Clay Minerals

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Ch 7 - Environmental Mineralogy

Mineral = a naturally occurring inorganic substance with a characteristic internal structure and a chemical composition that is either fixed or varies within certain limits.

Clay Minerals

Clay = any material less than 2 um in diameter Clay Mineral = a fine-grained hydrous silicate composed of layers of tetrahedrally and octahedrally coordinated cations

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Octahedral Sheets

Tetrahedral Sheets

Kaolinite Clay Structures (1:1 Clays)

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Structure of Kaolinite Clay Minerals

T O

Montmorillonite Clay Structures (2:1 Clays)

T O

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Structure of Montmorillonite Clay Minerals

T O T

Ionic Substitutions, p. 211

1. Ions of one element can substitute for those of another in a crystal structure if their radii differ by less than about 15%.

2. Ions that differ by one charge unit substitute readily for each other as long as charge neutrality is maintained. Note that this requires a coupled substitution, such as occurs in the plagioclase solid solution series. If the charges differ by more than one unit, substitution is limited.

+ + 2+ 3+

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Ionic Substitutions, p. 211

3. When two ions can occupy the same site in a electronegativity crystal structure, the ion with the higher ionic potential (I.P.) preferentially enters the site.

I.P. = charge radius in Angstroms

4. Even if the size and charge of the minor and major ion are similar, substitution may be limited for the minor ion if it has a very different electronegativity and forms a bond of very different character from that of the major ion.

EXAMPLE 7-3, p. 212

In galena (PbS, radius Pb = 1.26 Å) lead is in six-fold coordination with sulfur. Will any of the following trace elements – Mn2+, Ag+, or Np3+ - likely be found in galena? The ionic radius of S2- = 1.72 Å and the E.N. = 2.6.

EN % Ionic Ionic (Table Character Subst Ions radius I.P. % Diff. Radii 7-2) Metal-S bond . ? (App III) (Table 7-3)

+2/1.26= 2.6 - 1.8 = 0.8 Pb2+ 1.26 Å 1.8 ------1.6 15%

+2/0.75= 2.6 – 1.6 = 1.0 (1.26-0.75)/1.26 Mn2+ 0.75 Å 1.6 2.7 22% = 40%

+1/1.23 = 2.6 – 1.9 = 0.7 (1.26-1.23)/1.26 Ag+ 1.23 Å 1.9 0.81 12% = 2.4%

+3/1.10 = 2.6 – 1.3 = 1.3 (1.26-1.10)/1.26 Np3+ 1.10 Å 1.3 2.7 34% = 13%

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Ionic Substitutions in Clay Minerals

Kaolinite Structure –

Montmorillonite Structure –

tetrahedral sheets

octahedral sheets

Positive Charge Deficiency -

Table 7-5. Summary of the principal characteristics of the layered clay mineral groups Smectites Kaolinites lllites Vermiculites (Mont.’s ) Structure: Octaheral/ 1:1 2:1 2:1 2:1 Tetrahedral Cation exchange capacity Nil Low High High (CEC) in 3-5 10-40 80-150 100-150 meq/100 g clay + 3+ 2+ 2+ 3+ 2+ Formula Al2Si2O5(OH)2 K0.50.75Al2 M 0.7(Y ,Y )4-6 M 0.66(Y ,Y )6 little variation (Si,Al)2O10(OH)2 (Si,Al)8O20(OH)4 (Si,Al)8O20(OH)4 nH20 8H20

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Langmuir Isotherms – Ion Exchange Properties different metals are adsorbed more efficiently

Q KC C = o ads 1 + KC

mass adsorbed (mg/g) Cads = mass sorbent

Qo = maximum adsorbed (mg/g)

K = partition coefficient

C = solution concentration (mg/L)

Case Study 7-2, p. 224 Adsorption Characteristics of Clays for Various Heavy Metals

Cads = 5.5 Cads = 8.0 mg/g mg/g

Cads = 7.0 Cads = 2.5 mg/g mg/g

Figure 7-C2-1. Adsorption isotherms for sepiolite. Adsorbent dose = 10 g L-1, agitation time = 3 h, pH = 4, and T = 22oC.