site must remain vacant in order to maintain charge balance. The vacant sites
in adjacent rows are shifted so that vacancies are directly below two filled octahedral
sites. Take a look at these diagrams and make sure that you can see that each hydroxyl is
maintaining charge neutrality, except, of course, those at the edges. A good way to check
whether the octahedral sites are filled correctly is to draw a hexagonal ring with the filled
sites (see bottom diagram on the previous page). If a hexagonal ring does not result then
the filling was incorrectly diagrammed.
Forming and Linking Tetrahedral and Octahedral Sheets
The main class of soil minerals are formed by linking silica tetrahedra, to from
tetrahedral sheets, and either Al or Mg octahedra, to form octahedral sheets. The
tetrahedral sheets are formed by a sharing of 3 O, which form hexagonal rings, and give a
formula (Si2O5)2-. The octahedral. units are Me(OH)2 or Me(OH)3 (where Me is a metal
cation such as Al3+, Mg2+, or Fe2+). The octahedral sites in the oxygen (hydroxyl) lattice
can be completely filled by divalent cations or two out of every three sites can be filled by
trivalent cations, as we have just discussed. Since 3 out of 3 sites are filled in the divalent
cation case these minerals are termed trioctahedral; those with trivalent cations are
dioctahedral minerals. It turns out the Al and Mg octahedra have ligand positions very
closely aligned to those of the apical oxygens in the silica tetrahedra. As a consequence,
the tetrahedra and octahedra can link together to form a class of minerals known as the
phyllosilicates ('phyllo' being a Greek word for 'layered'). As shown on the following
page, the phyllosilicates can have varying ratios of tetrahedra to octahedra. The simplest
form is just a single octahedra; the next type of phyllosilicate would have a Si-tetrahedral
sheet added to 1 octahedral sheet (a 1:1 mineral). Then we move on to one composed of
2 tetrahedra and 1 octahedra (known as a 2:1 mineral). The ratio of tetrahedra to
octahedra is the dominant means of classifying the phyllosilicates. Additionally, this
class of minerals is further identified by the cation in the octahedral position and the layer
charge, which is our next topic.
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I-5, Charge Development
There are two primary processes by which minerals exert a charge: isomorphic
substitution and terminal ("broken") bonds.
Isomorphic substitution is the
replacement of one ion with another having a different charge but with no change in the
mineral structure. Examples are the substitution of Al3+ for Si4+ or Al3+ for Mg2+. This
charge is termed "permanent" since it will not vary as a function of the solution
composition. Terminal (“broken) bonds result from coordinately unsaturated central
cations or ligands that occur at terminal ends of many minerals. Mineral that are
composed of 2-dimensional or 3-dimensional growth patterns need to extend infinitely to
satisfy all their charges; this of course is not possible and so that at the surfaces of these
minerals a charge will emanate into the surrounding media. In solution such surfaces will
react with water and its components (H+ and OH-) yielding a pH dependent charge--this
is also termed a variable charged surface. Of the minerals we will discuss, the x-axis {the
(100) plane) and y-axis edges {the (010) plane} of the phyllosilicates and all faces of the
hydrous oxides will have variable charges.
The charges on the functional groups of variable charged surfaces can be
determined based on the knowledge of the bond valences and coordination number. For
example, suppose we are looking at the mineral gibbsite, Al(OH)3; Al enters octahedral
coordination (CN = 6), so that 6 OH- ions must surround each Al. We also know that Al
is trivalent, which leads to v = 3/6 = 1/2. From this we can also reason that each OHmust be bound to two Al ions to satisfy its charge (each bond only satisfies 1/2 a charge
and the OH- has 1 unit of charge so two bonds are needed to fully satisfy its charge).
