Soil Chemistry: Rocks, minerals, weathering. Prof. Dr. Karsten Kalbitz Institute of Soil Science and Site Ecology

Transcription

1 Soil Chemistry: Rocks, minerals, weathering Prof. Dr. Karsten Kalbitz Institute of Soil Science and Site Ecology Dresden, 2 and 9 November 2017

2 Schedule and topics

3 Soil minerals and weathering

4 S = f(cl,o,r,p,t) Hans Jenny UC Berkeley, humans Factores processes properties

5 Rocks to soils Knowledge of the parent material (rocks) information about fertility of the soil developing from this rock Information about rocks and minerals = crucial to assess soils

6 Soils as an environmental interface Wilding & Lin, 2006 Approximate proportions (by volume) of components found in a loam surface soil(good condition for plant growth) (Source: Brady and Weil, 2008)

7 Geology Petrology Mineralogy gneiss geology sandstone petrology granite quartz Geology, petrology and mineralogy determine the nutrient pool of the soil feldspar (orthoclase) KAlSi 3 O 8 mica (biotite) mineralogy

8 Geological map of Germany Relationship between geology and soil development Soil regions in Germany Bundesanstalt für Geowissenschaften und Rohstoffe soil development depends on the parent material

9 Rocks

10 Rocks Igneous Rocks - produced by solidification of molten magma from the mantle. extrusive or volcanic igneous rocks: magma solidifies at the Earth's surface (i.e. basalt). intrusive or plutonic igneous rocks: magma cools and solidifies beneath the surface (i.e. gabbro, granite). Sedimentary Rocks - formed by burial, compression, and chemical modification of deposited weathered rock debris or sediments at the Earth's surface (i.e. sandstone, limestone). Metamorphic Rocks - created when existing rock is chemically or physically modified by intense heat or pressure (i.e. gneiss, schist, marble). 12

11 Formation of intrusive (plutonic) and extrusive (volcanic) rocks

12 Rijsdijk, 2014 Metamorphic rocks

13 Rijsdijk, 2014 Metamorphic rocks

14 Metamorphic rocks Ortho metamorphic rocks: derived from magmatites (e.g. orthogneiss) Para metamorphic rocks: derived from sediments (e.g. marble, paragneiss) Rijsdijk, 2014

15 Igneous Rocks: intrusive and extrusive rocks Si-rich K-rich Fe-poor light Si-poor Ca-rich Fe-rich dark Extrusive: fine grained, porphyritic rhyolite basalt Volcano landscape; Hegau (SW-Germany) Intrusive rocks (coarse grained) granite gabbro Old bedrock; Black Forest (SW-Germany) Jahn, 2007

16 Classification of intrusive (plutonic) and extrusive (volcanic) rocks light dark intrusive extrusive coarse grained fine grained felsic intermediate mafic ultramafic granite diorite gabbro peridotite rhyolite andesite basalt komatite

17 Composition of the earth crust Element by mass (%) by volume (%) Oxygen O Silicon Si Aluminium Al Iron (III) Fe Iron (II) Fe Calcium Ca Magnesium Mg Sodium Na Potassium K

18 Naturally occurring, inorganic, solid Minerals Specific composition (e.g., silicates, quartz SiO 2, carbonates, sulfates, Fe oxides) Definite crystalline structure atoms are arranged in a specific pattern Specific chemical composition formula Sources of macro- and micronutrients in soils Minerals (and other inorganic material) are the basic constituents of rocks (rocks = mixture of minerals) plant macro nutrients: plant micro nutrients: K, Ca, Mg, N, P, S B, Cl, Mo, Cu, Fe, Mn, Zn

19 Soil minerals Silicates (salts of silicic acid - H 4 SiO 4 ) and quartz (SiO 2 ) > 95% of the earth crust quartz: 18% silicates: 78.5% Other minerals: 3.5% Tectosilicate: Plagioclase Iron oxide: Hematite

20 Soil minerals principle building blocks Si-Tetrahedron Al-Octahedron tetrahedron octahedron all primary silicates phyllosilicates

