Mineralogy – The Study of Minerals Chapter 3

Mineralogy – The Study of MineralsChapter 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


• 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


• 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


• 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


• 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


• 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


• 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

Metallicexample: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/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


• 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)

Mohs hardness scale - note the log scale of absolute hardness (Y axis)

e.g. quartz is 100 times harder than talc


• 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• 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



• Cleavage (1 direction):

Example: mica


• Cleavage (2 directions):

orthoclase

amphibole



halite

calcite



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 BondingBonding – Controls Mineral Structure and Properties

Mineral - Composed of Atoms of Specific ElementsArranged in a Specific Geometric Structure.

The elements involved and the type of bonds arewhat 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.


• 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


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.


• 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 ElectronegativityElectronegativity – 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–


Elements of very different electronegativity.

Ionic bonds are of moderatestrength.

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, Cl2


• 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

• 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

http://www.powow.com/samihossaily/ptr39p106.jpg

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 elementsand 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 TetrahedronSilica Tetrahedron (SiO(SiO44))4-4-

SILICATES

Common cations thatbond with the silica tetrahedron – an anionic complex

What are the most important minerals?

SiO4 - The Silica Tetrahedron

The Fundamental Building Block of Earth

Si4+

O2-

(SiO4)4-

Group Anionic Complex

– Oxides O2-

– Carbonates (CO3)2-

– Sulfides S2-

– Sulfates (SO4)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 (CO3)2- in common.

Calcite CaCOCO33

Siderite FeCOCO33

Malachite Cu2COCO33(OH)2

http://en.wikipedia.org/wiki/Image:Calcite_USA.jpg

http://en.wikipedia.org/wiki/Image:SideriteUSGOV.jpg

http://en.wikipedia.org/wiki/Image:Malachite_Zaire.jpg

Silicates are subdivided on the basis of crystal structure...Or simply, how tetrahedra are connected - or not.

Example: Olivinedark silicates (Fe-Mg)

Isolated Tetrahedron Silicates – Olivine Group

Always greenNo cleavage

ferromagnesian

Example: PyroxeneFerromagnesian / dark silicates (Fe-Mg)

Single Chain Silicates – Pyroxene Group

Black to dark green

2-directionsof cleavage(at ~90 degrees)

Augite

Example: AmphiboleFerromagnesian / dark silicates (Ca, Fe-Mg)

Black to light green

2-directionsof cleavage(~60 and 120 degrees)

Hornblende

Double Chain Silicates – Amphibole Group

Mica Group and Clay Mineralslight silicates (K, Al)

Sheet Silicates – Micas and Clays

1-directionof cleavageSilvery color

Muscovite

and dark (K, Fe, Al) silicates

Note: Biotite similar, but black

Feldspar Grouplight silicates (K-Na-Ca, Al)

3-D Framework Silicates

2-directionsof cleavage

(at ~90 degrees)

Orthoclase

Plagioclase

K-feldspar

Ca/Na-feldspar

Most common mineral group

Quartzlight silicates (pure SiO2)

no cleavage(conchoidal fracture)

hard, resistant to weatheringQuartz

3-D Framework Silicates

Oxides

Very simple minerals with oxygen (O2-) bonded to atoms (cations) of other elements.

Example: Hematite Fe2O3

Sulfides

Example: Pyrite (fools gold) FeS2

Very simple minerals with sulfur (S2-) bonded to atoms (cations) of other elements.

Sulfates

Example: Gypsum CaSO4.2H2O

Minerals with the sulfate anionic complex (SO4)2- bonded to atoms (cations) of other elements.

1) Olivinedark silicates (Fe-Mg)

Element Substitutions – How?

Ferromagnesian

Olivine can have a range of compositions from a pure Fe-silicateto a pure Mg-silicate or anything in between.

(Mg,Fe)2SiO4 Fe2SiO4 Mg2SiO4

Elements can freely substitute for one another in a mineralstructure if they are approximately the same size and have the same charge.

End MembersGeneral Formula

2) Plagioclase Feldsparlight silicates (Na-Ca, Al)

Element Substitutions – How?

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

(Na,Ca)Al2Si2O8 NaAl2Si2O8 CaAl2Si2O8

Elements can freely substitute for one another in a mineralstructure if they are approximately the same size and have the same charge.

End MembersGeneral Formula

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Mineralogy – The Study of Minerals Chapter 3