Flint: Formation, properties, and historical uses

The origin of flint is still not fully understood. Presumably, silicic acid-containing solutions provide for a displacement of carbonates during the diagenesis (reaction and transformation processes during rock formation). Relics of skeletons of siliceous sponges and diatoms in flint nodules prove the organic origin. Flint consists primarily of fibrous chalcedony, similar to jasper (a cryptocrystalline but granular quartz, with grain size less than 1 micron). Flint diagenesis generally proceeds via opal-A (amorphous), opal-CT (easily worked like chalk) to flint.

The dehydration of the silica takes place from the inside to the outside, whereby the flint nodules often have an onion-like structure. The porous light outer layer (the so-called rind or cortex) is often clearly visible. This is the diagenetic precursor to flint, (SiO2 - nH2O), called opal CT. This is easily worked. The transformation of opal-CT to flint requires millions of years. The outer layers can absorb water to a small extent, which favours weathering of the surface.

Flint has an isotropic or amorphous structure, which means that it lacks a preferred orientation. When great pressure is applied slowly increasing or abruptly to a point of the flint, the kinetic energy is absorbed by the rock and spreads out concentrically in a cone shape starting from the point of impact. If the impact energy is sufficiently high, the rock is split by the propagating impact waves. The resulting fracture front usually has a conchoidal shape, as can also be observed in broken glass.

In the area of a fracture, the flint also exhibits impact waves, the Wallner lines. They occur mainly with deliberately split off parts of the stone, which are called knock-offs.

Fresh flint usually has a black to grey colour. It becomes increasingly milky through weathering; yellowish to brownish discolorations due to iron oxide can also occur. Red flint is rather rare; it is found in Central Europe, for example, on the beaches of the dune of Helgoland. The red colouring is the result of inclusions of trivalent iron compounds (for example hematite). The red flint originates from submarine chalk rocks of the Turonian (Upper Cretaceous) in the vicinity of the island. On Helgoland the red Helgoland flint is processed as a gemstone, set and sold as a polished disc, cut as a ring stone (cabochon) or spherically strung as a necklace.

Flint deposits are found in numerous Jurassic and Cretaceous deposits. Mostly the nodules with a size of up to 30 cm diameter are embedded in chalk deposits. There are also slabs with thicknesses of up to 20 cm. Due to later rearrangement processes they can also be found detached from their original stratigraphic context of origin. Thus, flints are very common in glacial sediments as part of ground and terminal moraines as well as within meltwater deposits.

Distribution in Germany

In Germany, flint occurs in the parent rock, especially in the Cretaceous of Helgoland Düne, Rügen, Lägerdorf (Schleswig-Holstein) and Hemmoor (Lower Saxony). Secondarily redeposited it is found in the entire north-central European distribution area, there locally also extremely enriched (flint fields in the nature reserve Steinfelder in der Schmalen Heide and extension). In the mountainous region of southern Lower Saxony, flint is found as glacial till on the edge of the Harz Mountains and in the Leine Valley as far as Freden, north of the so-called flint line. It also occurs in white weathered form in Tertiary sands of the Miocene of the Solling and is known as chert from the Middle Muschelkalk (Göttingen to Einbeck), coral oolite/Heersum strata (Thüster Berg) and Hilssandstein.

The origin of this secondary rearrangement was the Elster Ice Age, whose ice masses penetrated as far as the northern edge of the Fahner Heights and carried large quantities of rock material from northern Europe with them. When the glacial ice melted, the erratic blocks and flints were left behind in the boulder clay. Erosion as well as the subsequent ice ages, Saale and Vistula ice ages, obliterated the original deposits, the flint line.

Flint also occurs in other areas of Germany, but more rarely, but was also found and used there. Flint blocks weighing tons, which were probably formed in the Knollenmergel, are found at Flinsberg near Oberrot, Baden-Württemberg.