Igneous Metamorphic Sedimentary Rocks

July 20, 2021

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Rocks are important components of the soil and form the basis for the formation of soil matter. The study of rocks and the earth in general is known as geology. According to geologists, a rock is defined as combination of minerals, rock fragments, as well as mineraloids. A rock must possess structural integrity and be characterized by binding. The most usual binding agents consist of minerals, as calcite and clay, mineraloids, such as glass and cherts, and others. However, in some rock compartments, no binding agent is required since crystals are inter-grown.

The process of identification and classification of rocks is quite important in the study of rocks, which helps categorize rocks with similar facets into common groups for study and economic purposes. This process is known as petrography, summarizing the features of the rocks that are useful for both present and future. Petrography explores the physical properties of the rocks besides conducting simple tests in the field and in laboratories. Petrography classifies rocks into three major components: sedimentary rocks, formed from compacted eroded particles from other rocks through the actions of minerals and mineraloids of water; metamorphic rocks, which are formed from distortion of existing rocks from either changes in pressure and temperature, while igneous rocks are formed from the cooling and subsequent crystallization of magmatic components on or beneath the earth’s surface (New Zealand & Curriculum Development Centre (Australia), 1980).

Part 1

Chart 1 below represents an igneous rock identification chart.

Composition/

Texture

UltramaficMaficFelsicIntermediate
Pegmatitic

 

Gabbro PegmatiteGranite PegmatiteDiorite Pegmatite
AlphaniticBasaltRhyoliteAndesite
PhaneriticDuniteGabbroGraniteDiorite
PorphyriticBasaltRhyoliteAndesite
GlassyObsidian
Vesicularpumice
PyroclasticVolcanic tuff

Resource: Adopted and modified from (Damm & Thiemeyer, et al. 2012).

The igneous rock is determined from a laboratory analysis of its chemical composition. However, this process may be very expensive and not critically necessary. Indeed, the feature of the rock color indicates the chemical composition of igneous rock. Igneous rock can be identified through its light colors. These include light gray, white, and pink, which indicates the presence of felsic materials, which are richly endowed with silica. On the other hand, the black and dark brown components show the presence of either mafic or ultramafic chemical components and, hence, igneous rock. Essentially, mafic rock is loosely endowed with silica but rich in both iron and magnesium. However, the intermediate rocks portray intermediate color composition, ranging between dark and bright mineral components (Damm et al., 2012).

The Color Index system is therefore the basis upon which igneous rocks can be identified. In exceptional cases, Obsidian rock, which is a rich felsic mineral, has very dark coloration. Additionally, the Dunite, also an igneous rock, is characterized by ultramafic components with olivine minerals and the color ranging from apple green to yellow green. With respect to texture, igneous rock does not display either rough or smooth texture. Instead, the rock can be identified by the size of its crystals. For instance, a pegmatitic texture is characterized by very large crystals, about 2-3 centimeters in width while a phaneritic texture is relatively small but visible. However, glassy igneous rocks are the most easily identified igneous rocks due to their visible glassy appearance. Consequently, the distinguishing feature of igneous rock, in contrast to other rocks, implies that igneous rock is generally composed of ultramafic rocks, while its chemical composition averages at about 35 percent of the feldspars (Damm et al., 2012). Chart 2 below represents the metamorphic rock identification chart.

TextureCompositionFoliationParent rockRock NameType
 

 

 

Foliated Rocks

micaslatyMudstoneSlateRegional
quartz, mica, chloritephylliticMudstonePhylliteRegional
mica, quartzschistoseSlateSchistRegional
Amphibole

plagioclase

schistoseBasalt or GabbroAmphiboliteRegional
feldspar, mica, quartzgneissic bandingSchistGneissRegional
 

 

 

Non-Foliated Rocks

carbonBituminous CoalAnthracite CoalContact or Regional
quartz, rock fragmentsConglomerateMetaconglomerateContact or Regional
calciteLimestoneMarbleContact or Regional
quartzSandstoneQuartziteContact or Regional

Resource: Adopted and modified from (Hefferan & O’Brien, 2010).

The identification and classification of metamorphic rocks is based on the mineral composition of the rocks with respect to the temperature and pressures at which the minerals forms. The identification process involves the analysis of rock texture and composition form a physical point of view. Essentially, metamorphic rocks can be divided into two main categories based on the texture that shows their planar facets. This implies that the mineral components in the rock are compactly aligned against each other with either flat slate or folds for the foliated textures. On the other hand, non-foliated textures of the rock have non-aligned mineral structures with random orientation (Hefferan & O’Brien, 2010).

With respect to foliated rocks, there are four main types of foliation: Slaty cleavage, Schistose, Gneissic banding, and phyllitic foliations. Among the latter, they are differentiated by size of plates with the gneissic banding having relatively large platy minerals and, perhaps, being the easiest to recognize. The non-foliated metamorphic rocks, on the other hand, have a missing planar feature. Furthermore, they may also be identified by composition. For instance, Anthracite coals are similar to bituminous types, both of which are black in appearance. The rock has an interlocking texture and a chemical composition of about 90 percent calcite (Hefferan & O’Brien, 2010).

Finally, in this context, the sedimentary rocks identification (Chart 3 below) can be used in identifying the two main categories of sedimentary rocks.

