Revised 8 / 06 (Monroe 6th ed.)

Minerals - Chapter 3

 

Including...

Introduction

Summary of physical reality

The surface of the Earth

Minerals

Crystalline solids

Chemical properties of minerals

Mineral classes

Mineral Identification

Click here for online mineral and rock ID charts

 

Introduction

Planet warmed up early - a big ball of very hot stuff

Cooled at surface where exposed to space

Formed crystalline solids composed of minerals

Minerals are combinations of elements

Which are made of smaller things

Start with some review

 

Summary of physical reality

Definition of terms

Matter: Anything which has mass and occupies space

Atomic structure: Protons, neutrons, electrons, etc.

Atoms very small

Half pound of lead has 1024 atoms

Salt: 4.5 X 1019 atoms per grain

And they're all little cubes!!!

Nucleus has 99.9% of mass but 1 X 10-9 of volume (one billionth)

Start with 1,600,000 pounds of steel

Remove all space = grain of rice

Sound familiar? (the solar system)

Mass, Volume, and Density: Describe and discuss interrelationships

Very fundamental, but so very important

Pay attention!

Elements: Discuss the Periodic table (on wall) (Monroe; fig. 3-4, pg. 74)

Only 8 make up 98.5% of the earth (Monroe; Fig. 3-11, pg. 80)

DIGRESS TO: Oxygen (47/94) and Silicon (28/does it matter?)

On average, 75% by weight of everything is these 2 elements!

And by volume? Greater than 95% minimum

Bonding: Chemical combination of elements

Three general types of bonding

All relate to how different nuclei relate to their electrons

Ionic bonding: swap electrons, positive/negative attraction

Cation (positive charge) and anion (negative charge) (Monroe; fig. 3-6, pg. 76)

Salt excellent example

Co-valent bonding: share electrons - the strongest

Graphite and diamond (Monroe; fig. 3-7, pg. 76)

Metallic bonding: excess electrons - very dense packing

Leads directly to electrical conductivity of the metals

Hydrogen bonding: found in water

Similar to Co-valent bonding

Oxygen lacks 2 electrons to fill outer shell

Hydrogen is easy and is happy to share

But they form a lop-sided molecule

Very important association - results in the di-polarity of the water molecule

More on this during G-102 (2nd term)

Magma: Liquid rock

Like water is to ice

Crystallization: Liquid to solid phase change

Phase change depends on:

Temperature - we already knew this

Pressure - VERY important in the generation of magma

Strength of bond

Minerals: Composed of elements in specific combinations

Lots more on this later

Rocks: Composed of minerals in specific combinations

To summarize solid matter:

who knows what --> subatomic particles --> p/n/e --> atoms --> elements --> minerals --> rocks

 

The surface of the Earth

Matter (elements) cannot be destroyed (or created)

Over the 4 billion years since the first proto-crust crystallized

Continuous, ever changing succession of minerals

Formed by crystallization from magma

Destroyed by physical & chemical attack at or near the surface

New minerals re-form, destroyed, re-form, destroyed, re-form

Responding to changing conditions

Attempt to maintain equilibrium

This succession of minerals forms our best (and only) link with the earth's past

Easy to get excited about large mineral specimens

Can be quite showy (and expensive!)

Minerals usually come in smaller, less exciting sizes

Form the rocks on which we live

Form the basis of our civilization - both ancient and modern

Weapons, tools, comforts and adornments

DIGRESS TO: name something which is NOT dependent upon minerals

Minerals were poorly understood through most of classic western history

European culture again discouraged scientific thought

Always been a "black market" for knowledge of minerals and their properties

A "need to know" basis

Also, mining was a base occupation, and miners were lower class citizens

Under the double yoke of ignorance and prejudice - little advance in mineral knowledge

At least in Europe - Orientals were probably more advanced, as usual

Some historical notes:

Democritus: 400 B.C.

Postulated atoms & gave them that name

All were similar and eternal (can't be created or destroyed?)

Aristotle: 350 B.C.

All matter on earth composed of Fire, Water, Air, or 'Earth'

Heaven composed of 'quintessence'

Albertus Magnus - 13th century

"Natural History" - 5 books devoted to minerals!

Was the authoritative work of the time

Described "valuable" stones

Amethyst - from amethustos (Greek for "not drunken")

Others gave wearer super powers, invulnerability

"Draconites" - from the head of a dragon

Georgius Agricola (1546) - German physician

"De Re Metallica" - beginning of modern mineral studies

Accurate descriptions of mining and minerals

Factual observations

The current central concept of mineralogy:

External form, physical properties and chemical behavior are the result of 2 things:

Chemical composition

Internal structure

 

Minerals

What is a mineral?

Solid

Natural substance - not man-made

Inorganic - not coal

Chemical element or compound with definite ratios and formulas

NaCl is always salt

Fixed internal structure

Graphite vs. diamond

DIGRESS TO: How about ice?

Chemistry and internal structure very important

Observations & measurements of these allow us to describe and classify different minerals

 

Crystalline solids

Easy to see in the big specimens - much tougher when small or not there

Some truly large minerals have been identified

Etta Mine - South Dakota

Spodumene "logs" over 10 feet long

Brazil: quartz crystals of several hundred pounds

Feldspar (forms 60% of earth's crust)

Norway: 7' X 12' X 30'

Ural Mtns.: A quarry was opened in a single crystal (30' X 30' X ?)

