Processos de forma o de rochas

1. 03/09/2012 Silvia F. de M. Figueir�a Processos de forma��o de rochas GM 861 - Mineralogia

4. O que � uma rocha? Rochas s�o agregados naturais multigranulares, uni ou poliminer�licos � isto �, constitu�dos de um �nico tipo ou de v�rios tipos de minerais. As rochas costumam ser analisadas e classificadas segundo o crit�rio gen�tico (isto �, de origem), que por sua vez est� relacionado aos diferentes ambientes de forma��o.

6. 03/09/2012 Silvia F. de M. Figueir�a Distribui��o e abund�ncia Em extens�o, as rochas sedimentares predominam, mas em volume s�o as rochas �gneas e metam�rficas as mais abundantes.

9. 03/09/2012 Silvia F. de M. Figueir�a MAGMA % de SiO2 � vari�vel e interfere nas caracter�sticas e no comportamento do magma: > % SiO2 ? > polimeriza��o > % SiO2 ? > viscosidade > % SiO2 ? < mobilidade

10. 03/09/2012 Silvia F. de M. Figueir�a A polimeriza��o tamb�m � influenciada pelos c�tions presentes em maior quantidade: N.C. 6 N.C. 8 N.C. 12 Fe Mg Ca Li Na K ? ? ? ? ? ? > polimeriza��o MAGMA

11. A viscosidade do magma depende: Temperatura: > T� ? < Viscosidade Composi��o: > SiO2 ? > Viscosidade Vol�teis: > vol�teis (rompem as liga��es Si- O e Al-O) ? < Viscosidade Press�o: > P ? > Viscosidade Mantendo-se o Vol magma constante, se P e T� aumentam simultaneamente ? < viscosidade (? T� � mais influente) MAGMA

15. 03/09/2012 Silvia F. de M. Figueir�a Mineral�gica: basicamente est�o presentes 6 grupos mineral�gicos (silicatos) - feldspatos, quartzo, olivinas, pirox�nios, anfib�lios, micas, com quantidades subordinadas de magnetita, ilmenita e apatita. A % dos minerais essenciais � o crit�rio b�sico para classifica��o das rochas �gneas. ROCHAS �GNEAS: CLASSIFICA��O

