Mineral Weathering and Secondary Mineral Formation weathering: chemical alteration of minerals (in soils, involves water, gases, acids, etc). Parent material.

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Presentation transcript:

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 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.)

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 Alkali metals and alkaline earths Ti group

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

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

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

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 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

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

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)

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