Flint

Flints

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.

Un niveau de silex

Flint

We distinguish generally two parts: a heart or a darker, grey or black pit, and a more clear cortex.

Distinction d'un cortex et d'un nucléus

Distinction of a cortex and a nucléus

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 %

For more detail of the origin and the training of the flint, consult the following pages:
Origin of the silica
Training of the flint
Or still this very brief video summary:


4 answers in Flint

  1. Souad Mrabet Says:

    I needed to have quickly an idea on the training of flints, your work answered it in a clear and fast way. Thank you for your effort.

  2. DALMAS Thierry Says:

    Hello

    Your document is interesting on the plan paléoécologique and mineralogical but brings little as for the question of the dia genesis, in particular on the crystalline thermodynamics. Avez-you determined the physico-chemical conditions of this dia genesis to verify and possibly confirm the hypotheses sédimentologiques?
    I work on the flint and am in search of a bibliography.
    Cordially.
    TD

    • Bhadmin Says:

      I have very few data on the stages and the conditions of the diagenèse. Thus your work will doubtless bring clarifications. Here are some readings which guided me, but which you already have to know:
      Clayton (1986),
      Zijlstra (1987, 1994),
      Maliva and Siever (1988),
      Fröhlich F. (2006),
      Madsen H.B. and Stemmerik L. (2010).

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