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Minerals Developed societies depend on mineral resources.
Metals – Iron, copper, lead, zinc, nickel, aluminum, etc.
Non-metals – Gypsum, limestone, aggregate, clay.
Minerals Economically important – Drive world economies.
Historically important – Dictated human history.
Iron.
Copper.
Gold.
Diamonds.
Gems.
Mineral Definition Geologic definition of a mineral is specialized:
Naturally occurring.
Solid.
Definite chemical
composition.
Ordered atomic
arrangement.
Mostly inorganic.
A mineraloid exhibits some of these properties.
Doesn’t include “minerals” in the nutritional sense.
Rocks Rocks are earth materials made from minerals.
Most rocks contain more than one kind of mineral.
Example: Granite
K-feldspar – Pink.
Quartz – Gray.
Hornblende – Black.
Some are monomineralic.
Limestone (Calcite)
Rock salt (Halite)
Glacial ice.
Atoms in a mineral are specifically ordered.
A solid with disordered atoms is called a glass.
Crystalline structure is based on atomic patterns.
Crystalline Structure
Crystals
Rare minerals displaying flat external faces.
Crystal faces form best in open cavities.
Crystals are often prized mineral specimens.
Crystals Constancy of interfacial
angles.
Different samples of the same mineral will have the same crystal faces.
Adjacent faces are always
oriented at the same angle.
Crystal faces reflect the
internal atomic order.
Crystal Lattice Ordered atoms in crystals form a 3-D lattice.
Lattices are patterns that repeat in three dimensions.
This internal pattern controls most mineral properties.
Crystal shape.
Symmetry.
Atomic Bonding
Lattice atoms are held in place by atomic bonds.
Bond characteristics also govern mineral properties.
Models depict atoms, bonds, and lattices.
Graphite
Polymorphs
Diamond
Minerals with the same composition; different structure.
Polymorphs reveal the importance of bond type.
Diamond and graphite are carbon polymorphs (C).
Diamond – Strong covalent bonds; hardest mineral.
Graphite – Weak van der Waals bonds; softest mineral.
Crystal Growth
Crystals grow as atoms attach to mineral surfaces.
Growth starts from a central seed crystal.
Growth expands outward as atoms accumulate.
Crystal Growth Outward crystal growth fills available space.
Resulting crystal shape is governed by surroundings.
Open space – Good crystal faces grow.
Confined space – No crystal faces.
Crystals grow by…
Solidification from a melt.
Precipitation from solution.
Solid-state diffusion.
Mineral Physical Properties
Pyrite
Characteristics determined by your five senses.
Used to ID minerals.
Properties depend upon…
Chemical composition.
Crystal structure.
Some are diagnostic.
Example: Pyrite (FeS2)
Cubic crystals, high specific gravity, striated crystal faces, black streak, metallic luster, dull brassy color, sulfur smell when crushed, erroneously mistaken for gold (fool’s gold).
Minerals have unique sets of physical properties.
Mineral Physical Properties
Needle-like crystal habit
Common properties of
minerals are...
Crystal form.
Crystal habit.
Luster.
Color.
Streak.
Hardness.
Cleavage.
Fracture.
Specific gravity.
Mineral Physical Properties
Calcite effervesces with acid
Magnetite crystals on a large magnet.
Less common physical properties are... Taste.
Smell.
Feel.
Magnetism.
Effervescence.
Diaphaneity.
Piezoelectricity.
Pyroelectricity.
Refractive index.
Malleability.
Ductility.
Color
Quartz – Many colors Malachite – Always green
Color is diagnostic for some minerals.
Olivine is olive green.
Azurite is always blue.
Some minerals may exhibit a broad color range.
Quartz (Clear, white, yellow, pink, purple, gray, etc).
Color varieties often reflect trace impurities.
Streak
Hematite – Red-brown streak
Mineral color crushed on an unglazed porcelain plate.
Streak is often a useful diagnostic property.
Congruent streak – Streak color the same as the mineral.
Magnetite – Black mineral; black streak.
Incongruent streak – Streak color differs from the mineral.
Chromite – Black mineral; greenish-brown streak.
Luster
Satin spar Gypsum – Satiny luster
Quartz – Vitreous luster
The way a mineral surface scatters light.
Two subdivisions.
Metallic – Looks like a metal.
Nonmetallic.
Vitreous (glassy).
Satiny.
Silky.
Resinous.
Pearly.
Earthy (dull).
Adamantine (brilliant).
Resistance to abrasion (scratching)
Hardness compared to the Mohs hardness scale.
Hardness
Glass - Steel 5.5
Fingernail 2.5
Copper Penny 3.5
Steel File 6.5
Pyrite
Specific Gravity
Potassium Feldspar
Related to density (mass per volume)
Mineral weight over weight of equal water volume.
Specific gravity is “heft”– How heavy it feels.
Pyrite – Heavy (SG 5.0)
Feldspar – Light (SG 2.6)
Pyrite “feels” heavier than feldspar.
