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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
How can galena and pyrite be distinguished?
Galena shows good cleavage Pyrite shows poor cleavage Also, galena can be scratched by a copper coin, pyrite can not
26
How can pyroxene and amphibole be distinguished?
Amphibole has 60-degree and 120-degree cleavage | Pyroxene has 90-degree cleavage
27
How can micas be distinguished?
Mica sheets can be peeled | Biotite is dark, Muscovite is colourless
28
What is the crystal system, colour, lustre, habit and cleavage for magnetite?
``` Crystal system: cubic Colour: grey/metallic Lustre: metallic Habit: massive/equant Cleavage: none ```
29
What is the crystal system, colour, lustre, habit and cleavage for hematite?
``` Crystal system: trigonal Colour: metallic red Lustre: metallic/non-metallic Habit: massive/botryoidal Cleavage: none ```
30
What is the crystal system, colour, lustre, habit and cleavage for pyrite?
``` Crystal system: cubic Colour: yellow/gold Lustre: metallic when fresh Habit: equant/cubic Cleavage: none ```
31
What is the crystal system, colour, lustre, habit and cleavage for galena?
``` Crystal system: cubic Colour: grey Lustre: metallic when fresh Habit: equant Cleavage: 3 mutually orthogonal ```
32
What is the crystal system, colour, lustre, habit and cleavage for gypsum?
``` Crystal system: monoclinic Colour: translucent Lustre: pearly Habit: prismatic/twinned Cleavage: one perfect, two well developed ```
33
What is the crystal system, colour, lustre, habit and cleavage for calcite?
``` Crystal system: trigonal Colour: translucent Lustre: pearly Habit: equant Cleavage: rhombohedral ```
34
What is the crystal system, colour, lustre, habit and cleavage for garnet?
``` Crystal system: cubic Colour: red to brown Lustre: vitreous Habit: facetted with cubic symmetry Cleavage: none ```
35
What is the crystal system, colour, lustre, habit and cleavage for olivine?
``` Crystal system: orthorhombic Colour: pale green Lustre: vitreous Habit: equant Cleavage: none ```
36
What is the crystal system, colour, lustre, habit and cleavage for augite (pyroxene)?
``` Crystal system: monoclinic Colour: black Lustre: vitreous Habit: equant/prismatic Cleavage: 2 x 90-degree ```
37
What is the crystal system, colour, lustre, habit and cleavage for hornblende (amphibole)?
``` Crystal system: monoclinic Colour: black Lustre: vitreous Habit: acicular/equant/prismatic Cleavage: 2 x 60-degree ```
38
What is the crystal system, colour, lustre, habit and cleavage for muscovite?
``` Crystal system: monoclinic Colour: white/silver, translucent in thin sheets Lustre: glistening Habit: micaceous Cleavage: perfect between sheets ```
39
What is the crystal system, colour, lustre, habit and cleavage for biotite?
``` Crystal system: monoclinic Colour: dark brown to blaack Lustre: glistening Habit: micaceous Cleavage: perfect between sheets ```
40
What is the crystal system, colour, lustre, habit and cleavage for quartz?
``` Crystal system: trigonal Colour: translucent Lustre: vitreous Habit: equant/prismatic Cleavage: none, conchodial fracture ```
41
What is the crystal system, colour, lustre, habit and cleavage for orthoclase feldspar?
``` Crystal system: monoclinic Colour: cream/pink Lustre: vitreous Habit: prismatic/tabular Cleavage: 2 good, 1 weak ```
42
What is the crystal system, colour, lustre, habit and cleavage for plagioclase feldspar?
``` Crystal system: triclinic Colour: white/pink/grey Lustre: vitreous Habit: prismatic/tabular Cleavage: 2 good, 1 weak ```
43
What is the extent of solution for orthoclase feldspars at high and low temperatures?
High: solution can have any mix of K and Na Low: solution can only have high K or high Na
44
What is the extent of solution for plagioclase feldspars at high and low temperatures?
