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?