Chalk
Chalk is a sedimentary rock containing almost exclusively of calcium carbonate CaCO 3 and a bit of clay. Chalk is permeable, porous and brittle.
Chalk is a limestone rather unusual. Generally very pure, but it can be marl (where the limestone and clay are roughly equal proportions), glauconitic (if it contains glauconite ), dolomite (if it contains recrystallization of dolomite as in chalk Vernon), to flint , etc..
The limestone is chalk micaceous sandy or fine-grained, white or sometimes yellowish cream, containing a few flakes of white mica ( muscovite ).
Summary |
The chalk was formed by accumulation of skeletons of marine microorganisms, coccoliths mainly to the geological age of the Cretaceous which it gave its name. As such, the chalk is part of the carbon sink geological and plays an important role in the calcium cycle. During the Cretaceous higher (from -100 to -65 million years), large sections of chalk were deposited on the current Paris Basin.
The layers of chalk are often aquifers. This is so the water table of the Artois or the gigantic, the Beauce , which covers 9000 square km.
Uses
In northern France the prehistoric men had dug chalk to remove large nodules of chert.
The chalk was then used to produce stone to fuel lime kilns or provide the stones that adorned the interior walls of fortifications ( Citadel de Lille for example). This is the origin of holt of Nord-Pas-de-Calais and Picardy.
Chalk was also used as amendment to the fields.
And finally, as the chalk rock hosted the chalk aquifer , one of the largest groundwater ( aquifer ) of Europe, which supplies water to millions of people in northern Paris basin in Belgium.
Pure chalk of Cretaceous is used as painting materials. More or less oily or dry, it comes in sticks or in the form of parallelepipeds. It is usually applied directly onto the paper. Chalk gray are often used for sketches, while colored chalk can work fairly close to pastels to oils.
The chalk is also working to pool to prevent slippage of the tip of the tail hit the ball, thus giving effects.
Materials like chalk
 New school chalk, chalk-based rock, but also cast |  French school Chalk |  German school chalk |
The chalk was used as a stick to write on the blackboards of classrooms in schools. Today, the chalk used for teaching are made of plaster , itself made from gypsum which is a rock salt, mixed with pigments for colored chalk.
Tailors chalk is made from talc.
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