Hornblende Andesite Rock Characteristics - Research Paper Example

Insert Hornblende Andesite Rock Hornblende andesite is a type of rock that is formed through igneous and metamorphic processes. The hornblende andesite is a major rock forming mineral that is found in different types of igneous and metamorphic environment. There are two varieties of the hornblende andesite rocks. One that is gray and slightly purplish in color, friable, fine-grained, and somewhat porous. The other type is dark gray in color, and much more compact and firm in texture. This paper examines the physical and chemical characteristics of the hornblende andesite. An understanding established for this type of rock is very crucial in determining where it can be used. A field research was done where the rock was identified, and physical examination done to substantiate its characteristics. The paper seeks to explain the various types of hornblende andesite and their location around the world. An understanding on the on the plate tectonics association is important in understanding the natural processes that necessitated the formation of the rocks in the various location regions in the world. The thin report section gives the complete picture on the physical and chemical composition of the hornblende andesite detailing its mineral composition. The three minerals that make up the hornblende are Plagioclase, Sanidine mineral, and the Hornblende mineral. A hand sample showing dark gray hornblende in a very fine-grained groundmass is also illustrated further to exhibit some of the physical characteristics of hornblende. Thin section report Hornblende is a common rock-forming mineral that is found in igneous and metamorphic rocks. Hornblende andesite has several different types of minerals at varied percentages (Kepezhinskas 246). The crystals vary in chemical composition, but they are all double chain in silicates with similar physical characteristics. The chemical composition of the minerals include metals, such as magnesium, chromium, calcium, sodium, iron, aluminum, silicon, fluorine, manganese, potassium, and hydroxyl (Kepezhinskas 246). The above chemical substances vary in percentages. It is composed of calcium, sodium, magnesium, iron aluminum, silicon, and Fluorine and hydroxyl groups. Different composition of these chemical substances makes the appearance or the optical properties of the rock to differ .The general optical characteristics of the hornblende andesite are: Optical properties of the hornblende Has refractive index na=1.543 to 1.554,nb=1.547 to 1.559 and ny=1.552 to 1.562 Has a birefringence=0.009 The 2v angle is measured from 76 to 83degrees It is distinctively pleochroic and colored with green, blue-green, brown and yellow green shades The amphibole cleavage on the (110) plane intersect at 124 degrees and 56 degrees. Its fragment shape is controlled by the cleavage and usually elongates parallel to the c axis. Has extinction angles of 34 degrees and 12 degrees in the sections that are parallel to (010) The extinction is between 14 and 25 degrees in most cases. Fig 1.1: A thin section specimen showing a dark gray, dark brownish hornblende in a very fine-grained groundmass Fig 1.1 shows a hand specimen showing a dark gray hornblende in the very fine-grained groundmass (Kepezhinskas 246). The hornblende andesite minerals consist of the different chemical composition of substances. As such, the properties of these metals depend heavily on the chemical composition. Description of the hornblende andesite minerals Plagioclase Plagioclase is a mineral whose origin is the igneous and metamorphic rocks especially at the volcanic regions. It is composed of calcium, sodium, silicon, and aluminum oxide. Different composition of these chemical substances makes the appearance or the optical properties of rock to differ. (Smith 127) Optical Properties of plagioclase Colorless in thin section The optical orientation varies in regular pattern with the composition and thus provides the basis for determining the composition. Extinction is inclined in almost the whole orientation The cleavage is noticeable in the grains along the edge of the slide Maximum interference colors in the thin section are gray or white. The cleavage (001) is perfect with a good (010) cleavage and a poor cleavage (110) as observed in the thin section. Sanidine mineral The sanidine mineral is composed of potassium, aluminum and silicon oxide. It originates from the igneous and metamorphic activities of rocks Optical properties of sanidine mineral Colorless in thin section High sanidine is monoclinic optical with optic plane that is parallel to (010) and 2v between zero(0) and 47 Has a birefringence of 0.007 There is a perfect cleavage on {001} and {010} good cleavage that intersects at 90 degrees. Hornblende mineral Hornblende is a mineral whose origin is the igneous and metamorphic rocks especially at the volcanic regions (Smith 127). It is composed of calcium, sodium, magnesium, iron aluminum, silicon, and Fluorine and hydroxyl groups. The crystal system of the hornblende is monoclinic with hardness varying between 5 and 6. Optical properties of the hornblende It is distinctively pleochroic and colored with green, blue-green, brown and yellow green shades The amphibole cleavage on the (110) plane intersect at 124 degrees and 56 degrees. Its fragment shape is controlled by the cleavage and usually elongates parallel to the c axis. Has extinction angles of 34 degrees and 12 degrees in the sections that are parallel to (010) The extinction is between 14 and 25 degrees in most cases. Geologic context Hornblende andesite formation process Hornblende andesite rock was formed by the metamorphosis and igneous process of rock formation. It is used to describe Ferro hornblende and magnesia hornblende including all the other calcium, aluminum amphiboles (Green 699). Its typical opacity and color are caused by the iron mineral inherent in its structure. Hornblende is a major rock forming mineral that is