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1A Earth > Practical Work - Rocks > Flashcards

Practical Work - Rocks Flashcards

1
Q

What features can be distinguished from quartz by eye?

A

Colourless

Waxy/glassy

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2
Q

What features can be distinguished from feldspars by eye?

A

Plagioclase: white, brick-shaped crystals
Orthoclase: pink, brick-shaped crystals

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3
Q

What features can be distinguished from olivine by eye?

A

Lime green if fresh, but easily altered

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4
Q

What features can be distinguished from pyroxene by eye?

A

Black

Typically elongate crystals

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5
Q

What features can be distinguished from amphibole by eye?

A

Dark green, typically needle-shaped crystals

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6
Q

What features can be distinguished from micas by eye?

A

Biotite: shiny black, flat, sometimes hexagonal crystals
Muscovite: shiny colourless, flat, sometimes hexagonal

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7
Q

What features can be distinguished from garnet by eye?

A

Red-brown

Good crystal shape

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8
Q

What features can be distinguished from calcite by eye?

A

White

Soft

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9
Q

Which minerals are found in granitic rocks?

A

Orthoclase feldspar
Biotite
Plagioclase feldspar
Quartz

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10
Q

Which mineral can sometimes be seen as large crystals in basaltic rocks?

A

Olivine

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11
Q

Why does grain size differ between layers in sedimentary rocks?

A

Wind strength

Source sediment

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12
Q

What are the features of bivalve shells, what is the explanation?

A

Thin concavo-convex shells, with concentric growth lines
Shell material may be present or dissolved away leaving a mould
Hard parts of fossil organisms preserved by burial

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13
Q

What are the features of ooids, what is the explanation?

A

Spherical/ellipsoidal grains
Often have a concentric internal structure
Fine-grained CaCO3 formed while being rolled around by marine currents

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14
Q

What are the features of crystalline calcite, what is the explanation?

A

White/colourless mineral with glinting cleavage planes

A carbonate cement, sticking other grains together

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15
Q

What are the features of mud, what is the explanation?

A

Fine-grained material, individual grains barely visible by eye
A common component of sedimentary rocks; often formed of clay minerals or CaCO3

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16
Q

What are the features of iron oxides and hydroxides, what is the explanation?

A

Very fine-grained rusty-brown material

Often formed by near-surface oxidation of iron-bearing minerals in the grain/cement of the rock

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17
Q

What is the term for a sedimentary rock with ooids and can have crystalline calcite and iron oxide?

A

Oolitic limestone

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18
Q

What is the term for a sedimentary rock with ooids and shells and can have crystalline calcite and iron oxide?

A

Shelly oolitic limestone

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19
Q

What is the term for a sedimentary rock with shells and mud?

A

Shelly mudstone

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20
Q

What is the term for a sedimentary rock with mud?

A

Mudstone

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21
Q

What are the non-silicate minerals you are expected to recognise in hand specimen?

A 
Magnetite
Hematite
Pyrite
Galena
Gypsum
Calcite
Halite
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22
Q

What are the silicate minerals you are expected to recognise in hand specimen?

A 
Quartz
Alkali feldspar
Plagioclase feldspar
Olivine
Garnet
Clinopyroxene
Orthopyroxene
Amphibole
Muscovite
Biotite
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23
Q

How can gypsum and calcite be distinguished using hardness?

A

Gypsum can be scratched by a fingernail, calcite can not

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24
Q

How can magnetite and hematite be distinguished?

A

Magnetite moves a compass

Hematite produces a red colour when scratched across a streak plate

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25
Q

How can galena and pyrite be distinguished?

A

Galena shows good cleavage
Pyrite shows poor cleavage
Also, galena can be scratched by a copper coin, pyrite can not

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26
Q

How can pyroxene and amphibole be distinguished?

A

Amphibole has 60-degree and 120-degree cleavage

Pyroxene has 90-degree cleavage

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27
Q

How can micas be distinguished?

A

Mica sheets can be peeled

Biotite is dark, Muscovite is colourless

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28
Q

What is the crystal system, colour, lustre, habit and cleavage for magnetite?

A 
Crystal system: cubic
Colour: grey/metallic
Lustre: metallic
Habit: massive/equant
Cleavage: none
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29
Q

What is the crystal system, colour, lustre, habit and cleavage for hematite?

A 
Crystal system: trigonal
Colour: metallic red
Lustre: metallic/non-metallic
Habit: massive/botryoidal
Cleavage: none
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30
Q

What is the crystal system, colour, lustre, habit and cleavage for pyrite?