Crystal Habit Crystal habit is the ideal shape of crystal faces.
Ideal faces require ideal growth conditions.
Many descriptive terms are used to characterize habit.
Cubes Hexagonal PrismsBlades
Dodecahedra
Octahedra
Tetragonal PrismsRhombohedraCompound Forms
Crystal Form
Amethyst
Minerals vary in crystal face development.
Euhedral – Good crystal faces; grown in open cavity.
Anhedral – No crystal faces; grown in tight space.
Subhedral – Between the two.
Face development indicates growth history.
Anhedral crystals common; euhedral less so.
Cleavage Tendency to break along planes of lattice weakness.
Cleavage produces flat, shiny surfaces.
Described by the number of planes and their angles.
Sometimes mistaken for crystal habit.
Cleavage is through going; often forms parallel “steps”.
Habit is only on external faces.
1, 2, 3, 4, and 6 cleavages possible.
Examples of Cleavage:
1 direction
2 directions at ~ 90º
2 directions NOT at 90º
Cleavage Muscovite Mica
Amphibole
Potassium Feldspar
Fracture
Obsidian Some minerals lack planes of lattice weakness.
Due to equal molecular bonds in all directions.
These minerals don’t cleave; they exhibit fracture.
Example: Quartz displays conchoidal fracture.
Shaped like the inside of a clam shell.
Breaks along smooth, curved surfaces.
Produces extremely sharp edges.
Mineral Compositions
74.3% of crustal minerals !!!
Only about 50 minerals are abundant.
98% of crustal mineral mass is from eight elements.
Oxygen O 46.6%
Silicon Si 27.7%
Aluminum Al 8.1%
Iron Fe 5.0%
Calcium Ca 3.6%
Sodium Na 2.8%
Potassium K 2.6%
Magnesium Mg 2.1%
All others 1.5%
Mineral Classes
Fluorite (Halide) Native CopperMalachite (Carbonate)
Minerals are classified based upon the dominant anion.
Silicates SiO24- Rock-forming minerals
Oxides O2- Magnetite, Hematite
Sulfides S- Pyrite, Galena
Sulfates SO42- Gypsum
Halides Cl- or F- Fluorite, Halite
Carbonates CO32- Calcite, Dolomite
Native elementsCu, Au, C Copper, Gold, Graphite
Silicate Minerals Silicates are know as “the rock-forming minerals.”
They dominate Earth’s crust.
Oxygen and silicon…
Make up 94.7% of crustal volume, and...
74.3% of crustal mass.
Silicate Minerals The anionic unit is the silica tetrahedron.
4 oxygen atoms are bonded to 1 silicon atom (SiO4
4-).
Silicon is tiny; oxygen is huge.
The silica tetrahedron has a net -4 ionic charge.
The silicate unit can be depicted by…
Spheres.
A ball-and-stick model.
Polyhedra.
Silicate Minerals
Type of Silicate Structure Formula Si:O Ratio
Independent Tetrahedra SiO4 0.25
Double Tetrahedra Si2O7 0.29
Ring Silicates Si6O18 0.33
Single Chains SiO3 0.33
Double Chains Si4O11 0.36
Sheet Silicates Si2O5 0.40
Framework Silicates SiO2 0.50
Silica tetrahedra link together by sharing oxygens.
More shared oxygen = lower Si:O ratio; governs…
Melting temperature.
Mineral structure and cations present.
Susceptibility to chemical weathering.
Independent Tetrahedra
KyaniteGarnet
Tetrahedra share no oxygens–they are linked by cations.
Olivine group.
High-temperature Fe-Mg silicate.
Small green crystals; no cleavage.
Garnet group.
Equant crystals with no cleavage.
Dodecahedral (12-sided) crystals.
Single-Chain Silicates
Pyroxene
Single-chain structures bonded with Fe and Mg.
Pyroxene Group.
Black-to-green color.
Two distinctive cleavages at nearly 90°.
Stubby crystals.
Augite is the most common pyroxene.
Double-Chain Silicates
Hornblende
Double chain of silica tetrahedra bonded together.
Contain a variety of cations.
Amphibole group - two perfect cleavages;
elongate crystals.
Sheet Silicates
Muscovite
Two dimensional sheets of linked tetrahedra.
Characterized by one direction of perfect cleavage.
Mica group – Biotite (dark) and Mucsovite (light).
Clay mineral group – Feldspar-weathering residue; tiny.
Framework Silicates
Potassium Feldspar
All four oxygens in the silica tetrahedra are shared.
Feldspar group – Plagioclase and potassium feldspar.
Silica (Quartz) group – Contains only Si and O.
Watermelon Tourmaline
Gems
Aquamarine Beryl
Minerals with special value.
Rarity.
Beauty.
Color.
Interaction with light.
Dispersion.
High refractive index.
Gems
Gems are cut and polished to be used in jewelry.
Facets are ground onto a gemstone by a machine.
Facets are not natural crystal faces.