21 Soil minerals chemical bonds Increase in electronegativity Metals: electron donors Nonmetals: electron acceptors Decrease in electronegativity electronegativity: attraction of electrons Formation of ionic bonds transfer of electrons Metal: cation Non-metal: anion Formation of covalent bonds share of electrons

22 Soil minerals chemical bonds Type of bonds depends on the difference in in electronegativity (ΔEN) Type of bonds ΔEN=0 ΔEN<0.7 Covalent Nonpolar covalent 0.7<ΔEN<1.7 Polar covalent ΔEN>1.7 ionic Character of bonds determines properties, weatherability and stability of minerals covalent bond = stronger than ionic Essington, 2004

23 Soil minerals principle building blocks and type of bonds Binding of tetrahedra with octahedra polyhydron bonds Si-O: 51% ionic bonds Al-O: 63% ionic Al Si O Tetrahedron Octahedron (ΔEN): 1.54 (ΔEN): 1.83 Essington, 2004

24 Hydrogen bonds Hydrogen bonds between O and H atoms Stabilizing effect in kaolinite high heat capacity of water Van der Waals forces between nonpolar neutral surfaces Weak bonds depending on the surface

25 Summary chemical bonds

26 Silicates Primary silicates formed out of cooling magma principle building blocks: SiO 4-4 distinction by charge per unit and by Si/O ratio in sand and silt fraction (>2 µm) Secondary silicates (clay minerals) formed by chemical weathering of primary silicates (chemically altered) distinction by charge and by alteration of Si-tetrahedral and Al-octahedral layers in clay fraction

27 Primary silicates - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates inosilicate (single chain) pyroxene amphibole inosilicate (double chain) Mica, (and clay minerals) neso-/sorosilicate olivine phyllosilicate principle building blocks tectosilicate ilicate ilicate quartz, feldspars most important silicate structures of rock forming minerals

28 Diversity of silicates Diversity of silicates 2 properties Degree of condensation: Differences how tetrahedra are linked together via oxygen: Si O Si (balance of the negative charge of the building blocks)

29 Primary silicates Class building block charge / Si Si/O ratio name Nesosilicate SiO Olivine Sorosilicate Si 2 O Akermanite Inosilicate SiO 2-3 (pyroxene) Augite Si 4 O 6-11 (amphibole) Tremolite Phyllosilicate Si 2 O Muscovite Tectosilicate SiO Anorthite Condensation results in decrease of charge Primary silicates with the lowest Si/O ratio have the highest weathering rate

30 Diversity of silicates Diversity of silicates 2 properties Degree of condensation: Differences how tetrahydra are linked together via oxygen: Si O Si Isomorphic substitution

31 Isomorphic substitution Mg 2+ - Al 3+ Al 3+ - Si 4+ permanent (-) charge Adsorption (+) cation Cation exchange capacity (CEC)

32 Isomorphic substitution isomorphic substitution in tetra/octahedrons -> permanent negative charge structure remains the same ( isomorphic ), charge changes in tetrahedrons: substitution of Si 4+ with Al 3+ in octahedrons: substitution of Al 3+ with Mg 2+ (Fe 2+, Fe 3+ )

33 Primary silicates source of nutrients primary silicates Neso-/ sorosilicates olivine (Mg, Fe) 2 SiO 4 Tectosilicates quartz SiO 2 feldspars orthoclase KAlSi 3 O 8 albite NaAlSi 3 O 8 anorthite CaAl 2 Si 2 O 8 Inosilicates pyroxene (augite) Ca, Mg, Fe - silicates Phyllosilicates: (sheet silicates) amphibole (hornblende) K, Ca, Mg, Fe - silicates primary: Mica biotite: K-Mg-Fe (hydroxo-silicates) muscovite: K-Al (hydroxo-silicates) Macro nutrients: K, Ca, Mg, N, P, S Micro nutrients: B, Cl, Mo, Cu, Fe, Mn, Zn

34 Nesosilicates - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates nesosilicates (orthosilicates) olivine (Mg, Fe) 2 SiO 4 olivine in basalt

