Minerals and Mineralogy

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Minerals and Mineralogy

• A/Prof John M. Worden
• DEC
• University of Southern Queensland

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Minerals and Mineralogy

• What is a mineral?
• A naturally occurring,
• -solid, inorganic crystalline substance,
• -with a specific composition,
• -and characteristic atomic structure.
• Approximately 100 minerals are common.
• Of these, about 30 comprise the rock forming
  minerals and make up most Crustal rocks.
• A select group of minerals are of economic
  interest and these are termed ore minerals.

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Minerals and Mineralogy

• Minerals are the building blocks of rocks.
• So, we need to briefly re-visit basic chemical
  concepts to understand both their chemistry and
  physical properties.
• Smallest units of matter are atoms that retain
  physical chemical properties of an element.
• Each atom has a dense nucleus at its centre,
  containing positively charged protons and
  neutral neutrons.
• The number of protons is constant for an
  element, and defines its atomic number (z)
• The atomic massis the ? ( protons
  neutrons).
• A cloud of moving, negatively charged
  electrons surround the nucleus.
• The number of electrons equals the number
  ofprotons, so an atom is electrically neutral.

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Minerals and Mineralogy

• Electrons occupy orbitals, or more simply,
  spherical shells around the nucleus.
• An atoms electronic structure determines its
  interactions with atoms of another element to
  form chemical compounds.
• We must examine the fine structure of electron
  orbitals to understand this process.
• Each orbital or electron shell can hold a
  specified maximum number of electrons (ie ,K 2
  L8 M 18 N 32 etc).
• A complete outermost shell is the most stable
  configuration
• In chemical reactions, ONLY the electrons in the
  outermost shells interact to attain maximum
  stability.

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Minerals and Mineralogy

• Electrons may be exchanged between atoms of two
  elements thereby forming a new compound.
• For Sodium (Na z 11) and Chlorine (Cl z17)
  atoms
• Na atoms surrender their single outermost
  electrons leaving the next inner shell complete
  ( L8), while Cl atoms accept electrons to
  complete part of their outermost shell (M8).
  Both become charged IONS.
• Cations (Na) and Anions (Cl-) are charged
  ions in which electron shells are either depleted
  or supplemented.
• Ionic bonds form between anions cations due to
  electrical attraction.
• They are the dominant type (gt90) of chemical
  bonding found in minerals.

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Minerals and Mineralogy

• Elements that do not readily gain or lose
  electrons share electrons, forming Covalent
  bonds
• Covalent bonds are stronger than ionic bonds.
• An example is Diamond, where each C atom (z6)
  covalently bonds with four surrounding C atoms so
  that L8, forming a regular tetrahedron.
• Metallic bonds are a type of covalent bond
  found in native metals (ie Cu Ag Au Te) and
  some sulfides.
• Van der Waals bonds result from weak
  electrostatic attraction between atoms.
• Complex ions, ie SiO4 are strongly-bonded
  covalent pairs that act as single ions.
• All minerals are combinations of these bonding
  types in regular, repeated arrays, or crystal
  lattices.

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Minerals and Mineralogy

• Minerals are grouped into 8 classes (Native
  elements Oxides Hydroxides Halides
  Carbonates Sulfates Silicates).
• By far the most dominant mineral group are the
  Silicates, based on the silicate ion, or SiO44-
  tetrahedron. The silicon ion is surrounded by
  four oxygen ions that can bond with other ions.
• Various bonding linkages result in isolated
  tetrahedra single or double chains sheets and
  3D frameworks.
• Oxygen-Oxygen bonds are very strong, whereas
  Oxygen-Cation bonds are much weaker.

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Minerals and Mineralogy

• The common rock-forming minerals are combinations
  of Al, Fe, Ca, Na, K, and Mg with SiO4.
• These elements are the most abundant in the
  Earths crust.
• Bonding types of various silicates reflect
  temperatures of crystallisation , so that
  isolated tetrahedra linked through cations form
  at highest temperatures, ie Olivine.
• As temperature declines, atom vibrational
  energies decline and more complex bonds result.
• Single chain silicate minerals form next,
  sharing two apical oxygens, ie Augite.
• Next, Double chain silicates, which share
  three apical oxygens crystallise,ie Hornblende.

