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QAPF diagrams are not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites). Instead, an alternate triangle plot diagram is used; (see Streckeisen diagram, lower right.) An exact name can be given only if the mineralogical composition is established, which cannot be determined in the field.
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions (or equivalently, oxygen fugacities) at which a rock forms and evolves can be important for interpreting the rock history.
Normative mineralogy is an estimate of the mineralogy of the rock. It usually differs from the visually observable mineralogy, at least as much as the types of mineral species, especially amongst the ferromagnesian minerals and feldspars, where it is possible to have many solid solution series of minerals, or minerals with similar Fe and Mg ratios substituting, especially with water (e.g ...
The TAS classification cannot be applied to all volcanic rocks—as discussed in detail by Le Maitre et al (2002). Certain rocks cannot be classified/named using the diagram. For others, additional chemical, mineralogic, or textural criteria must be used, as e.g. lamprophyres.
A "quadrangle" is a topographic map produced by the United States Geological Survey (USGS) covering the United States. The maps are usually named after local physiographic features. The shorthand " quad " is also used, especially with the name of the map; for example, "the Ranger Creek, Texas quad".
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The structure of gibbsite is interesting and analogous to the basic structure of the micas.The basic structure forms stacked sheets of linked octahedra.Each octahedron is composed of an aluminium ion bonded to six hydroxide groups, and each hydroxide group is shared by two aluminium octahedra. [4]
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