Lecture 3: Rocks and Minerals
References: Principles and applications of Geochemistry (Ch. 7 & 8)
Igneous Petrogenesis (Ch. 2)
How are rocks formed: the rock cycle
www.geolsoc.org.uk
Granite: intrusive igneous rock
quartz
hornblende
feldspar
What defines a mineral?
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Naturally Occurring
Inorganic
Solid
Specific composition (e.g., Gold - Au, Salt NaCl, quartz - SiO2)
• Definite crystalline structure – atoms are
arranged in a specific pattern
Mineral Identification
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Colour: caused by trace elements or impurities within a mineral
Lustre: how a mineral surface reflects light
Texture: how the mineral feels to the touch
Streak: the colour of a mineral when it is scratched on a streak plate
(i.e., colour when broken up)
Hardness (Moh’s scale: 1-10 – diamond is 10, talc is 1)
Cleavage: how a mineral breaks (typically along planes of weakness
– related to bonding
Fracture: splitting with no orientation
Density
Flame: colour under a flame
Special properties like double refraction, radioactivity,
taste, pleochroism, fluorescence
Mineral groups
1) Silicates (SiO4) – make up 96% of minerals, e.g., olivine
2) Carbonates (CO3): e.g, calcite CaCO3
3) Oxides: metal and oxygen (e.g., hematite, magnetite)
4) Sulfides: element + S2 (pyrite – FeS)
5) Sulfates: element + SO4 (gypsum – CaSO4nH2O)
6) Halides: element + halide (salt - NaCl)
7) Native elements: e.g., Cu, Au, Ag
pyrite
gypsum
Crystal Habit
• appearance – shape and size of crystals
stibnite
hematite
Dendritic: tree-like
Botryoidal: grape-like
Bladed
Crystal Form
Any grouping of crystal faces or facets that are arranged in the same
symmetry is referred to as a crystal's "form." There are approximately 48
unique crystal forms.
Atomic structure of crystals
The relative size of ions determine how atoms pack and
which ions can serve as substitutes.
Crystal structure:
• determined by radius size…
Silicates
Silicate tetrahedron
olivine, quartz
O2Si4+
Single chain structure
pyroxene
2O2- O2-O
Double chain structure
hornblende
SiO44-: although it is
geometrically balanced, it is
not charge balanced – needs
ions or other tetrahedra to
balance charge
Sheet silicate structure
micas
Framework silicate structure
Feldspars
How are minerals formed?
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2)
3)
Solution: if a solution is supersaturated,
minerals will precipitate
Magma: minerals form during cooling of a magma –
the slower a magma cools, the larger the crystals
Intrusive: cools slowly beneath Earth’s surface (e.g.,
basalt)
Extrusive: cools rapidly at Earth’s surface (e.g.,
granite)Metamorphism
Metamorphism: transformation due to changes in
pressure and temperature
Phase Diagrams
A phase diagram is common way to represent the various phases of a substance
and the conditions under which each phase exists.
A phase diagram is a plot of pressure (P ) vs temperature (T). Lines on the diagram
represent conditions (T,P) under which a phase change is at equilibrium. That is, at
a point on a line, it is possible for two (or three) phases to coexist at equilibrium. In
other regions of the plot, only one phase exists at equilibrium.
Phase diagram for water
Triple point: where 3 phases
coexist
Binary phase diagram for a solid solution of Olivine
fsolid
Fayallite (Fa)
fliq
% Fo (Mg2SiO4)
Forsterite (Fo)
Solidus: the temperature below which the substance is stable in the solid state
Liquidus: the temperature above which the substance is stable in the liquid state
Lever Rule: to determine quantitatively the relative composition of a mixture in a
two-phase region in a phase diagram
Magma: mixture of molten rock, gases and mineral phases,
produced by mantle melting
Mantle melts between ~800-1250ºC due to:
1) Increase in temperature
2) Decrease in pressure
3) Addition of volatile phases
Upwelling mantle
plumes – hotspots
Hawaii, Iceland
Magma: mixture of molten rock, gases and mineral phases,
produced by mantle melting
Mantle melts between ~800-1250ºC due to:
1) Increase in temperature
2) Decrease in pressure
3) Addition of volatile phases
Partial melting
Adiabatic rise of
mantle material with
no heat loss –
decompression
melting
Mid-Ocean Ridges
Magma: mixture of molten rock, gases and mineral phases,
produced by mantle melting
Mantle melts between ~800-1250ºC due to:
1) Increase in temperature
2) Decrease in pressure
3) Addition of volatile phases (e.g., water)
0
Depth (km)
50
Mantle solidus is depressed
by addition of water
100
150
Subduction zone settings
Wet mantle plumes
200
Mantle melting: endmember models
Batch melting: Melt remains in contact with residual crystals at all times, so
the bulk composition remains constant
Fractional melting: Melt leaves the system as soon as it is formed, so the
bulk composition of the residual solid changes continuously.
• Incompatible elements: preferentially partition into the melt phase (D<1)
• Compatible elements: preferentially partition into the solid phase (D>1)
• Partition or distribution coefficient (D) = Csolid/Cliquid
Spider diagram
showing depleted
MORB vs. enriched
OIB sources
Concentrations
normalized to
bulk earth, C1
chondrites, or
primitive mantle
Most incompatible
Less incompatible
Relating trace element concentrations to melt fraction (F)
Batch melting equation: Cliq/Csol = 1/(F+D(1-F))
Fractional melting equation: Cliq/Csol = (1/D)*(1-F)(1/D-1)
E. Klein, “The Crust”, T.I.G series
Spider diagram of crust vs mantle
Workman and Hart, 2005
Rare Earth Element diagrams
• REE are a group of 15 elements with atomic numbers ranging from
57 (La) to 71 (Lu) – LREE vs. HREE
• Although they are geochemically similar, they have different partition
coefficients so are sensitive tracers of source enrichment, the
degree of melting and/or fractional crystallization
Shaw et al., 2009
Oman ophiolite
Samples of the mantle
1) Ophiolites
– Slabs of oceanic crust and upper mantle
– Thrust at subduction zones onto edge of
continent
2) Dredge samples from oceanic fracture zones
3) Nodules and xenoliths in basalts
4) Kimberlites
– Diamond-bearing pipes blasted up from the
mantle carrying xenoliths from depth
Hacker
Mafic Rocks – Magnesium, Iron rich, usually dark coloured
Felsic or SiAlic Rocks – Silicon, Aluminum rich, usually light coloured