F1 Minerals and Rocks

• What is a mineral?

  • Naturally occurring, inorganic solid
  • Ordered atomic arrangements
  • Fixed chemical composition - can vary within certain limits and these variations are due to impurities. For example, impurities within quartz produce amethyst, rose quartz etc.
  • Minerals with the same composition but different structures are termed polymorphs
• What is a rock?

  • A rock is an aggregate of minerals - crystals of one or more minerals, fragments (clasts) of one or more minerals or clasts of pre existing rocks.
• Crystal Shape

  • Crystal shape: When there is space to grow, minerals will crystallise into characteristic crystal shapes.
  • Form: pattern of faces and angles between them (set of crystal faces)
  • Habit: external shape of an individual crystal
  • Cubic - cube shaped
  • Octahedral - 8 sides
  • Dodecahedral - 12 sides
  • Hexagonal - hexagonal prisms
  • Bladed
  • Botryoidal - kidney shaped
  • Fibrous
• Defects and Twinning

  • Virtually all crystals contain minute imperfections or defects
  • Several types: Point, Line, Planar
  • Point: involve missing or displace atoms in the crystal structure
  • Line: caused by rows of atoms that are out of place in the crystal
  • Planar: Crystals grow by the progressive addition of atoms onto a surface. Planar defects are ‘mistakes’ in the stacking of new planes with respect to previously formed planes. These can produce distinct regions called domains within a single crystal.
• • Twinning: Simple
  • Two mirror images of the crystals are effectively joined together - arrowhead shape
  • Twinning: Lamella
  • Twins stacked side by side
  • Twinning: Crosshatched
  • Crosshatched shape
• Colour

  • The appearance of the mineral in reflected/normal light
  • The issue with colour: The same mineral may have different colours and different minerals can have the same colour.
• Streak

  • Streak is the property whereby a mineral leaves a crushed powder on an unglazed porcelain plate
  • Streak is much more reliable than colour and mineral colour is not always the same as the streak.
• Hardness

  • Resistance to abrasion
  • Measured using the Moh’s scale
  • A fingernail scratches minerals with a hardness of 1-2
  • A copper coin scratches minerals with a hardness of 1,2,3
  • A steel nail scratches minerals of hardness 1,2,3,4,5
  • Minerals harder than 6.5 scratch glass
• Cleavage & Fracture

  • Cleavage: the tendency for a mineral to break along lattice planes with weaker atomic bonds
  • Fracture: the way that brittle minerals break or fracture when they do not follow any natural planes of separation
  • Cleavage planes are smooth and shiny, fracture planes are rough and dull
• Types of cleavage

  • 1 cleavage (Basal) - only one cleavage plane. Minerals with basal cleavage can sometimes be peeled. E.g. Mica minerals
  • 2 at 90 (Blocky) - 2 cleavage planes at 90 degrees to each other.
  • 2 not at 90 - 2 cleavage planes not at 90 degrees to each other
  • 3 at 90 (Cubic) - 3 at 90 degrees to each other
  • 3 not at 90 (Rhombohedral) - 3 cleavage planes not at 90 degrees to each other
  • 4 cleavage directions (Octahedral)
• Lustre

  • Describes the way that light reflects from a mineral - how shiny it is
• Density and Heft

  • The density is the mass per unit of volume of the mineral (g/cm3)
  • Heft is the estimate of the density of a sample by holding in hand - useful for metals
• Effervescence

  • Minerals containing carbonate will effervesce (fizz) when in contact with weak acid (HCl).
  • Carbon dioxide is released
• How do minerals form?

  • Cooling from magmas and lavas. Bigger crystals mean slower cooling
  • Precipitation of minerals that were dissolved in water e.g. gypsum and halite
  • Sublimation: transition from a gas phase to a solid without passing through the intermediate liquid phase
• • Crystals act as a seed for further mineral growth. Growth occurs as atoms attach to the outer surface. In an open cavity, crystal faces grow perfectly
  • Mineral growth is often restricted by space
• • Euhedral crystals form when crystals can grow into an open space, resulting in good crystal faces
  • Subhedral crystals have some good faces
  • Anhedral crystals have no good faces
• Density (p) = mass (g) / volume (cm3)
• • Goldschmidt’s classification helps to predict + explain how the layered structure of Earth formed and where elements are most likely to be found
  • Aids subdivisions of the earth based on geochemistry
• • Lithophile - found close to the Earth’s surface as they combine readily with oxygen. They form compounds that do not sink into the core.
  • Lithophile elements include: Aluminium, Potassium, Magnesium, Calcium, Sodium, Oxygen, Silicon, Titanium
• Siderophile - high density materials that tend to sink into the Earth’s core. They do not combine with oxygen and dissolve readily in iron as solid solutions or in the molten state.
• Chalcophile - combine readily with Sulphur and form compounds that do not sink into the core. More concentrated in mineral veins. They include copper and lead.
• Atmophile - Occur in gases and liquid close to the Earth’s surface. They include Carbon, Hydrogen, Nitrogen, Argon, Helium, Neon, Krypton and Xenon
• • Chondrites are formed when various types of dust and small grains present in early solar system accreted to form primitive asteroids
  • Chondrites are undifferentiated and contain silicates, nickel, iron
  • They therefore represent the bulk composition of the Earth and other planets in the inner solar system.
• • For example, we don’t find significant concentrations of iron and nickel in the crust, but they are present in the composition of the earth, so they must be present in the other layers of the Earth
  • Pairing this with other evidence such as seismic waves, mantle xenoliths and earth density data allows us to establish the different Earth layers and their compositions
• • Silicon atoms are cations - they have a charge of 4+
  • Oxygen atoms are anions - charge of 2-
  • The silica tetrahedron is the basic building block of silicate minerals
  • Each silicon atom is surrounded by 4 oxygen atoms
• • A silicon tetrahedron has a charge of -4
  • To balance the remaining charge, metal cations fill the spaces in between the tetrahedra
  • Silicates are categorised based on the arrangement of silica tetrahedra and other atomic components
  • There are five main types of tetrahedra
• Single tetrahedra

  • Independent tetrahedra with cations that fill the space balancing the charge
• Single chains

  • Silica tetrahedra that are linked up in a chain
  • They share two oxygen atoms - oxygens share electrons with two silicon atoms
  • They are weakly paired
  • Single tetrahedra exhibit a cleavage of 2 at 90 degrees
• Double chains

  • Form by sharing either two or three oxygens
  • For example, amphibole
  • They show two cleavage planes not at 90