found in different types of igneous and metamorphic environments. There are two varieties of the hornblende andesite rocks (Green 699). Fig 1.2 hornblende andesite granules One that is gray and slightly purplish in color, friable, fine-grained, and somewhat porous. The other type is dark gray in color, and much more compact and firm in texture. The two varieties contain the macroscopic hornblende and plagioclase, but under the microscope they are seen to be similar or identical with each consisting of a gray groundmass in which there is embedded deep reddish brown and strongly dichroic hornblende (Green 699). Other materials are the light green augites and the numerous crystals of the plagioclase feldspar. Sanidine is also present, iron oxides, which seem to be mostly magnetite and a very little apatite. The groundmass consists of a micro felsitic base, which carries colorless microlites, minute yellowish and greenish particles of hornblende and augite and the grains of opacity (Green 699). The light colored variety contains minute patches of nearly transparent glass whereas the dark variety seems felsitic throughout Non scientific aspects The lustrous, well-formed crystals originate from the Italian volcanoes at Monte Somma and The Montenegro Quarry, Latera, and Lazio Province. Andesite rock is the most common volcanic rock after the basalt. It is normally porphyritic as it consists of very coarse materials and crystals that are embedded in a granular and glassy matrix (Harford et al. 99). The name andesite originates from the Andes volcanic mountains chains of the North and South America. Dark green hornblende was found in Sweden, while the shiny hornblende crystal plates have come from the iron mines in Jersey Highlands in Passaic, New Jersey (Allen & Boettcher 312) Plate tectonic association During the volcanic eruption at the Subduction zones, the hot magma raises the vent up to the surface. The magma encounters friction with the crustal walls thus acquiring some minerals that result in the formation of the andesite rocks when the magma cools on the ground. The Adakites type of magma gives rise to High-Mg andesite (though different from boninites) where the ridge Subduction occurs, or mafic rocks that have been underplated at the crustal walls at the subduction zones (Harford et al. 97). Consequently, when the andesite rock formed from the magma coalesces with the hornblende mineral it forms the hornblende andesite rock. Andesite rock is the most common volcanic rock after the basalt. It is normally porphyritic as it consists of very coarse materials and crystals that are embedded in a granular and glassy matrix (Harford et al. 99). The name andesite originates from the Andes volcanic mountains chains of the North and South America. Andesite is very frequent in the mountain building rims of the Pacific Ocean. The transition from oceanic crust of the primary basin of the Pacific to the andesitic rocks that are around its perimeter is referred to as the andesite line. The andesite line, therefore, marks the geological border of the Pacific basin. There are different classes of andesine depending on the type and composition of various minerals. Andesine is the primary ingredient of the andesite. An andesite rock composed of hornblende mineral is thus referred to as hornblende andesite. (Harford et al. 98) Hornblende andesite location and other geological aspects Hornblende andesite is abundant and occurs worldwide. However, good display crystals are not commonly encountered. The lustrous, well-formed crystals originate from the Italian volcanoes at Monte Somma and The Montenegro Quarry, Latera, and Lazio Province. Dark green hornblende was found in Sweden, while the shiny hornblende crystal plates have come from the iron mines in Jersey Highlands in Passaic, New Jersey (Allen & Boettcher 312) Hornblende is a mineral whose origin is the igneous and metamorphic rocks especially at the volcanic regions. It is composed of calcium, sodium, magnesium, iron, aluminum, silicon, and fluorine and hydroxyl groups. Different composition of these chemical substances makes the appearance other physical properties of the rock to differ. Two varieties of the hornblende andesite rocks were examined. One that is gray and slightly purplish in color, friable, fine-grained, and somewhat porous. The other type is dark gray in color, and much more compact and firm in texture (Kepezhinskas 250) Other materials are the light green augites and the numerous crystals of the plagioclase feldspar. Sanidine is also present, iron oxides, which seem to be mostly magnetite and a very little apatite (Kepezhinskas 250) The groundmass consists of a micro felsitic base which carries colorless microlites, minute yellowish and greenish particles of hornblende and augite and the grains of opacity. The light colored variety contains minute patches of nearly transparent glass whereas the dark variety seems felsitic throughout. The two types contain the macroscopic hornblende and plagioclase, but under the microscope they are seen to be similar or identical to each consisting of a gray groundmass in which there is embedded deep reddish brown and strongly dichroic hornblende (Kepezhinskas 250) Work Cited Harford, C. L., et al. "The Volcanic Evolution of Montserrat Using 40ar/39ar Geochronology." Geological Society, London, Memoirs 21.1 (2002): 93-113. Smith, Ian EM. "High-Magnesium Andesites: The Example of The Papuan Volcanic Arc." Geological Society, London, Special Publications 385.1 (2014): 117-135. Allen, J. C., and A. L. Boettcher. "The Stability Of Amphibole In Andesite and Basalt At High Pressures." American Mineralogist 68.3-4 (1983):307-314. Kepezhinskas, P. K. "Origin Of The Hornblende Andesites Of Northern Kamchatka." International Geology Review 31.3 (1989): 246-252. Green, Nathan L. "Geology and petrology of Quaternary Volcanic Rocks, Garibaldi Lake Area, Southwestern British Columbia: Summary." Geological Society of America Bulletin 92.10 (1981): 697-702. Read More