Therefore, if at the surface of gibbsite a OH- ion is only bound to 1 Al then it will have a
charge of -1/2. This surface group could also react with water and become protonated
yielding a +1/2 charge.
mineral
surface
plane
protonation
(occurs at low pH)
positively charged surface
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OH
OH
-1/2
Al
Al
OH
+ 2 H+
OH
OH
-1/2
Al
OH
OH
+1/2
OH2
Al
+1/2
OH2
OH
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I-6, Crystal Planes and Faces (Miller Indices)
In many minerals, particularly the phyllosilicates, different crystal faces will have
vastly different properties than others. Accordingly, we need to have a nomenclature that
will allow us to identify these different mineral surfaces. To accomplish this we turn to a
notation called "Miller Indices".
The Miller Index uses 3 parameters in a Cartesian coordinate system to represent a
face or plane. Planes and faces are designated by: (hkl). In this system, hkl represents the
reciprocal of the specific axis intercept, where h is for the x-axis, k the y-axis, and l the zaxis. Furthermore, the Miller Indices are all designated to be within 1 unit value of each
axis. Why? Because for any mineral the length given by 1 unit value is that of the unit
cell (the smallest repeatable unit within the crystal). Since the unit cell can be repeated to
make up the crystal, moving to a second unit on an axis will simply repeat our first unit.
That is to say that the crystal is exactly the same from 0 to 1 as it is from 1 to 2 units of
the unit cell. Therefore, we only have to concern ourselves with faces and planes within 1
unit value of each axis.
For the phyllosilicate minerals, 3 faces are of concern for us: the (100), (010), and
(001). These 3 planes are represented schematically below. Looking back a couple of
pages you can see the usual orientation of the phyllosilicates. In this orientation the (100)
and (010) faces will have similar properties; the (001) plane, however, will be very
different. The (001) plane is composed of surface oxygen groups that are completely
charge (and coordinatively) satisfied, making them chemically unreactive. In contract,
the surface groups, either oxygens or hydroxyls, on the (100) and (010) faces are not
charge balanced or coordinatively satisfied. They will therefore be chemically reactive
and generate a charge simply from their unsatisfied coordination environment (terminal
bonds).
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I-7, Phyllosilicates
As already stated, the phyllosilicates are minerals composed of Si,Al-tetrahedral
and Mg- or Al-octahedra. The phyllosilicates are classified based upon:
1. The number of tetrahedra and octahedra in a layer
2. The octahedral site occupancy (i.e., octahedral composition)
3. The charge per formula unit for each layer.
The dominant minerals we will study are provided in the following table, based on the
specified criteria above.
General Classes (layer build-up) of Phyllosilicate Minerals:
Layer Type
Charge†
Trioctahedral
Dioctahedral
1 octahedra
0
brucite, Mg(OH)2
gibbsite, Al(OH)3
1 tet. : 1 oct.
0
serpentine, Mg3Si2O5(OH)4
kaolinite, Al2Si2O5(OH)4
2 tet. : 1 oct.
0
talc, Mg3Si4O10(OH)2
pyrophyllite, Al2Si4O10(OH)2
2 tet: 1 oct.
1
phlogopite
KMg3(AlSi3O10)(OH)2
muscovite
KAl2(AlSi3O10)(OH)2
1
biotite
KFe3(AlSi3O10)(OH)2
0.6-0.8
illite (hydrous mica)
K(Na,Ca) Al1.3Fe0.4Mn0.2Si3.4Al0.6O10(OH)2
0.6-0.9
vermiculite
0.25-0.6
smectite
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† The layer charge per formula unit
I-8, Mineral Descriptions
Non-expandable Clay Minerals
Brucite and Gibbsite: Gibbsite is a common secondary mineral, and it is a sink for
aqueous Al. Brucite is less commonly observed in soils but is found in those derived
from ultramafic parent materials.
Serpentine and Kaolinite: The structural sheets composed of 1 tet. to 1 oct. are held
together by H-bonds. Because of the numerous bonds these sheets are held rather
tightly together and are thus only slightly expandable.
Serpentine, like brucite, is found most commonly in soils derived from ultramafic
material; it exhibits a tubular morphology due to differences between the octahedral and
tetrahedral layers. (Incidently, asbestos is composed of serpentine minerals and is
hazardous to lung tissue because of it’s fiberous habit --a consequence of the tubular
morphology).
