Mineralogy Flashcards
- What is the definition of a mineral?
Minerals 1
Mineral: a crystalline, homogenous, inorganic solid with a defined chemical composition that occurs naturally.
- Define “crystalline” in minerals:
Minerals 1
Minerals have a crystal structure:
* Their building blocks (atoms, ions, molecules) are arranged in an ordered and repeated pattern.
* The unit cell is the smallest unit that still has the full symmetry of the crystal structure of a material.
* Repeating this unit cell over and over again forms a crystal.
- The ordered atomic network within a crystal can be simple or fairly complex.
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However: some minerals are not (fully crystalline).
- These are sometimes called a mineraloid (e.g, Opal [SiO2nH2O] can be microcrystalline or completely without any crystal structure)
- Others include:
- pearl,
- jet,
- obsidian.
- Define “homogenous” in minerals:
Minerals 1
Following from the indefinitely repeatable unit cell of the crystal structure, minerals should by definition be homogenous
- (minerals with uniform chemical composition and internal structure throughout)
- However:
- Zonations (i.e., “Watermelon” tourmaline)
- Crystal defects.
- Define “inorganic” in minerals:
Minerals 1
Minerals are inorganic substances:
- Inorganic being defined as “not organic”
Organic substances typically involve:
1. carbon-carbon and/or carbon-hydrogen bonds
1. molecules of C ± H, O, N (and P)
2. no carbon chains in mineralogy (except alcohol in beer/gin)
2. sugars, fats, oils, proteins and plastic
Instead, minerals typically are a COMBINATION OF SEVERAL OTHER ELEMENTS.
However:
1. Biominerals are formed by a living organism usually inorganic in composition but may contain organic material.
2. Amber is fossilised tree resin with an organic composition (gemstone)
- Define “solids” in minerals:
Minerals 1
Minerals are (crystalline) solids at Earth surface conditions:
However:
1. Mercury is liquid (melting point: -39C)
2. Ice is crystalline (melting point: 0C)
- Define “defined chemical composition” in minerals:
Minerals 1
In many minerals, one element can easily be substituted with another, and their relative proportions vary:
- The mineral is then referred to as a solid solution of/between two endmembers
However:
Same composition does not require same crystal structure:
- Polymorphs are minerals with the same composition, but different chemical structure.
- (i.e., graphite and diamond)
- Define “naturally” in minerals:
Minerals 1
Minerals typically form in natural processes, when atoms arrange themselves into a crystal structure by:
1. crystallisation of a magma due to cooling (effectively the same as freezing)
1. Magma cools below its liquidus, and starts to crystallise minerals. The mix of melt + minerals keeps on crystallising until it “hits” the solidus, now all melt has solidified.
2. precipitation from a solution, by evaporation, other change in solubility, or aided by an organism.
3. (solid state) rearrangement of elements due to changes in pressure and temperature
However:
There are synthetic minerals:
1. Diamond (C)
2. Ruby (Al2O3 (w/ traces of Cr2O3)
- What is the smallest building block?
Minerals 1
** *Atoms are the building blocks of matter.
* They have a dense core (or nucleus) of positively-charged protons and neutral neutrons.
* The core is surrounded by a cloud of negatively-charged electrons
* The number of electrons is equal to the number of protons, so their charges are balanced.
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Ions are atoms or molecules with a number of electrons that does not match with number of protons.
* They have a positive or negative charge.
* An ion with a negative charge (e- > p+) is called anion.
* An ion with a positive charge (e- < p+) is called cation.
- How do we bind atoms together?
Minerals 2
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Metallic bonds:
are a type of chemical bonds where there is an electrostatic attractive force between delocalised electrons and positively charged metal ions (in the form of sharing free electrons among a structure of cations)- In pure metals, the electrons are free to roam across the metal.
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Covalent bonds:
are a type of chemical bonds where two atoms share one or more of their electrons to form a stable molecule.- The covalent bonds don’t have to be between atoms of the same elements
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Covalent bonds are very strong, especially between atoms of the same element.
- All carbon in diamond is covalently bonded.
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Electronegativity:
Atoms of different elements have different strengths in attracting electrons: this is measured as electronegativity.- It is a function of the number of protons (→ size of the core) and how far the outermost electrons are from the core.
