Read the clay article in the reading tab. Write an essay answering:1. What is clay minerals?2. What are the building blocks of clay? What is strcutre of clay?3. How do clays develop charge?4. What are the different types of clays? Why do they have different net negative charge, surface area, and basal spaing?5. How do you identify different types of clays?6. Why is studying clay and their structure important?
CLAY MINERALS CD. Barton United States Department of Agriculture Forest Service, Aiken, South Carolina, U.S.A. A.D. Karathanasis University of Kentucky, Lexington, Kentucky, U.S.A. INTRODUCTION Clay minerals refers to a group of hydrous aluminosili- cates that predominate the clay-sized (<2 |xm)="" fraction="" of="" soils.="" these="" minerals="" are="" similar="" in="" chemical="" and="" structural="" composition="" to="" the="" primary="" minerals="" that="" originate="" from="" the="" earth's="" crust;="" however,="" transformations="" in="" the="" geometric="" arrangement="" of="" atoms="" and="" ions="" within="" their="" structures="" occur="" due="" to="" weathering.="" primary="" minerals="" form="" at="" elevated="" temperatures="" and="" pressures,="" and="" are="" usually="" derived="" from="" igneous="" or="" metamorphic="" rocks.="" inside="" the="" earth="" these="" minerals="" are="" relatively="" stable,="" but="" transform-="" ations="" may="" occur="" once="" exposed="" to="" the="" ambient="" conditions="" of="" the="" earth's="" surface.="" although="" some="" of="" the="" most="" resistant="" primary="" minerals="" (quartz,="" micas,="" and="" some="" feldspars)="" may="" persist="" in="" soils,="" other="" less="" resistant="" minerals="" (pyroxenes,="" amphiboles,="" and="" a="" host="" of="" accessory="" minerals)="" are="" prone="" to="" breakdown="" and="" weathering,="" thus="" forming="" secondary="" minerals.="" the="" resultant="" secondary="" minerals="" are="" the="" culmination="" of="" either="" alteration="" of="" the="" primary="" mineral="" structure="" (incongruent="" reaction)="" or="" neoformation="" through="" precipitation="" or="" recrystallization="" of="" dissolved="" constituents="" into="" a="" more="" stable="" structure="" (congruent="" reaction).="" these="" secondary="" minerals="" are="" often="" referred="" to="" as="" phyllosilicates="" because,="" as="" the="" name="" implies="" (greek:="" phyllon,="" leaf),="" they="" exhibit="" a="" platy="" or="" flaky="" habit,="" while="" one="" of="" their="" fundamental="" structural="" units="" is="" an="" extended="" sheet="" of="" sio4="" tetrahedra.="" structure="" of="" clay="" minerals="" the="" properties="" that="" determine="" the="" composition="" of="" a="" mineral="" are="" derived="" from="" its="" chemical="" foundation,="" geometric="" arrangement="" of="" atoms="" and="" ions,="" and="" the="" electrical="" forces="" that="" bind="" them="" together="" (1).="" given="" that="" there="" are="" eight="" elements="" that="" constitute="" over="" 99%="" of="" the="" earth's="" crust="" (table="" 1),="" the="" inclusion="" of="" these="" in="" the="" elemental="" makeup="" of="" soil="" minerals="" is="" understandable.="" notwithstanding,="" the="" prevalence="" of="" silicon="" and="" oxygen="" in="" the="" phyllosilicate="" structure="" is="" logical.="" the="" sic="">4 tetrahedron is the foundation of all silicate structures. It consists of four O2~~ ions at the apices of a regular tetrahedron coordinated to one Si4+ at the center (Fig. 1). An interlocking array of these tetrahedral connected at three corners in the same plane by shared oxygen anions forms a hexagonal network called the tetrahedral sheet (2). When external ions bond to the tetrahedral sheet they are coordinated to one hydroxyl and two oxygen anion groups. An aluminum, magnesium, or iron ion typically serves as the coordinating cation and is surrounded by six oxygen atoms or hydroxyl groups resulting in an eight-sided building block termed an octohedron (Fig. 1). The horizontal linkage of multiple octahedra comprises the octahedral sheet. The minerals brucite Mg(0H)2 and gibbsite A1(OH)3 are similar to the octahedral sheets found in many clay minerals; however, phyllosilicates may contain coordinating anions other than hydroxyls. Cations in the octahedral layer