Transcript Physical Properties of Minerals

Mineralogy – The Study of Minerals
Chapter 3
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What is a mineral?
What are the properties of minerals?
What are minerals composed of?
How do we know the atomic structure of minerals?
How do elements combine to form minerals?
What determines the physical properties of minerals?
What are the most common / important minerals?
Why Study Minerals???
• Rocks are made of one or more individual
minerals.
• Earth is made of rocks… of course!
• Geophysics – seismic (shock) waves traveling
through Earth are influenced by minerals.
• Volcanology – minerals crystallizing from a magma
(molten rock) influence eruptions.
• Economic Geology – metals and industrial
materials are extracted from minerals. (40,000 lbs. of
minerals per person per year in the U.S.!)
• Geochronology – some minerals contain
radioactive atoms, allow us to determine ages of
rocks.
• Food! Minerals control properties of soils,
determine whether, or not, soils are suitable for
agriculture.
Rocks - Composed of One or More Minerals
Definition - What is a mineral?
• A mineral is/has:
– a naturally occurring solid
– usually inorganic (note - some exceptions)
– distinctive chemistry - which can vary within limits
– an ordered internal structure at the atomic scale
– distinctive physical properties
Definition - What is a mineral?
• A mineral is/has:
– a naturally occurring solid
• This separates minerals from synthetic “mineral-like”
materials like cubic zirconia or synthetic ruby
• Must be a solid - liquid or gas does not have an
ordered atomic level structure
– usually inorganic (some exceptions)
– distinctive chemistry - which can vary within limits
– an ordered internal structure at the atomic scale
– distinctive physical properties
Definition - What is a mineral?
• A mineral is/has:
– a naturally occurring solid
– usually inorganic (some exceptions)
• Aside from a few exceptions (e.g., calcite and
aragonite in shells, apatite in bones) minerals are
natural inorganic solids
• Organic crystalline materials like, e.g., sugar are not
minerals
– distinctive chemistry - which can vary within limits
– an ordered internal structure at the atomic scale
– distinctive physical properties
Definition - What is a mineral?
• A mineral is/has:
– a naturally occurring solid
– usually inorganic (some exceptions)
– distinctive chemistry - which can vary within
limits
• All specimens of a mineral will fall within a definite
chemical range, e.g. plagioclase feldspar (Na-Ca)
– an ordered internal structure at the atomic scale
– distinctive physical properties
Definition - What is a mineral?
• A mineral is/has:
– a naturally occurring solid
– usually inorganic (some exceptions)
– distinctive chemistry - which can vary within limits
– an ordered internal structure at the atomic
scale
• Certain materials like obsidian (volcanic glass) may
be mistaken for a mineral, but they have no atomic
level structure, actually are just a frozen liquid
– distinctive physical properties
Definition - What is a mineral?
• A mineral is/has:
– a naturally occurring solid
– usually inorganic (some exceptions)
– distinctive chemistry - which can vary within limits
– an ordered internal structure at the atomic scale
– distinctive physical properties
• Because of their chemistry, types of bonding, and
atomic structure minerals posses characteristic
properties such as hardness, color, cleavage,
density, etc.
How do we identify minerals?
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Physical properties:
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Color
Streak
Luster
Hardness
Crystal shape
Cleavage
Specific gravity (density)
Atomic lattice structure
Physical Properties of Minerals
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Color
– Most obvious, but may be misleading
– Different colors may result from impurities
Example:
Quartz
Physical Properties of Minerals
• Luster
– How it reflects light
Metallic
example:
Galena
• metallic or non-metallic
• e.g., vitreous (glassy), dull, earthy, etc.
• Crystal faces
– Characteristic shapes
Note that the calcite
and quartz crystals are
both six-sided, yet very
different in shape
Fig 2.2
Physical Properties of Minerals
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Crystal shape (or form):
– external expression of a mineral’s internal
atomic structure
– planar surfaces are called crystal faces
– angles between crystal faces are constant for
a particular mineral
– results from growth of the mineral
Quartz
Pyrite
Physical Properties of Minerals
• Density – mass divided by volume
– we use grams per cubic centimeter or g/cm3
• Specific gravity – weight of mineral in air
divided by the weight of an equivalent volume
of pure water at 4oC (which is 1 g/cm3)
– A weight:weight ratio…so no units (dimensionless)
– Numeric values of density = specific gravity
• e.g., 2.65 g/cm3 ≈ 2.65 specific gravity
• common minerals range from ~2.5 to ~3.5
Physical Properties of Minerals
• Streak – color of a mineral in powdered
form
(mainly used for metallic minerals)
Obtained by scratching
a mineral on a piece of
unglazed porcelain
Example:
Hematite
Mohs Scale of Hardness
Hardest (10) – Diamond
Softest (1) – Talc
Common objects:
- Fingernail (2.5)
- Copper penny (3.5)
- Nail (4.5)
- Glass (5.5)
- Streak plate (6.5)
Physical Properties of Minerals
Mohs hardness scale - note the log scale of absolute hardness (Y axis)
e.g. quartz is 100 times harder than talc
Physical Properties of Minerals
• Cleavage – the way a mineral breaks into
smaller pieces of a characteristic shape.