16. Bowen's Reaction Series has two branches. They are: Discontinuous reaction series, from olivine to biotite,. Continuous reaction series, from Ca plagioclase to Na plagioclase. The discontinuous reaction series involves the dark-colored ferromagnesian minerals: olivine pyroxene amphibole biotite. As a magma cools, olivine crystallizes first. The olivine crystals react with the remaining magma to form pyroxene. Pyroxene reacts with the magma to form amphibole. Amphibole reacts with the magma to form biotite. Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure. As crystallization proceeds, the crystal structures become more complex (olivine has an isolated tetrahedral structure, pyroxene has a single chain structure, amphibole has a double chain structure, and biotite has a sheet structure). The series of minerals is called discontinuous because a series of different minerals is formed, each with a different crystal structure. The continuous reaction series involves the plagioclase feldspars. Plagioclase feldspars are an example of a "solid solution series", exhibiting gradations in chemical and physical properties. Chemically, this series consists of two "end members": albite or Na plagioclase (NaAlSi3O8), the sodium "end member", and anorthite or Ca plagioclase (CaAlSi2O8), the calcium "end member". There is a continuous chemical and physical gradation between the two end members. (Various plagioclase mineral names are given, based on the percentages of calcium and sodium present, including anorthite, bytownite, labradorite, andesine, oligoclase, and albite). Ca-plagioclase is the first to crystallize. It reacts with the melt to become more sodium rich. (If reaction is not complete, a zoned plagioclase crystal results which has a calcium-rich center and sodium-rich edges). This series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure. The minerals differ primarily in the proportions of calcium and sodium present. During the last stages of crystallization, potassium feldspar (KAlSi308) crystallizes. Muscovite may also form. If the remaining melt contains excess silica, quartz will crystallize. Bowen's Reaction Series helps us to understand why certain minerals tend to occur together in igneous rocks. For example, the mafic rocks, basalt and gabbro tend to contain olivine, pyroxene, and calcium-rich plagioclase feldspar. These are all minerals which crystallize at high temperatures. As another example, felsic or sialic rocks such as granite and rhyolite tend to contain quartz, potassium feldspar, sodium-rich plagioclase feldspar, and sometimes muscovite. These are minerals which crystallize at lower temperatures. The minerals that ultimately form are controlled by the initial composition of the magma. Bowen's Reaction Series also helps us to understand why certain minerals do NOT occur together in igneous rocks. For example, olivine and quartz are unlikely to occur in the same igneous rock, because olivine is a high temperature mineral, and quartz is a low temperature mineral. Bowen determined that specific minerals form at specific temperatures as a magma cools. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches (see below). The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium rich. The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. The remarkable thing that Bowen found concerned the discontinuous branch. At a certain temperature a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to amphibole, and then to biotite. Magmatic Differentiation: With this term we describe the process of separating the magma into several batches (separating crystals from melt) as it evolves and migrates upwards in the earth's crust. E.g. we might early on separate olivine and pyroxene (crystal mush), and would thus create a peridotite (more mafic than basalt). The remaining melt would crystallize pyroxene, hornblende, and plagioclase with intermediate Ca content (andesine), and might erupt from a volcano and form andesite. Or, if the material remained buried, the crystal fraction would form a diorite from which in turn a granitic melt would rise. The parental magma that Bowen envisioned was a "primitive" basalt with high Fe and Mg contents and fairly low SiO2 contents. This gradual change in mineral composition during differentiation is accompanied by a change in color (mafic rocks dark, felsic rocks light) and density of the respective rocks, and is the basis for most of the accepted classification schemes of igneous rocks. The original composition of the magma of course determines the variety of igneous rocks we can derive from it. An originally dioritic/andesitic magma for example could never give rise to a basalt, but may give rise to a granite. A magma may be modified by mixing with another magma that is further or less far evolved, and then rocks of unusual mineral composition may form. Another way in which a magma can be modified is by assimilation of wall rock during its rise through the crust. Bowen's Reaction Series has two branches. They are: Discontinuous reaction series, from olivine to biotite,. Continuous reaction series, from Ca plagioclase to Na plagioclase. The discontinuous reaction series involves the dark-colored ferromagnesian minerals: olivine pyroxene amphibole biotite. As a magma cools, olivine crystallizes first. The olivine crystals react with the remaining magma to form pyroxene. Pyroxene reacts with the magma to form amphibole. Amphibole reacts with the magma to form biotite. Each successive mineral, from olivine to biotite, has a different composition and a different silicate crystal structure. As crystallization proceeds, the crystal structures become more complex (olivine has an isolated tetrahedral structure, pyroxene has a single chain structure, amphibole has a double chain structure, and biotite has a sheet structure). The series of minerals is called discontinuous because a series of different minerals is formed, each with a different crystal structure. The continuous reaction series involves the plagioclase feldspars. Plagioclase feldspars are an example of a "solid solution series", exhibiting gradations in chemical and physical properties. Chemically, this series consists of two "end members": albite or Na plagioclase (NaAlSi3O8), the sodium "end member", and anorthite or Ca plagioclase (CaAlSi2O8), the calcium "end member". There is a continuous chemical and physical gradation between the two end members. (Various plagioclase mineral names are given, based on the percentages of calcium and sodium present, including anorthite, bytownite, labradorite, andesine, oligoclase, and albite). Ca-plagioclase is the first to crystallize. It reacts with the melt to become more sodium rich. (If reaction is not complete, a zoned plagioclase crystal results which has a calcium-rich center and sodium-rich edges). This series of plagioclase minerals is called continuous because all of the plagioclase minerals have the same crystal structure. The minerals differ primarily in the proportions of calcium and sodium present. During the last stages of crystallization, potassium feldspar (KAlSi308) crystallizes. Muscovite may also form. If the remaining melt contains excess silica, quartz will crystallize. Bowen's Reaction Series helps us to understand why certain minerals tend to occur together in igneous rocks. For example, the mafic rocks, basalt and gabbro tend to contain olivine, pyroxene, and calcium-rich plagioclase feldspar. These are all minerals which crystallize at high temperatures. As another example, felsic or sialic rocks such as granite and rhyolite tend to contain quartz, potassium feldspar, sodium-rich plagioclase feldspar, and sometimes muscovite. These are minerals which crystallize at lower temperatures. The minerals that ultimately form are controlled by the initial composition of the magma. Bowen's Reaction Series also helps us to understand why certain minerals do NOT occur together in igneous rocks. For example, olivine and quartz are unlikely to occur in the same igneous rock, because olivine is a high temperature mineral, and quartz is a low temperature mineral. Bowen determined that specific minerals form at specific temperatures as a magma cools. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches (see below). The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium rich. The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. The remarkable thing that Bowen found concerned the discontinuous branch. At a certain temperature a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to amphibole, and then to biotite. Magmatic Differentiation: With this term we describe the process of separating the magma into several batches (separating crystals from melt) as it evolves and migrates upwards in the earth's crust. E.g. we might early on separate olivine and pyroxene (crystal mush), and would thus create a peridotite (more mafic than basalt). The remaining melt would crystallize pyroxene, hornblende, and plagioclase with intermediate Ca content (andesine), and might erupt from a volcano and form andesite. Or, if the material remained buried, the crystal fraction would form a diorite from which in turn a granitic melt would rise. The parental magma that Bowen envisioned was a "primitive" basalt with high Fe and Mg contents and fairly low SiO2 contents. This gradual change in mineral composition during differentiation is accompanied by a change in color (mafic rocks dark, felsic rocks light) and density of the respective rocks, and is the basis for most of the accepted classification schemes of igneous rocks. The original composition of the magma of course determines the variety of igneous rocks we can derive from it. An originally dioritic/andesitic magma for example could never give rise to a basalt, but may give rise to a granite. A magma may be modified by mixing with another magma that is further or less far evolved, and then rocks of unusual mineral composition may form. Another way in which a magma can be modified is by assimilation of wall rock during its rise through the crust.

