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?

• 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