Download - Siliceous sediments
Siliceous SedimentsPreparation, Properties,
Problems
By Zeshan Ali and Rachael Pung
Preparation 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
Vanport FlintRed JasperBrown Chert
Preparation 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
Preparation
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)
Preparation and Changes in Properties
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
Factors affecting knapping Made up entirely of cryptocrystalline
silica Presence of pore spaces Size/Thickness Temperature Holding time
PropertiesPhysical 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
PropertiesGeotechnical propertiesDensity 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 µDCompressibility (for rock) Extremely strong, with a uniaxial
compressive strength of >250 Mpa and a point load index of >10 Mpa
Problems 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.
Potlidding, crazing and cracking on chert
Problems 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.
Problems 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
Problems 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.
References http://www.springerreference.com/docs/html/chapterdbid/76
579.html http://firearchaeology.com/firearchaeology/Direct_Effects_fil
es/Gregg_Grybush_1976.pdf http://www.rocscience.com/help/rocfall/webhelp/baggage/Ro
ck_Density_Table.htm http://
www.pugetsoundknappers.com/how_to/Heat%20Treating%20Guide%20with%20Table.html
http://geology.com/rocks/chert.shtml http://
geology.about.com/od/more_sedrocks/qt/About-Chert.htm
Questions?