Chemistry, Geochemistry, Minerals & Rocks Flashcards
Lectures 3.1-3.5
What is a crystal?
- Homogenous solid formed by repeating 3D patterns of atoms, ions or molecules
- Most of solid earth
What is a molecule?
Group of 2 or more of the same or different elements held together by chemical bonds. Electrically neutral.
What is a mineral?
Element or chemical compound, usually crystalline and formed via a geological process.
What is a group number?
Number of electrons in outermost shell.
What is the period number?
Number of shells.
What are valence electrons?
Number of electrons available for bonding.
What is the importance of transition metals?
- Can exist in variable oxidation states
- Donates or accept electrons easily
- Able to bond to a wide range of molecules/ions
What is ionisation energy?
- Energy to remove least strongly bound electron, creating a positively charged cation.
What is electronegativity?
- Ability of bonded atom to pull electrons
- Become a negatively charged ions
-Increases from the bottom left of the periodic table
Describe ionic bonds.
One or more electrons from one atom are taken, attached to another atom. Results in + and - ions that attract eachother.
Describe a covalent bond.
One or more pairs of electrons are shared by to atoms. Can be polar or non polar. This is what water uses as O has a higher attraction for electrons, as it is more electronegative.
Describe Van de Waals forces.
- Weak
- Due to a permanent dipole, a multiple multipole or spontaneously induced dipole
- Important for things like graphite
What is the importance of electronegativity in chemical bonding?
- As electronegativity increases:
Covalent non polar to
Covalent polar to
Ionic
Describe metallic bonds.
- Electrons in certain orbits are detached from parent atoms
- Metal bound by strong attraction between positive nuclei and de-localised electrons
- Hence ability to conduct electricity
Describe hydrogen bonds.
- Attraction between H atom and atom of high electronegativity (mostly N,O or F)
- In water, proteins and DNA
Who recognised a partitioning of elements according to their preferred host phases?
Victor Goldschmidt.
Describe the 4 partitioning of elements?
Atmophile: Volatile, form mainly atmospheric gases
Lithophile: Silicates and oxides - rock forming and dominate crust/mantle.
Chalcophile: Affinity for sulfide liquid phase
Siderophile: Affinity for metallic liquid phase
How did oxygen amounts determine the lithophile layer?
Used up all of the oxygen after all Si, Mg, Ca and Al were used up. So was just based on what was left.
How are minerals separated?
The principle ion (negative ion)
or
Anionic Group (negative molecule)
How does the environment in which carbon cools determine the polymorph it ends up as?
- Diamond: Each C bound to 4 other C via covalent bonds.
Graphite: Each C bonded to 3 other C, spare electron is delocalised. This also has Van de Waals forces.
How do we view inside crystals?
- X ray diffraction usually
But also electron microscopy can be used
How do we use TEMs to view inside crystals?
- Shoot a beam of electrons at a crystal
- Electrons pass through spaces, reaching a detector
- Electrons interacting with atoms don’t reach detector
- Dark and light pattern images crystal atoms
Describe the mineral class: Oxides
- Metal cations bonded to oxygen
- Includes magnetite
Describe the mineral class: Sulphides
- Metal cations bonded to sulfide anion
- Includes pyrite
Describe the mineral class: Sulphates
- Metal cation bonded to sulphate anionic group
- Includes gypsum
- Often evaporite deposits
Describe the mineral class: Halides
- Cl⁻ or F⁻
- Includes Halite
- Often evaporite deposits
Describe the mineral class: Carbonates
- CO₃²⁻
- Includes calcite and dolomite
What are native elements?
Pure masses of a single metal, such as gold.
Describe silicate mineral structure.
- SiO₄⁴⁻ : Silicon oxygen tetrahedron
- 4 O atoms bonded to Si
- Define corners of 4 sided geometric figure
- Divided into groups based on ratio of Si:O (important for melting temperature, mineral structure, susceptibility to chemical weathering, cations present)
What are silicate independent tetrahedra?
- Si:O = 1:4
- Share no oxygens
- Linked by cations
- Include garnet
What are single chain silicate minerals?
- Si:O = 1:3
- One shares two oxygens
- Pyroxenes
What are framework silicate minerals?
- Si:O = 1:2
- All 4 oxygens in each tetrahedra are shared
- Include feldspars and silica
What are sheet silicate minerals?
- Si:O = 2:5
- Silica tetrahedra share 3 oxygens
- Create 2D flat sheets of tetrahedra
- One direction of perfect cleavafe
- Include Micas like biotite and Clays
How does the melting of the continental crust create magma and its residue?
- When melting, only a portion of the rock melts. Si rich minerals melt first, so a partial melt creates:
- Felsic magma
- Mafic residue
What did N.L.Bowen experiment and find out?
- Experimented cooling melts, with the remaining melt being enriched in Si, Al % Na.
- He discovered minerals solidify in a specific series.
What are the two types of mineral solidification?
Continuous: Plagioclase (Al rich mineral in mafic rocks) changed from Ca to Na rich.
Discontinuous: Minerals start and stop crystallising.
How are metamorphic rocks created?
- Plate tectonic collisions that thicken the crust
- Create new minerals as shearing field rocks. These are then exposed by uplift and erosion.
- side note, this liberates CO2.
Define weathering.
Disintegration and decomposition of rocks and sediments mechanically or chemically in situ.
Define erosion.
Process in which primarily rock or soil are detached and transported by weather.
Describe frost wedging.
- Rain goes into crack
- Crack freezes and therefore expands
- Rock break
Describe Salt Wedging.
- Salt water gets into rock
- Dries, salt crystals expand
- Rock break
Describe Thermal expansion and contraction.
- Minerals and expand and contract due to extreme diurnal temperature ranges
Describe exfoliation.
- Spalling (removal) of surface layers
- Due to release of lithostatic pressure and expansion as rock is exhumed
- Due to chemical + physical weathering
Describe root wedging.
Biological weathering. Just roots in a joint growing.
Describe abrasion.
Wind action.
Describe dissolution.
- Reaction between minerals and acidic/alkaline water (mostly slightly acidic rainwater)
- Dissolves calcite in limestone
- Known as carbonation
- Also some is caused just by polarity of water
Describe Hydration weathering.
Absorption of water.
- Example being Anhydrite (CaSO⁴) accepts water to become gypsum.
Describe Hydrolysis.
- Reaction between H+ and OH- ions of water and the mineral.
- With silicate this is a negative feedback cycle as silicate weathering consumes CO2 which cools the climate and decreases chemical weathering.
What are the effects of metamorphism and volcanism have on CO2?
Metamorphism - liberates CO2
Volcanism - Releases Co2