Kaolinite is a common soil mineral and may be the most ubiquitous of all soil minerals.
It has a low cation exchange capacity value (vida infra) ranging from 10 to 100 mmol /
kg; the charge originates from unsatisfied bonds at the terminal ends of the sheets and is
pH dependent. The surface area is also low, typically between 7 and 30 m2/g.
Talc and Pyrophyllite: These minerals are the structural basis for the 2:1 clay minerals.
They, however, have no permanent charge. Adjoining layers are bonded via van der
Waals forces. As a consequence, the mineral is very easily cleaved along this bonding
plane and accounts for the slipperiness of these minerals; they have minimal CEC.
Micas (phlogopite, muscovite, and biotite): 1 Al substitutes for every 4th Si in the
tetrahedral sheet of the micas. This produces a charge of -1 per formula unit; the
additional charge is satisfied by K+ ions that resides in the interlayer cavities and is
bound such that it is not exchangeable. The high layer charge results in a strong
assemblage of the layers which are thus non-expandable; there is no appreciable CEC.
Potassium does have a role in this collapse; it is the perfect size to fit in the cavities
between layers and allows for the collapse of these layers.
The trioctahedral micas weather more rapidly than the dioctahedral ones. The K+ in the
interlayer is released during weathering—a source of nutrients in soils.
Expandable Clay Minerals
The layer charge of these minerals is reduced relative to mica. This occurs from a
number of processes including:
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i) Fe(II) oxidation to Fe(III)
ii) H+ incorporation into the structure
iii) exchange of Si4+ for Al3+ in the tetrahedral sheet
iv) cation (e.g., Al3+ for Mg2+) substitutions in the octahedral layer
Illite (hydrous mica): This mineral is derived from the weathering of the muscovite. It
differs in that a divalent cation substitutes in the octahedral layer for Al. Also, the
quantity of Si is increased in the tetrahedral layer. The CEC is approximately 300
mmol / Kg (30 meq/100g).
Vermiculite: Can occur in either di- or trioctahedral forms. Approximate formulas for
these are:
dioctahedral: Nax(Al,Fe)2(AlxSi4-x)O10(OH)2•4H2O
trioctahedral: Nax(Mg,Fe)2(AlxSi4-x)O10(OH)2•4H2O
-The charge originates primarily from isomorphic substitution in the tetrahedral layer
(Al for Si). The charge in the tetrahedral layer may be somewhat offset by Al3+
substitution for Mg2+ in the octahedral layer—a source of positive charge that
diminishes (slightly) the negative charge developed in the tetrahedral sheet. The cations
in the interlayer, which balance the layer charge, are exchangeable. Due to a relatively
high layer charge that originates from the tetrahedral layer, the layers are not very
expandable. Ions of the correct size to ‘fit’ the interlayer cavity can lead to a layer
collapse of the structure; such ions include K+, NH4+, Cs+, Rb+. Once these ions
collapse the structure they are no longer exchangeable. The CEC and surface area (SA)
are high, generally on the order of CEC = 1200-1500 mmol/Kg, SA = 600-800 m2/g.
Smectites: The smectite minerals are the most expandable of the clay minerals and
account for most of the observed shrink-swell properties in soils. This is due to their
moderately-low layer charge. Additionally, they have a very high water holding
capacity as a result of such swelling. Like the vermiculite minerals, smectites can be
found in both dioctahedral and trioctahedral forms.
Dioctahedral Forms:
Montmorillonite: ≈ Nax(Al2-xMgx)Si4O10(OH)2
Mg2+ is substituted
for Al3+ in the octahedral layer
x = 0.25-0.45
Beidellite: ≈ Nax(Al2)AlxSi4-xO10(OH)2
Al3+ substituted for Si4+ in
tetrahedral layer
x = 0.45 - 0.5
Trioctahedral Forms:
Saponite: ≈ Nax-y(Mg3-yAly)AlxSi4-xO10(OH)2
Al3+ substituted for Si4+ in
tetrahedral layer and for Mg2+ in
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octahedral layer
Hectorite: ≈ Nax(Mg3-xLiy)Si4O10(OH)2
The CEC ranges from 800 to 1200 mmol / Kg with a surface area of 600-800 m2/g.