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Ionic bonds:
are a type of chemical bonds where two atoms with sharply contrasting electronegativities form an electrostatic attraction that occur between a metal (gives the electron) and a non-metal (takes away the electron) to form a cation and anion.- A typical example for an ionic bond is halite (NaCl), or table salt.
- Sodium is a metal that readily can lose an electron to a non-metal like chlorine.
- A typical example for an ionic bond is halite (NaCl), or table salt.
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Other weaker bonds:
Not all bonds involve sharing or the transfer of electrons.- Intermolecular bonds can occur when a molecule has a slightly asymmetric charge.
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Mix the bonds:
Often, bonds are not purely ionic or covalent, but of ionic or covalent “character” (many minerals haave more than one type of bond)
- What are the definitions of coorddination and site?
Minerals 2
- Coordination: the number of direct neighbours that an atom/ion is bonded to in a crystal structure. Typically we talk about cations and their surrounding ion-neighbours.
- Site: a space in a crystal lattice that can be occupied by an atom/ion. It is typically named by its coordination.
- What are some examples of coordination?
Minerals 2
- Tetrahedral: or 4-fold coordination (Si by 4 O in quartz)
- Octahedral: or 8-fold coordiantion (Na by CI in halite)
- Cubic: or 8-fold coordination (Ca by F in fluorite)
- What are some examples of sites?
Minerals 2
- A crystal lattice made up of CI- (chloride ion) anions has gaps/holes that are surrounded by 6 CI-
- These octahedral sites can be occupied by cations:
- In halite, sodium chloride, they are occupied by Na+ (sodium)
- In slyvite, potassium chloride, they are occupied by K+ (potassium)
- In more complicated crystal lattices, it is not necessarily the case that all ions have the same coordination.
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Sites in a crystal structure have a certain size. Simply put, the size is determined by the ideal crystal sturcture of a mineral.
- The coordination also affects the site size:
- Bond distances are smaller when shared with less atoms (less sharing, stronger pull):
1. site size goes up, as coordination goes up.
2. tetrahedral is smaller than octahedral and so on.
- What is atomic/ionic radius?
Minerals 2
The atomic radius of a chemical element is a measure for the size of its atoms (and ions).
On the periodic table, atomic radii:
- decrease along a group (=row) because the charge increases (the core pulls more strongly on the electrons)
- increase within a group (=column) because of the number of protons and electrons (and electron shells) increases.
- What is compatibility?
Minerals 2
Since sites in a crystal lattice has a given size:
- Atoms/ions in a crystal lattice can be substituted by other elements, as long as their radius is similar.
- Ideally, their charge would also be same!
- If an element fits readily into a crystal structure, it is called compatible.
- What is an example of compatibility?
Minerals 2
Minerals that have Mg2+ (magnesium cation) in a certain site, can often also accommodate Fe2+ in the same site as an alternative, because their radius and charge is very similar.
What are sites?
Minerals 2
A space in a crystal lattice that can be occupied by an atom/ion. It is typically named by its coordination.
* A crystal lattice made up of CI- anions has gaps/holes that are surrounded by 6 CI-.
* These octahedral sites can be occupied by cations.
* In halite, sodium chloride, they are occupied by Na+.
* In sylvite, potassium chloride, they are occupied by K+.
Sites in a crystal structure have a certain size. Simply put, the size is determined by the ideal crystal structure of a mineral.
1. The coordination also affects the site size.
2. Bond distances are smaller when shared with less atoms (less sharing, stronger pull).
3. Site size goes up, as coordination goes up.
4. Tetrahedral is smaller than octahedral and so on.
What are atomic/ionic radius?
Minerals 2
The atomic radius of a chemical element is a measure for the size of its atoms (and ions).
On the periodic table, atomic radii:
- decrease along a period (=row) because the charge increases (the core pulls more strongly on the electrons)
- increase within a group (=column) because the number of protons and electrons (and electron shells) increases.
What is the (Nickel-) Strunz classification?
Minerals 2
Karl Hugo Strunz developed a classification scheme for minerals based on their anions:
- These anions can be:
- single ions: Cl- , S2-…
- polyatomic ions: [CO3] 2-, [SiO4] 4
Strunz mineral classes:
1. elements
2. sulfides and sulfosalts
3. halides
4. oxides, hydroxides and arsenites
5. carbonates and nitrates
6. borates
7. sulfates, chromates, molybdates and tungstates
8. phosphates, arsenates and vanadates
9. silicates and germanates
10. organic compounds