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Minerals and Mineralogy

• This continues with Sheetsilicates, (mica
  minerals) the Framework minerals forming
  last, ie Biotite Quartz.
• Crystal lattices control the external shape and
  surfaces of a mineral. The planar surfaces are
  termed crystal faces and their combinations and
  interfacial angles are specific for each
  mineral.
• A crystal will only possess perfect form if it
  crystallises in a fluid or in unconstrained
  space.
• Confinement means interlocking grain boundaries
  and the formation of aggregates.

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Minerals and Mineralogy

• Summary
• Minerals must be naturally formed.
• They must be inorganic.
• Minerals must be solid and crystalline.
• Have a specific chemical composition.
• They must possess a characteristic crystal
  structure.

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Minerals and Mineralogy

• Physical properties of minerals reflect the
  elements, bonding types and crystal lattices of
  each mineral.
• Physical properties include
• Colour Streak
• Lustre Diaphaneity
• Hardness Cleavage
• Fracture Tenacity
• Crystal Form Twinning
• Specific Gravity Other

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Minerals and Mineralogy

• Colour
• Very obvious
• Intrinsic
• May be due to impurities and highly unreliable,
  and
• Note the great diversity of colours exhibited by
  Quartz (SiO2). For example, amethyst, citrine,
  rose quartz, etc.
• Streak
• The colour of a powder of a mineral.
• More reliable than that of the massive mineral.
• Attributed to the grain size effect.

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Minerals and Mineralogy

• Lustre
• Quality and intensity of reflected light from a
  mineral surface.
• Subdivided into
• Metallic
• Non-metallic
• Non-metallic further subdivided into
• vitreous (glass-like)
• pearly (pearl-like)
• greasy- (as if covered with an oil film)
• resinous (like resin)
• dull (no discernible lustre)
• etc.

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Minerals and Mineralogy

• Diaphaneity
• Refers to the degree of Transparency.
• Transparent
• Translucent
• Opaque.
• Tenacity
• Manner in which a mineral breaks down or deforms
  under stress.
• Most minerals are brittle and shatter!
• The micas (biotite muscovite) are flexible and
  elastic.

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Minerals and Mineralogy

• Hardness
• A measure of the ability of a mineral to resist
  abrasion or scratching.
• Reflects the strength of the bonds between atoms.
• Mohs Hardness Scale from Talc (H1) to Diamond
  (H10).
• Cleavage
• The tendency of a mineral to break along smooth,
  parallel planes.
• Due to the presence of weak bonds between layers
  of atoms in crystals.

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Minerals and Mineralogy

• When describing cleavage note
• The number of cleavage sets
• The angles between the cleavage planes, and
• The developed quality/perfection of each cleavage
  set.
• Crystal Form
• Sets of crystal faces define a crystal form,
  which is a diagnostic property ofa mineral.
• Interfacial angles remain constant for any
  particular mineral.

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Minerals and Mineralogy

• Twinning
• If a crystal lattice alters orientation about a
  definite plane, the crystal is said to be
  twinned.
• May be a highly diagnostic property (i.e.
  plagioclase felspar, and calcite).
• Specific Gravity
• The ratio of the weight of a mineral to the
  weight of an equal volume of water.
• Heft method of specific gravity estimation
  used in the field.

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Minerals and Mineralogy

• Other Properties
• Effervescence Reaction to dilute Hydrochloric
  acid and release of gas (i.e. Calcite HCl?
  releases CO2 gas) .
• Feel/ Touch Greasy feel like soap (i.e. Talc).
• Magnetism Magnetic attraction or repulsion
  (i.e. Magnetite, Fe3O4) .
• These physical properties are used to identify
  rock-forming minerals in hand specimens and in
  rocks.