Transcript Chapter 2 - Minerals

Atoms and Minerals
Magnet and Iron and slide
Quartz SiO2 a common mineral
Minerals: Building blocks of
rocks
• Definition of a mineral:
• Naturally occurring
• Inorganic solid
• Ordered internal molecular structure
• Definite chemical composition
• Definition of a rock:
• A solid aggregate or mass of minerals
• Atomic structure
• Central region called the nucleus
– Consists of protons (positive charges) and neutrons
(neutral charges)
• Electrons
– Negatively charged particles that orbit around the nucleus
– Located in discrete energy levels called shells
Flattened structure of an atom
# protons (+) equals # electrons (-)
Electrons in shells
Number of outermost electrons
determine types of bonding
Outermost (Valence) shell
Argon
Some definitions:
• Atomic number: number of
protons in the nucleus
• Atomic Mass: total mass of
protons and neutrons within an
atom’s nucleus
• We can see these on a Periodic Table
Periodic Table of the Elements
# protons (+) equals # electrons (-)
Electrons in shells
Number of outermost electrons determine types of bonding
1
2
3 4
5 6
Shows atomic number (# protons) and atomic mass (# protons + neutrons).
Column shows # electrons in outermost shell
7
8
Electrons are in shells.
Octet Rule:
Atoms larger than Hydrogen and Helium need 8 electrons in their outer shell for stability
Neutral Atoms have #protons = # electrons
Oxygen has
6 electrons in its
valence shell
Silicon has 4 electrons in
Its outer shell
To satisfy the octet rule atoms can gain or lose electrons
In that state they are called IONS
They can combine with oppositely charged ions to form
neutral molecules
Oxygen, normally 6 valence electrons, wants 2 extra
Ions
Silicon, normally 4 valence electrons, would like to be rid of, or share, 4
Chemical Bonding 1: Ionic
• Chemical bonding
• Formation of a compound by combining two or
more atoms
• Ionic bonding
• Atoms gain or lose outermost (valence) electrons
to form ions
• Ionic compounds consist of an orderly
arrangement of oppositely charged ions
• Usually Columns I (alkali metals e.g. Na) and VII
(halogens e.g. Cl)
Halite (NaCl)- An Example of Ionic
Table
Bonding
Salt
Cl-
Na+
Na+
Cl-
Na+
Cl-
Na+
Na+
Halite small Na+ large Cl-
Crystalline structure of
Internal atomic arrangement is
NaCl
primarily determined by the size of
ions involved
(a)
Small Sodium ions between
large Chlorine ions
Covalent bonding – sharing of
valence electrons
Sharing Electrons in Outermost Shell
Cl2 Chlorine gas
Covalent Bonds in Water
H2O
Water is polar
Metallic Bonds
• Metallic bonding
– Valence electrons are free to migrate among
atoms
– Weaker than ionic or covalent bonds
Intermolecular Bonds 1
• Intermolecular bonding
– Hydrogen bonds- charged regions in water
Intermolecular Bonds 2
• Van der Waals bonds- opposite charges near gap
between close atoms
1
2
With close approach, the electron
clouds repel each other. When one
electron cloud (2) is further than the
opposite nucleus (1), the other electron
cloud (1) is attracted to the opposite
nucleus (2). A weak momentary bond is
present.
Isotopes
• Atomic mass is the total mass of neutrons
plus protons in an atom
• An elements isotopes are all atoms with the
same # protons but different #neutrons
• Some isotopes have unstable nuclei, emit
particles: “radioactive” decay.
• 12C 13C stable 14C radioactive all 6 protons
Structure of minerals
• Polymorphs
• Two or more minerals with the same
chemical composition but different
crystalline structures
• Diamond and graphite (both carbon) are
good examples of polymorphs
» The transformation of one polymorph to
another is called a phase change
» Example: Graphite in a High Pressure
Cell Makes Diamond
• Some polymorphs make good PT* indicators
*We can measure the Temperature and Pressure a mineral experienced
when it formed in the past
Diamond and graphite –
polymorphs of carbon
Physical properties of minerals
• Crystal Form
• External expression of the orderly internal
arrangement of atoms
The mineral garnet
sometimesexhibits good
crystal form:dodecahedra
Physical properties of minerals
• Luster
• Appearance of a mineral in reflected light
• Two basic categories
– Metallic
– Nonmetallic
• Terms are used to further describe
nonmetallic luster are vitreous (glassy),
pearly, silky, earthy (like dirt), adamantine
(greasy)
Galena PbS displays
metallic luster
Valuable ore of Lead
Physical properties of
minerals
• Color
• Generally an unreliable diagnostic
property to use for mineral identification
• Often highly variable for a given mineral
due to slight changes in mineral chemistry
• Exotic colorations of some minerals
produce gemstones
• But we use it anyway
Quartz (SiO2) exhibits
a variety of colors
Physical properties
of minerals
Example: Hematite
has a reddish streak
• Streak
Streak
• Color of a mineral in its powdered form
• Helpful in distinguishing different
minerals with similar appearance
• Hardness
• Resistance of a mineral to abrasion or
scratching
• All minerals are compared to a standard
scale called the Mohs scale of hardness
• Cleavage
• Tendency to break along planes of weak
bonding
• Produces flat, shiny surfaces
• Described by resulting geometric shapes
– Number of planes
Micas have one perfect cleavage
– Angles between adjacent planes
Biotite
Mica
Three directions of perfect cleavage
– fluorite, halite, and calcite
Fluorite, CaF2
4 planes of
cleavage,
octahedron
Calcite, CaCO3, 3 planes of
cleavage not at 90o
Salt, NaCl, 3 planes
of cleavage, 90o
Each Cleavage Plane is paired
Cleavage
This Feldspar has two planes of good
cleavage about 90o apart.