My experience in the Pala District

Stewart Lithia Dike: 40' long perthite crystal

White Queen: quartz crystal as roof support

Tourmaline Queen: my mining experience

External crystal form a reflection of the internal order

DIGRESS TO: internal and external order

The "unit cell" is the smallest subdivision of a mineral

Microscopic building blocks of the earth

Example: Halite

Unit cell: 4 atoms of each

In a grain the size used at the table there are 5.6 X 1018 unit cells!

Six possible crystal systems are possible (Monroe; fig. 3-8 pg. 78)

With many variations due to interference and impurities

Quartz: hexagonal system (Monroe; fig. 3-9, pg. 78)

Halite: cubic system

External form only evident if mineral was allowed to crystallize in open space

Euhedral: perfect external crystal form (Monroe; fig. 3-1, pg. 72)

Subhedral: some external form

Anhedral: no visible external crystal form

Remember: internally, it's always there and helps define what the mineral is

 

Chemical properties of minerals

What they are made of - at the elemental level

Definite chemical formulas

Halite (ordinary table salt): NaCl

Composed of sodium (reactive solid) & chlorine (poisonous gas)

If it's not NaCl, it's not halite

 

Mineral classes

To review...

There are 92 naturally occurring elements

Only 8 elements account for 98% of the crust (Monroe; Fig. 3-11, pg. 80)

Obviously, these 8 make up most of the minerals we see

There are well over 4000 minerals identified to date

DIGRESS TO: lumpers and splitters

Fortunately, only a couple dozen make up the vast majority of the rocks

Mineralogist have classified them into groups

Most mineral groups based on common elements found in the mineral (Monroe; Table 3-1, pg. 81)

Silicates

The vast majority of the crust is igneous rock

Most of the important igneous rock forming minerals are silicates

Silicates are the most common mineral class in igneous rocks (95%)

And possibly in the mantle, too

Discuss the SiO4 tetrahedron (Monroe; fig. 3-13, pg. 82)

Basic building block of the crust

Oxygen composes 46.6% of the crust by weight

And nearly 94% by volume!

The crust is essentially a boxwork of oxygen joined together by silica and a few other elements

Several major divisions within the silicates

Based on how the tetrahedron lattice is arranged (Monroe; fig. 3-13, pg. 82)

Ferromagnesian and non-ferromagnesian minerals (Monroe; Fig. 3-14, pg. 83)

DIGRESS TO: Mafic vs. felsic

Very important - pay attention to this!

Ferromagnesian (Monroe; Fig. 3-14, pg. 83)

Mafic: olivine, pyroxene, amphibole, biotite mica

Also some plagioclase feldspar

Non-ferromagnesian (Monroe; Fig. 3-14, pg. 83)

Felsic: quartz, orthoclase feldspar, muscovite mica

Also some plagioclase feldspar

Quartz & Feldspar - lots and lots in the crust

REVIEW the importance of oxygen and silicon

Feldspars makes up approx. 60% of the crust

Know them or die!

Orthoclase vs. plagioclase

Oxides - here's oxygen again

Hematite, magnetite

Carbonates

Limestone, dolomite

Sulfides (Monroe; fig. 3-15, pg. 84)

A metal combined with sulfur

Pyrite the most commonly recognized sulfide (FeS2)

Includes many of the major ore minerals

Chalcopyrite, sphalerite, galena

Very important to military/industry/economy

Not all available locally in sufficient quantities

There are several others

Native elements: gold, silver

Halides: Halite, fluorite (Monroe; fig. 3-15, pg. 84)

Sulfates: Gypsum, anhydrite (Monroe; fig. 3-15, pg. 84)

Arsenides

Tellurides

We will concentrate on the major rock-forming minerals (silicates)

 

Mineral Identification

Click here for online mineral and rock ID charts

Hard for the normal mortal to deal with the chemistry and internal structure

And the external form is so rare

Need to rely on other observations

Physical properties of minerals

Luster

The first main decision in most mineral ID charts

Quantity and quality of light reflected from surface

Can be tough to use

Many minerals have a range of lusters

Color: Obvious but not always definitive

Sulfur is (almost) always yellow, and there are a few others

But not many

Small amounts of impurities can drastically change a mineral's color

Streak: Can be definitive (ex. hematite)

Hardness (Monroe; Table 3-2, pg. 89)

Can vary due to impurities but usually definitive

Breakage pattern: very important, but often the hardest to determine (sorry)

Fracture vs. cleavage

Irregular surface vs. mirror surface

Controlled by internal crystalline order

Fracture: It just breaks

Uneven breakage - no mirror flash

Most are irregular but some special cases

Ex.: Conchoidal fracture (quartz and glass)

Cleavage (Monroe; figs. 3-17/18, pg. 88)

The ability of a mineral to split along closely spaces parallel planes

Usually parallel to crystal planes (if present)

Can have 1, 2, 3, 4, or 6 planes of cleavage

Can be obscured, but definitive when present

Perfect, Good in 2 directions, Poor, etc.

Can also be tough to distinguish from external crystalline form

Shame to break a good "crystal" when checking for cleavage

Specific Gravity

Defined as "The weight of a specific volume of a mineral divided by the weight of an equal volume of water (at 4°C.)"

Since water is always 1.0, it's the same number as density

Can vary due to impurities but usually definitive

Effervescence: the Fizz test

Carbonates react in dilute hydrochloric acid

Some may need to be powdered before reaction can take place

Need to increase surface area

Magnetism: Magnetite (Monroe; fig. 3-19, pg. 89)

Taste: Halite, chalcanthite

Double Refraction

Smell: Sulphur

Click here for online mineral and rock ID charts

 


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