35 Inosilicates - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates augite Soils formed on rocks containing pyroxenes and amphiboles are often quite fertile! Hornblende (Ca, Mg, Fe 2+, Fe 3+, Ti, Al) 2 [(Si, Al) 2 O 6 ] pyroxene (chain-like) Ca 2 (Na, K) 0,5-1 (Mg, Fe 2+ ) 3-4 (Fe 3+, Al) 2-1 [(O, OH, F) 2 /Al 2 Si 6 O 22 ] amphibole (tape-like)

36 Inosilicates - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates pyroxene (augit) amphibole (Hornblende) pyroxene-basalt basalt

37 Phyllosilicates - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates 1:1 (two-sheet) silicates: serpentine kaolinite, hydrohalloysite 2:1 (three-sheet) silicates: micas (muscovite, biotite) illite, smectite, vermiculite Essington, 2004

38 Example - phyllosilicate Structural unit 2:1 layer (3 sheet mineral)

39 Example - phyllosilicate Muscovite KAl 2 (Si 3 Al)O 10 (OH) 2 {interlayer}{octahedron}{tetrahedron}o 10 (OH) 2 {K} {Al 2 } {Si 3 Al} O 10 (OH) 2 Interlayer: neutralization of charge by K + Tetrahedral sheet: 3x Si, 1x Al = -1 negative charge Octahedral sheet: 2 Al 3+ Si/O = (3+1)/10 = 0.4 Biotite K(Fe II 1.5Mg 1.5 )(Si 3 Al)O 10 (OH) 2

40 Phyllosilicates - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates biotite (dark mica) thin sheets of muscovite muscovite (light mica)

41 - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates Phyllosilicates biotite muskovite mica (muscovite) quartz granite potassium feldspar

42 - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates Phyllosilicates biotite muscovite mica (biotite) diorite feldspar

43 - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates Phyllosilicates biotite muscovite mica (biotite) quartz dark stripes in the rock = usually mica gneiss potassium feldspar

44 Tectosilicates: quartz - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates: quartz amethyst milky quartz rock crystal ( german diamond ) most abundant quartz specimen - SiO 2 coarsely crystalline

45 Tectosilicates: quartz - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates: quartz - chemical composition: SiO 2 - resistant to weathering and acid - abundant in acid magmatites and metamorphites - abundant in sediments (quartz sands, mechanically rounded quartz grains) - not relevant for plant nutrition - important for soil texture (sand and silt fraction) - important for soil structure -> meso- and macropores SiO 2 SiO 2 SiO 2

46 Tectosilicates: feldspar - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates: feldspars K-feldspar KAlSi 3 O 8 (orthoclase) plagioclase (Na,Ca)Al 2 Si 2 O 8 albite NaAlSi 3 O 8 anorthite CaAl 2 Si 2 O 8 Essington, 2004

47 Tectosilicates: feldspar - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates: feldspars K-feldspar KAlSi 3 O 8 (orthoclase) plagioclase (Na,Ca)Al 2 Si 2 O 8 quartz granite feldspar 06. Nov /50

48 Tectosilicates: feldspar - neso-/sorosilicates - inosilicates - phyllosilicates - tectosilicates: feldspars K-feldspar KAlSi 3 O 8 (orthoclase) plagioclase (Na,Ca)Al 2 Si 2 O 8 quartz porphyry (rhyolithe) K-feldspar 06. Nov /50

49 silicates

50 Temperature dependency of mineral formation during cooling of magma

51 Igneous rocks: mineral composition

52 Other minerals non-silicates Carbonates Calcite CaCO 3 Dolomite CaMg(CO 3 ) 2 Sulfates Gipsum CaSO 4 x 2 H 2 O Sulfides Pyrite ( fool`s gold ) FeS 2 desert rose (Morocco)

53 Other minerals sulfides non-silicates carbonates sulfates sulfides phosphates Pyrit FeS 2 fool`s gold weathers easily: Fe 2+ -> Fe 3+ rust formation = limonite oxidation of sulfide to sulfate plant available occurrence: usually finely spread microscopic grains in many magmatic, metamorphic and sedimentary rocks

54 Other minerals phosphates non-silicates carbonates sulfates sulfides phosphates apatite Ca 5 [(F, Cl, OH)/(PO 4 ) 3 ] accessory (finely spread) in all magmatic and metamorphic rocks not macroscopically visible in rocks only identifiable when measuring the phosphorous content in rocks and soils primary phosphorous source in soils guano, bones and teeth largely consist of hydroxylapatite!

