Revised 10 / 17 (Monroe 6th ed.)
Including...
Click here for online mineral and rock ID charts
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
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
Four 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
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:
earth --> rocks --> minerals --> elements --> atoms --> p/n/e --> subatomic particles --> who knows what
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
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
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
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
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
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)
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
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
GeoMan's Home Page | RCC Index | High School Geology Index
You are GeoManiac number since April 1, 1997