High: solution can have any mix of Na and Ca Low: solution can have any mix of Na and Ca
45
What is the reason for the discrepancy in the extent of solution for orthoclase feldspars at different temperatures?
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
What kind of twinning is there in alkali/orthoclase feldspars?
Carlsbad twinning
47
What kind of twinning is there in plagioclase feldspars?
Albite twinning
48
Quartz and feldspar are colourless in thin section, how can they sometimes be distinguished though, and why?
Feldspars are cloudy and quartz is clear | Feldspars are altering to clay minerals
49
Define accessory phase
Phases which only form a few vol% of a rock
50
Grains of orthoclase and quartz both show low relief, how can they be distinguished by the Becke line test?
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
Which rotation effect can help you recognise carbonates in the thin section?
Twinkling | A succession of high and low relief positions
52
What is the pleochroic effect?
Change in colour of a mineral grain as it is rotated
53
What is the optical path difference equation?
Δ=tδn | where δn = ε-ω (birefringence)
54
For a quartz wedge, as the thickness increases, what is the effect on the light?
Optical path difference increases
55
What is seen when a sensitive tint plate is placed on top of the quartz wedge and rotate 90 degrees?
The sensitive tint turns black | The rest turns white
56
Which minerals are seen in a basalt?
Pyroxene Olivine Plagioclase feldspar
57
How are plagioclase feldspars identified in thin section with the analyser in?
Albite/lamellar twinning
58
What is undulose extinction?
In waves | Doesn't all go extinct at once
59
What are the four minerals seen in acid igneous rocks?
Quartz Orthoclase Plagioclase Microcline (K-feldspar)
60
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)?
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
How could you demonstrate the presence of glass in a thin section?
Optically isotropic so will be black
62
Which minerals are seen in rhyolite obsidian? | Describe them
Plagioclase: acicular crystals Biotite: brown-yellow pleochroism, straight extinction
63
In rhyolitic obsidian, what can be seen aside from the minerals, why do they look the way they do?
Gas pockets with a teardrop shape | Aligned with the bands and flow
64
What is the difference between peridotite, dunite and pyroxenite?
Dunite is mostly olivine Peridotite is mostly olivine and pyroxene Pyroxenite is mostly pyroxene
65
How can olivine often be distinguished?
Serpentine cracks | High birefringence
66
What are cleaved rocks?
Rocks that split along closely spaced, regular, parallel, secondary foliation surfaces
67
How does rock cleavage arise?
Weakness due to the alignment of many platy minerals such as fine-grained micas
68
What are slates?
Fine-grained, cleaved rocks Rock cleavage cutting across the bedding Splitting property called fissility
69
What are phyllites?
Coarser-grained than slates Have a sheen/lustre on the foliation surface Sheen due to crystallisation of new minerals, mostly chlorite and muscovite
70
What are schists?
Medium- to coarse-grained, foliated and/or lineated rocks Foliation called schistosity Rock cleavage tends to irregular
71
What are gneisses?
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
What are migmatites?
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
What can be seen in mudstones?
Tiny quartz grains Low birefringent background of clay minerals Tiny mica flakes with random orientation
74
For a slate, if the colour changes on the z-axis, where would you define the bedding and cleavage planes?
Bedding on x y plane | Cleavage on x z/y z plane
75
How does a slate differ from a mudstone?
Mica flakes are oriented parallel to cleavage rather than bedding
76
How can you tell a foliation has been kinked in the thin-section?
Lines bend around the minerals
77
What are porphyroblasts?
Large crystals | Grown in a rock during metamorphism
78
Outline the properties of an amphibolite
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
What produces a lineation? | What produces a foliation?
Lineation: rock is stretched Foliation: rock is squashed
80
How is an amphibolite generally formed?
Basalt + water
81
How can pyroxene and amphibole be distinguished in hand specimen?
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
Which minerals are present in eclogite?
Garnet | Pyroxene
83
How does an eclogite form?
Conversion of basalt directly to eclogite Dehydration of amphibolite Amphibole + plagioclase = garnet + pyroxene + water

1A Earth (15 decks)

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