Quartzite is sandstone that has been metamorphosed. Unlike sandstone, quartzite breaks through, not around, the quartz grains, producing a smooth surface instead of a rough and granular one. Quartzites are snowy white, less often pink or gray. They yield a thin and very barren soil and, because they weather slowly, tend to project as hills or mountain masses. Many prominent ridges in the Appalachian Mountains are composed of highly resistant tilted beds of quartzite.
The term quartzite implies not only a high degree of hardening (induration), or "welding," but also a high content of quartz. Most quartzites contain 90 percent or more quartz, but some contain 99 percent and are the largest and purest concentrations of silica in the Earth's crust. Pure quartzites are a source of silica for metallurgical purposes and for the manufacture of brick. Quartzite is also quarried for paving blocks, riprap, crushed stone, railroad ballast, and roofing granules.
In microscopic section the clastic structure of some quartzites is well preserved; the rounded sand grains are seen with quartz overgrowths deposited in crystalline continuity, so that optical properties of the grains are similar to those of the material surrounding them. In some cases a line of iron oxides may indicate the boundary of the original sand grain. Many quartzites, however, have been crushed, and the quartz largely is a mosaic of small, irregularly shaped crystalline fragments with interlocking margins.
Sandstones turn into quartzite in two different ways. The first way involves low pressure and temperature, where circulating fluids fill the spaces between sand grains with silica cement. When this rock is broken, the fractures go right through the original grains, not around them. This kind of quartzite, orthoquartzite, is not strictly speaking a metamorphic rock because the original mineral grains are still there, and bedding planes and other sedimentary structures are still evident.
Under the high pressures and temperatures of deep burial, the mineral grains recrystallize and all traces of the original sediments are erased. The result is a true metamorphic rock, called metaquartzite. This boulder is probably a metaquartzite.
Quartzite is a very strong stone but is difficult to work. Because quartzite has a limited color range and is not often found in large bodies suited for quarrying, the building industry prefers granite for demanding applications.
METAMORPHIC ROCKS GENERAL
Metamorphic rocks result from mineralogical and structural adjustments of solid rocks to physical and chemical conditions differing from those under which the rocks originally formed. Changes produced by surface conditions such as compaction are usually excluded. The most important agents of metamorphism are temperature, and pressure. Equally as significant are changes in chemical environment that result in chemical recrystallization where a mineral assemblage becomes out of equilibrium due to temperature and pressure changes and a new mineral assemblage forms.
Three types of metamorphism may occur depending on the relative effect of mechanical and chemical changes. Dynamic metamorphism, or cataclasis, results mainly from mechanical deformation with little long-term temperature change. Textures produced by such adjustments range from breccias composed of angular, shattered rock fragments to very fine-grained, granulated or powdered rocks with obvious foliation and lineation termed mylonites. Contact metamorphism occurs primarily as a consequence of increases in temperature where differential stress is minor.
A common phenomenon is the effect produced adjacent to igneous intrusions where several metamorphic zones represented by changing mineral assemblages reflect the temperature gradient from the high-temperature intrusion to the low-temperature host rocks; these zones are concentric to the intrusion. Because the volume affected is small, the pressure is near constant. Resulting rocks have equidimensional grains because of a lack of stress and are usually fine-grained due to the short duration of metamorphism. Regional metamorphism results from the general increase of temperature and pressure over a large area. Grades or intensities of metamorphism are represented by different mineral assemblages. Regional metamorphism can be subdivided into different pressure-temperature conditions based on observed sequences of mineral assemblages. It may include an extreme condition, where partial melting occurs, called anatexis.
Other types of metamorphism can occur. They are retrograde metamorphism, the response of mineral assemblages to decreasing temperature and pressure; metasomatism, the metamorphism that includes the addition or subtraction of components from the original assemblage; poly-metamorphism, the effect of more than one metamorphic event; and hydrothermal metamorphism, the changes that occur in the presence of water at high temperature and pressure which affect the resulting mineralogy and rate of reaction.
METAMORPHIC ROCKS MORE DETAILS
Metamorphic rock are the result of the transformation of a pre-existing rock type, the protolith, in a process is called metamorphism. The nature of the protolith can be either sedimentary, igneous or older metamorphic rocks.
Metamorphic rocks can be classified according texture or mineral assembly (metamorphic facies).
Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i.e. without melting, in response to new conditions of pressure and/or temperature, and/or introduction of fluids.
Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.
LIMITS OF METAMORPHISM
The temperature lower limit of metamorphism is considered to be between 100-150°C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic. But, see below, some types of metamorphism can occur at extremely lower pressures.
The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The temperature interval is between 700-900°C, with pressures that depend on the composition of the rock. Migmatites are rocks formed on this borderline. They present both melting and solid-state features.
KINDS OF METAMORPHISM
This type of metamorphism occurs over broad areas of the Earth's crust. Regionally metamorphosed rocks are originated in the core of mountain belts, formed during an orogenic event. These mountains are later eroded exposing the metamorphic rocks. Regional metamorphic rocks are usually strongly deformed. Structural geology is largely based in studies of these rocks because they contain useful information. Regional metamorphism can be described metamorphic zones. It can also be separated into Barrovian- or Buchan-type metamorphism, according to the pressure-temperature gradients recorded in the rocks.
Contact metamorphism occurs typically around igneous intrusive rocks, as a result of the temperature increased caused by the igneous body. Pressures are usually low because the contrasting temperature effect is more effective at shallow crustal depths. The area surrounding the igneous rock where the contact metamorphism effects are present is called metamorphic aureole. As expected, the contact metamorphism effects are greater in the vicinity of the intrusive rock and fade away to the exterior of the aureole. Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Rocks formed by contact metamorphism do not present signs of strong deformation and are usually fine grained. Contact metamorphic rocks are usually known as hornfels. Skarns are another example of contact metamorphism and can have great economic interest.
Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of distinct composition. The difference in composition between protolith and fluid triggers a set of methamorphic reactions. This kind of metamorphism is responsible for many economic metal deposits. Convection circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism.
This knd of metamorphism occurs when an extraterrestrial object (a meteorite for instance) collides with the Earth's surface, or, during an extremely violent volcanic eruption. Impact metamorphism is, therefore, chracterised by ultrahigh pressures conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite) and textures are characteristic of these conditions.
CLASSIFICATION OF ROCKS