Rock properties - Geotechnical Investigation and Design Tables - page 104

9.4 Silica in igneous rocks
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Silica has been used to distinguish between groups as it is the most important
constituent in igneous rocks.

Table 9.4 Silica in igneous rocks (Bell, 1992).

Igneous rock group Silica

Acid/Silicic

>

65 %

Intermediate 55-65 %

Basic/mafic 45-55 %

Ultra-basic/ultramafic 45 % <

9.5 Hardness scale
•

The rock hardness is related to drillability, but is not necessarily a strength
indicator.

•

Each mineral in scale is capable of scratching those of a lower order.

•

Attempts to deduce hardness by summing hardness of rock minerals by its relative
proportion has not proved satisfactory.

Table 9.5 Moh's hardness values.

Material Hardness Common objects scratched

Diamond 10 -

Corundum 9 Tungsten carbide

Topaz 8

Quartz 7 Steel

Orthoclase 6 Glass

Apatite 5 Penknife scratches up to 5.5

Fluorspar 4

Calcite 3 Copper coin

Gypsum 2 Fingernail scratches up to 2.5

Talc 1

9.6 Rock hardness
•

Rock Hardness depends on mineral present.

9.7 Mudstone - shale classification based on
mineral proportion

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Shale is the commonest sedimentary rock - characterised by its laminations.

•

Mudstones are similar grain size as shales - but non laminated.

•

Shale may contain significant quantities of carbonates.