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1:1 Clay Minerals Repeat TO layers bonded with weak electrostatic bonds 1 Cations with +2 and +3 charge Gibbsite Layer: Trioctahedral - All three out of three octahedral sites are occupied by a divalent ion Brucite layer Dioctahedral - Only two out of three octahedral sites are occupied by trivalent ions 2 2:1 Clays General Structure TOT Structure Dioctahedral +3 cation -1 Hydroxyl +4 Silicon -2 Oxygen c b 3 2:1 Clay Minerals 2:1 Phyllosilicate Clay Minerals 4 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 5 2:1 Clay, Illite tetrahedral octahedral tetrahedral K+ K+ K+ K+ tetrahedral Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+. octahedral tetrahedral 6 MAJOR CLAY MINERAL GROUPS Group Layer Type Layer Charge (x) Typical Chemical Formulaa Kaolinite 1:1 <0.01 [Si4]Al4O10(OH)8·nH2O) Illite 2:1 1.4-2.2 Mx[Si6.8Al1.2]Al3Fe0.25Mg0.75O20(OH)4 Vermiculite 2:1 1.2-2.0 Mx[Si7Al]Al3Fe0.5Mg0.5O20(OH)4 Smectite 2:1 0.5-1.2 Mx[Si8]Al3.2Fe0.2Mg0.6O20(OH)4 Chlorite 2:1 with hydroxide interlayer Variable (Al(OH)2.55)4·[Si6.8Al1.2]Al3.4Mg0.6 O20(OH)4 7 Iron Oxide and Hydroxide Minerals Very common weathering products 8 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 9 Weathered Surfaces Weathering reactions are surface reactions Via growth of itch pits 10 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) + 10HCO311 Primary Weathering Products Soluble constituents removed from the weathering site Residual primary minerals little affected by weathering reactions: Na+, Ca2+, K+, Mg2+, H4SiO4, HCO3-, SO42-, Cl- 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 12 3+ Fe and 3+ Al 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) 13 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– 14 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? 15 Weathering products vary with varying rainfall Increasing Cation concentration Importance of Climate Increasing Si concentration Use of Stability field diagrams: Degree of flushing 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 16 17 Weathering products: Impact of Climate For areas with low rainfall (and a source of Mg2+), For areas with moderate rainfall, 3NaAlSi3O8(albite) + 2H2O + Mg2+ 2Na0.5Al1.5Mg0.5Si4O10(OH)2 (Smectite) + 2Na+ + H4SiO40 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 18 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) 19 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 20 Quatz and amorphous silica At low pH values, the solubility of quartz is ~10 ppm A ph >9, silicic acid dissociates slightly H4SiO4 H+ + SiO4Increases the solubility of quartz Most dissolved silica comes from other weathering reactions Determining biogenic opal in sediments 21 EXPERIMENTAL RATES OF MINERAL WEATHERING Mean lifetime of a 1 mm crystal at pH = 5 and 298 K Mineral Lifetime Mineral Lifetime Quartz 34 Ma Enstatite 8.8 ka Muscovite 2.7 Ma Diopside 6.8 ka Forsterite 600 ka Nepheline 211 a K-feldspar 520 ka Anorthite 112 a Albite 80 ka Source: Lasaga (1984) 22 Factors affecting weathering rates Rainfall, relief Mean annual temperature (affect dissolution rate and microbial activity) Vegetation (organic acid production) 23 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 24 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 25 Surface Complexation by Ligands Ligand* attack is a three-step process: 1) A fast ligand adsorption step O L O O H H k 1 + 2 + O M M M M 2 k 1 O L O O H 2) A slow detachment process: O O L H k 2 + + ( + O M M M 2 2 sl o O O L *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. 26 Surface Complexation by Ligands 3) Fast protonation to restore the initial surface: O O k 3 + + M M fa O O 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. 27 Surface Complexation by Ligands Another example: Organic-ligand forms a complex with surface hydroxide and weakens internal bonds. 28 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 COO-OOC C OOMeta l C OO- CH2 CH2 N C H2 -OOC CH2 CH2 N CH2 C OO- 29 Carbonate dissolution and reprecipitation Decomposition of organic matter yields carbonic acid (H2CO3) H2CO3 + CaCO3 → Ca2+ + 2HCO3H2CO3 + CaMg(CO3)2 (dolomite) → Ca2+ + Mg2+ + 2HCO3- When water degas (loss dissolved CO2), CaCO3 reprecipitate Cave deposit (stalactites, stalagmites etc.) Carbonate nodules in soils 30 31 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, Kfeldspar 32 33 AMD (Acid Mine Drainage) (Abandoned Mine Drainage) 34 AMD What is Acid Mine Drainage (AMD)? What is Abandoned Mine Drainage (AMD)? Any water discharge from a mine. Typically high in dissolved metals Not necessarily acidic How is AMD formed? Drainage flowing from or caused by surface mining, deep mining or refuse piles that is typically highly acidic with elevated levels of dissolved metals. 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 35 Pyrite in Coal Pyrite (FeS2) is disseminated in coal as fine-grained particles generally less than 10 µm 36 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 37 Common Sulfide minerals Pyrite FeS2 Fool’s gold Galena PbS Ore of lead Sphalerite ZnS Ore of zinc Chalcopyrite CuFeS2 Ore of copper 38 PA WV 39 40