The Biogeochemistry of Soils: Soils from Stars Composition of soils on earth is arguably unexpected Soils, and Earth, not reflective of chemistry of Universe Soils reflect chemical fractionation processes since beginning of universe: –Big Bang –Subsequent star formation/collapse –Chemical differentiation during formation of solar system –Chemical differentiation during formation of Earth –Late cometary additions to Earth

Chemistry of Solar System Exponential decline in abundance w/ atomic number (number of protons) Sawtooth pattern Elements from Fe have passed through stars Solar system is dominantly H and He

Crust vs. Solar System Depleted in volatiles (as other inner planets) Noble gases (group VIIIA) H, C, N Core formation depleted crust in siderophile elements (group VIIIB..) Crust also reflects late stage cometary additions of light elements, etc. including water

Soil vs. Crust Soil enriched in biochemically impt elements (C, N, S, Se) Soil depleted in alkali and alkine earths, Si, …. Date normalized to a relatively immobile element (Zr)

Methods of (reasons for) Normalization to Index Element

Weathering Losses of Elements from Soils As might be expected, water enriched relative to crust via chemical reactions Relative concentration related to chemical nature of elements and their reactivity in water and type of bonds they form in crust

Plant Composition and Soil Chemistry Plants reflect water chemistry (with some selectivity) and photosynthesis/N fixation

Soil Biogeochemistry Highlights Biological group Alkali/alkaline earths Halogens Rare earths Ti group Si, Al, Fe, P

Soil Mineralogy: Primary Minerals Minerals are associations of elements Mineralogical composition a function of elemental behavior and abundances –O 474,000 mg/kg –Si 277,000 –Al 82,000 –Fe 41,000 –Ca 41,000 –Na 23,000 –Mg 23,000 –K 21,000 Relative abundance and behavior leads to reality that soils are dominated by aluminosilicates (O,Si, Al).

Structure of Silicates Silica tetrahedron –Net charge –Role of Al Covalent bonds (Si-O, Al-O) vs. ionic bonds (cations-O) –Bond type based on electronegativity differences and tendency to attract electrons Big differences lead to ionic bonds Similar electronegativities lead to covalent bonds Linage of tetrahedra dictate classes of silicates and their chemical behavior – Nesosilicates –Inosilicates –Phyllosilicates –Tectosilicates

Electronegativities of the Elements Electonegativities dictated by position on table: elements with outer shells almost filled highly electonegative, those just starting new shell not. Si-O form mainly covalent bond

The Silica Tetrahedron 1 Si, 4 O = -4 net charge Tetrahedra can be linked by sharing O, thereby reducing net negative charge. Class of silicate is determined by number of shared O, and need for cations to neutralize net negative charge

Nesosilicates: Singe Tetrahedra Linked with Cations Foresterite Single tetra linked with Mg +2 Other minerals in group have all Fe +2 Highly susceptable to chemical weathering via ejection of cations by acid (H + ) Products then form secondary silicates and oxides

Inosilicates: Chains Diopside: Single chains Tremolite: Double chains

Phyllosilicates: Sheets Muscovite ‘dioctahedral w/ Al +3 Phlogopite ‘trioctahedral’ w/ Mg +2 K + strongly adsorbed in cavities

Tectosilicates: Framework Anorthite (Ca) 50% Al for Si substition Albite (Na) 25% Al substition Quartz No substition/O charge

Primary Silicate Summary

Mineralogical Composition of Igneous Rocks

Stability of Primary Minerals in Soils Increasing Si/O ratio increases stability –More covalent bonds –Fewer ionic bonds –Less susceptable to acids Decreasing Si/Al ratio reduces stability –Al creates charge imbalance and need for cations Presence of Fe +2 reduces stability –Fe +2 oxidizes to +3 –Size and charge altered and Fe is expelled