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