How/Why does Flint form within Chalk?

I am curious that two so different materials seem to be so closly linked.. what causes a lump of flint to be seemingly randomly within some Chalk?

My (rough) understanding of flint nodule formation in chalk deposits is pretty much what the Wikipedia article says: organic life rich in silicates that die on the sea floor get compressed into flint over the ages.

I've been down into an ancient flint mine:

http://www.english-heritage.org.uk/server/show/nav.12454/chosenImageId/3

Says here sponges caught in the fossil-bearing layer become flint:

http://www.yourgemologist.com/flint.html

BUMP

anyonesel

BUMP

anyonesel

What is deficient in the above answers?

Well I imagine the thus far proposed creation point of flint seems far to common. If flint were to form around the about examples.. there wouldn't be room for the chalk!

Hm, I ought to know this (I'm studying geology), but I am not actually sure. Despite this I think I can hazard a guess: Chalk is a calcium carbonate mineral. It is formed when microscopic creatures with shells die off. Flint on the other hand, is quartz, SiO2. Quartz is very common, and the wiki article states that flint is sedimentary. I would therefore think that during the formation of chalk, some of the chalk was weathered away, or simply replaced outright by quartz, probably in the form of sand. What happens then is less easy to guess, but presumably the chemical environment of the chalk allowed flint to form, rather than causing it to go over into quartz crystals.

I live in Norfolk which is made of chalk.

The formation of flints in chalk is a complex and subtle process which is still not yet fully understood.

However the flints are not entirely randomly distributed in chalk.

They are concentrated at particular strata levels, and there is some evidence of equidistant distribution within rock strata levels.

Brine water solutions percolating through proto-chalk layers under the sea may contain dissolved silica. Over many years, the silica may start to selectively precipitate out of solution. Changes in pressure, PH, temperature and other minerals in solution may trigger this. The initial precipitation occurs particularly at places where there may be an initial silica nucleus from an atypical silicon fossil water and/or where the percolation of the silica rich brine is disrupted e.g. by large fossils or by tunnels left by burrowing animals.


It is understood that once a silica micro substrate has formed, further silica is preferentially deposited there.

It is not clear whether the equidistant locations is due to concentration around a silica fossil representing extinct animals that would in life have spaced themselves apart OR merely that once a silica substrate had formed silica would concentrate there and not form near another substrate.

Thankyou. It was a mild curiosity for me :D