Ch. 5: Minerals

Intro

mineral: naturally occurring, solids made of specific elements, arranged in a particular repeating 3D structure

  • “naturally occurring”: minerals can be formed from substances and under conditions found in nature, not by other forces
  • “solid”: solid at 25 degrees Celsius   * some exceptions, such as ice and mercury
  • “specific elements”: minerals have a specific chemical formula or composition
  • solid solution: substitutions of an element in a molecule that can be allowed   * ex. olivine can range from having Fe or Mg
  • lattice: a specific, repeating 3D structure
  • All minerals are crystals, because of this lattice
  • ex. halite (NaCl) always appears in the same crystal lattice and always have NaCl

anthropogenic: man-made materials

  • materials made in this way are NOT counted as minerals

mineraloids: mineral-like substances that fulfill the other requirements of being a mineral, but do not have a crystalline structure

5.1 Atoms

Atoms are made up of protons, neutrons, and electrons

nucleus: formed by protons and neutrons

isotopes: forms of an element with different # of neutrons

atomic number: number of protons in an element

mass number: total number of all subatomic particles (usually protons and neutrons)

As # of protons increase, the proton to neutron ratio is no longer 1:1 in order to stabilize the atom

Electrons control the way atoms interact with other atoms

energy levels: shells of the atom where electrons orbit

inert: non-reactive, have a full outer shell

  • ex. noble gases (helium, neon, argon, krypton, radon)

5.2 Bonding and Lattices

ionic bonds: a bond between two atoms where one is positively charged, and the other is negatively charged and therefore attracted to each other.

ion: atom or molecule with a charge

cation: positive-charged ion

anion: negative-charged ion

covalent bond: sharing electrons rather than giving them away, regardless of charge

Van der Waals forces: weak intermolecular forces that are dependent on the distance between atoms or molecules

hydrogen bonds: dipole-dipole attraction between molecules with hydrogen present

metallic bonding: occurs in metallic elements where outer electrons are relatively loosely held

  • results in dissociated electrons that can move freely from one atom to another
  • gives two properties for metals:   * electrical conductivity   * malleability

5.3 Mineral Groups

oxide minerals: have oxygen (O^2-) as their anion

  • does not include anion groups with other elements, like carbonate or sulphate
  • hydroxide: mineral with hydroxyl anion (OH^-)

sulphide minerals: has sulfur (S^2-) as their anion

sulphate minerals: has sulphate (SO4^2-) as their anion

halide minerals: has halogens (F, Cl, etc.) as their anion

carbonate minerals: has carbonate (CO3^2-) as their anion

phosphate minerals: has phosphate (PO4^3-) as their anion

silicates: has silicate (SiO4^4-) as their anion

native element minerals: minerals made up of a single element

5.4 Silicate Minerals

silicate tetrahedron: SiO4^-4, four-sided pyramid shape with oxygen at each corner, and silicon in the center. Building block of many important minerals in the crust and mantle

In silicate minerals, the tetrahedra are arranged and linked together in lots of ways:

  • single units
  • chains
  • rings
  • other complex frameworks

With simpler structures, like single units, the melting temperature is comparatively higher than other silicates.

As the Si:O increases, the lower the melting point becomes.

olivine: the simplest silicate structure composed of isolated tetrahedra bonded to iron and/or magnesium ions

Iron and magnesium can be switched out for each other because they are a similar radius.

single-chain silicate: silicate where one oxygen from each tetrahedron is shared with the next tetrahedron, with a Si:O of 1:3, since the ends share an oxygen atom to complete the tetrahedra

  • ex. pyroxene (NaFeSi2O6)

double-chain silicate: silicate where the Si:O is higher than single-chain silicates (like 4:11), where fewer cations are needed to balance the charge

  • ex. amphibole

sheet silicates: silicates where the tetrahedra are arranged in continuous sheets, where each tetrahedron shares three oxygen atoms, and even fewer cations are needed to balance the charge

  • ex. micas (biotite), clay minerals (kaolinite, illite, smectite)
  • typically occur in clay-like fragments

framework silicates: silicates where tetrahedra are connected to each other in 3D structures rather than 2D chains or sheets

  • ex. feldspars, quartz   * feldspars: group of very abundant framework silicates in Earth’s crust   * quartz: framework silicates, where silica tetrahedra are the only ones to be present (SiO2)

5.5: How Minerals Form

Has following criteria in order for mineral crystals to grow:

  • the elements needed must be present in sufficient

  quantities AND in appropriate proportions

  • physical and chemical conditions must be favorable   * includes factors like temperature, pressure, amount of oxygen available, pH, presence of water
  • sufficient time for atoms to become arranged into a lattice   * most important factor! if not enough time, mineral grains may be very small

Most minerals that make up rocks in crust and mantle formed through cooling and freezing of melted rock

Minerals can also grow in other ways:

  • precipitation from a solution   * from hot water flowing underground, or when evaporation concentrates ions in a lake or inland sea
  • precipitation from a gas   * from vents releasing volcanic gases
  • metamorphism
  • weathering   * by wind, other rock, water
  • organic formation   * shells, teeth, bones

5.6: Mineral Properties

color: …the color of the mineral

  • some of the same type have multiple colors they can present as   * ex. clear quartz, rose quartz, smoky quartz, amethyst, citrine, etc…

streak: grinding a small amount of the mineral sample to observe the color of the powder

streak plate: a piece of unglazed porcelain used to observe the streak of a mineral

Streak is helpful because similar-looking minerals (ex. hematite and galena) can be distinguished by their streak.

luster: the way light reflects off the surface of a mineral, and how much light penetrates that mineral

  • metallic luster: mostly reflects the light, and doesn’t transmit through it.
  • non-metallic luster: absorbs the light or transmit it in a non-metallic way   * glassy luster: shiny, reflective surface   * earthy luster: dull and non-reflective surface   * resinous luster: honey-like minerals with slightly high refractive indices

hardness: determines whether or not a mineral can be scratched by a particular material

Friedrich Mohs: created hardness scale, with references of hardness by being scratched by a natural fingernail, copper, a steel nail, a glass plate / knife blade, streak plates, or a hardened steel file

As the toughness of a material goes up, fewer materials are able to scratch that material.

  • ex. glass cannot scratch quartz, since it doesn’t have the hardness to withstand scratching it. However, corundum can scratch quartz, since corundum can withstand scratching quartz.

crystal habits: the distinctive formation pattern of minerals when formed in rocks, IF not crowded by other pre-existing minerals

How a mineral breaks can also describe a mineral.

  • cleavage: a clean break along a plane or planes
  • fracture: irregular break
  • can sometimes be difficult to tell if minerals are small, the cleavage is not perfectly flat, or knowing if a flat surface is a cleavage plane, a crystal face, or a surface that just turned out to be flat.

density: mass/volume

  • cannot assess density in minerals that are a small part of a rock with other minerals in it

Other properties not listed:

  • calcite reacts with dilute acid to form CO2 bubbles
  • magnetite is strongly magnetic
  • sphalerite gives off a sulphur-like smell when drawn across a streak plate
  • halite tastes salty
  • talc feels soapy
  • plagioclase feldspar has striations, and when light hits it at the right angle in some varieties, can show an array of colors

\