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Most rock forms at elevated temperature and pressure, and the minerals making up the rock are often chemically unstable in the relatively cool, wet, and oxidizing conditions typical of the Earth's surface. Chemical weathering takes place when water, oxygen, carbon dioxide, and other chemical substances react with rock to change its composition.
This order is controlled by the temperature of the melt and its composition. Because earlier crystallizing minerals are more stable at higher temperatures and pressures, these weather the fastest under surface conditions. Saponite is a common weathering product of ultramafic and mafic rocks.
It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify the rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ; in that context, basalt is defined as volcanic rock with a content of between 45% ...
The proportion of silica in rocks and minerals is a major factor in determining their names and properties. [7] Rock outcrop along a mountain creek near Orosí, Costa Rica. Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size.
Due to incomplete replacement of the earlier formed minerals under changing P-T conditions, [16] minerals formed at various P-T environments can be found in the same rock specimen. [ 16 ] [ 1 ] As different minerals have different optical characteristics and textures, determination of the mineral compositions in metamorphic rocks is made possible.
In sedimentary geology, maturity describes the composition and texture of grains in clastic rocks, most typically sandstones, resulting from different amounts of sediment transportation. A sediment is mature when the grains in a sediment become well-sorted and well-rounded due to weathering or abrasion of the
The other minerals in the rock are termed accessory minerals, and do not greatly affect the bulk composition of the rock. Rocks can also be composed entirely of non-mineral material; coal is a sedimentary rock composed primarily of organically derived carbon. [34] [38]
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. [ 1 ]