A 
Crystal system: cubic
Colour: yellow/gold
Lustre: metallic when fresh
Habit: equant/cubic
Cleavage: none
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31
Q

What is the crystal system, colour, lustre, habit and cleavage for galena?

A 
Crystal system: cubic
Colour: grey
Lustre: metallic when fresh
Habit: equant
Cleavage: 3 mutually orthogonal
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32
Q

What is the crystal system, colour, lustre, habit and cleavage for gypsum?

A 
Crystal system: monoclinic
Colour: translucent
Lustre: pearly
Habit: prismatic/twinned
Cleavage: one perfect, two well developed
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33
Q

What is the crystal system, colour, lustre, habit and cleavage for calcite?

A 
Crystal system: trigonal
Colour: translucent
Lustre: pearly
Habit: equant
Cleavage: rhombohedral
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34
Q

What is the crystal system, colour, lustre, habit and cleavage for garnet?

A 
Crystal system: cubic
Colour: red to brown
Lustre: vitreous
Habit: facetted with cubic symmetry
Cleavage: none
35
Q

What is the crystal system, colour, lustre, habit and cleavage for olivine?

A 
Crystal system: orthorhombic
Colour: pale green
Lustre: vitreous
Habit: equant
Cleavage: none
36
Q

What is the crystal system, colour, lustre, habit and cleavage for augite (pyroxene)?

A 
Crystal system: monoclinic
Colour: black
Lustre: vitreous
Habit: equant/prismatic
Cleavage: 2 x 90-degree
37
Q

What is the crystal system, colour, lustre, habit and cleavage for hornblende (amphibole)?

A 
Crystal system: monoclinic
Colour: black
Lustre: vitreous
Habit: acicular/equant/prismatic
Cleavage: 2 x 60-degree
38
Q

What is the crystal system, colour, lustre, habit and cleavage for muscovite?

A 
Crystal system: monoclinic
Colour: white/silver, translucent in thin sheets
Lustre: glistening
Habit: micaceous
Cleavage: perfect between sheets
39
Q

What is the crystal system, colour, lustre, habit and cleavage for biotite?

A 
Crystal system: monoclinic
Colour: dark brown to blaack
Lustre: glistening
Habit: micaceous
Cleavage: perfect between sheets
40
Q

What is the crystal system, colour, lustre, habit and cleavage for quartz?

A 
Crystal system: trigonal
Colour: translucent
Lustre: vitreous
Habit: equant/prismatic
Cleavage: none, conchodial fracture
41
Q

What is the crystal system, colour, lustre, habit and cleavage for orthoclase feldspar?

A 
Crystal system: monoclinic
Colour: cream/pink
Lustre: vitreous
Habit: prismatic/tabular
Cleavage: 2 good, 1 weak
42
Q

What is the crystal system, colour, lustre, habit and cleavage for plagioclase feldspar?

A 
Crystal system: triclinic
Colour: white/pink/grey
Lustre: vitreous
Habit: prismatic/tabular
Cleavage: 2 good, 1 weak
43
Q

What is the extent of solution for orthoclase feldspars at high and low temperatures?

A

High: solution can have any mix of K and Na
Low: solution can only have high K or high Na

44
Q

What is the extent of solution for plagioclase feldspars at high and low temperatures?

A

High: solution can have any mix of Na and Ca
Low: solution can have any mix of Na and Ca

45
Q

What is the reason for the discrepancy in the extent of solution for orthoclase feldspars at different temperatures?

A

The large size difference between K and Na
At higher T, ions can move more easily so can dissolve more easily
The mix of K and Na at low T is unstable

46
Q

What kind of twinning is there in alkali/orthoclase feldspars?

A

Carlsbad twinning

47
Q

What kind of twinning is there in plagioclase feldspars?

A

Albite twinning

48
Q

Quartz and feldspar are colourless in thin section, how can they sometimes be distinguished though, and why?

A

Feldspars are cloudy and quartz is clear

Feldspars are altering to clay minerals

49
Q

Define accessory phase

A

Phases which only form a few vol% of a rock

50
Q

Grains of orthoclase and quartz both show low relief, how can they be distinguished by the Becke line test?

A

Orthoclase: as distance increases, the line moves into the medium so has RI lower than 1.54
Quartz: as distance increases, the line moves into the crystal so has RI greater than 1.54

51
Q

Which rotation effect can help you recognise carbonates in the thin section?

A

Twinkling

A succession of high and low relief positions

52
Q

What is the pleochroic effect?

A

Change in colour of a mineral grain as it is rotated

53
Q

What is the optical path difference equation?