– flat cleavage faces (not a crystal face!)
– smaller pieces resemble larger ones
Physical Properties of Minerals
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Cleavage vs. Fracture:
– Both are the way a mineral breaks, but….
– Cleavage: tendency of a mineral to break
along planes of weakness – flat surfaces
– Minerals that do not exhibit cleavage are said
to fracture – irregular surfaces
• Do not confuse cleavage planes with crystal faces!
• Crystal faces are growth forms and cleavage planes
are “breakage” forms, cleavages repeat over and over as
a mineral is broken smaller and smaller….
Physical Properties of Minerals
• Quartz fractures into irregular pieces
• Calcite cleaves into rhombohedrons
• Both often grow as 6-sided crystal forms
Physical Properties of Minerals
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Cleavage (1 direction):
Example: mica
Physical Properties of Minerals
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Cleavage (2 directions):
orthoclase
amphibole
Physical Properties of Minerals
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Cleavage (3 directions):
halite
calcite
Physical Properties of Minerals
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Cleavage (4 directions):
fluorite
What Are Minerals Composed Of?
We must recognize different levels of
organization of physical matter….
Atom  Element  Compound  Mineral  Rock
What Are Minerals Composed Of?
• Particles that make up an atom:
– Protons: positive (+) charge
– Neutrons: no charge
– Electrons: negative (-) charge
Protons + neutrons comprise the nucleus of an atom.
Layers of electrons that orbit around the nucleus are in orbitals
or energy-level shells.
Only the outermost electrons (valence electrons) are involved in
chemical bonds between atoms.
Atomic Structure – Controls Chemical Bonding
Bonding – Controls Mineral Structure and Properties
Mineral - Composed of Atoms of Specific Elements
Arranged in a Specific Geometric Structure.
The elements involved and the type of bonds are
what controls this structure.
How do we know the atomic
structure of minerals?
• Two ways of looking at small scale matter… there are
others (e.g. X-ray diffraction yields an accurate
measurement of spacing between atoms in a
mineral).
– Optical microscope has ~1000x limit
• Limit of 0.001 mm – much too large for atoms
– TEM – Transmission Electron Microscope
• 10,000,000x magnification, so has a limit of ~0.000001
mm
• Can resolve atoms, but edges of electron cloud appear
indistinct, thus yields a fuzzy image
How do we know the atomic structure of minerals?
Representation of how a
transmission electron
microscope (TEM) images
atoms in a mineral.
How do we know the atomic
structure of minerals?
• Most of the volume of an atom is the electron
cloud - which is:
– much less dense than the nucleus
– denser close to the nucleus and less dense farther
out from it
• Imaging electrons from a TEM are blocked
more towards an atom’s center than at the
edge
– this leads to the “fuzzy” images that we see
How do we know the atomic
structure of minerals?
The atoms “shadows” in TEM images are proportional to the mass and size
of the element involved as shown in the TEM of dolomite above.
Each mineral has a distinctive atomic level arrangement of atoms, and thus a
distinct TEM image – this is dolomite.
How do elements combine to form minerals?
• Definition:
– A chemical compound consists of elements
that combine in a specific ratio.
Examples:
NaCl
H2O
• The smallest quantity of a compound is called a
molecule.
• Molecules are held together by the various forms
of chemical bonding.
• A molecule may represent the chemical formula
of a mineral – what about the two above?
How do elements combine to form minerals?
• Chemical bonding:
– formation of a compound by combining two or
more elements
– manner in which electrons are distributed
among atoms
– only the outer shell valence electrons interact
• In bonded atoms, electrons may be lost,
gained, or shared.
• 4 types of bonding:
ionic
covalent
metallic
van der Waals
Low Electronegativity     High Electronegativity
Electronegativity – the tendency of an atom to pull electrons
away from neighboring atoms during chemical bonding.
How do elements combine to form minerals?
• Ionic bonding:
– Electrons are transferred between atoms forming
electrostatically attracting ions (e.g., NaCl which is
the mineral halite).
Cl–
Na+
Elements of very different
electronegativity.
Ionic bonds are of moderate
strength.
Most common bond type.
How do elements combine to form minerals?
• Covalent bonding:
– Electrons are shared between atoms.
Chlorine gas molecule, Cl2
– Elements of similar
electronegativity.
– Are generally very
strong bonds.
(e.g., diamond, pure C)
How do elements combine to form minerals?
• Metallic bonding:
– Electrons drift around from atom to atom
(e.g., copper, gold, silver).
