1:1 Clay Minerals Repeat TO layers bonded with weak electrostatic bonds

Cations with +2 and +3 charge Brucite layer Dioctahedral - Only two out of three octahedral sites are occupied by trivalent ions Gibbsite Layer: Trioctahedral - All three out of three octahedral sites are occupied by a divalent ion

2:1 Clays General Structure

2:1 Clay Minerals 2:1 Phyllosilicate Clay Minerals

Smectite Group (e.g., Montmorillonite) The charged double layers are held together by interlayer cations Ca and Na which are surrounded by one to two layers of water molecules. Cations exchangeable with those is water Because variable amounts of water can be held between the layers, the layer spacing can expand and contract depending on the hydration. This causes a great deal of structural damage to buildings sited on soils with a high smectite clay content. Al2Si4O10(OH)2• nH2O

2:1 Clay, Illite tetrahedral octahedral Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+. tetrahedral K+ K+ K+ K+ tetrahedral octahedral tetrahedral

MAJOR CLAY MINERAL GROUPS This table outlines the properties of five major clay mineral groups. The column titled “Layer Type” refers to the proportion of octahedral to tetrahedral sites in the structure. In the 1:1 layer type, the basic structural unit is composed of one tetrahedral sheet and one octahedral sheet that are tightly bound to one another. This basic unit is then repeated indefinitely. In the 2:1 layer type, we have one octahedral sheet sandwiched between two tetrahedral sheets as the basic structural unit. In some clay mineral groups, Al3+ may substitute for Si4+ in the tetrahedral sheets, and some divalent metal ions may substitute for Al3+ in the octahedral sheets. This results in a charge imbalance, with the layers having an excess negative charge. This negative charge can be balanced by interlayer cations such as NH4+, K+ and Na+. The column titled “Layer Charge” gives the amount of excess negative charge per formula unit that must be balanced by interlayer cations. Finally, the last column gives the chemical formula for each clay type, with some structural information. For example, the tetrahedral cations are given in square brackets, the interlayer cations are given in front of the square brackets, and the octahedral cations after the square brackets. Chlorite has an additional twist. Together with the tetrahedral-octahedral-tetrahedral sandwich, there is an aluminum hydroxide interlayer.

Iron Oxide and Hydroxide Minerals Very common weathering products

Mechanisms of silicate weathering Grain Surface Features affecting Dissolution Points of fast weathering Point Defects Dislocations Microfractures Kinks Grain or twin boundaries Corners Edges and ledges

Weathered Surfaces Weathering reactions are surface reactions Via growth of itch pits

Weathering reagent and products Carbonic acid (H2CO3) is the most common weathering reagent in natural waters MgSiO4 (forsterite) + 4H2CO30  2Mg2+ + 4HCO3- + H4SiO40 CaAl2Si2O8(anorthite) + 2H2CO30 + H2O(l)  Ca2+ + 2HCO3- + Al2Si2O5(OH)4(kaolinite) 2NaAlSi3O8(albite) + 2H2CO30 + 9H2O(l)  2Na+ + 2HCO3- + Al2Si2O5(OH)4(kaolinite) + 4H4SiO40 2K[Mg2Fe]AlSi3O10(OH)2(biotite) + 10H2CO30 + 0.5O2 + 6H2O  Al2Si2O5(OH)4(kaolinite) + 4H4SiO40 + 2K+ + 4Mg2+ + 2Fe(OH)3(s) (iron hydroxide) + 10HCO3-

Primary Weathering Products Soluble constituents removed from the weathering site Na+, Ca2+, K+, Mg2+, H4SiO4, HCO3-, SO42-, Cl- Residual primary minerals little affected by weathering reactions: Quartz, zircon, magnetite, ilmenite, rutile, garnet, titanite, tourmaline, monazite New, more stable minerals produced by the reactions Kaolinite, smectite, illite, chlorite, gibbsite, amorphous silica, hematite, goethite, boehemite, diaspore, pyrolusite

Fe3+ and Al3+ Ferric iron (Fe3+) and Al3+ have very low solubilities, so when silicates containing these metals are weathered, Fe and Al oxides form Overall, weathering removes, alkalis and alkaline earths, but leaves behind Fe and Al in soil (recall dust derived from soils contain high abundance of Fe and Al) As we have seen previously, incongruent dissolution is the dissolution of one mineral which takes place simultaneously with the precipitation of another mineral. Aluminosilicates dissolve incongruently because of the high insolubility of Al minerals. Thus, when aluminosilicate minerals weather, Si, Ca, Mg, Na and K are largely released to solution, whereas Al is retained in secondary minerals such as clays. Iron behaves similarly. Under the oxidizing conditions near the Earth’s surface, ferric iron (Fe3+) is the predominant form of iron. Ferric iron is even less soluble than Al, and so also tends to remain behind as secondary minerals, in this case oxyhydroxides.

