The three major categories of rocks. The Rock Forming minerals The most abundant minerals in the...
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Transcript of The three major categories of rocks. The Rock Forming minerals The most abundant minerals in the...
The three major categories of rocks
The Rock Forming minerals
• The most abundant minerals in the earth’s crust and the most common in rocks are:-
• Quartz
• Feldspar
• Mica
• Pyroxene
Igneous Rocks
• Formed by the cooling and solidification of melted rock (molten silicate liquid) called magma or lava.
Classification of igneous rocks
Texture – crystal size
All large crystals
- Produced by relatively slow cooling deep underground.
- Found in intrusive igneous rocks.
Classification of igneous rocks
Crystal size
- Small crystals and/or glass
- Produced by rapid cooling, usually at or near the earth’s surface.
Typical of extrusive rocks.
Occasionally in intrusive rocks close to the surface.
Classification of igneous rocks
Other texturesBubbleswhich are trapped in igneous rocks are
called vesicles. E.g Pumice.Broken fragmentsIf there is enough gas, expanding bubbles
cause lava to break into fragments explosively. E.g. volcanic ash and pumice.
Sediments and Sedimentary rock
There are different types of sediment
1. Clastic sediments (also called detrital sediment).
2. Chemical sediment
3. Biochemical sediment
Some examples
Textures of Sedimentary rock
Clastic sediment
Maximum grain size – fine, medium and coarse.
Grain shape – angular and rounded.
Sorting – well or poorly sorted.
Well and poorly sorted sediment
Clastic rocks
By texture
• Course and rounded – conglomerate
• Course and angular – breccia
• Medium - sandstone
• Fine – siltstone, mudstone, claystone or shale.
Sandstone by composition
• Pure quartz sand – quartz sandstone
• Sand and fine matrix – graywacke sandstone.
• Sand with 20% rock fragments – lithic sandstone.
Chemical and biochemical Sedimentary Rocks
Calcite – Limestone
Plant remains – Coal or peat
Silica – Chert
Halite – Rock Salt
Gypsum – Rock Gypsum
Sedimentary structures
• Layering
• Ripples
• Cross-bedding
• Mud craks
• Burrows and tracks (fossils)
Metamorphism
• Includes process which change the minerals and texture of rock without melting it
Causes of metamorphism
Pressure
Different minerals are stable at different pressures.
Changing the pressure can change the types of minerals in rocks.
Causes of metamorphism
Temperature
Different minerals are stable at different temperatures.
Changing the temperature can cause the types of minerals present in the rock to change.
Causes of metamorphism
Hydorthermal fluids
Usually water and/or CO2 plus dissolved materials.
Can change the chemical composition of rocks by adding or removing materials.
Many important ore deposits have been formed by metamorphic deposits.
Other types of metamorphism
• Deformation – along fault lines
• Hydrothermal – associated with igneous intrusions along ocean ridges
• Burial – caused by deep burial.
• Shock – caused by sudden high pressures caused during impacts
Metamorphic rocks
Composition and texture are used to decribe these rocks, just like the previous ones.
Metamorphic rocks
Texture
Foliated rocks and this refers to the preferred orientation of the mineral grains in rocks as a result of directed pressure.
Non-foliated rocks have no mineral orientation.
Foliated metamorphic rocks
Slate
Schist
Gneiss
Phyllite
Non-foliated metamorphic rocks
• Quartzite
• Marble
• Skarn
• Hornfels
Sedimentary Rocks
Metamorphic Rocks
The Rock Cycle
Sediments
Magma
Sediments
SedimentaryRock
Metamorphic Rock
IgneousRock
Magma
Cooling
Melting
Heat and pressure
Uplift
UpliftUplift
Heat and pressure
Burial andlithification
Deposition in oceans andon continents
Weathering and erosion
Minerals
Make up the lithosphere, mantle and the core.
• Over 4000 are known.
• Minerals are used
extensively by humans.
• Minerals record information about the
processes that formed them.
Minerals the building blocks of rocks
• Definition of a mineral.Homogenous (uniform throughout)
Naturally occurring (no synthetics)
Inorganic (no biological stuff)
Solid (no liquid or gas)
Ordered internal molecular structure
Definite chemical composition
Rocks
• An aggregate of minerals
• Most rocks have
more than one
Mineral.
• Some rocks are mono mineralic
Granite is an igneous rock made up of several minerals
Quartz FeldsparHornblende
Crystals
• A single continuous mineral with crystal faces.
• The faces indicate growth inan open space.• Crystals of the same mineralshave similar faces, which reflectthe ordered atomicarrangement.
Polymorphs
These are minerals with the same chemical composition.
But different crystalline structures
Diamond and graphite are polymorphs of (C).
Mineral Physical Properties
The characteristics determined by your 5 senses.
• Used to ID minerals• Depend upon…
Chemical Composition
Crystal Structure
Physical Properties
• Crystal Form• Crystal Habit• Luster• Color• Streak• Hardness• Cleavage• Fracture• Specific Gravity
Crystal Form
• Ideal shape displayed by crystal faces. These are variable and depend on the chemicals involved.
Crystal Form
• Minerals may exhibit a range
of crystal face development• Euhedral – Well-developed• Anhedral – No visible crystal faces.
Usually from growth in a confined space.• Subhedral – Between the two.
Crystal Form
Crystals need an open space to grow, or they take on a different form.
Quartz crystals from an open and closed space.
Luster
Appearance in reflected light.Two categories
MetallicNon-metallic
Vitreous (glassy)SilkyResinousEarthy (dull)Adamantine (brilliant)
Colour
• Often unreliable for mineral ID.
• May vary due to impurity elements.
• Many gemstones are brightly colored.
Streak
Color of a mineral when scratched (crushed) on an unglazed porcelain plate.
Hardness
• Resistance of a mineral to abrasion or scratching.
• All minerals are compared to a standard scale
• called the Mohs scale of hardness.
Moh’s scale of hardness10 Diamond
9 Corundum
8 Topaz
7 Quartz
6 Feldspar
5 Apatite Glass
4 Flourite
3 Calcite Coin
2 Gypsum Fingernail
1 Talc
Hardest
Softest
Cleavage
• Tendency to break along planes of lattice weakness.
• Cleavage produces flat, shiny surfaces.• Described by resulting geometric shapes.• Number of planes• Angles between adjacent planes• Be careful to discriminate from crystal form
Cleavage
• Cleavage in one direction
More Cleavage
• 2 directions at 90º
• 2 directions NOT at 90º
• Example: Amphibole Example: Amphibole
• Example: Potassium Feldspar Example: Potassium Feldspar
• Cleavage
•
More Cleavage
• 3 directions at 90º
• 2 directions NOT at 90º
Conchoidal Fracture
• Some minerals lack planes of weakness.
• Molecular bonds are equally strong in all directions. They don’t cleave; they fracture.
Conchoidal fracture - curved surfaces, sharp edges.
Specific Gravity
• Ratio of the weight of a mineral to the weight
of an equal volume of water• Average crustal value is 2.7 (quartz)• Average mantle value is 3.3 (olivine)
SG is reflected in “heft” – how heavy the mineral feels.• Galena – Heavy Feldspar - Light