CLAY MINERALS - USDA

1 clay MINERALSCD. BartonUnited States Department of Agriculture Forest Service, Aiken,South Carolina, KarathanasisUniversity of Kentucky, Lexington, Kentucky, MINERALS refers to a group of hydrous aluminosili-cates that predominate the clay -sized (<2 |xm) fraction ofsoils. These MINERALS are similar in chemical and structuralcomposition to the primary MINERALS that originate fromthe Earth's crust; however, transformations in thegeometric arrangement of atoms and ions within theirstructures occur due to weathering. Primary MINERALS format elevated temperatures and pressures, and are usuallyderived from igneous or metamorphic rocks.

2 Inside theEarth these MINERALS are relatively stable, but transform-ations may occur once exposed to the ambient conditionsof the Earth's surface. Although some of the most resistantprimary MINERALS (quartz, micas, and some feldspars) maypersist in soils, other less resistant MINERALS (pyroxenes,amphiboles, and a host of accessory MINERALS ) are prone tobreakdown and weathering, thus forming secondaryminerals. The resultant secondary MINERALS are theculmination of either alteration of the primary mineralstructure (incongruent reaction) or neoformation throughprecipitation or recrystallization of dissolved constituentsinto a more stable structure (congruent reaction).

3 Thesesecondary MINERALS are often referred to as phyllosilicatesbecause, as the name implies (Greek: phyllon, leaf), theyexhibit a platy or flaky habit, while one of theirfundamental structural units is an extended sheet of OF clay MINERALSThe properties that determine the composition of a mineralare derived from its chemical foundation, geometricarrangement of atoms and ions, and the electrical forcesthat bind them together (1). Given that there are eightelements that constitute over 99% of the Earth's crust(Table 1), the inclusion of these in the elemental makeupof soil MINERALS is understandable.

4 Notwithstanding, theprevalence of silicon and oxygen in the phyllosilicatestructure is logical. The SiC>4 tetrahedron is the foundationof all silicate structures. It consists of four O2~~ ions at theapices of a regular tetrahedron coordinated to one Si4+ atthe center (Fig. 1). An interlocking array of thesetetrahedral connected at three corners in the same planeby shared oxygen anions forms a hexagonal networkcalled the tetrahedral sheet (2). When external ions bond tothe tetrahedral sheet they are coordinated to one hydroxyland two oxygen anion groups. An aluminum, magnesium,or iron ion typically serves as the coordinating cation andis surrounded by six oxygen atoms or hydroxyl groupsresulting in an eight-sided building block termed anoctohedron (Fig.)

5 1). The horizontal linkage of multipleoctahedra comprises the octahedral sheet. The mineralsbrucite Mg(0H)2 and gibbsite A1(OH)3 are similar to theoctahedral sheets found in many clay MINERALS ; however,phyllosilicates may contain coordinating anions other thanhydroxyls. Cations in the octahedral layer may exist in adivalent or trivalent state. When the cations are divalent(Mg, Fe2+), the layer exhibits a geometry similar tobrucite, such that electrical neutrality is maintained. In thisarrangement the ratio of divalent cations to oxygens is 1:2and all three possible cation sites in the octahedron areoccupied.

6 This configuration and the respective sheetformed from an array of such as octahedral are referred toas trioctahedral . When the cations are trivalent (Al, Fe3+),the charge balance is maintained by leaving one of everythree octahedral cation sites empty. Under this configur-ation, the ratio of trivalent cations to oxygens is 1:3 and thelayer exhibits a gibbsite-like dioctahedral arrangement. Acombination of tetrahedral and di- or trioctahedral sheetsbound by shared oxygen atoms forms aluminosilicatelayers that comprise the basic structural units ofphyllosilicates (Fig. 2). Sheet arrangement within thealuminosilicate layers varies between clay mineral typesresulting in variable physical and chemical properties thatdifferentiate the clay mineral of Soil ScienceCopyright 2002 by Marcel Dekker, Inc.

7 AH rights 1 Common elements in Earth's crust and ionic radiusElemento2-Si4+Al3+Fe2+Mg2+Ca2+Na2+ (AdapCed fromCrustal average(gkg~') 1.)Ionic radius(nm) (%) SUBSTITUTIONThe structural arrangement of the elements describedabove forms the template for the silicate clay , the composition varies frequently due tosubstitution 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 respectivetetrahedral and octahedral sheets (Table 1). Consequently,Si4+ may be replaced by Al3+ in the center of thetetrahedron without changing the basic structure of thecrystal.

8 Moreover, cations such as Fe3+/2+ and Zn2+ (ionicradius = nm) may replace Al3+ and Mg2+ in theClay Minerals1:1 clay mineral (kaolinite)TetrahedralOctahedral2:1 clay mineral (pyrophyllite)TetrahedralOctahedralTetra hedral* Hidden oxygen* Oxretn* AluminumFig. 2 Diagrammatic sketch of a 1:1 clay mineral consistingof one tetrahedral sheet bonded to an octahedral sheet(kaolinite); and a 2:1 clay mineral consisting of an octahedralsheet bound between two tetrahedral sheets (pyrophyllite).(From Ref. 6.)Tetrahedron Oxygen Silicon Aluminum or MagnesiumFig. 1 The basic structural components of clay MINERALS ; asingle four-sided tetrahedron, and a single eight-sided octa-hedron.

9 (From Ref. 6.)octahedra. The process of replacing one structural cationfor another of similar size is referred to as isomorphoussubstitution. This replacement represents the primarysource of both negative and positive charges in clayminerals. For example, the substitution of one Al3+ for aSi4+ in the tetrahedron results in a gain of one , replacement of a lower valence cationby one with a higher valence (Fe2+ by Fe3+) results in again of one positive charge. Some clay MINERALS exhibitsubstitutions that result in both positive and negativecharges. A balance of electron loss and gain within thestructure determines the net charge of the mineral .

10 Inmost soils, however, substitutions that result in netnegative charge exceed those producing a MineralsTable 2 Properties of clay mineral groups189 GroupKaoliniteFine-grained micaSmectiteVermiculiteChloriteLayer type1:12:12:12:12:1:1 Net negative charge(cmolkg"1)2-515-4080-120100-18015- 40 Surface area(mV)10-3070-100600-800550-70070-100 Basal spacing(nm) (Adapted from Ref. 5.) clay mineral CLASSIFICATIONClay MINERALS are generally classified into three layer typesbased upon the number and arrangement of tetrahedral andoctahedral sheets in their basic structure. These are furtherseparated into five groups that differ with respect to theirnet charge (Table 2).