55 Summary geology and mineralogy strongly influence the formation and properties of the soil Variation of silicates depends on condensation of Si tetrahedra and Al octahedra and isomorphic substitution Isomorphic substitution determines permanent negative charge (cation adsorption): soil fertility Type of bonds and degree of isomorphic substitution determine stability of minerals Mineral composition of rocks determine soil properties, e.g. potential nutrient content soil minerals are fascinating and beautiful -> Terra Mineralia in Freiberg

56 Soil Chemistry: Rocks, minerals, weathering Prof. Karl-Heinz Feger Prof. Dr. Karsten Kalbitz Institute of Soil Science and Site Ecology Dresden, 2 and 9 November 2017

57 Schedule and topics

58 Rocks

59 Chemical weathering introduction atmosphere soi l biosphere Weathering: change of rocks and minerals in contact with the atmosphere, biosphere and hydrosphere Precondition for soil forming Physical and chemical weathering lithosphere hydrosphere

60 Physical weathering Fragmentation of rocks no chemical change (temperature, frost, salts) Physical and chemical weathering

61 Sediments Brady & Weil 2008

62 Sediments Hadean Sedimentary rocks in the Grand Canyon Age: about Ma

63 Biogenic sediments: limestone (Muschelkalk, shell limestone) Jenzig close to Jena, Germany

64 Other minerals carbonates non-silicates carbonates sulfates sulfides phosphates calcite CaCO 3 Coral reef haptophytes (calcite algae) biogenely formed calcite calcite shells of mussels, snails

65 Sediments: aeolian Saharan dust storm aeolian sediment Dust storm, Texas, Oct. 17, 2011

66 Sediments: aeolian Loess an aeolian deposit

67 Sediments: fluvial Alluvial fan

68 Examples Sediments: glacial Terminal moraine, German Baltic coast. Ice transported sediment Soil formation after the retreat of the glacier about years ago Relatively young soils in comparison with soils of some parts of Africa or Australia 98

69 Examples Sediments: marine Coastal marine sediments Baltic Sea Soil formation North Sea (at low tide)

70 atmosphere lithosphere soi l Chemical weathering introduction biosphere hydrosphere Weathering: change of rocks and minerals in contact with the atmosphere, biosphere and hydrosphere Precondition for soil forming Physical and chemical weathering Chemical weathering Formation of new minerals by chemical changes (and dissolution) of old minerals Most important chemical reactions: hydrolysis and oxidation Stability of minerals Clay minerals Oxides

71 Chemical weathering Chemical weathering Weathered residues and products have a different chemical composition Weathering rate depends on: mineral composition of rocks composition of water climate (temperature and precipitation)

72 Water can dissolve minerals Chemical weathering: water Without water no chemical weathering CaSO 4 2H 2 O (s) Ca 2+ (aq) + SO 4 2- (aq) + 2H 2 O (l) Water contains protons that may react with minerals FeOOH (s) + citrate 3- (aq) + 3H + (aq) Fe-citrate 0 (aq) + 2H 2 O (l) Water as transport medium: leaching of minerals Mg 2 SiO 4 (s) + 4H + (aq) 2Mg 2+ (aq) + H 4 SiO 4 0 (aq)

73 Hydrolysis and general principles of chemical weathering Dissolving of minerals by reaction with water containing protons (water or acids) Protons replace metal cation from bonds with O-ions at the surface of the mineral Weathering of silicates: Si O M + H + Si OH + M + H + is being used: Rate of reaction depends on H + -concentration, so rate increases at low ph Buffer (acid neutralizing) potential! General principles of chemical weathering: Consumption of H + Production of base cations and H 4 SiO 4 0 Forming of metal oxides