17. The reason for this "stepped" evolution of minerals is that with dropping temperature we have decreasing thermal vibration of molecules, and that allows silica to form more complex structures.� Thus, olivine with its isolated silica tetrahedrons forms at the highest temperatures, and as temperatures drop silica tetrahedrons first manage to join together in chains (pyroxenes), then in ribbons (amphiboles), and then sheets (micas).�� Finally, at the lowest temperatures the two branches merge and we get the minerals that are common to felsic rocks - muscovite mica, orthoclase feldspar,and quartz (3D frameworks).The reason for this "stepped" evolution of minerals is that with dropping temperature we have decreasing thermal vibration of molecules, and that allows silica to form more complex structures.� Thus, olivine with its isolated silica tetrahedrons forms at the highest temperatures, and as temperatures drop silica tetrahedrons first manage to join together in chains (pyroxenes), then in ribbons (amphiboles), and then sheets (micas).�� Finally, at the lowest temperatures the two branches merge and we get the minerals that are common to felsic rocks - muscovite mica, orthoclase feldspar,and quartz (3D frameworks).

18. This diagram shows the main groups of igneous rocks, their main mineral constituents and their intrusive (cooling in the crust) and extrusive (cooling as lava flow) equivalents.� For example: granitic magmas solidify to granite if they cool in the crust (intrusive), but are called rhyolites if they cool down after they reach the Earth's surface as lava flows (extrusive).� Both, rhyolites and granites, are composed of K-feldspar, Quartz, Sodium Plagioclase, and Biotite.� Peridotite is the name for rocks of the upper mantle, and Komatiite is the name for extrusive lavas that are essentially of Peridotite composition.� The latter are found primarily in very old rocks (Archean) that formed soon after the formation of the first crust (crust was thin, very mobile, and convection was vigorous). This diagram shows the main groups of igneous rocks, their main mineral constituents and their intrusive (cooling in the crust) and extrusive (cooling as lava flow) equivalents.� For example: granitic magmas solidify to granite if they cool in the crust (intrusive), but are called rhyolites if they cool down after they reach the Earth's surface as lava flows (extrusive).� Both, rhyolites and granites, are composed of K-feldspar, Quartz, Sodium Plagioclase, and Biotite.� Peridotite is the name for rocks of the upper mantle, and Komatiite is the name for extrusive lavas that are essentially of Peridotite composition.� The latter are found primarily in very old rocks (Archean) that formed soon after the formation of the first crust (crust was thin, very mobile, and convection was vigorous).

19. 03/09/2012 Silvia F. de M. Figueir�a Composi��o qu�mica de rochas �gneas (alguns exemplos � Nockolds, 1954)

20. 03/09/2012 Silvia F. de M. Figueir�a Em termos de composi��o qu�mica, o crit�rio fundamental � a % SiO2 (essa varia��o se reflete quase que diretamente na colora��o das rochas): > 65% = �cidas < 65% e > 55% = intermedi�rias < 55% e > 45% = b�sicas < 45% = ultrab�sicas ROCHAS �GNEAS: CLASSIFICA��O Quest�es: Porque na S�rie de Bowen que representa a sequ�ncia de cristaliza��o de minerais a partir de um magma, a olivina e o quartzo provavelmente n�o ir�o coexistir em uma mesma rocha �gnea? R. Olivina quartzo n�o devem ocorrer na mesma rocha �gnea pois apresentam temperaturas de fus�o muito difirentes. A olivina na sequ�ncia de cristaliza��o magm�tica � um mineral de alta temperatura, enquanto o quartzo tende a formar-se a T mais baixas.Quest�es: Porque na S�rie de Bowen que representa a sequ�ncia de cristaliza��o de minerais a partir de um magma, a olivina e o quartzo provavelmente n�o ir�o coexistir em uma mesma rocha �gnea? R. Olivina quartzo n�o devem ocorrer na mesma rocha �gnea pois apresentam temperaturas de fus�o muito difirentes. A olivina na sequ�ncia de cristaliza��o magm�tica � um mineral de alta temperatura, enquanto o quartzo tende a formar-se a T mais baixas.