Incongruent and congruent weathering Incongruent weathering of silicate mineral 2NaAlSi3O8 + 2H2CO3 + 9H2O = 2Na+ + 2HCO3– + 4H4SiO4 + Al2Si2O5(OH)4 (Albite, Na-feldspar) (Kaolinite) Congruent weathering of calcite CaCO3 + H2CO3 = Ca2+ + 2HCO3–

Formation of Al ore deposit Incongruent kaolinite weathering: Al2Si2O5(OH)4 (kaolinite) + 5H2O  Al2O3•3H2O (gibbsite) + 2 H4SiO4 Gibbsite (or more often bauxite, a gibbsitelike mineral) is ore for Al What conditions favor the formation of bauxite?

Importance of Climate Increasing Cation concentration Weathering products vary with varying rainfall Increasing Cation concentration Increasing Si concentration Use of Stability field diagrams: High rainfall removes Si from the solution, promoting the conversion of Kaolinite to gibbsite. Most tropical and subtropical soils contain Kaolinite as the major clay mineral. In poorly drain soils (e.g., aemiarid climate), however, smectite is the characteristic soil mineral Degree of flushing

Weathering products: Impact of Climate For areas with low rainfall (and a source of Mg2+), 3NaAlSi3O8(albite) + 2H2O + Mg2+  2Na0.5Al1.5Mg0.5Si4O10(OH)2 (Smectite) + 2Na+ + H4SiO40 For areas with moderate rainfall, 2NaAlSi3O8(albite) + 2H2CO30 + 9H2O(l)  2Na+ + 2HCO3- + Al2Si2O5(OH)4 (kaolinite) + 4H4SiO40 For areas with higher rainfall, silicic acid is removed efficiently to allow: NaAlSi3O8(albite) + H2CO30 + 7H2O(l)  Na+ + HCO3- + Al(OH)3 (gibbsite) + 3H4SiO40

Biotite Weathering Reaction: formation of iron oxide 2K[Mg2Fe][AlSi3]O10(OH)2 (biotite) + 10H+ + 0.5O2 + 6H2O  Al2Si2O5(OH)4 (kaolinite) + 2K+ + 4Mg2+ + 2Fe(OH)30 (amorphous iron oxide) + 4H4SiO40 Over time, the amorphous iron oxide will convert to common, stable iron mineral goethite (α-FeOOH)

Dissolution of quartz Quartz: Adsorption of H2O molecules on middle Si-O bond Hydrolysis reaction breaks Si-O bond Further adsorption and bond breaking H4SiO4 molecule forms and goes into solution SiO2 + H2O  H4SiO4

Quatz and amorphous silica At low pH values, the solubility of quartz is ~10 ppm A ph >9, silicic acid dissociates slightly H4SiO4  H+ + SiO4- Increases the solubility of quartz Most dissolved silica comes from other weathering reactions Determining biogenic opal in sediments

EXPERIMENTAL RATES OF MINERAL WEATHERING The data in this table illustrate Goldich’s series very well. The data represent how long it would take for a crystal with a dimension of 1 mm to completely dissolve at pH = 5 and 298 K. These calculations are based on experimental measurements of dissolution rates. Under these conditions, as expected from Goldich’s series, quartz lasts the longest (34 Ma). On the other hand, minerals that tend to crystallize early from a silicate melt, such as enstatite, diopside, nepheline and anorthite, dissolve the most rapidly (on the order of 100’s to 1000’s of years) in the weathering environment.

