A rock is an assemblage of mineral grains.....
Silicate minerals are the most important mineral class because they are by far the most abundant rock-forming minerals. This group is based on the silica (SiO4) tetrahedron structure, in which a silicon atom is covalently bonded to 4 oxygen atoms at the corners of a triangular pyramid shape.
Each tetrahedron may be isolated from one another or they may be bonded together covalently by sharing oxygen atoms between adjacent tetrahedra. In this way they may form single chains, double chains, sheets, and 3-dimensional networks of interlocking tetrahedra.
Each of these covalently bonded structural groups (except framework) is bonded to its neighboring structural group (e.g. single chain to single chain) by ionic bonds with intervening cations (K+, Na+, Ca2+, Mg2+, Fe2+, etc.).
The silicate minerals vary in the amount of silica (silicon & oxygen) vs. metal cations. The more silica the fewer cations and vice versa. The more silica, the more covalent bonding and the greater the stability to chemical weathering. The more cations and ionic bonding, the lower the weathering stability.
Quartz (framework), feldspars (framework), and muscovite mica (sheet) are light colored silicate minerals with very high to high weathering stability due to their abundance of silica and therefore covalent bonds.
Biotite mica (sheet), amphibole (double chain) (e.g., hornblende), and pyroxenes (e.g., augite) (single chain) are dark colored, iron and magnesium cation rich, silicate minerals with low to very low weathering stability.
Igneous rocks crystallize from a cooling silicate melt (magma or lava). They are composed of intergrown crystals of minerals such as quartz, feldspars, micas, amphiboles, pyroxenes, and olivine.
Felsic igneous rocks are silica rich and contain abundant potassium feldspar and quartz. Felsic igneous rocks are light in color. Mafic igneous rocks are iron and magnesium rich and contain abundant pyroxene, and calcium rich plagioclase feldspar. Mafic igneous rocks are dark or black in color.
Extrusive, or volcanic, igneous rocks cool and crystallize quickly at the Earth's surface resulting in very small, microscopic crystals. Intrusive, or plutonic, igneous rocks cool and crystallize very slowly beneath the Earth's surface allowing large, visible crystals to grow. Obsidian, volcanic glass forms where lava cools so quickly (e.g., due to quenching in water) that crystals with their orderly arrangement of atoms do not have enough time to form.
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Sedimentary rocks are formed from sediments derived from the breakdown of pre-existing rocks (of any kind). First, pre-existing rocks are weathered, yielding sediments such as gravel, quartz sand and silt, clays, and dissolved ions such as calcium, iron, silica, and sodium. These sediments are transported, usually by flowing water. Eventually the different sediments will be deposited as layer upon layer of horizontal strata. As the weight of overlying layers increases the lower-lying sediments become compacted. Finally, cements such as calcite (calcium carbonate), hematite (iron oxide), and silica are precipitated from the dissolved ions in the water forming coatings around the sediment grains that stick them together thereby forming a rock.
common sedimentary rocks
Silicastic sedimentary rocks are composed principally of the solid weathering products of preexisting silicate rocks (predominantly clays and quartz). Sedimentary rock formed from clay-size particles (too small to be seen with a light microscope) are called shale. Rock formed from silt-size particles (visible under a microscope) are called siltstone. Sand-sized grains (visible to the naked eye) range from 1/16 mm to 2 mm. Rock formed from these are called sandstone. Sedimentary rock containing "gravel"-sized grains ranging from > 2 mm granules to very large boulders are called conglomerate.
Limestone, a carbonate rock, is formed from the biogenic or physical precipitation of calcite (CaCO3) directly from calcium and bicarbonate ions dissolved in water solution. The calcite is most commonly formed as shells of marine organisms of visible or microscopic size. Much limestone has apparently been altered over the ages into a related carbonate rock dolomite, which is a calcium magnesium carbonate (Ca Mg(CO3)2).
Metamorphic rocks form when some pre-existing rock is exposed to very high pressures and/or temperatures, for example when shallow crustal rocks are deeply buried in a continent-continent collision zone. There the rock re-crystallizes very slowly while remaining in the solid state, forming new minerals that are stable under the high temperature and pressure conditions. If there are platy or elongate minerals in the resulting metamorphic rock they will show a preferred alignment or foliation.
Foliated Metamorphic Rocks
Non-foliated Metamorphic Rocks
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