Notice the other surface is rough, not a plane.
non metallic, usually light in color, streak clear to white, hardness ~6, usually
scratches glass
Physical properties of
minerals
• Fracture
• Absence of cleavage when a mineral is
broken. Shown: conchoidal fracture in Quartz
• Specific Gravity
• Ratio of the weight of a mineral to the
weight of an equal volume of water
• Average value is approximately 2.7
• Simply hefting a mineral works too.
Physical properties of
minerals
• Other properties
• Magnetism
• Reaction to hydrochloric acid
• Malleability
• Double refraction
• Taste
• Smell
• Elasticity
Is it calcite or dolomite?
Classification of Minerals
• Nearly 4000 minerals have been
identified on Earth (We discuss a few)
• Rock-forming minerals
• Common minerals that make up most of
the rocks of Earth’s crust
• Only a few dozen members
• Composed mainly of the 8 elements that
make up 98% of the continental crust
Commonly formed Ion charges
often called “oxidation state”
Metals can form more than one Ion. Fe+2 is name Ferrous, Fe+3 is named Ferric
Classification of Minerals
• Silicates
• Most important mineral group
– Comprise most of the rock-forming minerals
– Very abundant due to large amounts of
silicon and oxygen in Earth’s crust
• Basic building block is the silicon-oxygen
tetrahedron molecule
– Four oxygen ions surrounding a much
smaller silicon ion
The Component Atoms
Oxygen has
6 electrons in its
valence shell
Silicon has 4 electrons in
Its outer shell
Remember: atoms can gain or lose electrons
They then combine with oppositely charged ions to form
neutral molecules
Ions
Anion (negative)
Cation (positive)
O2 -
The Silicate Tetrahedron
2_25
Si4+
O2 -
O2 The basis of most rock-forming
minerals, charge - 4
O2 -
Silicate Bonding I
• Oxygen O atoms may obtain electrons
from Si atoms, producing the SiO4 -4 Ion.
• The negative charge is balanced by
positive metal ions.
• This occurs in Olivine, (Fe,Mg)2SiO4, a
high temperature Fe-Mg silicate. Forms of
this mineral are stable 100’s of kilometers
below Earth’s surface.
• Sort of Ionic Bond
Example OLIVINE
Positive ion
Fe and Mg
SiO4 -4 Ion
Tetrahedron
facing down
Tetrahedron
facing up
Independent tetrahedra
Silicate Bonding II
• Alternately, the oxygen atoms may
complete their outer electron shells by
sharing electrons with two Silicon atoms
in nearby silicon tetrahedra.
• A sort of covalent bond
A Pyroxene
Single chains weakly paired
Two good cleavages at ~90o, non metallic, dark green to black, H ~ 6
2_26c
An Amphibole
Positive
ion
Cleavages 56 and 124 deg
Double chains
(c)
Example: Mica
One excellent cleavage plane
Sheet silicates
(d)
Clay Minerals
(at high magnification)
note sheet structure
Very small crystals
Kaolinite
(hand specimen)
Clays are also Sheet Silicates, just as Micas are
Vietnam Anecdote
Example: Quartz
SiO2
2_26e
Framework silicates
(e)
(3-D, also the Feldspars)
More 3-D Framework Minerals: Feldspars
Classification of Minerals
• Common Silicate minerals
• Feldspar Group
– Most common mineral group
– two directions of perfect cleavage at 90 degrees
– In Feldspars, some of the Silicon atoms (oxidation
state +4) are replaced by Aluminum (oxidation state
+3)
– Ion is not symmetrical
– Pearly Luster
A Potassium Feldspar
A Potassium Feldspar
Here is a flattened model of a 3-D framework Potassium Feldspar,
similar to Orthoclase KAlSi3O8 . In some of the Tetrahedra an
Aluminum ion Al+3, is in the center instead of Silicon. An additional
Potassium, K+, completes the charge balance.
Feldspars that use Calcium (Ca) or Sodium (Na) metals to balance
the SiO4 - 4 and AlO4 -5 charges are called:
Plagioclase feldspar
Note the Twinning, seems to have ‘stripes’
Summary
Rocks are made of minerals. Many are silicate minerals.
This granite, an igneous rock, has Quartz, an amphibole
called Hornblende, a pink potassium feldspar,
and a white Plagioclase feldspar
Silicate Mineral
Examples
Mica
Feldspar
Olivine
Quartz
Pyroxene
Asbestos
Classification of Minerals
• Important non-silicate minerals
• Many non-silicate minerals have economic
value
• Examples
– Hematite (oxide mined for iron ore)
– Halite (halide mined for salt)
– Sphalerite (sulfide mined for zinc ore)
– Native Copper (native element mined for copper)
Nonsilicate Mineral
Examples
Spinel
(Oxide)
Halite
(Halide)
Gypsum
(Sulfate)
Galena
(Sulfide)
Pyrite
(Sulfide)
Calcite
(Carbonate)
Hematite
(Oxide)
Classification of Minerals
• Important non-silicate minerals
• Carbonates
– Primary constituents in limestone and dolostone
– Calcite (calcium carbonate) and Dolomite
(calcium-magnesium carbonate) are the two most
important carbonate minerals
Calcite showing cleavage faces
Classification of Minerals
• Important non-silicate minerals
• Several major groups exist including
– Oxides
– Sulfides
– Sulfates
– Native Elements
– Carbonates
– Halides
– Phosphates
Hematite, an oxide, Fe2O3
An important Iron ore
Galena, PbS, a Sulfide
• An important ore of Lead
Gypsum, a Sulfate
.
CaSO4 2H2O
Perfect cleavage 1 dir, poor in 2 others, H2, transparent, non-elastic
• An important evaporite mineral
Native Copper
Cu