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 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 H2O shell, soluble in H2O (Na, Ca, etc) Z/R=3-~9.5 ion so strongly attracts H2O that insoluble oxides/hydroxides form (Al, Fe) Z/R=>~9.5 soluble oxyanions form (S, C, etc.) Ionic potential of important elements •Red arrow indicates decreasing attaction to H2O within a group of elements •Decreasing attraction is reflected in weathering losses… Element loss varies with ionic potential N 1.5 Ti group C I Br ) Alkali metals and alkaline earths Zr /crust0.5 Zr S Zr Ti Li Fe Si Al P Cl log (soil -0.5 Rb Sr Tm Dg Yb Nd Lu Eu Tb Cs Pr Sm Gd Hf Ba Em Ce Ho Th U KCa Mg F -1 Na Be -1.5 20 40 60 atomic number 80 100 Mineral Particle Size and Mineralogy Gravel > 2mm (primary) Sand >= 0.05 to 2.0 (primary) Silt [...]...Other secondary mineral groups: oxides Al oxides (gibbsite) •Results of vigorous chemical weathering (desilication) Non-silicate secondary minerals: oxides Fe oxides 1 Geothite • Yellowish brown • Acidic, OM-rich envir 2 Hematite • Bright red • Warm, dry environments Non-silicate secondary minerals: carbonates Calcite •Ca is released from some weathering... 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) Secondary Minerals in California Soils: Sierra Nevada Soil Mineralogy vs Elevation