Transcript Minerals and Mineralogy - University of Southern Queensland

Minerals and Mineralogy
A/Prof John M. Worden
DEC
University of Southern Queensland
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”.
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 mass”is the  (# protons + # neutrons).
 A cloud of moving, negatively charged “electrons”
surround the nucleus.
 The number of electrons equals the number of
protons, so an atom is electrically neutral.
Minerals and Mineralogy
 Electrons occupy orbitals, or more simply, spherical shells around the
nucleus.
 An atom’s 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; L=8; 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.
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; z=17) atoms:
 Na atoms surrender their single outermost electrons leaving the next inner
shell complete ( L=8), while Cl atoms accept electrons to complete part of
their outermost shell (M=8). 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 (>90%) of chemical
bonding found in minerals.
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 (z=6) covalently bonds with
four surrounding C atoms so that L=8, 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.
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 [SiO4]4- 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.
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 Earth’s 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.
Minerals and Mineralogy
 This continues with “Sheet”silicates, (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.
Minerals and Mineralogy
 Summary
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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.
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
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.
Minerals and Mineralogy
 Lustre:
 Quality and intensity of reflected light from a mineral surface.
 Subdivided into:
• Metallic
• Non-metallic
 Non-metallic further subdivided into:
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vitreous – (glass-like)
pearly – (pearl-like)
greasy- (as if covered with an oil film)
resinous – (like resin)
dull – (no discernible lustre)
etc.
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.
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 (H=1) to Diamond (H=10).
 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.
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 of
a mineral.
Interfacial angles remain constant for
any particular mineral.
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.
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.