5.minerals

Question 1

What is a mineral?

Answer 1

Minerals are the building blocks of rocks.
It must be naturally occurring.
Inorganic
Solid
With an ordered crystalline structure (the atoms are arranged in an ordered, repetitive manner).
And a characteristic chemical composition.

Question 2

What is an isotope?

Answer 2

• Equal number of protons & electrons but different number of neutrons
  Isotopes of an element react the same way chemically - Different isotopes of one element can be part of the same mineral
  Many isotopes have instable cores - they disintegrate through radioactive decay to stable isotopes of other elements - energy and particles are emitted.
  The rate of decay can be measured - dating of material (ex: dating of organic compounds).

Question 3

How different atomic bondings affect the mineral properties?

Answer 3

• Ionic bonding: Transference of electrons. Weak bonds. Form minerals with low to moderate degrees of hardness and density, moderate melting points, high degree of crystal symmetry, poor conductors of heat.
• Covalent bonding: Sharing electrons. Strongest type of bonds. Forms hard, insoluble minerals with high melting point, nonconductive, “low” degree of crystal symmetry.
• Metallic bonding: Forms yield minerals that are soft, ductile / malleable, highly conductive due to easily mobile electrons - metals. Non-directional bonding produces high crystal symmetry.
• Residual bonding (van der Waals bonding): created by weak bonding of oppositely dipolarised electron clouds. Commonly around covalently bonded elements. It forms solids that are soft, very poor conductors, have low melting points, low crystal symmetry
• Hydrogen bonding: Water molecules have a positive charge near H because of a concentration of electrons and a negative charge at the other side of the molecule. It forms weak bonds between water molecules. Example: ice

Question 4

Why some minerals have a higher density than others?

Answer 4

Density depends on the atomic mass of a mineral’s ions and how closely they are packed in its crystal structure.
Example: The density of the iron silicate olivine, 4.4 g/cm3, is lower than that of magnetite for two reasons. First, the atomic mass of silicon, one of the elements that make up olivine, is lower than that of iron. Second, this olivine has a more openly packed structure than minerals of the spinel group. The density of magnesium olivine is even lower, 3.32 g/cm3, because magnesium’s atomic mass is much lower than that of iron.

Question 5

Why do the colours of the same mineral can vary? .

Answer 5

The color of a mineral is imparted by light—either transmitted through or reflected by crystals or irregular masses—or a streak.
Color is determined both by the kinds of ions found in the pure mineral and by trace impurities

Question 6

What are the properties that can be used to describe and identify minerals? -

Answer 6

-Colour: the colour you see. Red, green, blue, grey (and other) colour shades may - in some cases - be indicative for a specific mineral. “Colourless” also is an option.
- Lustre: Vitreous (glass-like), fat (greasy), pearly, dull, resinous, silky, adamantine, metallic,…
- Streak: the color of the fine deposit of mineral dust left on an abrasive surface, such as a tile of un-glazed porcelain, when a mineral is scraped across it.
- Transparency: Is the mineral opaque or is it possible to see through it? This property is NOT the same as colour.
- Hardness (H): In Moh’s scale of hardness the
softest material has H = 1 and the hardest has H = 10. Different minerals define the numbers 1 to 10.
- Development of the cleavage: Is the cleavage obvious or only weakly
developed?
- Cleavage angles: Does the cleavage occur in more than one direction? If yes: which angle do the cleavages have to each other?
- Fracture: Look at fractured (broken) surfaces - is the fracture conchoidal (musslig), fibrous, or hackly (tagget)?
- Tenacity: rigid, brittle, ductile, etc.
- Crystal planes: What angles do the crystal faces
have to each other (interfacial angles)? You may draw a sketch in addition to the text description.
- Habit: Describe the shape of the crystals. Ex: acicular (needle-like), columnar, tabular, cubic, platy, foliated.
- Twins: Sometimes mineral crystals form twins - similar to Siamese twins.
- Density: Compare the density to another mineral - is the density higher or lower? Do not confuse “density” with “weight” - if you compare two minerals with very different sizes, the larger one will (in most cases) be the heavier one, irrespective of their density differences.
- Magnetic effect: Chose a mineral that is magnetic. Is the magnetic effect weak or strong?
- Solubility: Chose a mineral that dissolve with HCl. Is the effect strong or weak?
- Surface roughness: Feel with your fingertip on the mineral surface. Is the sensation soft or rough? Make a subjective scale from very soft to very rough.
- Taste, solubility in water, elasticity, plasticity, double refraction, smell, or something else

Question 7

What are the main mineral classes? (9)

Answer 7

Silicates
Oxides
Hydroxides
Sulfides
Sulfates
Carbonates
Phosphates
Halides
Native elements

Question 8

What is the difference between different types of silicates?

Answer 8

• Nesosilicates: Isolated single tetrahedra (olivine, garnet)
• Sorosilicates: Isolated double tetrahedra (epidote)
• Cyclosilicates: Closed ring of 6 tetrahedra (tourmaline)
• Single-chain inosilicates: interlocking single chains of tetrahedra (augite (pyroxene))
• Double-chain inosilicates: interlocking double chains of tetrahedra (hornblende (amphibole))
• Phyllosilicates: continuous sheets of tetrahedra (Biotite, Muscovite, Glauconite, Illite, Kaolinite, Chlorite)
• Tectosilicates: 3D framework of tetrahedra - all 4 O bond to an other tetrahedra (Alkalifeldspar, Plagioclase, quartz)

Question 9

How (silicate) minerals can form?

Answer 9

Formation of silicate minerals
1. Magmatic processes
- Most silicates crystallize from molten rock.
- The chemical composition of the magma + P & T determine which mineral forms
- Examples of crysallisation temperatures:
Olivine (nesosilicate - simple crystal structure) crystallises at 1200°C
Quartz (tectosilicate - complex crystal structure) crystallises at 700°C
2. Weathering and diagenesis
- Some silicate minerals form at the Earth’s surface as weathered products of other silicate minerals (e.g., feldspar - clay minerals)
3. Metamorphism
- Some silicate minerals need high P to form (e.g., epidoite).
- Metamorphic minerals form from other minerals (e. g., clay minerals, chlorite)