Secondary minerals 2014-2 Dörte Lehsten Weathering of primary minerals results in secondary minerals When carbonic acid attacks a silicate rock ... i.e. weathering of albite (Na-feldspar) 2NaAlSi3O8 + 2H2CO3 + 9H2O → 2Na+ + 2HCO3 + 4H4SiO4+ Al2Si2O5 (OH) 4 formation of a secondary clay mineral KAOLINITE When carbonic acid attacks a silicate rock ... i.e. weathering of albite (Na-feldspar) 2NaAlSi3O8 + 2H2CO3 + 9H2O → 2Na+ + Si:Al=3:1 2HCO3 + 4H4SiO4+ Al2Si2O5 (OH) 4 Dominant anion in stream water Loss of Si to stream waters Si:Al=1:1 Exchange of 2 H+ with 2 Na+ Soil fluid acidity becomes less Na+ available for uptake by vegetation Incongruent dissolution of weathering primary sediments: Not all constituents are released. ( in Kaolinite Si fixed) Further weathering is possible Under tropical humid conditions kaolinite turns into Gibbsite Al2Si2O5 (OH) 4 + 5H2O → 2H4SiO4 + Al2O3 *3H2O Congruent dissolution of weathering primary sediments: All constituents are released. No further weathering (release of Kations) is possible . Olivine (FeMgSiO4) undergoes congruent dissolution in water: Releasing Fe, Mg and Si. Mg and Si are lost in run-off water to streams. Fe reacts with O2 in precipitating iron(III)oxide. Similar to Olivine, also pyrite undergoes (FeS2) undergoes congruent dissolution in water: + 4 FeS 2 + 8 H 2 O + 15O2 → 2 Fe 2 O3 + 16 H + 8SO The release of high amount of protons by iron oxidation is reason for acid run-off by mining 2− 4 Secondary minerals • By-products of chemical weathering • Precipitate near the parent mineral • In temperate forest soils dominated by layered (silicate) clay minerals Clay minerals • Less tan 0.002 mm • Layers held together by shared O2ions • 2 types of crystalline layer structure – Silicon layers – Layers dominated by Fe, Al, and Mg Clay minerals • Moderately weathered soils: – Ratio of Si to metal dominated layers 2:1 • More strongly weathered soils – Ratio of Si to metal dominated layers 1:1 Clay minerals • 2:1 clay minerals: montmorillonite (Mg instead of Al) illite (K fixed in clay structure) Structure of the clay minerals leads to a negative charge on the surface Clay minerals • 1:1 clay minerals: kaolinite Si tetrahedron layer Al octahedron layer Shared oxygen atom Clay minerals • elements in secondary minerals often fixed to their structure Î not available for plant uptake • Mg fixed in crystal of montmorillonite • K fixed in illite Clay minerals • Weathering of secondary minerals leads to release of sometimes large quantities of elements • E.g. Ammonium (NH4) by weathering of 2:1 clay minerals, a vegetation growth limiting mineral Clay minerals • Cations are than in solution of soil water which – either percolates downwards, washing plant available nutrients into stream water – Will be taken up by vegetation Clay minerals • Exception is Al and Fe – downwards percolation only in form of chelation (only if fulvic acid occurs, which is produced be microbes during litter decomposition) – Otherwise oxidation of Al and Fe into insoluble oxides and hydrous oxides, which results in near complete weathering of secondary minerals Iron and alluminium oxides Iron Alluminium occur in well weathered soil (mainly in tropical regions) Geothite α-FeO(OH) Gibbsite Al(OH)3 Hematite α-Fe2O3 Boehmite γ-AlO(OH) Cation exchange capacity • N+ • P+ • K+ • Na+ • Ca+ • mfl+ Layered 2:1 minerals posses a negative charge due to: ionic substitution of less positive charged element with higher charged element within the crystalline structure of the clay mineral (K+ with Al3+) Cation exchange capacity pH dependent cation exchange Layered 2:1 minerals posses a negative charge due to: exposed hydroxide (-OH) groups at the edges of clay minerals while corresponding H+ protons are dissociated. Dissociation depends on the pH of soil water. (remember: pH depends on concentration of H+ in the soil water NOT on the concentration of other cat ions.) As higher pH as easier are H+ dissociated, leading to a negative charge of clay minerals. Hence pH depending cation exchange Cation exchange capacity • N+ • P+ • K+ • Na+ • Ca+ • mfl+ Layered 2:1 minerals posses a negative charge which: attracts and holds cat ions dissolved in the soil water solution near the clay surface. Binding is reversible. Cat ions are available for plant uptake. Cation exchange capacity pH dependent cation exchange For example Ca2+, K+ NH4+ Cation exchange capacity Total negative charge or cation exchange capacity (CEC) Expressed in mEq/100g (mol equivalent) or cmol(+)/kg of soil. Cation exchange is a chemical mass balance with the soil solution. Functions and models are given in the literature. In general cations held on the exchange side and displace one another in the sequence: Strong Al3+>H+>Sr2+>Ca2+>Mg2+>Rb+>K+>NH4+>Na+>Li+ weak With an overload of weak cations, stronger can be released and washed out. Cation exchange capacity Base saturation (BS) Except for Al and hydrogen cations form bases in solution. Ratio of total CEC occupied by these base cations is termed Base Saturation. Under initial soil development CEC and BS increase, followed by a decrease under further soil development and is higher in temperate soils with 2:1 clay minerals than those with 1:1 minerals. What about tropical soils? CEC and soil organic matter • Phenolic (-OH) and organic (COOH) acids are able to dissociate the corresponding H+ depending on the pH of soil solution. • Hence, it can be substituted with cations as well. • In regions with low amount