Wk. 8 Minerals and Rocks: Formation, identification and ...€¦ · Mineral Properties • Mineral ... • Thousands of minerals occur in rocks, but only a few are common and called

Earth Sciences and Geography

Palaeontology and Stratigraphy William Smith Building Room 10A [email protected] 01782 583162

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This is Planet Earth /

Earth Story

Wk. 8

Minerals and Rocks:

Formation, identification and classification



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Earth Materials – Minerals

• Gemstones and other minerals, such as gold, have fascinated people for thousands of years and have been supposed to have “mystical” or “curative” powers.

• Minerals have many essential uses in industrial societies.

• Minerals are the basic units that make up most of Earth’s materials.



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• What are rocks?

– are solid aggregates of minerals.

• Rocks find many uses...

– rocks crushed for aggregate in cement and for roadbeds.

– sawed and polished rocks for tombstones,

monuments, mantle pieces and counter tops

– Even the soils we depend on

• for most of our food

• formed by alteration of rocks



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What are Minerals?

– naturally occurring.

– crystalline solid.

crystalline means that minerals have an ordered internal arrangement of their atoms.

– minerals have a narrowly defined chemical composition.

– and characteristic physical properties such as

density

hardness

color...



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Matter and its Composition

• Matter

– is anything that has mass and occupies space.

– exists as solids, liquids, and gases.

– consist of atoms.

• Element

– is a chemical substance.

– cannot be chemically decomposed into simpler

substances.

– is composed of tiny particles called atoms.



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What are Atoms?

• Atoms are the smallest units of matter

retain the characteristics of the element

• Atoms have

a compact nucleus containing

• protons – particles with a positive electrical charge

• neutrons – electrically neutral particles

particles orbiting the nucleus

• electrons – negatively charged particles



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Structure of an Atom

• The dense nucleus of an atom

– consisting of protons and

neutrons

– is surrounded by a cloud of

orbiting electrons.



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Structure of an Atom:

Example Sodium (Na)

Na23

11

Nucleus: 11 protons +

12 neutrons.

Cloud: 11 electrons



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Atoms: Atomic number & Atomic mass number

• Atomic number:

= the number of protons

• Atomic mass number:

= number of protons + number of neutrons

The number of neutrons in an atom may vary ...



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Isotopes

• The different forms of an element’s atoms with varying

numbers of neutrons are called isotopes.

• Different isotopes of the same element

have different atomic mass numbers

behave the same chemically

• Isotopes are important in radiometric dating.



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Example: Carbon Isotopes



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Electrons and Shells

• Electrons orbit the nucleus in one or more shells.

• The outermost shell participates

in chemical bonding

contains up to 8 electrons.

• Noble gas configuration of 8 electrons

or 2 for Helium.

completes the outermost shell.

• Other atoms attain a noble gas configuration in the process of bonding.



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Bonding and Compounds

• Bonding:

the process whereby atoms join to other atoms.

• Compound:

a substance resulting from the bonding of two or

more elements.

Example:

Oxygen (O2) is an element.

Ice is a compound made up of hydrogen and oxygen

(H2O).

• Most minerals are compounds.



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Ionic Bonding

• Ion– atom that has gained or lost one or more electrons

negative or positive charge.

• One way for atoms to attain the noble gas configuration is by transferring electrons

producing ions

• Ionic bonding attraction between two ions of opposite charge.



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Covalent Bonding

• Another way for atoms to attain the noble gas

configuration is by sharing electrons:

• Covalent bonding

results from sharingelectrons

shared electrons



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Minerals—The Building Blocks of Rocks

• A mineral’s composition is shown by a chemical

formula

a shorthand way of indicating how many atoms of

different kinds it contains.

Quartz: SiO2

Ratio: 1: 2

– Quartz consists of 1 silicon atom for every 2 oxygen atoms

– Orthoclase consists of 1 potassium, 1 aluminum, and 3 silicon for every 8 oxygen atoms

KAlSi3O8

1: 1: 3: 8



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Native Elements

• A few minerals consist of only one element.

