Tuesday, May 27, 2014

Significance of rock fragments and mineral grains in river sands.



River sands of Ranchi plateau consists quartz, quartzite, feldspar, mica, coals, etc.

by
Dr. Nitish Priyadarshi




Sand grains of Ranchi river showing rock particles and other minerals.


Sand has become a very important mineral for the expansion of society. Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. River sand is one of the world’s most plentiful resources (perhaps as much as 20% of the Earth’s crust is sand) and has the ability to replenish itself. River sand is vital for human well being & for sustenance of rivers.

Rivers are hydraulically complicated and can vary greatly in source materials. Therefore, nearly all river sands show a broad spectrum of textures within the same deposit. Sand deposits of the rivers are accumulations of rock fragments and mineral grains, which have been derived from the weathering and erosion of hard rocks by glacial and river action. Water action is an effective mechanism for wearing away weaker particles, as well as separating different size fractions. The properties of sand largely depend on the properties of the rocks from which they were derived. Most sand is composed of particles that are durable and rich in silica (quartz, quartzite and flint), but other rock types, mainly limestone, may also occur, including negative impurities such as lignite, mudstone, chalk and coal. The sands of the local rivers of Ranchi plateau consists quartz, quartzite,  feldspar, mica, coals, etc. The look of the rocks varies from angular to round. Main source of these rock particles in the river sediments are the surrounding rocks like granite, granite gneiss and amphibolites with subordinate schist. 


Different size of rock particles in rivers of Ranchi.


Rock particles consisting of quartz and feldspar.


Polished rock fragment.

 Angular rock particles in Ranchi rivers.



Rounded Quartz 


Rounded to angular quartz.



Rock fragment.


 Rounded Quartz.


 Rocks of different colors and size.



Samples were collected from Swarnrekha river, Kanchi river, and from Jonha falls.

In Ranchi the predominant rock type is the Chotanagpur granite gneiss within which bands and enclaves of  mica schist, feldspathised mica schist, quartzite, calc-silicate rock, epidiorite etc. occur.  

The study of individual grains can reveal much historical information as to the origin and kind of transport of the grain. Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. It is called grus in geology or sharp sand in the building trade where it is preferred for concrete, and in gardening where it is used as a soil amendment to loosen clay soils. Sand that is transported long distances by water or wind will be rounded, with characteristic abrasion patterns on the grain surface. Desert sand is typically rounded. The composition of the sand fraction, including the percentage of quartz present, will vary from river to river depending on the sediments, rocks, or combination of these sources present in the area drained by the river, its climatic history, tectonic history, and other factors.

Sands may be divided into three major groups: terrigenous, carbonate, and pyroclastic. Terrigenous sands are those produced by weathering and breakdown of pre existing rocks. They are transported, sorted, and modified by moving fluids-both air and water- and derived from sources external to the basin of deposition. All terrigenous sands are quartz rich.

Carbonate sands are far the most part marine, and are primarily skeletal grains, oolites, locally derived detrital carbonate intraclasts. These constituents are produced within the basin of deposition and are not the debris formed by breakdown of preexisting rocks.

Pyroclastic sands are those produced by volcanic explosions. They may be deposited in diverse environments-in air or in water.   

Since sand comes from a "Mother" or source rock, it is possible to determine what type of rock produced the "baby rocks." The composition and general color of both the "baby" and "mother" rock are often very similar.

Geologists understand the significance of sand grains in rocks. The size, shape, and roundness help to explain the sandstone’s "life history."

This lab emphasizes two points. First, sand composition reflects the sand’s source. Second, the roundness of individual sand particles reflects how far the sand traveled and for how long. Sand is usually created when water and/or wind break off small pieces of pre-existing rock. If the particle is "newly" broken off, it tends to have an angular look, but if the particle has traveled by water or wind for a long time it becomes rounded.
The river's water carries the rocks along the bottom of the river bed. The pebbles are eroded by abrasion. The longer a pebble stays in the river bed, the more rounded it will become. The more angular a pebble the shorter the period of time it has been moving.







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