Mineral
Naturally occurring solid, crystalline structure with a definite chemical composition. Mostly inorganic.
Formation of Minerals
1. Crystallization: cooling of a melt.
2. Precipitation from a dissolved state in water or from a gas.
3. Chemical reactions at high pressures and temps.
4. Organisms create minerals: bones (apatite), shells (aragonite).
5. Solid-state diffusion
Crystalline Structure
Atoms are arranged in a specific order.
Glass
Not a mineral. Lacks crystalline structure. Disordered atoms.
Crystal
- A solid substance in which the atoms or ions are arranged in an orderly pattern that repeats over and over again.
- Atoms held in place by chemical bonds.
- Physical Properties depend upon: Identity of atoms, arrangement of atoms, nature of atomic bonds
Bonding and Mineral Properties
Stronger Bonds: harder/higher melting points.
Weaker Bonds: softer, lower melting points.
Bonds May Vary by Direction
-Faster growth where bonds form more easily.
- The direction of weaker bonds controls breakage.
Polymorph
One of two or more alternative possible crystal structures for a single chemical compound; for example, diamond and graphite are carbon polymorphs.
Diamonds
Originate under extremely high pressure (150 km deep in the upper mantle. Found in kimberlite pipes.
Ion
atom charged due to gain or loss of an electron.
Cation
positive ion due to loss of electrons.
Anion
negative ion due to gain of electrons.
Ionic Size
depends on number of electrons.
Atomic Packing
Packing configurations define a geometric shape.
Large central cation - Larger number of anions.
Small central cation- Smaller number of anions.
X-ray Diffraction (XRD)
probes crystal lattices. Unique lattice spacing is used to ID minerals.
Transmission Electron Microscope (TEM)
- Shoots a beam of electrons at a crystal.
- Electrons pass through spaces reaching a detector.
- Electrons that interact with atoms don't reach the detector.
Dark and ligt pattern images the atomic crystal lattice.
Formation of Crystals
Open space (Euhedral)- good crystal faces grow.
Confined space (Anhedral)- no crystal faces. More more common.
Mineral Destruction
Melting- heat breaks atomic bonds.
Dissolving- solvents (mostly water).
Chemical Reaction
Fracture
Minerals break in ways that reflect atomic bonding.
Fracturing implies equal bond strength in all directions.
Cleavage
Tendency to break along planes of weaker atomic bonds.
1,2,3,4, and 6 cleavages are possible.
Silicates
Most important mineral class. Rock forming minerals. Composition of the crust and mantle.
Mineral Classes
Oxides: Metal cations bonded to oxygen (Ex: Magnetite, Hematite, Rutile)
Sulfides: Metal cations bonded to sulfide anion. (Ex: Pyrite, Galena,
Sulfates: Metal cation bonded to a sulfate anionic group. Form from evaporation of seawater. (Ex: gypsum, Anhydr
Native Elements
a mineral that is composed of only one element (i.e. copper (Cu), gold (Au).)
Only about 50 out of 4000 are abundant.
...
98% of crustal mineral mass is from 8 elements. Oxygen and Silicon are most abundant.
...
Silicate Minerals
Tetrahedral.
4 atoms are bonded to an Si atom.
Si:O Ratio
Controls:
Melting temperature
Mineral structure and cations present
Susceptibility to chemical weathering
Independent Tetrahedra
Si:O 1:4
Share no oxygens.
Linked by cations.
Ex: Olivine and Garnet
Single Chains
Si:0 1:3
Silica tetrahedra link to share 2 oxygens.
Ex: Pyroxenes
Double Chains
Si:O 2:7
Silica Tetrahedra alternate sharing 2 or 3 oxygens.
Ex: Amphiboles
Sheet Silicates
Si:O 2:5
Silica tetrahedra share 3 oxygens
2 dimensional flat sheets.
Ex: Biotite and Muscovite
Framework Silicates
Si:O 1:2
All 4 oxygens in each silica tetrahedra are shared.
Ex: Feldspars and Silica