Transcript SGES 1302 Lecture13

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 13: Minerals
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Lecture 13:
Introduction to Minerals
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Minerals
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Almost every manufactured product contains minerals. They are essential
for the development of a modern society.
All geological processes on earth such as volcanic eruption, earthquake,
landslide, and erosion involve earth materials (and minerals are the building
blocks of earth materials). Basic knowledge of earth materials is essential to
the understanding of all geological phenomena.
Geological defination of mineral: any naturally occurring inorganic solid that
possess an orderly crystalline structure and definite chemical composition.
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Naturally ocurring – formed by natural geologic processes. Exclude all synthetic
materials produced in the lab.
Solid within the temperature ranges normally experienced at Earth’s surface. Ice
is a mineral but water is not.
Orderly crystalline structure – atoms are arranged in an orderly, repetitive
manner. Forming regularly shaped objects : crystals. Volcanic glass is not
considered as mineral.
Definate chemical composition. Most minerals are chemical compounds made up
of 2 or more elements. Quartz = SiO2; Calcite = CaCO3; Gold = Au; Sulphur = S
Generally inorganic. Sugar = crystalline organic compound is not a mineral.
Organic compounds such as those in shells and coral reefs are generally
considered minerals.
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Example of a mineral: salt or NaCl
Sodium and chloride ions are arranged in an orderly and repetitive manner into a
building block having a cubic shape.
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Rock
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Defination: any solid mass of mineral, or mineral-like matter that occur
naturally as part of the Earth.
Most rocks occur as aggregate of several types of minerals
(eg. granite  quartz, K-feldspar, plagioclase, etc.)
Some rocks are composed of only 1 mineral (eg. limestone  calcite)
Few rocks are composed of non-mineral matter
(eg. pumice  volcanic glass; coal  organic matter)
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Properties of Minerals
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Each mineral has a definite chemical composition and crystalline structure,
which give it a unique set of physical properties. These properties are used
in the identification of the minerals.
Primary properties
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Crystal form
Lustre
Colour
Streak
Hardness
Cleavage
Fracture
Specific gravity
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Properties of Minerals
Crystal form
 The internal orderly arrangement of the atoms of a mineral is reflected in its
crystal form
 When a mineral forms without space restriction it will develop individual
crystals with well-formed crystal faces
 Crystal growth is often severly constrained, stunted due to competition for
space, forming small intergrowth of crystals or aggregates of crystals
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Properties of Minerals
Lustre: The appearance or quality of light reflected from the surface of a
mineral
 Metallic appearance of metals – metallic lustre, opaque
 Submetallic lustre – imperfect metallic lustre
 Adamantine lustre – diamond
 Vitreous lustre – appearance of a broken glass (most common in minerals)
 Resinous lustre – resin-like
 Greasy lustre – perfectly smooth appearance
 Pearly lustre – pearl-like due to reflection of light
 Silky lustre – due to reflection off small parallel fibers
 Earthy lustre – dull, lack of lustre
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Properties of Minerals
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Colour – Not a reliable diagnostic property
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Properties of Minerals
Streak – colour of a mineral in its powdered form. It is much more reliable
indication of colour.
 It is obtained by rubbing the mineral across a piece of hard, unglazed
porceline called a streak plate
 Even if a mineral occurs in more than one colour, its streak usually shows
the same colour
 Minerals with metallic lustre have a dense dark streak
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Properties of Minerals
Hardness: a measure of the resistance of a mineral to abrasion or scratching.
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One of the most useful diagnostic properties. Determined by rubbing the
mineral to be identified against another mineral of know hardness.
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Standard hardness scale: Mohs scale (10:hardest – 1:softest)
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Properties of Minerals
Cleavage - is breakage along planar surfaces, which are parallel to possible
external faces on the crystal. This is because bonds between layers of
atoms aligned in certain directions are weaker than bonds between
different layers.
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In some minerals, a single direction of weakness exists, but in others,
two, three, four, or as many as six may be present. Where more than one
direction of cleavage is present, it is important to determine the angular
relation between the resulting cleavage surfaces: are they perpendicular
to each other (right angle), or do they meet at an acute or obtuse angle?