may exist in a divalent or trivalent state. When the cations are divalent (Mg, Fe2+), the layer exhibits a geometry similar to brucite, such that electrical neutrality is maintained. In this arrangement the ratio of divalent cations to oxygens is 1:2 and all three possible cation sites in the octahedron are occupied. This configuration and the respective sheet formed from an array of such as octahedral are referred to as trioctahedral. When the cations are trivalent (Al, Fe3+), the charge balance is maintained by leaving one of every three octahedral cation sites empty. Under this configur- ation, the ratio of trivalent cations to oxygens is 1:3 and the layer exhibits a gibbsite-like dioctahedral arrangement. A combination of tetrahedral and di- or trioctahedral sheets bound by shared oxygen atoms forms aluminosilicate layers that comprise the basic structural units of phyllosilicates (Fig. 2). Sheet arrangement within the aluminosilicate layers varies between clay mineral types resulting in variable physical and chemical properties that differentiate the clay mineral classes. Encyclopedia of Soil Science Copyright © 2002 by Marcel Dekker, Inc. AH rights reserved. 187 188 Table 1 Common elements in Earth's crust and ionic radius Element o2- Si4+ Al3+ Fe2 + Mg2 + Ca2+ Na2+ (AdapCed from Crustal average (gkg~') 466.0 277.2 81.3 50.0 20.9 36.3 28.3 25.9 Ref. 1.) Ionic radius (nm) 0.140 0.039 0.051 0.074 0.066 0.099 0.097 0.133 Volume (%) 89.84 2.37 1.24 0.79 0.60 1.39 1.84 1.84 ISOMORPHOUS SUBSTITUTION The structural arrangement of the elements described above forms the template for the silicate clay minerals. However, the composition varies frequently due to substitution of ions within the mineral structure. Weath- ering allows for the substitution of Si4+, Al3+, and Mg2+ with cations with comparable ionic radii in their respective tetrahedral and octahedral sheets (Table 1). Consequently, Si4+ may be replaced by Al3+ in the center of the tetrahedron without changing the basic structure of the crystal. Moreover, cations such as Fe3+/2+ and Zn2+ (ionic radius = 0.074 nm) may replace Al3+ and Mg2+ in the Clay Minerals 1:1 Clay Mineral (kaolinite) Tetrahedral Octahedral 2:1 Clay Mineral (pyrophyllite) Tetrahedral Octahedral Tetrahedral * Hidden oxygen * Oxretn * Aluminum Fig. 2 Diagrammatic sketch of a 1:1 clay mineral consisting of one tetrahedral sheet bonded to an octahedral sheet (kaolinite); and a 2:1 clay mineral consisting of an octahedral sheet bound between two tetrahedral sheets (pyrophyllite). (From Ref. 6.) Tetrahedron © Oxygen • Silicon © Aluminum or Magnesium Fig. 1 The basic structural components of clay minerals; a single four-sided tetrahedron, and a single eight-sided octa- hedron. (From Ref. 6.) octahedra. The process of replacing one structural cation for another of similar size is referred to as isomorphous substitution. This replacement represents the primary source of both negative and positive charges in clay minerals. For example, the substitution of one Al3+ for a Si4+ in the tetrahedron results in a gain of one negative charge. Alternatively, replacement of a lower valence cation by one with a higher valence (Fe2+ by Fe3+) results in a gain of one positive charge. Some clay minerals exhibit substitutions that result in both positive and negative charges. A balance of electron loss and gain within the structure determines the net charge of the mineral. In most soils, however, substitutions that result in net negative charge exceed those producing a positive charge. Clay Minerals Table 2 Properties of clay mineral groups 189 Group Kaolinite Fine-grained mica Smectite Vermiculite Chlorite Layer type 1:1 2:1 2:1 2:1 2:1:1 Net negative charge (cmolkg"1) 2-5 15-40 80-120 100-180 15-40 Surface area (mV) 10-30 70-100 600-800 550-700 70-100 Basal spacing (nm) 0.7 1.0 1.0-2.0 1.0-1.5 1.4 (Adapted from Ref. 5.) CLAY MINERAL CLASSIFICATION Clay minerals are generally classified into three layer types based upon the number and arrangement of tetrahedral and octahedral sheets in their basic structure. These are further separated into five groups that differ with respect to their net charge (Table 2). 