Physical Properties of Minerals - UNLV Geoscience

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Mineralogy


The Study of Minerals

Chapter 3


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?


Physical properties:


Color


Streak


Luster


Hardness


Crystal shape


Cleavage


Specific gravity (density)


Atomic lattice structure

Physical Properties of Minerals


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 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

Metallic

example:

Galena

Physical Properties of Minerals


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


Density


mass divided by volume


we use grams per cubic centimeter or g/cm
3



Specific gravity


weight of mineral in air
divided by the weight of an equivalent volume
of pure water at 4
o
C (which is 1 g/cm
3
)


A weight:weight ratio…so no units (dimensionless)


Numeric values of density = specific gravity


e.g., 2.65 g/cm
3

≈ 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

Physical Properties of Minerals

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)

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

Physical Properties of Minerals


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….


Quartz fractures into irregular pieces


Calcite cleaves into rhombohedrons


Both often grow as 6
-
sided crystal forms

Physical Properties of Minerals

Physical Properties of Minerals


Cleavage (1 direction):

Example:

mica

Physical Properties of Minerals


Cleavage (2 directions):

orthoclase

amphibole

Physical Properties of Minerals


Cleavage (3 directions):

halite

calcite

Physical Properties of Minerals


Cleavage (4 directions):

fluorite

What Are Minerals Composed Of?

We must recognize different levels of
organization of physical matter….



Atom



Element


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副捫

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

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 we know the atomic

structure of minerals?


Definition:


A
chemical compound
consists of elements
that combine in a specific ratio.

Examples:

NaCl H
2
O


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

How do elements combine to form minerals?

Low Electronegativity









High Electronegativity

Electronegativity


the tendency of an atom to pull electrons

away from neighboring atoms during chemical bonding.


Ionic bonding:


Electrons are
transferred

between atoms forming
electrostatically attracting ions (e.g., NaCl which is
the mineral halite).

Na
+

Cl


How do elements combine to form minerals?

Elements of very different
electronegativity.


Ionic bonds are of moderate

strength.


Most common bond type.


Covalent bonding:


Electrons are
shared

between atoms.










Elements of similar


electronegativity.


Are generally very


strong bonds.


(e.g., diamond, pure C)

Chlorine gas molecule, Cl
2

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.

How do elements combine to form minerals?

e.g., Native Gold, Copper


Van der Waals bonding:


Sheets of covalently bonded atoms held together
by weak residual electrostatic forces.


Very weak bonds.


examples:

graphite, mica

How do elements combine to form minerals?

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

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

Polymorph
: A mineral which has the same chemical composition
as another mineral, however, their atoms are arranged differently.

Atomic Level Structure Controls


-

Growth Forms


-

Cleavage Forms


-

Hardness

What determines the physical properties of minerals?

Galena (PbS) Has Similar Atomic Structure as Halite (NaCl)

Table salt


magnified 10x

Galena

Atomic lattice structure

(including type of elements

and bonds) controls the

physical properties of

minerals!

Element abundances in the Earth’s crust (wt.%)

All others: 1.5%

What are the most important minerals?

All others: 1.5%

Element Abundances

Silica Tetrahedron


(SiO
4
)
4
-

SILICATES

Common cations that

bond with the silica

tetrahedron


an

anionic complex

What are the most important minerals?

SiO
4

-

The Silica Tetrahedron

The Fundamental Building Block of Earth

Si
4+


O
2
-


(SiO
4
)
4
-


Group



Anionic Complex


Oxides



O
2
-


Carbonates


(CO
3
)
2
-


Sulfides



S
2
-


Sulfates


(SO
4
)
2
-


Halides


Cl
-

or F
-


Native elements

(single elements, Au, Cu)

The Major Mineral Groups


Silicates
(most abundant and common, ~92%)


Non
-
silicates
(~8% of Earth’s crust)


Minerals are classified primarily on the basis of chemistry.

For example, these are all carbonate minerals that have
the carbonate anionic complex (CO
3
)
2
-

in common.

Calcite Ca
CO
3

Siderite Fe
CO
3

Malachite Cu
2
CO
3
(OH)
2


Silicates are subdivided on the basis of crystal structure...

Or simply, how tetrahedra are connected
-

or not.


Example: Olivine

dark silicates (Fe
-
Mg)

Isolated Tetrahedron Silicates


Olivine Group

Always green

No cleavage



ferromagnesian

Example: Pyroxene

Ferromagnesian / dark silicates (Fe
-
Mg)

Single Chain Silicates


Pyroxene Group

Black to dark green


2
-
directions

of cleavage

(at ~90 degrees)

Augite

Example: Amphibole

Ferromagnesian / dark silicates (Ca, Fe
-
Mg)

Black to light green


2
-
directions

of cleavage

(~60 and 120 degrees)

Hornblende

Double Chain Silicates


Amphibole Group

Mica Group and Clay Minerals

light silicates (K, Al)

Sheet Silicates


Micas and Clays

1
-
direction

of cleavage

Silvery color

Muscovite

and dark (K, Fe, Al) silicates

Note: Biotite similar, but black

Feldspar Group

light silicates (K
-
Na
-
Ca, Al)

3
-
D Framework Silicates

2
-
directions

of cleavage

(at ~90 degrees)

Orthoclase

Plagioclase

K
-
feldspar

Ca/Na
-
feldspar

Most common mineral group

Quartz

light silicates (pure SiO
2
)

no cleavage

(conchoidal fracture)

hard, resistant to weathering

Quartz

3
-
D Framework Silicates

Oxides

Very simple minerals with oxygen (O
2
-
) bonded

to atoms (cations) of other elements.

Example: Hematite Fe
2
O
3

Sulfides

Example: Pyrite (fools gold) FeS
2

Very simple minerals with sulfur (S
2
-
) bonded

to atoms (cations) of other elements.

Sulfates

Example: Gypsum CaSO
4
.
2H
2
O




Minerals with the sulfate anionic complex (SO
4
)
2
-


bonded to atoms (cations) of other elements.

1) Olivine

dark silicates (Fe
-
Mg)

Element Substitutions


How?



Ferromagnesian

Olivine can have a range of compositions from a pure Fe
-
silicate

to a pure Mg
-
silicate or anything in between.



(Mg,Fe)
2
SiO
4

Fe
2
SiO
4

Mg
2
SiO
4


Elements can freely substitute for one another in a mineral

structure if they are approximately the same size and

have the same charge.

End Members

General Formula

2) Plagioclase Feldspar

light silicates (Na
-
Ca, Al)

Element Substitutions


How?

Plagioclase can have a range of compositions from a pure Na
-
Al
-
silicate

to a pure Ca
-
Al
-
silicate.



(Na,Ca)Al
2
Si
2
O
8

NaAl
2
Si
2
O
8

CaAl
2
Si
2
O
8


Elements can freely substitute for one another in a mineral

structure if they are approximately the same size and

have the same charge.

End Members

General Formula