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Mica is a good example – breaking along very closely spaced flat planes
that yield thin "sheets."
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Properties of Minerals
Fracture: When a mineral without cleavage breaks, the fracture surfaces can
be used for their identification
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Conchoidal fracture is smooth curved surface like broken glass
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Subconchoidal fracture is similar to conchoidal, just not as curved, but still
smooth
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Splintery fracture is a fracture type that occurs in fibrous or finely acicular
minerals
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Earthy fracture is a fracture that produces a texture similar to broken clay
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Jagged or hackly fracture has sharp points or edges that catch on a finger
that's rubbed across the surface.
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Properties of Minerals
Specific gravity (G) is defined as the ratio between the weight of a substance
and the weight of an equal volume of water at 4° C. Thus a mineral with a
specific gravity of 2 weighs twice as much as the same volume of water.
Since it is a ratio, specific gravity has no units.
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The specific gravity of a mineral depends on the atomic weights of all its
constituent elements and the manner the atoms are arranged.
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Most minerals, including all the common rock-forming minerals, have a
specific gravity of 2.5 - 3.5. Metallic minerals have higher specific gravity.
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Other properties: fluorescence (response to ultraviolet light), magnetism,
radioactivity, tenacity (response to mechanical induced changes of shape
or form), piezoelectricity and reactivity to dilute acids.
Mineral
G
Quartz (SiO2)
2.65
Calcite (CaCO3)
2.7
Pyrite (FeS2)
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Copper (Cu)
8.9
Gold (Au)
19.3
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Mineral Groups
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There about 4,000 minerals described.
<100 are abundant, and they make up most of the rocks in the Earth’s crust. They are
classified as rock-forming minerals.
The bulk of these minerals are made up of 8 elements.
The most common rock-forming minerals are silicates (90%): formed by the
combination of oxygen, silicon and one or more metals.
Other minerals are grouped under nonsiliates. Many are important economically.
Elemental Composition of the Earth’s crust
Element
Wt %
Vol %
Oxygen, O
46.6
93.77
Silicon, Si
27.7
0.86
Aluminium, Al
8.13
0.47
Iron, Fe
5.00
0.43
Magnesium, Mg
2.09
0.29
Calcium, Ca
3.63
Sodium, Na
Abundance of minerals on Earth’s
surface
Mineral
Area %
Feldspars
30
Quartz
28
Clay minerals & mica
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1.03
Calcite
9
2.83
1.32
Iron oxides
4
Potassium, K
2.59
1.82
Others (trace)
1.43
0.01
Others
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Mineral Group
native elements
Anion or Anionic Complex Representative Minerals
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sulfur, gold, silver, copper,
diamond, graphite
sulfides
S-2
pyrite, galena, sphalerite,
chalcopyrite
oxides
O-2
hematite, magnetite,
chromite
halides
Cl -1, F-1
halite, flourite
sulfates
(SO4)-2
anhydrite, gypsum, barite
carbonates
(CO3)-2
calcite, dolomite
phosphates
(PO4)-3
apatite
silicates
(SiO4)-2
quartz, feldspar
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Silicate Minerals
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Silicate minerals are the most common rock-forming minerals. Their atomic structure
is based on silica tetrahedron (SiO4-2), in which four oxygen atoms are bond to each
silicon (Si) atom.
The mineral structures are constructed by sharing of oxygen between Si atoms
producing linkages of tetrahedra. This sharing of oxygen between Si tetrahedra
produces chains and other 3D Si tetrahedra structures, which are themselves linked
together through bonds between O and other atoms (e.g., Al, Mg and Fe).
Chemically, silicate minerals can be separated into two major types:
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ferromagnesian (iron/magnesian) - olivine, pyroxenes, amphiboles, biotite
felsic (silica/aluminum) - quartz, plagioclase, potassium feldspars
Silicate minerals, however, are typically classified on the basis of their silica
tetrahedra polymerization. The simplest silicate mineral structures have isolated Si
tetrahedra linked together through bonds between oxygen and cations other than
silicon. More complicated structures involve tetrahedra linked together to form rings
(beryl), single chains (pyroxenes), double chains (amphiboles), sheets (micas and
clay minerals), and 3D frameworks or networks (quartz and feldspars).
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