These minerals have very small particle sizes and are highly expandable. They are the
major components of soils that limit water movement in dispersed soils.
Montmorillonite is commercially available under the name "Bentonite", which is used
as an non-permeable material for ponds or earthen damns.
Integrades: There is a potential for cationic metals such as Al3+, Cr3+, and Fe3+ to
precipitate within the interlayers of the 2:1 minerals that have a permanent charge; these
precipitates may be generally termed 'hydroxy interlayers'. Interlayer precipitates are in
essence an additional octahedral layer, forming a 2:1:1 type mineral, if complete
interlayer occupancy is achieved. Usually, full occupancy of the interlayer is not reached
and so the resulting phase is termed a 2:1 integrate. Integrades decrease the CEC of the
mineral but often increase the anion exchange capacity (AEC). The interlayer precipitates
also decrease the phyllosilicate's surface area, expandability, and water-holding capacity.
They occur most commonly in vermiculite but are also found in some of the smectite
minerals.
Chlorite: True chlorite is a primary mineral with a 2:1:1 layer structure (also called 2:2)
and has the formula (AlMg2(OH)6)x {Mg3(Si4-xAlx)O10(OH2)}. Note that chlorite is a
trioctahedral mineral (both octahedra have Mg as the primary cation) with some Al
substituting in the additional octahedral sheet. The surface are is 70 to 150 m2/g and the
CEC 100-400 mmol/Kg. Again some positive charge is associated with the Al
substituted octahedral layer giving rise to anion exchange. Pedogenic chlorites are
actually end-members of the intergrades discussed above.
Tectosilicates (Frame-work silicates)
Extending the layer silicates to a three dimensional structure, so that every
tetrahedra shares all of its 4 oxygens with neighbors, results in the class of minerals
known as the tectosilicates. The most common and well known member of this group is
quartz (SiO2). As should be expected by its abundance in the surface environment, the
structural arrangement of the tectosilicates leads to a very stable configuration that resists
weathering. Another well known member of this group are the Feldspars. While
abundant is some soils, the tectosilicates do not significantly contribute to the chemical or
electrostatic properties of soils; they are relatively inert due to their stable configuration.
Zeolites are also tectosilicates; they are unique minerals having a 3-D
aluminosilicate framework that forms channels and cavities of distinct size. These
channels can act like molecular sieves and account for the use of zeolites as catalysts of
petroleum products. The aluminosilicate tetrahedra are connected by their vertices and
are composed of 4 to 12 member rings linked together; in the center of the rings are the
channels.
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Accessory Minerals: The Hydrous Oxides
In well developed, highly weathered soils the phyllosilicate clay fraction begins to
diminish giving way to Al and Fe oxides, hydroxides, and oxyhydroxides (collectively
referred to as hydrous oxides). Because these materials form late in the weathering
sequence and at the expense of other minerals, they are also referred to as secondary
minerals. Due to their high surface areas, ability to coat other particles, and reactivity,
hydrous oxides can exhibit a marked influence on the soil chemical properties even when
composing only a small fraction of the total solids. Most of the secondary minerals do
not have permanent charges, but they do have a very active variable charge. Because of
their structure, all surface planes will be coordinatively unsaturated and consequently
develop a variable charge. In addition, this results in the surfaces having a strong
CHEMICAL affinity for many ions, which leads to a very strong retention of ions on such
surfaces.
Silica: In addition to the very stable tectosilicate forms, hydrated amorphous forms with
very high surface areas are also common in soils. These materials are not rigid structures,
rather they form a gel-like amorphous phases. They can contribute to the cementation of
clay rich layers. The ZPC is low (at pH 2-3) so that the dominant electrostatic interaction
will arise with cations.
