What is the meaning of Petrography? Concept and Definition of Petrography

Definition of petrography


1. Concept of petrography

Petrology is responsible for the origin, the appearance, structure and history of the rocks, in particular of the igneous and the metamorphic. The study of sediments and sedimentary rocks Petrology is known as sedimentary Petrology.

The petrography, related discipline, trafficking in the description and characteristics of crystalline rocks determined by microscopic examination with polarized light. The petrologists study the changes taking place spontaneously in the masses of rock when magma solidifies, when rocks melt totally or partially, or sediments undergo physical or chemical transformations.

Those who work in this field concerned with the crystallization of minerals with the solidification of glass since matter fused at high temperatures (igneous processes) of the recrystallization of minerals at high temperature without the mediation of a molten phase (metamorphic) processes, the exchange of ions between minerals rocks and migratory fluid phases (metasomaticos or diagenetic processes) and sedimentation processesincluding weathering, transportation and deposit.

The rocks are agglomerates of one or more minerals. The processes that give rise to the rocks are different, depending on your location on Earth. As well as vary the values of pressure (P), temperature (T) and composition (X) key in different places of the Earth, also vary the processes that lead to the development of the rocks. By studying the rocks, try to use its chemical, physical and geometrical characteristics to bring us closer to understanding the processes and environments that formed them (Tolson, 2005)

Another definition indicates that you calling any material formed as a natural aggregate formed by one or more mineral rock. An aggregate is a solid cohesive. Rocks are generally formed by several mineralogical species (rocks composed), but there are also rocks consisting of a single mineral (rock monomineralicas). Rocks tend to be hard, but it can also be soft, as happens in the case of argillaceous rocks or sands. In everyday language, but not the scientist, rock also means fragment or block Rocky.

The rock cycle

A human scale, the rocks seem to us to be indestructible, from geological times, however, the rocks are in a continuous and permanent transformation. If we consider an example that for many years, some of the ancient civilizations, considered that the granite was indestructible, was a symbol of eternity. Today we know that granite is broken and eroded by different agents, releasing minerals such as quartz and feldspars, generating sands, clays and other types of rocks.

James Hutton about 200 years ago first proposed the concept of geological cycle (cycle of the rocks), which are considered the relationships that exist between the surface and the interior of the Earth as a cyclical process, through which the different types of rock are generated. The geological cycle diagram illustrates the interaction between sedimentation, sinking, deformation, magmatism, hoist and weathering.

The geological cycle display on first major environments where rocks are formed: magmatic / sedimentary, igneous and metamorphic. Also displays the major processes affecting rocks (Griem and Griem-Klee, 1999).

2. Meaning of petrography

Microscopic observation of thin sections of rocks is an often crucial instrument in petrography. Only when the rock is very coarse-grained and variable mineralogical composition, this type of observation will be incomplete.
In Petrography, the end of the microscopic examination of a rock is the precise determination of their constituent minerals and its structure, i.e., the way these elements are associated. This test requires the appreciation of a number of visible characters, some ordinary or natural light, and others with the help of polarized light.
Transparent bodies, in which the light can be overzealous, are divided into:
• Isotropic or monorrefringentes bodies, in which light propagates with a speed independent of direction. In them the index of refraction is equal in all directions. Belong to this group the amorphous minerals and the crystallized in the regular system.
• Anisotropic bodies or birrefrigentes, in which the speed of light is in them and their direction of propagation function the refractive index varies according to the direction. The mineral crystal systems of crystallization, except the regular system are birefringent.
On microscopic petrography is used the polarizing petrographic microscope whose most important features are the own a lens called the polarizer, between the light source and the preparation, which allows observations with polarized light; have a rotating stage with aiming degree indicator which allows you to turn the preparation and measure the angles that this is taking; and having inserted into the tube, between the objective and the eyepiece, another Analyzer called lens which has a plane of polarization perpendicular to the polarizer, and that may be entered, or not, at will.

Properties of rocks under the microscope.

The vitreous paste, which form part of the rocks, in some cases may have black color, which is called a melanocrata paste either have shades of clear or white colorations, called in this case pasta leucocrata.
Sedimentary rocks are readily appreciates the homogeneous structure of sedimentation in which all the crystals have a very similar size, and even similarly. Within this type of structure the structure stands psammitica in which all crystals are nearly identical in diameter and are small in size.
Different types of structure of igneous rocks are made very patent in the microscopic observation: vitreous structure, in which virtually all the rock mass is vitreous paste, except for some isolated phenocryst or some microlites; porphyritic structure, with absence of vitreous paste and in which small crystals contain phenocrysts equidimensional, either, when the phenocrysts are abundant, the microcrystals appear to simply filling in gaps and interstices; and granular, composed of phenocrysts only structure and presents like variety structure pegmatitic, in which small crystals included within other larger which form the set of rock can be seen.
Metamorphic rocks are identified more by structure, by the shaly texture with a constantly. This type of texture is recognized because, in it, the ferromagnesiano is oriented in the same direction, which can be seen easily without a scanner.
The texture of some escoriacea effusive rocks as and the liparita, is characterized by the presence of many holes in the rock mass.

3. What is petrography

The specialty of geology that is devoted to the description and analysis of the rocks called petrography. It is an area of knowledge that is complementary to the Petrology, whose study object is something different.
The petrography specializes in analyzing the composition of minerals, the structure and the descriptive section of the rocks. The petrography, instead, is focused on the origin of rocks and the relationships that they develop each other.
Arguably, therefore, petrography and Petrology are complemented the development of geology, which is the science dedicated to study the subjects that make up the terrestrial globe and to analyze the way of this.
The petrography, ultimately, works with different minerals forming rocks. According to the conditions and the State of the minerals that is, rocks can be classified as metamorphic (originating in the interior of the planet, being modified by temperature and pressure), sedimentary (created from the collection of the different minerals) or igneous (created from eruptions and processes linked to magma).
Should be noted that the concept of petrography is formed by two Greek terms: petra (which can be translated as rock or stone) and graphos (with reference to the description). Since the name of this specialty, it is clear that the purpose of the petrography is studying the rocks in order to develop a thorough description based on scientific criteria.

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