1:1 Clay Minerals The 1:1 layer minerals contain one tetrahedral and one octahedral sheet in their basic structural unit (Fig. 2.) This two-sheet mineral type is represented by the kaolin group, with the general formula Al2Si205(0H)4. Kaolinite, the most common mineral in this group, is dioctahedral, exhibiting Al3+ octahedral and Si4+ tetrahedral coordi- nation. The sheets are held together by van der Waals bonds between the basal oxygens of the tetrahedral sheet and the hydroxyls of the octahedral sheet. Layers are held together tightly by hydrogen bonding, which restricts expansion and limits the reactive area to external surfaces. Isomorphic substitution for Si4+ and Al3+ in this mineral is negligible. As such, soils dominated by 1:1 minerals exhibit a low capacity for adsorbing cations and have low fertility. The serpentine group, with the general formula Mg3Si2O5(OH)4, represents the trioctahedral version of the 1:1 layer minerals. 2:1 Clay Minerals The joining of two tetrahedral sheets (one from each side) to one octahedral sheet produces a three-sheet mineral type, which is called 2:1 and is represented by the mica, smectite, and vermiculite groups. Talc [Mg3Si4Oio(OH)2] and pyrophyllite [Al2Si4Oio(OH)2] are typical representa- tives of electrically neutral 2:1 type minerals in which adjacent layers are joined to each other by van der Waals bonds (Fig. 2). Although these two minerals are found infrequently in soils (2), their structure serves as a model for discussing transitions leading to the formation of other more common 2:1 clay minerals. The true micas have a similar structure to that of talc and pyrophyllite, except that substitution of Al3+ for Si4+ in every fourth tetrahedral site results in an excess of one negative charge per formula unit. The negative charge is satisfied by monovalent cations, primarily K+, that reside on interlayer sites between the 2:1 layers. The interlayer cation forms a strong bond between adjoining tetrahedral sheets, which limits expansion of the mineral. The mica group is subdivided into tri- and dioctahedral minerals according to cation substitutions in the octahedral sheet and within the interlayer. The trioctahedral group of micas contains interlayer K+ cations and is represented by phlogopite [KMg3(AlSi3O10)(OH)2], with Mg 2+ occupy- ing the octahedral sites, and biotite, which contains both Fe2+ and Mg2+ in the octahedron. Muscovite [KAl2(AlSi3_ O1{))(OH)2] is a dioctahedral mica containing Al 3+ in the octahedral sheet and K+ in the interlayer, while paragonite exhibits a similar dioctahedral coordination with interlayer K+ and Na+ cations. In most soils, micas are generally inherited from the parent material and occur in a relatively unweathered state in the sand and silt fractions. Mica in the clay fraction usually exhibits poorer crystallinity, lower K+ content, higher water content, and possible substi- tutions of Fe2+ and Fe3+ in the octahedral sheets and Ca2+ in the interlayer. Manganese, vanadium, lithium, chro- mium, titanium, and several other cations are also known to occur in varying amounts in these fine-grained or clay- sized micas (1). Illite and glauconite (dioctahedral iron illites) are commonly associated with the clay-sized micas; however, the structures of these minerals are poorly defined and likely to be representative of a mixture of weathered micas. Expandable 2:1 clay minerals exhibit a similar layer structure to that described for mica, but vary widely in layer charge and interlayer spacing due to the 190 Clay Minerals presence of weakly bound cations, water, or polar organic molecules in the interlayer region. Smectites generally refer to a group of expandable dioctahedral 2:1 minerals with a charge of 0.2-0.6 per formula unit. Montmorillonite, the most common member of this group, derives its charge from the octahedral substitution of Mg2+ for Al3+. Beidellite and nontronite, which are less abundant in soils, derive much of their charge from tetrahedral substitutions. Nontronite is distinguished from beidellite by the presence of iron in the octahedral sheet. The 2:1 layers in smectites are held together by van der Waals bonds and weak cation-to-oxygen linkages. The presence of exchangeable cations located between water molecules in the interlayer allows for expansion of the crystal lattice as the mineral hydrates. When the mineral is saturated with water, the basal spacing between layers can approach