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The most commonly observed is the oxidation of Fe 2+ by oxygen and water to form Fe 3+ oxides and hydroxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock.
Mineral alteration refers to the various natural processes that alter a mineral's chemical composition or crystallography. [1]Mineral alteration is essentially governed by the laws of thermodynamics related to energy conservation, relevant to environmental conditions, often in presence of catalysts, the most common and influential being water (H 2 O).
The redox state of a rock affects the relative proportions of the oxidation states of these elements and hence may determine both the minerals present and their compositions. If a rock contains pure minerals that constitute a redox buffer, then the oxygen fugacity of equilibration is defined by one of the curves in the accompanying fugacity ...
Banded iron formation from the Barberton Greenstone Belt, South Africa. A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe 3 O 4) or hematite (Fe 2 O 3), alternating with bands of iron-poor chert, often red in color, of similar thickness.
For example, oxidation of manganese in surface rocks by atmospheric oxygen leads to further reactions that oxidize chromium. The heavier 53 Cr is oxidized preferentially over the lighter 52 Cr, and the soluble oxidized chromium carried into the ocean shows this enhancement of the heavier isotope.
The important sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (), being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. [1]
Some examples of minerals in iron-rich rocks containing oxides are limonite, hematite, and magnetite. An example of a mineral in iron-rich rock containing carbonates is siderite and an example of minerals in an iron-rich rock containing silicate is chamosite. [2] They are often interbedded with limestones, shales, and fine-grained sandstones.
The mineralogical and chemical compositions of laterites are dependent on their parent rocks. [6]: 6 Laterites consist mainly of quartz, zircon, and oxides of titanium, iron, tin, aluminum and manganese, which remain during the course of weathering. [6]: 7 Quartz is the most abundant relic mineral from the parent rock. [6]: 7