Allophane: A generic term for amorphous hydrated aluminosilicate with a Si:Al ratio of
≈1-2. Aluminum enters octahedral coordination and Si is present in tetrahedral
coordination. They are commonly observed in volcanically derived soils and sediments.
The exchange capacity of allophanes is very large; values of up to 1500 mmol/Kg have
been reported. As might be expected, their surface area is also large (70-300 m2/g).
Allophane forms a spheroid surface topography and has short to mid-range atomic
ordering.
Imogolite: (SiO2•Al2O3•2.5H2O) Similar to allophane, imogolite is also a common
material found in soils derived from volcanic parent material. It has a tubular
morphology, in contrast to the spherical morphology of allophane. It also has a very high
CEC (1350 mmol/Kg).
Hydrous Al Oxides: The most abundant of the secondary minerals is gibbsite, Al(OH)3. It is a very stable mineral at low temperatures and is the building block for
other phyllosilicates (the dioctahedral class). In gibbsite, hexagonal sheets are bound by
H-bonds. It, like the other hydrous Al oxides, has a very high ZPC that ranges from 8 to
9.5. The surface area of this mineral is moderate, 5-20 m2/g. It is found ubiquitous in
well developed soils, particularly of the tropic. Another Al phase found in soils and
sediments is boehmite (-AlOOH). It is the analog to lepidochrosite, an iron
oxyhydroxides.
Hydrous Fe Oxides: Ferric hydrous oxides are abundant in many soils and due to their
strong pigmentation they are easily recognized; the yellow and red soil colors are due to
this class of minerals. Based on radius ratios, Fe(III) should enter octahedral
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coordination, and this is observed in nature. Accordingly, the Fe oxides are similar to the
Al-oxides.
Goethite (-FeOOH) is the most abundant of the HFOs but is closely followed by
hematite (-Fe2O3).
Goethite exhibits a yellowish color and is common in lower
temperature, higher moisture soils. Hematite forms a bright pinkish color, and is favored
in high temperature low moisture areas. Additionally, high pH favors the formation of
hematite relative to goethite. The surface are of goethite is slightly higher than for
hematite (10-50 m2/g for goethite and 5-20 m2/g for hematite). Ferrihydrite is another
common form of Fe is soil, and although not as stable as either hematite or goethite it
forms rapidly from Fe(III) precipitation. Given enough time ferrihydrite will transform to
either goethite or hematite. Iron oxides have a strong affinity for many metal ions and
consequently are used in wastewater treatments to partition metals out of the aqueous
phase. They also form concretions and coatings in many soils.
Manganese Oxides: The manganese oxides are an important fraction of the soil because
they are one of the strongest oxidants of naturally occurring material. They also have a
very high affinity for cationic metals such as Fe(III), Al, Zn, Cu, etc. In marine systems,
nodules composed of concentric rings alternating between Fe- and Mn-oxides are
common. These interesting noodles result from redox transformation and tend to be rich
in other metals such as Co as well.
The most prevalent Mn-oxide in soils is the mineral birnessite (a disordered MnO2 phase). Manganese oxides form coatings on many particles but are also observed
as discrete nodules; their strong black pigment often dominates the soil color-occasionally this is misidentified as arising from organic matter. These materials usually
have a low ZPC, high surface area, and are very reactive. They are also one of the first
materials lost to reductive dissolution under oxygen limiting conditions.
Titanium Dioxides: A residual from the primary minerals, the titanium dioxides are
found in lesser amounts in soils. They do have both an anion and cation exchange
capacity due to a moderate ZPC near 5-6. There are two polymorphs (minerals with the
same chemical composition but differing structures) of TiO2, rutile and anatase. Both are
found in soils, but rutile a little more commonly. Both of these minerals are strong white
pigments and account for the white color of paints. In fact, anatase and rutile are
extensively mined as pigments for paints.