Geomechanics 1: Week 2
Rock: A naturally occurring solid material comprised of many minerals (eg., granite) or a
single mineral (eg., quartzite)
– Igneous, sedimentary and metamorphic rocks
Clay: A clay is comprised of particles < 2 microns size (individual particles are not visible
without magnification)
Rock forming minerals
Ø Minerals:
• - Naturallyoccurringsolidsubstances
• - Havespecificchemicalcompositions(eg.,AlSi2O8)&
specific crystal structure (e.g. face centred cubic)
• - Primary(e.g.feldspar)andsecondary(e.g.ferrihydrite)
minerals
Ø Rock forming minerals: Minerals that combine to form rocks
Ø Primary: minerals formed from the molten state
Ø Secondary: minerals formed from the conversion of primary minerals through
various processes (eg., chemical weathering)
Examples of minerals
Rock à Granite
àComposed of quartz, feldspar, mica minerals àminerals formed during the cooling of
magma
Rock à Sandstone
àComposed of quartz, calcite, goethite minerals àCrystals of quartz cemented together by
crystals of calcite and/or goethite
Weathering
Rocks are broken down into smaller particles and/or individual components by
Ø Physical weathering: caused by freeze / thaw, wet / dry, disintegration of cementing
minerals (Example: Graniteàbreaks apart into gravel-, then sand-, then silt-, then clay-sized
mineral grains)
, Ø Chemical weathering: caused by dissolution of outer surfaces or by transformation of a
mineral to secondary minerals (Example: Sandstoneàbecomes ingredients for various
secondary silicates, aluminosilicates, and hydrous oxides)
Ø Biological weathering: caused by bacteria, fungi, plants (Example: Coastal sediments
(Quartz, clays, biogenic pyrite)àwhen pyrite exposed to oxygen, rFeS2+sO2 +tH2Oà
sulphuric acid (uH2SO4) productionàlow pH, dissolution of minerals – transport of metals.
Clay minerals
Some important definitions:
Ø ClayàSize < 2 μm diameter
Ø ClaysàSpecific minerals of clay size (goethite, opal)
Ø Clay Minerals àSpecific minerals that may or may not be clay sized (smectite, kaolinite,
vermiculite, mica)
Clay mineralogy
ØChemical weathering produces layered aluminosilicates plus other metallic ions from rock
minerals
Ø Each mineral consists of two basic structural units:
• Silica tetrahedron (4-O, 1-Si)
• Al/Mg octahedron (6-OH/O, 1-Al/Mg)
ØAl and OH produce gibbsite octahedral sheet
ØMg and OH form brucite octahedral sheet
Common clay minerals (schematic)
Rock: A naturally occurring solid material comprised of many minerals (eg., granite) or a
single mineral (eg., quartzite)
– Igneous, sedimentary and metamorphic rocks
Clay: A clay is comprised of particles < 2 microns size (individual particles are not visible
without magnification)
Rock forming minerals
Ø Minerals:
• - Naturallyoccurringsolidsubstances
• - Havespecificchemicalcompositions(eg.,AlSi2O8)&
specific crystal structure (e.g. face centred cubic)
• - Primary(e.g.feldspar)andsecondary(e.g.ferrihydrite)
minerals
Ø Rock forming minerals: Minerals that combine to form rocks
Ø Primary: minerals formed from the molten state
Ø Secondary: minerals formed from the conversion of primary minerals through
various processes (eg., chemical weathering)
Examples of minerals
Rock à Granite
àComposed of quartz, feldspar, mica minerals àminerals formed during the cooling of
magma
Rock à Sandstone
àComposed of quartz, calcite, goethite minerals àCrystals of quartz cemented together by
crystals of calcite and/or goethite
Weathering
Rocks are broken down into smaller particles and/or individual components by
Ø Physical weathering: caused by freeze / thaw, wet / dry, disintegration of cementing
minerals (Example: Graniteàbreaks apart into gravel-, then sand-, then silt-, then clay-sized
mineral grains)
, Ø Chemical weathering: caused by dissolution of outer surfaces or by transformation of a
mineral to secondary minerals (Example: Sandstoneàbecomes ingredients for various
secondary silicates, aluminosilicates, and hydrous oxides)
Ø Biological weathering: caused by bacteria, fungi, plants (Example: Coastal sediments
(Quartz, clays, biogenic pyrite)àwhen pyrite exposed to oxygen, rFeS2+sO2 +tH2Oà
sulphuric acid (uH2SO4) productionàlow pH, dissolution of minerals – transport of metals.
Clay minerals
Some important definitions:
Ø ClayàSize < 2 μm diameter
Ø ClaysàSpecific minerals of clay size (goethite, opal)
Ø Clay Minerals àSpecific minerals that may or may not be clay sized (smectite, kaolinite,
vermiculite, mica)
Clay mineralogy
ØChemical weathering produces layered aluminosilicates plus other metallic ions from rock
minerals
Ø Each mineral consists of two basic structural units:
• Silica tetrahedron (4-O, 1-Si)
• Al/Mg octahedron (6-OH/O, 1-Al/Mg)
ØAl and OH produce gibbsite octahedral sheet
ØMg and OH form brucite octahedral sheet
Common clay minerals (schematic)
