GEOS102 Labs PT1

Mineralogy

Describes minerals that form the grains in the rock

Texture

Describes shape, size and arrangement of those grains

Cleavage

Way a mineral breaks along planes of weakness with the crystal structure

Cleavage description

• prescence or abscence

• number of directions

• angle between planes

• degree

Fracture

Way a mineral breaks when it does not break along a plane

Conchoidal fracture

Minerals break with smooth, curved surfaces

Hardness

Resistance that a surface of a mineral offers to scratching

Mohs Hardness Scale

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Feldspar

7. Quartz

8. Topaz

9. Conundrum

10. Diamond

Fingernail hardness

2.5

Copper hardness

3.5

Stainless steel hardness

5.5

Glass hardness

6.5

Talc hardness

1

Gypsum hardness

2

Calcite hardness

3

Fluorite hardness

4

Apatite hardness

5

Feldspar hardness

6

Quartz hardness

7

Topaz hardness

8

Conundrum hardness

9

Diamond hardness

10

Specific Gravity

Ratio of the weight of any volume of a mineral to the weight of an equal volume of water

Light specific gravity

1-3

Medium specific gravity

3-6

Heavy specific gravity

>6

Diaphaneity

ability of a mineral to transmit light

Lustre

Appearance of a mineral in reflected light

Non-metallic lustres

• Glassy/Vitreous

• Pearly/Nacreous

• Waxy/Greasy

• Fibrous

• Silky

• Adamantine

• Earthy/Dull

Streak

Colour of the powdered mineral

Detrial (Clastic)

Sedimentary rocks made up of bits of other rocks

Non-detrial (Chemical)

Sedimentary rocks made up of dissolved chemicals in water precipitating to solid form

Breccia

• Detrial

• Coarse, angular particles >2mm, embedded in finer matrix

Conglomerate

• Detrial

• Coarse, round particles >2mm, embedded in finer matrix

Sandstone

• Detrial

• Grains visible but <2mm

Siltstone

• Detrial

• Grains too fine to see, gritty on tooth/fingers

Mudstone

• Detrial

• Extremely fine clay minerals, smooth on teeth

Limestone

• Non-detrial

• Comprises of shell fragments or calcium carbonate, fizzes with HCl, scratches with knife blade

Chert

• Non-detrial

• Microcrystalline quartz, will not scratch with knife, conchoical fracture

Dolomite

• Non-detrial

• Similar to limestone but is a Mg carbonate, does not fizz as dramatically as calcium carbonate

Coal

• Non-detrial

• Derived from ancient organic matter

Boulder

• >256mm

• Gravel

• Conglomerate

Cobble

• 64-256mm

• Gravel

• Conglomerate

Pebble

• 4-64mm

• Gravel

• Conglomerate

Granule

• 2-4mm

• Gravel

• Conglomerate

Sand

• 0.062-2mm

• Sand

• Sandstone

Silt

• 0.0004-0.062mm

• Silt (Mud)

• Siltstone (Mudstone)

Clay

• <0.0004mm

• Clay (Mud)

• Claystone (Mudstone)

Form

Can be determined by measuring length of short, long and intermediate axes and calculating ratios between these axes

• disc

• spheroid

• blade

• roller

Roundness

• 1= Very angular

• 6 = Well rounded

Surface Texture

Physical state of clast surfaces

Younging direction

Which way was up when the rock was deposited

Steno’s 3 Laws

• Law of Superposition

• Law of Original Horizontality

• Law of lateral Continuity

Law of Superposition

In a sequence of sedimentary rocks, oldest is at bottom and youngest is at top

Law of Original Horizontality

Sedimentary layers are usually deposited in an approximately horizontal positon

Law of Lateral Continuity

Sedimentary layers are formed as flat sheets that continue until they run into something

Other Principles

• Law of cross-cutting relationships

• Principle of Faunal Succession

• Principle of Inclusions

• Walther’s Law

Principle of Cross Cutting Relationships

A geological feature that cuts across another geological feature is younger than the feature it cuts through

Principle of faunal succession

Fossil organisms appear, exist for a period of time, go extinct in an evoloutionary ordered succession

Principle of inclusions

Rock fragment of included rock found in another rock must be older than the rock enclosing it

Walther’s Law

Rock strata conformably overlying another MUST have been deposited in an adjacent depositonal environment

Conformable contacts

Rocks thar are adjacent because they grade into each other

Unconformable contacts

Rocks that are adjacent but where one is eroded or cut into

Unconformaties

• Nonconformity

• Angular unconformity

• Disconformity

• Paraconformity

Nonconformity

Separates non sedimentary rocks (below) from younger sedimentary rocks (above)

