-DIMENSION (Granulometry)
The sands are particles with dimensions between the silts and the ballasts, have five categories: Very fine, thin, medium, coarse and very coarse.
In dimensional terms, they have a size between 0.063 and 2 millimeters.
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Dimensional limits of sedimentary particles and designations of different geometric classes, according to Wentworth, C.K. (1992)
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-COSPOSITION OF GRAINS
Any type of rock on the surface of the Earth's crust can give rise to sand.
A grain of sand may be a fragment of rock (lithoclast), mineral (mineraloclasts) or an organism (bioclast).
The color that the sands present is directly related to this composition.
The most common are light-colored quartzites, which present quartz (the most abundant mineral in nature) as the predominant component. It has great resistance to the actions of external agents.
in some sands, there may also be other minerals such as more or less altered felsdspars, micas among others.
However, there are sands that are mostly composed of ferromagnesian minerals, such as olivines, pyroxenes, amphiboles, and therefore have a dark color, or by lithic components (fragments of limestone, basalt, etc.)
The sands originated in volcanic materials are usually dark because of the presence of ferromagnesian silicates typical of basaltic rocks, especially pyroxenes, amphiboles and olivine, black oxides (magnetite and ilmenite or even lithoclasts)
Siliceous sands are white, when pure, as are calcareous sands
The calcareous sands, as well as those whose constitution enters carbonate shells or fragments of these, make effervescence with the hydrochloric acid, this test is very used in the field for being fast and allow to evaluate qualitatively the presence or absence of these components.
Sands rich in organic matter or in manganese compounds are black in color
The iron compounds give the sands a yellowish or greenish color
-CALIBRATION
It is a parameter that is related to the granulometric amplitude (variety of grain sizes) and is influenced by erosive agents (energy variations of the transport agent), namely water and wind.
The better the calibration the smaller the geometric amplitude, ie the grain size variation. Worse calibration, greater grain size range (grain sizes vary greatly)
The larger the grains, the greater the energy of the transport agent (the more difficult it is to transport). Therefore, the rivers, seas, glaciers and the wind, act as selecting agents. Transporting the sediments according to their size and depositing those too large and heavy. Thus, the greater the transport capacity of an agent, the greater its particle size range of the transported sediments.
Wind is the agent with the lowest transport capacity, the wind deposits are therefore the ones with the lowest calibration. The transport capacity of the rivers varies greatly, along their route and time: it tends to decrease as the distance from the source increases and the proximity of the mouth increases. The transport capacity in the marine environment depends on a number of factors such as tidal regime, currents and rippling.
If the sediments present the same approximate size they are classified as well calibrated, otherwise they are classified as poorly calibrated sediments
When they stand out from the parent rock, fractured by physical weathering, the sand grains are generally very angular and vary in size, depending on the texture of the parent rock and the spacing of the fracturing of the massif in erosion.
It is at the beginning of this process that the grains have the largest dimensions, before being eroded, that is, transport and wear.
As they are transported by water or wind, the grains are becoming smaller and rounder due to wear and tear caused by erosion agents along the transport. In this way it can be affirmed that if the sediments are very rolled it means that they have been transported for a long time, undergoing severe surface wear. otherwise, if they are too angular, the transport has not been prolonged. However, the initial shape of the grains remains generally recognizable.
The degree of rolling is still influenced by the granulometry and the type of agent involved in the transport. For example, gravel and blocks become rounded more easily than sands. As for the type of transport agent, it is verified that fluvial and mainly marine sediments tend to be more rolled than wind sediments, these tend to be more angular and faceted as a consequence of mechanical shocks during transport. Sediments of glacial origin often have characteristic striated polishing.
FORM OF GRAINS - SPHERE
The study of the shape of the sand grains and the degree of rolling can present interesting data about the history of its formation. these two parameters are closely related to the means of transport, with the duration and distance traveled. Also the nature of the material being transported interferes with the shape and dimension presented by the grains.
Depending on the agent (water, wind) that transports them, the sediments move through different processes (suspension, saltation, drag) and present different aspects regarding caliber, brightness, rolling. However, the formed grains is more a reflection of its composition, for example a grain of shale will never acquire the spherical shape, presenting the grain always flattened. On the other hand, a quartz grain can easily get the spherical shape.
BRIGHTNESS - SURFACE STATE
The gloss that the grains can present results in the appearance of the grain surface that depends mainly on the transport agent and the sedimentation environment. From this we can find grains:
Brilliant - When transport is carried out in water, shocks that occur between the particles are damped by the fluid, only the most prominent areas of the grains (sharp edges) are removed, leading to a very smooth polishing of the surface.
Low brightness - Transport independently of the agent promotes fracturing of its surface. However, the younger the grain, the more numerous the scars of mechanical collisions with a concave aspect, since there has not yet been a rounding time, often the surface covered by clays and iron oxides, with a "dirty" appearance. Sometimes the iron oxides cover all the quartz grains giving them orange shade.
Bumps - When the transport agent is wind, frequent shocks occur between the grains, promoting small splinters or cracks. Checking perforated, very irregular appearance. This irregularity makes it difficult to reflect the light, making the grains foggy, because the surface is unpolluted. Under these circumstances it is easy to accumulate impurities in the depressed areas, giving a "dirty" appearance to the grain.
Source
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Questions:
1- Based on this information of the text, perhaps with a magnifying glass, describe the sands of Copacabana: Coloration, composition, grain size, calibration, opacity, etc. To do so, analyze samples from different points on the beach, more or less distant from the shore.
2- If you take a photo to put in the log we would be very happy (OPTIONAL), but remember to give a tip to the sculptor