A

Δ=tδn

where δn = ε-ω (birefringence)

54
Q

For a quartz wedge, as the thickness increases, what is the effect on the light?

A

Optical path difference increases

55
Q

What is seen when a sensitive tint plate is placed on top of the quartz wedge and rotate 90 degrees?

A

The sensitive tint turns black

The rest turns white

56
Q

Which minerals are seen in a basalt?

A

Pyroxene
Olivine
Plagioclase feldspar

57
Q

How are plagioclase feldspars identified in thin section with the analyser in?

A

Albite/lamellar twinning

58
Q

What is undulose extinction?

A

In waves

Doesn’t all go extinct at once

59
Q

What are the four minerals seen in acid igneous rocks?

A

Quartz
Orthoclase
Plagioclase
Microcline (K-feldspar)

60
Q

For the four minerals seen in acid igneous rocks, what is the twinning seen, and what is the RI compared to 1.54 (the medium)?

A

Orthoclase: RI < 1.54 & no lamellar twinning
Quartz: RI > 1.54 & no lamellar twinning
Plagioclase: lamellar twinning, for Na-rich RI < 1.54 and for Ca > 30% RI > 1.54
Microcline: RI < 1.54 & cross-hatched twinning

61
Q

How could you demonstrate the presence of glass in a thin section?

A

Optically isotropic so will be black

62
Q

Which minerals are seen in rhyolite obsidian?

Describe them

A

Plagioclase: acicular crystals
Biotite: brown-yellow pleochroism, straight extinction

63
Q

In rhyolitic obsidian, what can be seen aside from the minerals, why do they look the way they do?

A

Gas pockets with a teardrop shape

Aligned with the bands and flow

64
Q

What is the difference between peridotite, dunite and pyroxenite?

A

Dunite is mostly olivine
Peridotite is mostly olivine and pyroxene
Pyroxenite is mostly pyroxene

65
Q

How can olivine often be distinguished?

A

Serpentine cracks

High birefringence

66
Q

What are cleaved rocks?

A

Rocks that split along closely spaced, regular, parallel, secondary foliation surfaces

67
Q

How does rock cleavage arise?

A

Weakness due to the alignment of many platy minerals such as fine-grained micas

68
Q

What are slates?

A

Fine-grained, cleaved rocks
Rock cleavage cutting across the bedding
Splitting property called fissility

69
Q

What are phyllites?

A

Coarser-grained than slates
Have a sheen/lustre on the foliation surface
Sheen due to crystallisation of new minerals, mostly chlorite and muscovite

70
Q

What are schists?

A

Medium- to coarse-grained, foliated and/or lineated rocks
Foliation called schistosity
Rock cleavage tends to irregular

71
Q

What are gneisses?

A

Medium- to coarse-grained rocks
Lacking a well-developed schistosity due to the low abundance of micas
Commonly made of granular layers, breakage takes place across these layers

72
Q

What are migmatites?

A

Veins of granite set in a gneissose matrix
Rocks formed at the boundary between magmatism and metamorphism
Rock was just beginning to melt or was close to

73
Q

What can be seen in mudstones?

A

Tiny quartz grains
Low birefringent background of clay minerals
Tiny mica flakes with random orientation

74
Q

For a slate, if the colour changes on the z-axis, where would you define the bedding and cleavage planes?

A

Bedding on x y plane

Cleavage on x z/y z plane

75
Q

How does a slate differ from a mudstone?

A

Mica flakes are oriented parallel to cleavage rather than bedding

76
Q

How can you tell a foliation has been kinked in the thin-section?

A

Lines bend around the minerals

77
Q

What are porphyroblasts?

A

Large crystals

Grown in a rock during metamorphism

78
Q

Outline the properties of an amphibolite

A

Dominated by amphibole, mostly hornblende
Elongate habit
Commonly has a fabric
Plagioclase can be up to 50%, may look like salt and pepper in hand specimen

79
Q

What produces a lineation?

What produces a foliation?

A

Lineation: rock is stretched
Foliation: rock is squashed

80
Q

How is an amphibolite generally formed?

A

Basalt + water

81
Q

How can pyroxene and amphibole be distinguished in hand specimen?

A

Amphibole has a 60-degree cleavage (done by noting the angle between flashes as the crystal is rotated)
Amphiboles tend to be more elongate

82
Q

Which minerals are present in eclogite?

A

Garnet

Pyroxene

83
Q

How does an eclogite form?

A

Conversion of basalt directly to eclogite
Dehydration of amphibolite
Amphibole + plagioclase = garnet + pyroxene + water

1A Earth (15 decks)

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