• They are not compounds.

• They are known as nativeelements.

• Examples:

gold – formula: Au

diamond – formula: C



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Crystalline Solids

• By definition, minerals are crystalline solids

– with atoms arranged in a specific 3D framework.

• If given enough room to grow freely,

minerals form perfect crystals with

planar surfaces, called crystal faces

sharp corners

straight edges.



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Narrowly Defined Chemical Composition

• Some minerals have very specific compositions

– examples are halite (NaCl) or quartz (SiO2).

• but others have a range of compositions

because one element can substitute for another if the atoms of the two elements have

• the same electrical charge

• and are about the same size

Example: olivine

• (Mg,Fe)2SiO4

• iron and magnesium substitution in any proportion



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Mineral Properties

• Mineral properties are controlled by:

Chemical composition

Crystalline structure

• Mineral properties are particularly useful

for mineral identification and include:

• cleavage

• fracture

• hardness

• specific gravity

• color

• streak

• luster

• crystal form



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How Many Minerals Are There?

• More than 3500 minerals are known.

• Only about 2 dozen are particularly common.

• Many others are important resources.

• Mineral groups:

– minerals with the same negatively charged ion or ion

group belong to the same mineral group.

• Most minerals in the crust belong to the group called

silicates.



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Silicates

• Silicates are minerals containing silica

Si and O

• They make up perhaps 95% of Earth’s crust and

account for about 1/3 of all known minerals.

• The basic building block of silicates is the silica

tetrahedron

• which consists of one silicon atom

• surrounded by four oxygen atoms



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Types of Silicates

• Silica tetrahedra can be

– isolated units bonded to other

elements

– arranged in chains (single or

double)

– arranged in sheets

– arranged in complex

3D networks



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Types of Silicates

• Ferromagnesian silicates

contain iron (Fe), magnesium (Mg), or both.

• Nonferromagnesian silicates

do not contain iron or magnesium



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Ferromagnesian Silicates augite, a pyroxene.

Hornblende, an

amphibole.

biotite mica

olivine



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Nonferromagnesian Silicates

Quartz Potassium feldspar

Plagioclase feldspar Muscovite



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Other Mineral Groups

• Carbonates

– minerals with carbonate ion (CO3)-2

calcite (CaCO3),

• found in limestone

dolomite [CaMg(CO3)2],

• found in dolostone

• Other mineral groups are important, but more as

resources than as constituents of rocks.



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Rock-Forming Minerals

• Most rocks are solid aggregates of one or more minerals.

• Thousands of minerals occur in rocks, but only a few are

common

and called rock-forming minerals.

• Most rock-forming minerals are silicates, but carbonates are

also important.

• Accessory minerals are present in small amounts and are

ignored in classifying rocks.



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―The Rock Cycle‖

• The rock cycle is a pictorial representation of events

leading to

the origin, destruction, change

and reformation of rocks

• Rocks belong to 3 major families

– igneous

– sedimentary

– metamorphic

• The rock cycle shows how these rock families are

interrelated and can be derived from one another.



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Pyroclastic

material

Lava



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Igneous Rocks

• All igneous rocks

cool and crystallize from magma,

solidify from lava,

or consolidate from pyroclastic materials.

• Magma is molten material, below the surface.

• Lava is molten material on the surface

• Pyroclastic materials are particles such as volcanic ash



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Pyroclastic

material

Lava

Igneous Part of the Rock Cycle



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Categories of Igneous Rocks:

• Extrusive or volcanic rocks

– formed at the surface

– from lava or pyroclastic materials

• Intrusive or plutonic rocks

– formed from magma injected into the crust

– or formed in place in the crust

• Plutons are intrusive bodies

consisting of plutonic rock



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Plutons



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Igneous Rock Textures:

• Texture

– is the size, shape and arrangement of crystals, grains and other constituents of a rock.

• Igneous rocks have 4 different textures

relate to cooling rate of magma or lava.