74 Sources of protons in the soil Sources of protons CO 2 CO 2 production by respiration of micro-organisms (decomposition of organic matter) and plant roots Root exudation of organic acids Dissolving CO 2 from the atmosphere Dissolving CO 2 in water CO 2 (gas) + H 2 O H 2 CO 3 Dissociation of carbonic acid H 2 CO 3 + H 2 O H 3 O + + HCO 3 K a,1 = HCO 3 + H 2 O H 3 O + + CO 3 2 K a,2 = Hydrolysis of Al and Fe (will come later)

75 Chemical weathering: dissolution of lime by carbonic acid H 2 O + CO 2 H 2 CO 3 H + + HCO 3 CaCO 3 + H + + HCO 3 Ca(HCO 3 ) 2 Soil development always starts with decalcification (calcareous rocks) foto: Jahn CO 2 - concentr. [Vol-%] mg CaCO 3 /L water CaCO 3 -solubility depending on CO2 concentration

76 Field test: decalcification

77 Field test: decalcification Detection of carbonates with hydrochloride acid: CaCO HCl CaCl 2 + H 2 O + CO 2 Intensity of gas emission (forming of foam) is a measure of carbonate content Karst landscape in China

78 Weathering and the long-term C cycle Use of olivine to sequester CO 2 Horwath, 2007

79 Oxidation Oxidation of iron: Fe 2+ in minerals to Fe 3+ by oxygen: Fe 2+ (aq) O 2 (g) + H + (aq) Fe 3+ (aq) + 0.5H 2 O (l) Fe 2+ (aq) Fe 3+ (aq) + e - Oxidation 0.25O 2 (g) + H + (aq) + e - 0.5H 2 O (l) Reduction 2Fe 2+ (aq) + 0.5O 2 (g) + 3H 2 O (l) 2FeOOH (s) + 4H + (aq) Fe 3+ in minerals disturbs electrostatic neutrality other cations leave the crystal Fe 2+ in solution will be oxidized to FeOOH Muscovite KAl 2 (Si 3 Al)O 10 (OH) 2 Biotite K(Fe II 1.5Mg 1.5 )(Si 3 Al)O 10 (OH) 2 photograph: Yang

80 Chemical weathering stability of minerals Stability against weathering increases: with decrease in water solubility: Salt < Gypsum < Calcite < Dolomite << Silicates Increase in stability

81 Chemical weathering stability of silicates Stability against weathering increases: With increasing number of bonds (condensation; Si/O ratio) between polyhedra Nesosilicate Sorosilicate Cyclosilicate, inosilicate - pyroxene Inosilicate - amphibole Phyllosilicate Tectosilicate Increase in stability With decreasing isomorphic substitution (charge) in tectosilicates Substitution (%) Ca plagioclase (Anorthite) (Ca, Na) and (Na, Ca) plagioclase Na plagioclase (Albite) Orthoclase (K feldspar) quartz 50% % 25% 25% 0%

82 Chemical weathering stability of silicates Stability against weathering increases: With decreasing contents of ions (Fe 2+ ) that can be oxidized Muscovite > Biotite Little Fe 2+ Much Fe 2+ Much Fe 2+ = electrostatic disturbance and disturbed octahedra by smaller radius of Fe 3+ Increase in stability

83 Stability of primary silicates unstable Contains many base cations Low Si/O ratio Formation at high temperature stable contains almost no base cations High Si/O ratio Formation at low temperature Olivine, (Fe,Mg) 2 SiO 4 Muscovite, KAl 2 (Si 3 Al)O 10 (OH) 2 Ca plagioclase, CaAl 2 Si 2 O 8 Quarty, SiO 2 The temperature of formation of most stable minerals is close to soil temperature

84 Chemical weathering summary Hydrolysis and oxidation = most important reactions Properties of minerals that promote weathering rates: High solubility in water Low degree of condensation of polyhedra High degree of isomorphic substitution High concentrations of ions that can be oxidized (Fe 2+ ) Increase in weathering rate at low ph, high water fluxes The temperature of formation of most stable minerals is close to soil temperature

85 Secondary silicates = clay minerals Weathering of primary phyllosilicates Phyllosilicate + H 2 O + H 2 CO 0 3 clay + base cations + HCO H 4 SiO metal oxide Formation of clay minerals (particles <2 μm, = secondary silicates) Layered structure large specific surface area important for adsorption and cation exchange From J.E. Andrews et al., An Introduction to Environmental Chemistry (2004) Blackwell Publishing, Oxford, UK, ISBN , p. 90.