– Good conductors of electrical current.
– Generally weaker, less common than other bonds.
e.g., Native Gold, Copper
How do elements combine to form minerals?
• Van der Waals bonding:
– Sheets of covalently bonded atoms held together
by weak residual electrostatic forces.
– Very weak bonds.
examples: graphite, mica
Where is the cleavage plane at?
Diamond - 3 dimensional network
of strong covalent bonds.
Mohs Hardness = 10
Graphite - 2 dimensional layers of
strong covalent bonds held together
by weak Van der Waals bonds.
Mohs Hardness = 1.5
Polymorph: A mineral which has the same chemical composition
as another mineral, however, their atoms are arranged differently.
Polymorphs have the same formula, but can have very different properties.
Diamond and Graphite are examples of polymorphs – they are both made of
pure carbon, so have a very simple chemical formula – C
What determines the physical properties of minerals?
Atomic Level Structure Controls
- Growth Forms
- Cleavage Forms
- Hardness
Table salt – magnified 10x
Atomic lattice structure
(including type of elements
and bonds) controls the
physical properties of
minerals!
Galena
Galena (PbS) Has Similar Atomic Structure as Halite (NaCl)
What are the most important minerals?
Element abundances in the Earth’s crust (wt.%)
All others: 1.5%
What are the most important minerals?
Silica Tetrahedron
(SiO4)4-
Element Abundances
SILICATES
Common cations that
bond with the silica
tetrahedron – an
anionic complex
All others:
1.5%
Si4+
O2-
(SiO4)4-
SiO4 - The Silica Tetrahedron
The Fundamental Building Block of Earth
The Major Mineral Groups
• Silicates (most abundant and common, ~92%)
• Non-silicates (~8% of Earth’s crust)
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Group
Anionic Complex
Oxides
O2Carbonates
(CO3)2Sulfides
S2Sulfates
(SO4)2Halides
Cl- or FNative elements (single elements, Au, Cu)
Minerals are classified primarily on the basis of chemistry.
For example, these are all carbonate minerals that have
the carbonate anionic complex (CO3)2- in common.
Calcite
CaCO3
Siderite
FeCO3
Malachite
Cu2CO3(OH)2
Silicates are subdivided on the basis of crystal structure...
Or simply, how tetrahedra are connected - or not.
Isolated Tetrahedron Silicates – Olivine Group
Example: Olivine
dark silicates (Fe-Mg)
 ferromagnesian
Always green
No cleavage
Single Chain Silicates – Pyroxene Group
Example: Pyroxene
Ferromagnesian / dark silicates (Fe-Mg)
Augite
Black to dark green
2-directions
of cleavage
(at ~90 degrees)
Double Chain Silicates – Amphibole Group
Example: Amphibole
Ferromagnesian / dark silicates (Ca, Fe-Mg)
Hornblende
Black to light green
2-directions
of cleavage
(~60 and 120 degrees)
Sheet Silicates – Micas and Clays
Mica Group and Clay Minerals
light silicates (K, Al) and dark (K, Fe, Al) silicates
Muscovite
Note: Biotite similar, but black
1-direction
of cleavage
Silvery color
3-D Framework Silicates
Feldspar Group
K-feldspar
light silicates (K-Na-Ca, Al)
Most common mineral group
Orthoclase
Plagioclase
2-directions
of cleavage
(at ~90 degrees)
Ca/Na-feldspar
3-D Framework Silicates
Quartz
light silicates (pure SiO2)
no cleavage
(conchoidal fracture)
hard, resistant to weathering
Quartz
Oxides
Very simple minerals with oxygen (O2-) bonded
to atoms (cations) of other elements.
Example: Hematite Fe2O3
Sulfides
Very simple minerals with sulfur (S2-) bonded
to atoms (cations) of other elements.
Example: Pyrite (fools gold) FeS2
Sulfates
Minerals with the sulfate anionic complex (SO4)2bonded to atoms (cations) of other elements.
Example: Gypsum CaSO4.2H2O
Element Substitutions – How?
Elements can freely substitute for one another in a mineral
structure if they are approximately the same size and
have the same charge.
1) Olivine
dark silicates (Fe-Mg)
 Ferromagnesian
Olivine can have a range of compositions from a pure Fe-silicate
to a pure Mg-silicate or anything in between.
(Mg,Fe)2SiO4
General Formula
Fe2SiO4
Mg2SiO4
End Members
Element Substitutions – How?
Elements can freely substitute for one another in a mineral
structure if they are approximately the same size and
have the same charge.
2) Plagioclase Feldspar
light silicates (Na-Ca, Al)
Plagioclase can have a range of compositions from a pure Na-Al-silicate
to a pure Ca-Al-silicate.
(Na,Ca)Al2Si2O8
General Formula
NaAl2Si2O8
CaAl2Si2O8
End Members