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8 May 2013

Soemtimes called a rock, sometimes a mineral, flint is usually grey or black (but can also be red or brown when affected by iron oxides, in Aylesbury Vale for example) or even blueish hues. Flint is one of the few rocks or minerals mentioned in the Bible (the Book of Psalms for example, Deuteronomy, Isaiah, etc). In Ireland it is only found in situ in the NE, in the chalk of Co Antrim. However, flint (transported by water and ice) also crops up as far south as Leinster (and as pebbles on the beaches near Dublin). Flint is made from almost pure silica – and is harder than steel (but unlike steel can be struck to shape, or shatter into flakes).

Chert, which is similar, is found in limestones, but flint is only found in chalk (or as drift, pebbles etc). Flint occurs commonly in Europe and Asia (in many places) and in America and Africa (less so) but it is still wide open as to quite how it was formed. The puzzle has exercised the minds of several generations of geologists with no convincing explanation, in a uniformitarian manner, dreamed up so far. One theory that you will find oft quoted is that it originated from the silica skeletons of sponges. Engulfed in the chalk sediment at the bottom of shallow seas the silica dissolved in the water circulating through that sediment, an idea that is at odds to the fact that in normal conditions silica is negligibly soluble in salt water. Peculiar conditions are required for the silica to form from solution – but what were those conditions? Down to Earth 83 ends by saying, perhaps some aspiring young geologist will discover the origins of flint.

Often flint is found with a white crust or patina. This casing is white silica and has no connection to the white chalk within which it is found. The crust can be quite thick.

Flint occurs either as lumps or nodules, or it forms as a tabular slab like vein running through the chalk, a flat layer like jam spread between two layers of Victoria Sponge cake. This is usually known as laminated flint. It can also occur in a third form, less commonly, as largish barrel shaped or cylindrical lumps lying at right angles to the bedding plains of the chalk, and is known as Paramondras, a name derived from the Gaelic = sea pears. These can be as big as a metre in length.

See http://geosupplies.co.uk

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