Flints are siliceous accidents which appear in calcareous rocks, in particular some chalk. They are of irregular shape: in kidneys, in tubers, in columns, in coats, in network, in veins. A descriptive terminology was never proposed really.
We distinguish generally two parts: a heart or a darker, grey or black pit, and a more clear cortex.
Various forms of the silica compose the flint:
- The hydrated silica or the partially crystallized, rare opal C-T,
- The fibrous calcédonite [chalcedony is established by an assembly of "fibers", themselves proceeding of the pile of cristallites of quartz of some tens of nanometers]
- The quartz aggrégats cryptocristallins, sometimes called quartzine.
The passage of the siliceous cortex in the chalk is rough for the flint, it is what differentiates him of the chert for which the passage is gradual. The opal is little plentiful and present in the periphery of the cortex.
The cortex is essentially made of calcédonite microporous scattering the light, which gives its white tint. In the polarizing microscope, the network is made opaque by total reflection of the light on the walls of cavities. In the MEB (Fröhlich, on 2006), we distinguish a very dense network of more or less spherical cavities, in which the cristallites of quartz is big and organized well, sometimes there sphérolites.
The heart, or the pit, is established by a compact matrix, within which the "fibers" of chalcedony are very badly expressed, in polarized light. In the MEB, the cristallites of quartz is untidy arranged clearly, leaving between them a nanoporosity.
The heart of the flint is hard (hardness 7 on the scale of Mohs: lines the glass), possess a curved break and esquilleuse (said break conchoïdale), give cutting brightness and resists acids (except the acid fluorhydrique).
The growth of the flint is made in a centrifugal way, it is the external part which is least evolved and the internal part which is most evolved. The normal evolution of the silica is opal > calcédonite microporous > calcédonite compact > quartz. Certain flints are hollow and show the growth of quartz crystals.
Atypical flints sometimes meet:paramoudras or vertical flints, oblique or horizontal , veinsbreaches of flint training modes of of which are more complex.
Certain black flints contain in their heart a chalky dust qualified as "flour of flint" very rich in microfossils (ostracodes, foraminifères planctoniques, spicules).
The content in flint of various chalks of the Country of Callus was estimated (Interreg IIIa program "beaches with risks", final scientific report):
- Cénomanien: 0,5 in 20 %
- Turonien lower and more average: 0 in 1 %
- Turonien superior: 3 in 5 %
- Coniacien: 8 in 13,5 %
- Santonien: 10 in 16 %