1. Mineral Weathering and Secondary Mineral Formation weathering: chemical alteration of minerals (in soils, involves water, gases, acids, etc). Parent material soil Desilication via weathering Parent Material=primary silicates formed from igneous/metamorphic processes Soil= secondary silicates, oxides, carbonates, etc.formed from weathering processes

2. Behavior of Elements During Chemical Weathering Soils are depleted in elements relative to parent material Element loss/depletion is determined by elements position on periodic table (which column or group of columns) AND the element’s ionic potential Z/R = ionic potential z=charge, r=radius Classes: Z/R= 0-3 ion surrounded by H 2 O shell, soluble in H 2 O (Na, Ca, etc) Z/R=3-~9.5ion so strongly attracts H 2 O that insoluble oxides/hydroxides form (Al, Fe) Z/R=>~9.5 soluble oxyanions form (S, C, etc.)

4. Ionic potential of important elements Red arrow indicates decreasing attaction to H2O within a group of elements Decreasing attraction is reflected in weathering losses…..

5. Element loss varies with ionic potential Alkali metals and alkaline earths Ti group

6. Mineral Particle Size and Mineralogy Gravel > 2mm (primary) Sand >= 0.05 to 2.0 (primary) Silt <0.05 to 0.002 (primary + secondary) Clay < 0.002 (secondary) Most secondary mineral are silicates, and most secondary silicates are phyllosilicates.

9. 1:1 phyllosilicates: kaolinite One layer of Si tetrahedra One layer of Al octahedra Individual minerals are held to another via H bonds

10. 2:1 Phyllosilicates: di and trioctahedral Dioctahedral (smectites) Substitution of +2 for +3 in octahedral layer (called isomorphous substitution) Creates a net negative charge (and property of cation exchange capacity) Results in expandable layers Trioctahedral (vermiculite) Substitution of +3 for +4 in tetrahedral layer Also has CEC, but little or no expansion

11. Other secondary mineral groups: oxides Al oxides (gibbsite) Results of vigorous chemical weathering (desilication)

12. Non-silicate secondary minerals: oxides Fe oxides 1. Geothite Yellowish brown Acidic, OM-rich envir. 2. Hematite Bright red Warm, dry environments

13. Non-silicate secondary minerals: carbonates Calcite Ca is released from some weathering source Forms in arid to semi- arid environments when soil solution becomes saturated Presence in upper 1m related to MAP Depth of carbonate layer related to MAP

14. Geographical distribution related to climate Greater than 100cm/yr removes carbonate Below 100cm, depth~MAP

15. Non-silicates: sulfates (gypsum) Presence of sulfates in soils usually occurs in hyperarid climates (or sites with high water table and evaporative enrichment of salts)

16. Secondary Minerals in California Soils: Sierra Nevada

17. Soil Mineralogy vs. Elevation