If we examine fresh (unweathered) outcrop in a road cut, rock often appears hard and shiny. Examination with a hand lens reveals that minerals have well-defined boundaries and generally sharp outlines. They may show good cleavage or crystal faces. Minerals may have their normal diagnostic colors: quartz is clear, feldspars are white or pink, muscovite is silvery and sparkly, magnetite appears metallic, and biotite and other mafic minerals appear black.
The picture is not the same if we examine outcrops exposed to weathering for a long time. After weathering, rock and most minerals have a dull or drab appearance. Grain boundaries and cleavages are obscured. Oxidation (rusting) and hydration may produce reddish, yellow, brown, or gray hues. Sometimes a layer of clay or other material coats all surfaces, obscuring diagnostic minerals. The photo shown here (Figure 7.6) shows granite that is being replaced by kaolinite (a clay) during weathering. The once solid crystalline granite is now a dull earthy mass.
Mafic silicates weather to create secondary clay minerals and iron oxides. Feldspars of all sorts weather to become clay minerals and dissolved material. Quartz is usually unchanged by weathering. Calcite weathers by dissolution producing dissolve ions. And aluminous minerals weather to gibbsite or other aluminum hydroxides. The table below lists weathering products for the most common minerals. Clays and limonite (a general term describing for a mix of hydrated Fe-oxides and hydroxides) dominate the list. Quartz and aluminous minerals are quite common too. While creating these secondary minerals, weathering also produces dissolved cations (especially alkalis and alkali earths) and anions, which may have a significant impact on water chemistry and quality.
|mineral (composition)||residual minerals||dissolved ions|
|halite (Na chloride)||Na+, Cl–|
|gypsum (hydrated Ca sulfate)||Ca2+, (SO4)2-|
|calcite (Ca carbonate)||Ca2+, (HCO3)–|
|plagioclase (Ca-Na-Al silicate)||clay minerals||Ca2+, Na+, (SiO4)4-|
|alkali feldspar (K-Na-Al silicate)||clay minerals||K+, Na+, (SiO4)4-|
|olivine (Fe-Mg silicate)||limonite, hematite, clay||Mg2+, (SiO4)4-|
|pyroxene (Ca-Fe-Mg silicate)||limonite, hematite, clay||Ca2+, Mg2+, (SiO4)4-|
|amphibole (Ca-Fe-Mg silicate)||limonite, hematite, clay||K+, Mg2+, (SiO4)4-|
|biotite (K-Fe-Mg-Al mica)||limonite, hematite, clay||K+, Mg2+, (SiO4)4-|
|muscovite (K-Al mica)||clay||K+|