86 Specific surface area

87 Secondary silicates = clay minerals Weathering of primary phyllosilicates Phyllosilicate + H 2 O + H 2 CO 0 3 clay + base cations + HCO H 4 SiO metal oxide Formation of clay minerals (particles <2 μm, = secondary silicates) Layered structure large specific surface area important for adsorption and cation exchange Classification based on: Layer type tetrahedra:octahedra (1:1 of 2:1) Permanent charge as a result of isomorphic substitution Swelling and shrinking properties From J.E. Andrews et al., An Introduction to Environmental Chemistry (2004) Blackwell Publishing, Oxford, UK, ISBN , p. 90.

88 Clay minerals: Kaolinite group 1:1 clay minerals Hydrogen bonds

89 Clay minerals: Kaolinite group 1:1 clay minerals Little isomorphic substitution Stable because of hydrogen bonds between 1:1 layers Low CEC (1-10 cmol c /kg) Small specific surface area (10-20 m 2 /g)

90 Clay minerals: Mica and illite groups 2:1 minerals Isomorphic substitution mostly in tetrahedron: ionic bonds between layers and cations (K + ) in interlayer Small specific surface area ( m 2 /g) Low CEC (10-40 cmol c /kg) No swelling

91 Clay minerals: Vermiculite group 2:1 clay minerals Isomorphic substitution mostly in tetrahedron: ionic bonds between layers and cations in interlayer Swelling in water Large specific surface area ( m 2 /g) High CEC ( cmol c /kg)

92 Clay minerals: Smectite group 2:1 clay minerals Isomorphic substitution in tetrahedron and octahydron: ionic bonds between layers and cations in interlayer Swelling in water Large specific surface area ( m 2 /g) High CEC ( cmol c /kg)

93 Clay minerals: Chlorite group Primary and secondary 2:1 minerals Interlayers of metal (hydr)oxides strong bonds - stable No swelling Small specific surface area ( m 2 /g) Variable charge Low CEC (10-40 cmol c /kg)

94 Clay minerals: Hydroxy-interlayered vermiculite secondary 2:1 minerals Weathering of vermiculite - chloritisation Al migrates into the interlayer and hydrolyses Mineral comparable with chlorite but interlayer is interrupted

95 Classification of clay minerals Layer type Group Distance of layers (nm), interlayer Charge CEC cmol c /kg Swelling? 1:1 Kaolinite 0.7 nm ca no 1:1 Halloysite 1.0 nm (H 2 O) ca 0 no 2:1 Illite (Mica) 1.0 (K, not hydrated) ca no Vermiculite (cations +/- hydrated) Smectite (cations +/- hydrated) Chlorite (2:1:1) yes Yes, strongly 1.4 (Al-hydroxides) variable no

96 Weathering of silicates and formation of clay minerals and oxides Brady, 2002

97 Metal oxides / hydroxides Weathering products: oxides, and hydroxides of Fe III, Al, Mn en Si In clay size fraction and as coating on clay minerals Responsible for high reactivity in soil surfaces amphoteric behavior depending on ph

98 Metal oxides / hydroxides Gibbsite, Al(OH) 3 most abundant in soils: Variable specific surface area crystalline Al(OH) 3 : m 2 /g amorphous Al(OH) 3 : 600 m 2 /g voor Charge depends on ph Point of Zero Charge (PZC): Low ph PZC + H + + OH - High ph [Me(OH) 2 ] + + H 2 O Me(OH) 3 [MeO(OH) 2 ] - + H 2 O Point of Zero Charge (PZC) for a given mineral surface is the ph at which the surface has a neutral charge

99 Point of zero charge (PZC)

100 Point of zero charge (PZC)

101 Metal oxides / hydroxides Gibbsite, Al(OH) 3 most abundant in soils: Variable specific surface area crystalline Al(OH) 3 : m 2 /g amorphous Al(OH) 3 : 600 m 2 /g Charge depends on ph Point of Zero Charge (PZC): Low ph PZC + H + + OH - High ph [Me(OH) 2 ] + + H 2 O Me(OH) 3 [MeO(OH) 2 ] - + H 2 O PZC = point of zero charge ph pzc = 8.9 ph < 8.9 positive charge exchange of anions ph > 8.9 negative charge exchange of cations

