SOIL FORMATION AND CHARACTERIZATION
2.7 STRUCTUR E O F CLAY MINERAL S
Clay mineral s ar e essentiall y crystallin e i n natur e thoug h som e cla y mineral s d o contai n material which is non-crystalline (for example allophane). Two fundamental building blocks are involved in the formation of clay mineral structures. They are :
1. Tetrahedra l unit.
2. Octahedra l unit.
The tetrahedral unit consists o f four oxygen atoms (or hydroxyls, if needed t o balance the structure) placed a t the apices of a tetrahedron enclosing a silicon atom which combines together t o form a shell-like structur e with all the tips pointin g in the sam e direction . Th e oxygen at the bases of all the units lie in a common plane .
Each of the oxygen ions at the base is common to two units. The arrangement is shown in Fig. 2.2. The oxygen atoms are negatively charged with two negative charges each and the silicon with four positiv e charges. Eac h of the three oxygen ions at the base shares it s charges wit h the
Table 2.3 Cla y mineral s
Name of minera l Structura l formula
I. Kaoli n group
1. Kaolinite Al 4Si4O10(OH)g
2. Halloysite Al 4Si4O6(OH)16
II. Montmorillonit e grou p
Montmorillonite Al 4Si8O20(OH)4nH2O III. Illit e group
Illite K y(Al4Fe2.Mg4.Mg6)Si8_y
Aly(OH)4020
adjacent tetrahedra l unit . The sharin g of charge s leave s thre e negativ e charge s a t th e bas e pe r tetrahedral unit and this along with two negativ e charges a t the apex make s a total of 5 negative charges to balance the 4 positive charges of the silicon ion. The process of sharing the oxygen ions at the base with neighboring units leaves a net charge of -1 per unit.
The second building block is an octahedral unit with six hydroxyl ions at apices of an octahedral enclosing an aluminum ion at the center. Iron or magnesium ions may replace aluminum ions in some units. These octahedral units are bound together in a sheet structure with each hydroxyl ion common to three octahedral units. This sheet is sometimes called as gibbsite sheet. The Al ion has 3 positive charges and each hydroxyl ion divides its -1 charg e with two other neighboring units. This sharing of negative charge with other units leaves a total of 2 negative charges per unit [(1/3) x 6]. The net charge of a unit with a n aluminu m ion a t th e cente r i s +1 . Fig . 2. 3 give s th e structura l arrangement s o f th e units.
Sometimes, magnesiu m replace s th e aluminu m atoms i n th e octahedra l unit s in thi s case , th e octahedral sheet is called a brucite sheet.
Formation o f Mineral s
The combination of two sheets of silica and gibbsite in different arrangement s and conditions lead to the formation of different cla y minerals as given in Table 2.3. In the actual formation of the sheet silicate minerals , th e phenomeno n o f isomorphous substitution frequentl y occurs. Isomorphou s (meaning same form) substitution consists of the substitution of one kind of atom for another.
Kaoiinite Minera l
This i s th e mos t commo n minera l o f th e kaoli n group . The buildin g blocks o f gibbsit e an d silica sheet s ar e arranged a s shown in Fig. 2.4 to give the structure of the kaolinite layer . The structure i s compose d o f a singl e tetrahedra l shee t an d a singl e alumin a octahedra l shee t combined i n unit s s o tha t th e tip s o f th e silic a tetrahedron s an d on e o f th e layer s o f th e octahedral shee t form a common layer. All the tips of the silica tetrahedrons poin t in the same direction and towards the center of the unit made of the silica and octahedral sheets. This gives rise to strong ionic bonds between the silica and gibbsite sheets . The thickness of the layer is about 7 A (on e angstro m = 10~8 cm) thick . The kaolinit e minera l i s formed b y stackin g th e layers one above the other with the base of the silica sheet bonding to hydroxyls of the gibbsite sheet b y hydroge n bonding . Sinc e hydroge n bond s ar e comparativel y strong , th e kaolinit e
(a) Tetrahedral unit (b) Silica sheet
Silicons Oxygen ]_ Symboli c representation
of a silica sheet
Figure 2. 2 Basi c structural unit s in the silico n shee t (Grim , 1959 )
Soil Formatio n and Characterizatio n 13
(a) Octahedral uni t (b) Octahedral shee t
0 Hydroxyl s
I Symboli c representatio n of a octahedral shee t
Aluminums, magnesium or iron
Figure 2.3 Basi c structural unit s in octahedral sheet (Grim , 1959 )
crystals consist of many sheet stackings that are difficult t o dislodge. Th e mineral is therefore, stable, an d wate r canno t ente r betwee n th e sheet s t o expan d th e uni t cells . Th e latera l dimensions of kaolinite particles rang e from 100 0 t o 20,000 A and the thickness varie s fro m 100 to 100 0 A . In the kaolinite mineral there is a very small amount of isomorphous substitution.
