lecture 3: rocks and minerals references: principles and applications of geochemistry (ch. 7 &...
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Lecture 3: Rocks and Minerals
References: Principles and applications of Geochemistry (Ch. 7 & 8) Igneous Petrogenesis (Ch. 2)
Granite: intrusive igneous rock
quartz hornblende feldspar
What defines a mineral?
• 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• 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 groups1) 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, Agpyrite
gypsum
Crystal Habit
• appearance – shape and size of crystals
Botryoidal: grape-like
Bladed
Dendritic: tree-like
stibnite
hematite
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
Single chain structure pyroxene
Double chain structure hornblende
Sheet silicate structure micas
Framework silicate structure Feldspars
O2-
O2- O2-O2-
Si4+
SiO44-: although it is
geometrically balanced, it is not charge balanced – needs ions or other tetrahedra to balance charge
How are minerals formed?
1) Solution: if a solution is supersaturated, minerals will precipitate
2) 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
3) Metamorphism: transformation due to changes in pressure and temperature
Phase DiagramsA 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
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
fliqfsolid
Fayallite (Fa) Forsterite (Fo)% Fo (Mg2SiO4)
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, Icelandgeothermsolidus
liquidus
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
Adiabatic rise of mantle material with no heat loss – decompression melting
Mid-Ocean Ridges
Partial melting
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)
Mantle solidus is depressed by addition of water
Subduction zone settings
Wet mantle plumes
0
200
100
150
50
Dep
th (
km)
Mantle melting: endmember modelsBatch 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
Most incompatible Less incompatible
Concentrations normalized to bulk earth, C1 chondrites, or primitive mantle
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
Samples of the mantle
1) Ophiolites– Slabs of oceanic crust and upper mantle– Thrust at subduction zones onto edge of
continent2) Dredge samples from oceanic fracture zones3) Nodules and xenoliths in basalts4) Kimberlites
– Diamond-bearing pipes blasted up from the mantle carrying xenoliths from depth
Hacker
Oman ophiolite
Mafic Rocks – Magnesium, Iron rich, usually dark coloured
Felsic or SiAlic Rocks – Silicon, Aluminum rich, usually light coloured