silicious sediment presentation focussed on chert.

1. Preparation, Properties, Problems
By Zeshan Ali and Rachael Pung

2.  Flint contains a few percent of water
 Should not be exposed to excessive heat, as from a fire.
 Due to the stone's molecular makeup and the presence of
water, flint may fracture, sometimes violently
Brown Chert Red Jasper Vanport Flint

3.  Desirable changes, in terms of knappability,
takes place in chert and many other siliceous
stones if they are heated slowly, at relatively low
temperatures and out of direct contact with
intense heat.
Poorly ordered, strongly
interlocking
cryptocrystalline fabric of
the unheated samples
Equigranular and better
crystallized with thermal
treatment

4. Types of Siliceous Sediment Temperature Range for Different
Rock Treatment
Flint and Hornstone 350 – 500°F (typically 200°C)
Agate and Chalcedony 400 – 600°F (typically 230°C)
Jasper 400 – 750 °F (typically 290 °C)
Chert 400 – 700 °F (typically 315°C)

5. Properties Changes in Properties
Elastic Constant Slight, consistent increase in the
Young’s modulus of elasticity when
heated at 300, 400 and 500 °C
Compressive Strength Did not show consistent changes, vary
substantially with the raw material
Tensile Strength Did not show consistent changes, vary
substantially with the raw material
Fracture Toughness Marked reduction after heating to 300
and 400 °C
 Material tested: Flint, chert, jasper, chalcedony
and agate

6. Made up entirely of cryptocrystalline silica
Presence of pore spaces
Size/Thickness
Temperature
Holding time

7. Physical
Properties
Description
Lustre Waxy, dull for freshly broken
surfaces
Transparency Translucent to opaque
Colour Grey, white, black, brown and
other colours due to staining
Streak White or lightly coloured
Hardness (Mohs) 6.5 - 7
Tenacity Brittle
Cleavage None observed
Fracture Splintery, Conchoidal, sub-conchoidal
Chert Specimen from
Mojave Desert

8. Geotechnical properties
Density Chert SpG range from 2.24 – 2.74.
Flint which has about 1% water within
it has a SpG that varies from 2.57 –
2.64
Porosity 0.59 - 3.46%
Permeability 0.001 - 0.033 μD
Compressibility (for rock) Extremely strong, with a uniaxial
compressive strength of >250 Mpa
and a point load index of >10 Mpa

9. Same piece of rock may have different
responses to heat treatment.
• Outer surface of flint, jasper and chalcedony nodules:
high quality stone, require little or no heat treatment
• Interior portion: opaque, more coarsely grained and
may require high temperatures for heat treatment.
• The outer edges of a stone may be very Knappable
following a moderate application of heat, while the
interior remains tough.
• Higher temperatures may treat the interior of the stone
effectively, but create potlidding in the outer portion of
the rock.

11.  While flint may be used in fire-lighting, it should not be
exposed to excessive heat, as from a fire. Due to the
stone's molecular makeup, flint may fracture, sometimes
violently, when different parts of the stone expand to
different degrees. This tendency, to fracture, is enhanced
by the fact that most samples of flint contain impurities
that may expand to a greater or lesser degree than the
surrounding stone.

12.  Deeply weathered chert develops surface pop-outs when used in concrete
that undergoes freezing and thawing because of the high porosity of
weathered chert.
 To solve the pop-out problem of weak cherts, the percentage of the low
density materials (SpG less than 2.45 is kept at a minimum.)
 Reducing the maximum allowable size of the coarse aggregate.
 Smaller pieces are less prone to freeze thaw as they are able to expel all the
water prior to freezing

13.  Cherts undergo an alkali-silica reaction with high-alkali cements. This
reaction leads to cracking and expansion of concrete and ultimately to failure
of the material
 To prevent the alkali-silica reaction problem, 3 procedures can be followed:
• (1) use low alkali cement (<0.65 alkali as Na2O).
• (2) Use nonreactive aggregate or add non-reactive aggregate to reudce the reactive portion by
dilution to a level below the percentages mentioned in the previous paragraph.
• (3) Add pozzolans, which include certain volcanic rocks and fly ash or ash collected from
smoke stacks where coal is burned. These finely ground high-silica materials yield a non-expansive
reaction product with the alkali in the cement.

14.  http://www.springerreference.com/docs/html/chapterdbid/76579.html
 http://firearchaeology.com/firearchaeology/Direct_Effects_files/Gregg
_Grybush_1976.pdf
 http://www.rocscience.com/help/rocfall/webhelp/baggage/Rock_Den
sity_Table.htm
 http://www.pugetsoundknappers.com/how_to/Heat%20Treating%20
Guide%20with%20Table.html
 http://geology.com/rocks/chert.shtml
 http://geology.about.com/od/more_sedrocks/qt/About-Chert.htm