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4 Cooling-Rate Textures:

• phaneritic,

– with visible grains

• cooled slowly

• aphanitic,

– with grains too small to see without magnification

• cooled quickly

• porphyritic,

– with larger grains surrounded by a finer-grained groundmass

• cooled slowly first, then more quickly

• glassy,

– with no grains

• cooled too quickly for minerals to grow



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Other textures reveal further details of the formation of the

rock:

• Vesicular texture, with holes (vesicles),

– indicates the rock formed as water vapor and other

gases became trapped during cooling of lava.

• Pyroclastic or fragmental texture,

– containing fragments,

– formed by consolidation of volcanic ash

– or other pyroclastic material.



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Rapid cooling

Aphanitic

texture

Slow cooling

Phaneritic

texture

2-stage cooling

Porphyritic

texture



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Glassy texture

cooling was too

rapid for

mineral growth

Vesicular texture

gasses trapped in

cooling lava

Pyroclastic texture

particles fragmented

during eruption



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Classifying Igneous Rocks:

• Texture and composition are the criteria used to classify

most igneous rocks.

• Composition categories are based on silica content:

felsic (>65% silica)

intermediate (53-65% silica)

mafic (45-52% silica)

• More felsic magmas have higher Na, K, Al.

• More mafic magmas have higher Ca, Fe, Mg.



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Common Igneous Rocks:

Basalt Gabbro

Andesite Diorite



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Common Igneous Rocks:

RhyoliteGranite



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Classifying Igneous Rocks with Special Textures:T

extu

re

Vesicular

Glassy

Pyroclastic or

Fragmental

Composition

Pumice

Obsidian

Volcanic breccia

Tuff/welded tuff

MaficFelsic



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Igneous Rocks with Special Textures:

Tuff has pyroclastic texture.Pumice is glassy and

extremely vesicular.



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Sedimentary Rocks:

• Sedimentary rocks form by the lithification of sediment

• In the rock cycle, sediment originates when:

– mechanical and chemical weathering

• breaks rocks down into smaller particles

• and into solution

Transport removes sediment

• from its source area

• and carries it elsewhere

– Running water, glaciers, wind and waves

• transport sediment

Deposition involves settling of particles,

• and chemical and biological extraction of minerals from solution



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Sedimentary Part of the Rock Cycle



ESC-10020


Earth StoryLithification:

• Lithification means turning loose sediment into rock

• Lithification occurs by burial

• when additional sediment accumulates on top

compaction

• reduction of the amount of pore space between particles

• because of the weight of overlying sediment

cementation

• precipitation of minerals within pores

• that effectively binds sediment together

– calcium carbonate (CaCO3) cement is common

– silica (SiO2) cement is common

– iron oxide (Fe2O3) cement is less common



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Categories of Sedimentary Rocks:

• Detrital sedimentary rocks

– consist of solid particles

– derived from preexisting rocks (detritus)

• Chemical sedimentary rocks

– consist of minerals derived from materials in solution and extracted by either

• inorganic chemical processes

• or by the activities of organisms

– subcategory biochemical sedimentary rocks, for which

• the activities of organisms are important.



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Detrital Sedimentary Rocks:

• are composed of fragments or particles

known as “clasts” = Clastic texture

• These rocks are defined primarily by the size of clasts

• conglomerate

– composed of gravel (>2mm)

– rounded clasts

• sedimentary breccia

– also composed of gravel (>2mm)

– but clasts are angular

• sandstone

– composed of sand



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• Mudrocks consist of particles < 1/16 mm

– siltstone

• composed of silt-sized particles - 1/16-1/256 mm,

• feel slightly gritty,

• but not visible without magnification

– mudstone

• composed of a mixture of silt- and clay-sized particles

– claystone

• composed of clay-sized particles

– <1/256 mm, feel smooth even to the teeth

– shale

• mudstone or claystone that

• breaks along closely spaced parallel planes (fissile)



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Chemical Sedimentary Rocks:

• Recall that these rocks result

– when inorganic chemical processes

– or organisms extract minerals from solution

• This can result in different textures

– Crystalline texture

• has an interlocking mosaic of mineral crystals

• results from chemical precipitation

– Clastic texture

• has an accumulation of broken pieces of shells



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Chemical Sedimentary Rocks:

• Limestone – carbonate rock made of calcite precipitated chemically or by organisms

• Dolostone – carbonate rock made of dolomite usually formed from limestone

• Evaporites formed by

inorganic chemical precipitation during evaporation

– Rock salt – evaporite made of halite

– Rock gypsum – evaporite made of gypsum

• Chert – compact, hard, fine grained silica, formed by chemical or biological precipitation (some consisting of microscopic shells of silica-secreting organisms)

• Coal – made of partially altered, compressed remains of land plants accumulated in swamps



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Common Sedimentary Rocks:

ShaleQuartz sandstone

Sedimentary brecciaConglomerate



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Common Sedimentary Rocks:

Rock salt

Rock gypsum

Fossiliferous limestone

Chert

Coal



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Metamorphic Rocks:

• Metamorphic rocks

– result from transformation of other rocks

– in the solid state, without melting

• Changes resulting from metamorphism:

compositional:

• new minerals form

textural:

• minerals become reoriented

• minerals recrystallize

– or both.



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Agents of Metamorphism:• Heat provides new conditions

– where different minerals may be stable – and increases the rate of chemical reactions

• Pressure– Lithostatic pressure provides new conditions

• where different minerals may be stable • and forms smaller denser minerals

– Differential pressure• exerts force more intensely from one direction • causing deformation • and development of foliation.

• Fluid activity enhances metamorphism – by increasing the rate of chemical reactions – by transporting ions in solution



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Types of Metamorphism:

• Contact metamorphism

– heat

– chemical fluids

– from an igneous body

– alter rocks adjacent to the magma

• Regional metamorphism

– large, elongated area

– tremendous pressure

– elevated temperatures

– fluid activity

– occurs at convergent and divergent plate boundaries



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Metamorphic Textures:

• Foliated texture

– platy and elongate minerals aligned parallel to one

another

– caused by differential pressure

• Nonfoliated texture

– mosaic of roughly equidimensional minerals

– or platy and elongate minerals

– arranged in a helter-skelter fashion

– with random orientations



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Formation of Foliation:

• When rocks are subjected to differential pressure

minerals typically rearrange or grow parallel to each other



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Formation of Foliation:

Microscopic view of a

metamorphic rock

with foliation

showing the parallel

arrangement of

minerals



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Foliated Metamorphic Rocks:

• Slate

– very fine-grained, breaks in flat pieces

• Phyllite

– fine-grained (coarser than slate but grains are still

too small to see without magnification)

– breaks in flat pieces

• Schist

– clearly visible platy and/or elongate minerals

• Gneiss

– alternating dark and light bands of minerals



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Nonfoliated Metamorphic Rocks:

• Marble

– made of calcite or dolomite from limestone or dolostone

• Quartzite

– made of quartz from quartz sandstone

• Greenstone

– made of green mafic igneous rock

• Hornfels

– results from contact metamorphism

• Anthracite

– made of black lustrous carbon from coal



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Common Metamorphic Rocks:

Slate Schist

Gneiss Marble Quartzite



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Plate Tectonics and the Rock Cycle:

• The atmosphere, hydrosphere and biosphere act on earth materials and cause weathering erosion and deposition

• Earth’s internal heat aids melting and metamorphism

• Plate tectonics recycles Earth materials

– heat and pressure at convergent plate boundaries

• lead to metamorphism and igneous activity

– resulting deformation makes mountains

• that in turn weather and erode to form sediment



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End of Lecture

Documents

Wk. 8 Minerals and Rocks: Formation, identification and ...€¦ · Mineral Properties • Mineral ... • Thousands of minerals occur in rocks, but only a few are common and called