clay minerals organic matter accounts for CEC (percentual) Soil buffering • Cation exchange acts to buffer acidity If H+ is added it exchange for instance for Ca2+as it is stronger than Ca2+ which subsequently released freely into the soil solution Hence acid will be buffered regarding the CEC. Soil buffering The lime potential If H+ is added to the soil Ca2+ ions increase in the soil water solution. Hence acid will be buffered regarding the CEC. pH – ½ (pCa) = k; (p stands for negative logarithmic ’concentration’ ) K remains constant within the CEC. Lime potential of a soil is defined as constant ratio between H+ and Ca2+ concentration in the soil water solution within a given range of pH (sufficient base saturation of at least 15%) Soil buffering in acid soils Low CEC Buffering by aluminum and Reactions are reversible Al3+ + H 2 O ↔ Al(OH) 2+ + H + Al (OH ) 2 + + H 2O ↔ Al (OH ) 2+ + H + Al (OH ) 2+ + H 2 O ↔ Al (OH ) 3 + H + Gibbsite dissolve to Al3+ under acid conditions, which is toxic for aquatic organisms like fish and amphibians Anion adsorption capacity (AAC) In tropical soils under acid conditions: Anions become adsorbed by positive charge minerals, (oxides and hydroxides of Al and Fe). H+ from soil solution will be associated to the surface hydroxide group, resulting in positive charge of these minerals. Increase in pH reduces AAC until a zero point of charge is reached. At higher pH, these soils posses then CEC (in low amount) For gibbsite ZPC is at pH9. =>under acid condition AAC high and present in most tropical soils. For soils in temperate climate ZPC is at pH2, practical not present Anion adsorption capacity AAC greater on smaller minerals (poorly crystalline forms of Al and Fe bonds) Anion’s strengths follow the sequence: Strongest PO43- > SO42- > CL- > NO3- weakest Hence, low availability of phosphorus. Soils reach in organic matter less efficient in anion adsorption Special attention to Phosphorus limiting factor of plant growth. Apatite contains phosphorus (only primary mineral which contains Phosphorus). Under carbonation weathering releasing of P Ca5 ( PO4 ) 3 OH + 4 H 2 CO3 ↔ 5Ca 2 + + 3HPO 42 − + 4 HCO3− + H 2 O Phosphoric acid: P in solution ⇒Plant uptake available Special attention to Phosphorus limiting factor of plant growth. Apatite contains phosphorus (only primary mineral which contains Phosphorus). Under carbonation weathering releasing of P Ca5 ( PO4 ) 3 OH + 4 H 2 CO3 ↔ 5Ca 2 + + 3HPO 42 − + 4 HCO3− + H 2 O Phosphoric acid: pH<7 P precipitates with Al or Fe ⇒ not available for Plant Special attention to Phosphorus limiting factor of plant growth. Apatite contains phosphorus (only primary mineral which contains Phosphorus). Under carbonation weathering releasing of P Ca5 ( PO4 ) 3 OH + 4 H 2 CO3 ↔ 5Ca 2 + + 3HPO 42 − + 4 HCO3− + H 2 O Phosphoric acid: pH>7.5 P precipitates with Ca ⇒ not available for Plant Special attention to Phosphorus Apatite weathering, P loss and P forms in time during soil development (example from New Zealand) Special attention to Phosphorus Apatite weathers rapidly: decline of total P by stream loss Phosphorus held on the surface of soil minerals (AAC) With time P accumulates in occluded forms (unavailable for organisms) Plant available P Apatite weathering, P loss and P forms in time during soil development (example from New Zealand) Accelerating processes of soil development in region of warm humid climate. www.komar.de Vegetation and microbiological activities in the soil accelerate chemical weathering by CO2 release and production of organic acids. Even though carbonate acid is main driver for chemical weathering on a global scale: Vegetation and soil organisms produce and release a number of other (organic) acids: Acetic and citric acid from vegetation Phenolic acid ( e.g. tannins) during litter decomposition Fulvic and humic acid from soil microbes Oxalic acid by fungies Often these acids dominate the acidity of the upper soil profile. Acetic and citric acid from vegetation Fulvic and humic acid from soil microbes Weather biotite mica; releasing potassion (K). Biotite mica K(Mg,Fe)3AlSi3O10(F,OH)2 Fulvic and humic acid from soil microbes Weather minerals which contain elements such as phosphorus (P). Ca5 (PO4 )3 OH Apatite Additionally organic acids speed up weathering reaction in the soil by combining with some weathering products and transportation into lower soil layers. Chelation: no equilibrium between dissolved products and mineral forms Example: Fe and Al combine with fulvic acid which make them mobile. They get transported by percolation. Fe+ solution percolation and following precipitation as Fe2O3 visible by brown horizon. Different impact of organic acid on soil weathering depending on climate Cool temperate climate: slow and incomplete decomposition of litter => More organic acid occurs which dominates weathering processes Different impact of organic acid on soil weathering depending on climate Warm humid climate: fast and complete decomposition of litter => Organic acid is restricted, carbonate acid dominates the weathering processes Weathering produces cations CaCO3 + H2CO3 → Ca 2+ + 2HCO34 FeS 2 + 8 H 2 O + 15O2 → 2 Fe2 O3 + 16 H + + 8SO42− 2NaAlSi3O8 + 2H2CO3 + 9H2O → - 2Na+ + 2HCO3 + 4H4SiO4+ Al2Si2O5 (OH) 4 Mainly all cations go over in solution and will be washed out by perculating soil water through the soil if not used by vegetation. Exceptions: Al and Fe are insoluble if not in chelation, accumulate as oxide in the soil.