Why is chalk white?

Qcmbr

I was reading some pro flood info and I came across an intriguing point regarding chalk - the fact that it is composed almost entirely of shells. Now I have no desire to start a pro flood blah blah debate. This is a sincere non religious question for our marine biologists / geologists - why is chalk white and not layered with silt and sand?

xjwms

The boards are green?

EvilForce

Chalk is a soft, white, porous form of limestone composed of the mineral calcium carbonate. It is relatively resistant to erosion and slumping compared to the clays that it is usually associated with, and so forms tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle.
Because chalk is porous, chalk downland usually holds a large water table, providing a natural reservoir that releases water slowly through dry seasons.
Chalk has been quarried from prehistory, providing building material and marl for fields. In southeast England, deneholes are a notable example of ancient chalk pits.
The Chalk Formation is a European stratigraphic unit in the upper Cretaceous period. This includes the famous White cliffs of Dover of Kent in England, which formed entirely of chalk deposits.
Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Blackboard chalk, often supplied in sticks about 5 cm long, is not actually made from the mineral chalk but from gypsum (calcium sulfate). Similarly, the "chalk" used by tailors is usually made from talc (magnesium silicate).

kls

Oh Evil, i just love a man that can talk chalk Chalk one up for Evil ,hee hee

xjwms

LOL kls

I think EvilForce is very wise.

I would have never come up with that.

Qcmbr

Hi I wasn't referring to blackboard chalk - I was actually taking about the huge chalk deposits.If / as these are formed over millions of years how come they are so relatively pure?

dh

Hi I wasn't referring to blackboard chalk - I was actually taking about the huge chalk deposits.If / as these are formed over millions of years how come they are so relatively pure?

well, it is assumed in nature that all blackboards are impure, hence the need for white chalk, even chalk that isn't for blackboards... this brings balance to the force.

doogie

Hi I wasn't referring to blackboard chalk - I was actually taking about the huge chalk deposits.If / as these are formed over millions of years how come they are so relatively pure?

chalk is super porous limestone. it's white because it's almost nothing but calcium carbonate. (wikipedia says 98%) so that accounts for its color. because it's so porous, it's continuously flushing out 'impurities' thus making it purer and whiter.
the only way chalk's color relates to its age is the whiter the chalk, the older it (probably) is. edited to add: i googled for the chemical composition of chalk and it seems that "calcium carbonate" and "chalk" are basically interchangeable with purity levels between 98 and 99.9%. Q, if by your question you really meant why is calcium white, i think we're actually moving into chemistry, which i'll have to leave to someone else.

AlanF

Try this link: http://www.geologyshop.co.uk/chalk.htm

AlanF

Scully

tsk tsk EF.... you really ought to cite your sources... people might start to think you really are that smart!

http://www.absoluteastronomy.com/encyclopedia/c/ch/chalk.htm

naughty naughty Dr Evil(Force)!! <insert spanking emoticon>

Why is chalk white?

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