Inorganically land-formed Sedimentary Rocks
TextureCompositionGrain sizeRemarksName of the Rock
 

 

Fragmental (Classic)

Often feldspar, quartz and clay minerals contain a mixed fragments from other rocksCobbles & pebbles embedment in clay, sands and siltRound particlesConglomerate
Angular particlesBreccias
Silt (about 0.006cm)Very fineSilt Stone
Sand (about 0.2 cm)Fine-CourseSandstone
Clay ( < 0.004cm)Compacted; ease of splittingShale
Chemically Formed Sedimentary Rocks
TextureCompositionGrain sizeRemarksName of the Rock
 

Crystalline

HaliteDifferentiatedCrystals arise from chemical evaporites and precipitatesRock Salt
GypsumDifferentiatedRock gypsum
DolomiteDifferentiatedDolostone
 

Bioclastic

Extremely fine to courseCalciteFirm shell fragments biological precipitatesLimestone
DifferentiatedCarbonflora wastesCoal

Resource: Adopted and modified from (Scholle & American Association of Petroleum Geologists, 2003).

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From the sedimentary rock identification chart above, it is obvious that the sedimentary rocks have interlocking grains with close fittings. Furthermore, the grains are clastic with round appearances resembling billiard balls. Furthermore, due to pressure, there is certain cement-like material that strongly holds the rock materials together. Furthermore, the rocks may be formed of uniform grains or consisting of a mixture of grains with poor assortment. Consequently, sedimentary rocks can be distinctively identified through a clear clastic texture with refined fossil making capacity. Chemically, the rock has more than 90 percent of quartz mineral in clastic texture and about 85 percent calcite (Scholle & American Association of Petroleum Geologists, 2003).

Part 2

Identifying specific Metamorphic, Sedimentary and Igneous Rocks

Igneous rocks emerge from the fast cooling rocks of igneous types. Essentially, granite is regarded as a durable stone of high economic importance through its use in both artistic and architectural designs. According to the American Society for Testing and Materials, granite can be identified from its granular appearance with colors ranging from pink to light grey. Furthermore, granite consists of mineral quartz and feldspar with some traces of dark minerals. However, some components of granite such as basalt, diabase, anorthosite, and gabbro are deficient of quartz and feldspar minerals (Scholle & American Association of Petroleum Geologists, 2003).

Besides that, Granite may also be identified through its composition of mica and pyroxene with occasional horneblend element. Contrary to other rocks, such as calcareous sandstones and limestones, granite is not acid-soluble and resists cleansing agent reactions. Consequently, granite can be identified from its inert properties besides that they portray low deterioration rates and water absorption capacities. Physically, granite is very hard compared to other igneous rocks, such as limestone (Hefferan & O’Brien, 2010).

On the other hand, metamorphic rocks appear in two distinct forms. These are the foliated and non-foliated forms. The foliated metamorphic rocks include Gneiss, while the non-foliated metamorphic rocks include marbles. The process of identification of gneiss involves the scrutiny of physical features with regard to texture and composition that may result in distinct coloration. Gneiss is characterized with light and dark color bands, which indicate the chemical composition of the rock. In particular, the appearance arises from the light pale phaneritic minerals, which lead to appearance in both dark and lightened layers. Basically, the light color bands constitute quartz and feldspar, while the dark bands portray traces of black pyroxene, garnets, and micas (Scholle & American Association of Petroleum Geologists, 2003).

On the other hand, marbles are the non-foliated types of metamorphic rocks. In particular, marbles can be identified by the different color inter-phase. Additionally, majority of calcite minerals contained in marble have dolomite features. However, the marble rock is often colored white and may at times have a blending of green, pink, and purple pigments, making the rock easy to identify by physical approach. As a matter of fact, metamorphic rocks of both foliated and non-foliated origin are compact (Scholle & American Association of Petroleum Geologists, 2003).

Finally, in this context, the sedimentary rocks that can be identified through the processes discussed above include arkose and dolomite. Sedimentary rocks fall into two major categories: clastic, or organic, and chemical sedimentary rocks. In particular, arkose is a chemically-formed sedimentary rock, while dolomite is a chemically-formed sedimentary rock. Arkose rock can be identified from its distinct features, commonly, the physical aspects. In this regard, arkose is identified from its red-colored appearance with considerable amount of feldspar. Furthermore, the rock may contain some traces of rock fragments, which are important identification features. On the other hand, dolomite can be identified from its physical hardness of level three. Additionally, the rock fizzes at the point of scratching by use of a knife. Essentially, dolomite has a zero capacity to form fossils (Scholle & American Association of Petroleum Geologists, 2003).

Conclusion

Finally, the process of rock identification and classification involves a systematic procedure, requiring conciseness and accuracy with respect to descriptions regarding to physical characteristics of the rocks within petrography. Consequently, the geologist employs petrographic analysis in communicating essential characteristics of rocks through writing. Furthermore, petrographic analysis provides summarized information, which is quite essential in the future study of rocks in closed precincts. The process of rock identification should be guided by the availability of sufficient information regarding the features of the rock in order to distinctively classify it in a large pool of differentiated rocks through the use of distinct features of independent rocks.

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In geology, the most important identification procedure involves the physical properties of rocks besides superficial tests that can be effectively carried out in the field. Consequently, this depends on the distinctive physical features with respect to the colors and fateful deductions from the knowledge of chemical characteristics, which results in a number of colors describing the chemical component of the rocks. The formation process of the rocks underlies the type of the rock that results. For instance, metamorphic rocks may form from the advanced formation processes of both sedimentary and igneous rock.