Halloysite Minera l
Halloysite minerals are made up of successive layers with the same structural composition as those composing kaolinite. In this case, however, the successive units are randomly packed and may be separated by a single molecular layer of water. The dehydration of the interlayers by the removal of the wate r molecule s lead s t o change s i n th e propertie s o f th e mineral . An importan t structural feature of halloysite is that the particles appear to take tubular forms as opposed to the platy shape of kaolinite.
Ionic bond
Hydrogen bon d
Gibbsite sheet Silica shee t
T
7A7A
7A
Figure 2.4 Structur e o f kaolinit e laye r
Montmorillonite Minera l
Montmorillonite i s th e mos t commo n minera l o f th e montmorillonit e group . Th e structura l arrangement o f thi s minera l i s compose d o f tw o silic a tetrahedra l sheet s wit h a centra l alumin a octahedral sheet . All the tips of the tetrahedra point in the same direction and toward the center of the unit. The silica and gibbsite sheets are combined in such a way that the tips of the tetrahedrons of each silica sheet and one of the hydroxyl layers of the octahedral shee t form a common layer . The atom s common t o both the silica and gibbsite layer become oxyge n instead of hydroxyls. The thickness of the silica-gibbsite-silica unit is about 10 A (Fig. 2.5). In stacking these combined units one above the other, oxyge n layer s o f eac h uni t ar e adjacen t t o oxyge n o f th e neighborin g unit s wit h a consequence tha t ther e i s a very wea k bon d an d a n excellent cleavag e betwee n them . Wate r ca n enter between th e sheets, causing them to expand significantly and thus the structure can break into 10 A thic k structural units. Soils containin g a considerable amoun t o f montmorillonit e mineral s will exhibit high swelling and shrinkage characteristics. The lateral dimensions of montmorillonite particles rang e fro m 100 0 t o 500 0 A wit h thicknes s varying fro m 1 0 to 5 0 A. Bentonit e cla y belongs t o th e montmorillonit e group. I n montmorillonite , there i s isomorphou s substitutio n of magnesium and iron for aluminum.
Illite
The basi c structura l uni t o f illit e is simila r t o tha t o f montmorillonit e excep t tha t som e o f th e silicons are always replaced b y aluminum atoms and the resultant charge deficienc y is balanced by potassium ions . Th e potassiu m ion s occu r betwee n uni t layers . Th e bond s wit h th e nonexchangeable K+ ions are weaker than the hydrogen bonds, but stronger tha n the water bond of montmorillonite. Illite , therefore , doe s no t swel l a s muc h i n th e presenc e o f wate r a s doe s montmorillonite. Th e latera l dimension s o f illit e cla y particle s ar e abou t th e sam e a s thos e o f montmorillonite, 100 0 t o 500 0 A , bu t th e thicknes s o f illit e particle s i s greate r tha n tha t o f montmorillonite particles, 50 to 500 A. The arrangement of silica and gibbsite sheets ar e as shown in Fig. 2.6 .