102 Summary Charge Permanent charge: Isomorphic substitution in 2:1 clay minerals Negative charge, not depending on ph Cation exchange variable charge: Dissociation of H + and OH - (1:1 clay minerals, oxides, organic matter) ph < PZC: positive, exchange of anions ph > PZC: negative, exchange of cations

103 Determine the color of soils! Iron oxides

104 Iron oxides Goethite, α-feooh Most abundant, formation by slow supply of Fe 3+ Fe 3+ :weathering in weakly acidic soils Fe 3+ concentrations low because of complex formation Temperate zone, not too wet Yellow-brown soils ph pzc = 9.0 (Goethite)

105 Iron oxides Ferrihydrite, 5Fe 2 O 3.9H 2 O, Fe 5 HO 8.4H 2 O Weakly crystallized, dark brown Fe(hydr)oxide Formation Fast supply of Fe 3+, e.g. contact of water with high concentrations of Fe 2+ with air Disturbance of crystallization by anions (silicates, phosphate, organic anions) Transformation into Goethite just after complete dissolution Transformation into Hematite by dewatering in soils in warm climates gleysols

106 Iron oxides Lepidocrocite, γ-feooh Orange brown Fe oxide Formation during slow supply and oxidation of Fe 2+ typical for stagnosols (deeper horizons) foto: Jahn

107 Iron oxides Hematite, α-fe 2 O 3 Dry and warm climate Formation from Ferrihydrite at higher temperatures, characterisitc for subtropical and tropical climate No direct transformation of Goethite into Hematite Red tropical and subtropical soils also indication of past tropical / subtropical climate Less stable than Goethite Zech en Hintermaier-Erhard, 2002

108 Iron oxides Hematite, α-fe 2 O 3 Fe 2 O 3 hematite causes the reddish color in a sandstone (sediment rock) almost all reddish colors in rocks and soils hematite (haima. greek: blood)

109 Iron oxides rock colors give hints on the mineral composition of the rocks Iron hydroxides and Beispiele oxides cause für färbende the Fe-Hydroxide coloring of these und Oxide sandstone in klastischen Sedimenten sediments

110 Manganese oxides Occurring in soils with wet and dry periods High reactivity (5-360 m 2 /g) although just low contents in soils negatively charged because of low PZC Binding of heavy metals Birnessite

111 Allophane and imogolite Allophane, Al 2 O 3 (SiO 2 ) H 2 O Imogolite, Al 2 SiO 3 (OH) 4 Oxides of Al and Si with little structure Formed at high concentrations of soluble Al and Si in volcanic soils Variable reactivity depending on Si:Al ph pzc = 2.9 for SiO 2 ph pzc = 8.9 for Al 2 O 3 Foto: Geoforschungsinstitut Potsdam

112 Assessing the weathering status of a soil Degree of weathering little moderate strong Very strong Important minerals Gipsum, Olivine, Calcite Biotite, Illite, Smectite Kaolinite Gibbsite, Hematite, Goethite

113 Summary soil minerals and weathering Composition of minerals (i.e. parent material) determines weathering (rate) and the formation of secondary minerals (clay minerals, oxides) Close correlation between chemical properties of minerals with the stability of minerals against weathering Composition of minerals reflects soil formation

114 Cambisols (Braunerde): processes Fe 2+ containing silicates (e.g. augite, olivine, biotite According to Hintermaier- Erhard and Zech, 1997 Decalcification, chem. weathering, hydrolysis (ph<7) Readily soluble ions (leaching of K +, Ca 2+ ) Silicic acid (H 4 SiO 4 ) + Al hydroxide Formation of clay minerals (enrichment in clay content) Partly leaching of components Oxidation of Fe 2+ to Fe 3+ oxides/ hydroxides (Goethite) Formation of brownish color Coatings of minerals with Fe oxides Formation of soil structure Formation of organic-fe complexes (leaching)