Factors affecting weathering rates Rainfall, relief Mean annual temperature (affect dissolution rate and microbial activity) Vegetation (organic acid production)

Attack by Organic Acids Many weathering reactions in the subsurface and soils are due to the presence of organic acids created by bacterial degredation of organic material. These acids include humic, fulvic and oxalic, among many others Organic acid reactions may be approximated by using carbonic acid. This is because organic acids rapidly breakdown and are found in much lower concentration than carbonic acids in ground and river waters Fulvic Acid Humic Acid

Attack by Organic Acids Reaction of albite and oxalic acid in upper soil zones 2H2C2O4 (oxalic acid) + 4 H2O + NaAlSi3O8 (albite)  Al(C2O4)+ + Na+ + C2O42- + 3H4SiO40 As the products of this reaction pass through the soil, Al(C2O4)+ and C2O42- are bacterially degraded. Al is released and will usually precipitate. Therefore, 4H2C2O4 (oxalic acid) +2O2 + 7H2O + 2NaAlSi3O8 (albite)  Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40 + 6CO2

Surface Complexation by Ligands Ligand* attack is a three-step process: 1) A fast ligand adsorption step 2) A slow detachment process: Just as in the case of protonation, the detachment step itself is much slower than the initial ligand adsorption step. Once the metal ion leaves the surface, the surface is now negatively charged and coordinatively unsatisfied. It therefore grabs the nearest proton to bond with, and the surface is reprotonated. Once again, the entire process can now be repeated. *Ligand: A compound with electron donating functional groups (e.g. ethylenediamine [H2NCH2CH2NH2] capable of bonding to a metal cation. In soils these are often derivatives of Oxalic, Humic, and Fulvic acids.

Surface Complexation by Ligands 3) Fast protonation to restore the initial surface: In this case, formation of the M-L bonds weakens the M-O bonds and allows the metal to leave the surface. Once the metal ion leaves the surface, the surface is now negatively charged and coordinatively unsatisfied. It therefore grabs the nearest proton to bond with, and the surface is reprotonated. the entire process can now be repeated.

Surface Complexation by Ligands Another example: Organic-ligand forms a complex with surface hydroxide and weakens internal bonds.

The effect of complex formation Increase the solubility over non-complex systems Some metals are present in natural waters almost completely complexed. Cu2+, Hg2+, Pb2+, Fe3+, U4+ Adsorption / desorption is greatly affected by complexation, e.g., carbonate, sulfate, floride, phosphate complexes Toxicity, bioavailability of species. Cu2+ is toxic to fish, but is unavailable when it is complexed. Similarly for other metal cations, Cd2+, Zn2+, Ni2+, Hg2+, Pb2+. In general, the most toxic species is the free ion. Thus, toxicity is reduced due to complexation

Carbonate dissolution and reprecipitation Decomposition of organic matter yields carbonic acid (H2CO3) H2CO3 + CaCO3 → Ca2+ + 2HCO3- H2CO3 + CaMg(CO3)2 (dolomite) → Ca2+ + Mg2+ + 2HCO3- When water degas (loss dissolved CO2), CaCO3 reprecipitate Cave deposit (stalactites, stalagmites etc.) Carbonate nodules in soils

Weathering and groundwater composition The differences in water composition between groundwater and rainwater are due to rock weathering and plant uptake Mobility of ions into groundwater: Ca > Na > Mg > Si > K > Al = Fe Because the most rapidly weathered silicates are Na-Ca silicates (plagioclase feldspars), Mg-containing silicates (pyroxenes, amphiboles), K is contained in less rapidly weathered minerals, e.g., biotite, muscovite, K-feldspar

AMD (Acid Mine Drainage) (Abandoned Mine Drainage)

AMD What is Acid Mine Drainage (AMD)? Drainage flowing from or caused by surface mining, deep mining or refuse piles that is typically highly acidic with elevated levels of dissolved metals. What is Abandoned Mine Drainage (AMD)? Any water discharge from a mine. Typically high in dissolved metals Not necessarily acidic How is AMD formed? AMD is formed by a series of complex geo-chemical and microbial reactions that occur when water comes in contact with pyrite (iron disulfide minerals) in coal, refuse or the overburden of a mine operation. The resulting water is usually high in acidity and dissolved metals. The metals stay dissolved in solution until the pH raises to a level where precipitation occurs. Where is AMD found? Anywhere Coal or metal-bearing rocks have been disturbed by mining or quarrying

Pyrite in Coal Pyrite (FeS2) is disseminated in coal as fine-grained particles generally less than 10 µm

Oxidative-Reductive Dissolution (attack by microorganisms) Weathering of Pyrite 4FeS2 (pyrite) + 14H2O + 15O2  4Fe(OH)3 + 16H+ + 8SO42- Acid Mine Drainage (AMD): more discussions on mechanisms when we discuss oxidation-reduction reactions

Common Sulfide minerals Pyrite FeS2 Fool’s gold Galena PbS Ore of lead Sphalerite ZnS Ore of zinc Chalcopyrite CuFeS2 Ore of copper

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