Angular unconformity

Separates two sequences of sedimentary rocks, the lower sequence tilted and eroded prior to deposition of a younger sequence

Disconformity

Separates two sequences of sedimentary rocks, where the lower sequence has been eroded BUT not tilted, prior to deposition of a younger sequence

Paraconformity

Separates two sequences of sedimentary rocks of different ages with no erosion appearant and the unconformity is parrallel to bedding

Approx how much of minerals in earths crust are slicates

90%

Quartz vs Feldspars diaphaneity

• Quartz is transparent

• Plagioclase and orthoclase opaque

Quartz vs felspars cleavage

• Quartz has no directions

• Plagioclase and orthoclase has 2 directions

Quartz vs Feldspars colour

• Quartz clear/white

• Plagioclase white

• Orthoclase brown/orange/red

Most distinguishing features between quartz and feldspar

• cleavage and fracture

• twinning

• mineral association

Distinguishing Orthoclase vs Plagioclase

• Plagioclase has multiple twinning on one cleavage surface

• Mineral association - plagioclase present in mafic and felsic rocks, orthoclase is rarely found in mafic rocks

Quartz vs Feldspars hardness

• Quartz has a hardness of 7

• Orthoclase and plagioclase have hardness of 6-6.5

Biotite vs Muscovite

• Biotite dark, muscovite light

• Biotite may weather but retains cleavage

• Both felsic

Pyroxene vs Amphibole

• hard, 5-6

• black

• Pyroxene 90^o cleavage, amphibole 120/60^o cleavage

• Pyroxene short, amphibole elongate

• Pyroxene more mafic, amphibole more intermediate/felsic

Olivine

• Green

• glassy

• Common in mafic and ultramafic rocks

• Sourced from mantle

Garnet

• variable in colour, mostly red

• typically present in metamorphic rock

Calcite (rarity/type? cleavage? colour? crystal shape? HCl?)

• Most common carbonate mineral

• 3 directions of cleavage

• transparent/translucent

• cubic crystal shape

• effervescent

Petrographic microscope components

• Eyepiece

• analyser

• objectives

• stage

• polarizer

• light source

• coarse and fine focus knobs

Thickness of thin sections

0.03mm (30 microns)

Polarizer

• polaroid beneath stage constraining light to vibrate in an E-W direction

Analyser

• polaroid above stage, only permits light travelling N-S

Analyser out

plane polarised light (PPL)

Analyser in

cross polarised light (XPL)

Extinction

• Anisotropic: extinct every 90^o, 4 times per 360^o rotation

• Can measure extinction angle

Paralell Extinction

• Cleavage trace is parralell to a crosshair when the grain goes extinct

Inclined (oplique) extinction

• Cleavage trace is not parralell to a crosshair when the grain goes extinct

Interference Colours (Birefringence)

• Indicates differences in velocity of the two light waves after they’ve passed through a mineral

• XPL

• eg. olivine bright and colourful, quartz grey . . .

Grain relief

• velocity of light through grains differ, causes appearance of high vs low relief

• PPL

• eg. garent in quartz

Isotropic minerals

• Minerals transparent under PPL but dark under 360^o rotation under XPL

• eg. garnet, glass

Pleochroism

• Minerals showing colour changes with rotation under PPL

• eg. biotite

Twinning

• Occurs when lattice of crustal grows in different orientations

  • Multiple twinning - stripes, eg plagioclase

  • Cross hatched twinning - cross stripes eg microcline

Distinguishing Mafic vs Felsic in thin sections (relief? pleochroism/colour? interference colours?)

• PPL: Mafic higher relief than plagioclase & quartz

• PPL: Mafic pleochroic/slightly coloured

• XPL: Mafic brighter interference colours

Distinguishing Mafic vs Mafic in thin section (colours? directions of cleavage? shape/cleavage?)

• PPL: Biotite and amphibole stronger colours than pyroxene

• Biotite sheet silicate, 1 direction of cleavage. Pyroxene and amphibole chain silicates, 2 directions of cleavage.

• Amphibole elongate and 120/60^o cleavage, pryroxene stubby and 90^o cleavage

Distinguishing Felsic vs Felsic in thin sections (twinning?)

• Quartz does not twin, feldspar does

• Plagioclase can have multiple twins, orthoclase does not

Plutonic grains texture

• crystalline granular (coarse or fine)

Volcanic grains texture

• Porphyritic

• Glassy

• Vesicular

• Pyroclastic

Crystalline granular texture

• Large grain size and mutual interference of growing crystals

• can be coarse or fine