Practical Work - Rocks Flashcards
What features can be distinguished from quartz by eye?
Colourless
Waxy/glassy
What features can be distinguished from feldspars by eye?
Plagioclase: white, brick-shaped crystals
Orthoclase: pink, brick-shaped crystals
What features can be distinguished from olivine by eye?
Lime green if fresh, but easily altered
What features can be distinguished from pyroxene by eye?
Black
Typically elongate crystals
What features can be distinguished from amphibole by eye?
Dark green, typically needle-shaped crystals
What features can be distinguished from micas by eye?
Biotite: shiny black, flat, sometimes hexagonal crystals
Muscovite: shiny colourless, flat, sometimes hexagonal
What features can be distinguished from garnet by eye?
Red-brown
Good crystal shape
What features can be distinguished from calcite by eye?
White
Soft
Which minerals are found in granitic rocks?
Orthoclase feldspar
Biotite
Plagioclase feldspar
Quartz
Which mineral can sometimes be seen as large crystals in basaltic rocks?
Olivine
Why does grain size differ between layers in sedimentary rocks?
Wind strength
Source sediment
What are the features of bivalve shells, what is the explanation?
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
What are the features of ooids, what is the explanation?
Spherical/ellipsoidal grains
Often have a concentric internal structure
Fine-grained CaCO3 formed while being rolled around by marine currents
What are the features of crystalline calcite, what is the explanation?
White/colourless mineral with glinting cleavage planes
A carbonate cement, sticking other grains together
What are the features of mud, what is the explanation?
Fine-grained material, individual grains barely visible by eye
A common component of sedimentary rocks; often formed of clay minerals or CaCO3
What are the features of iron oxides and hydroxides, what is the explanation?
Very fine-grained rusty-brown material
Often formed by near-surface oxidation of iron-bearing minerals in the grain/cement of the rock
What is the term for a sedimentary rock with ooids and can have crystalline calcite and iron oxide?
Oolitic limestone
What is the term for a sedimentary rock with ooids and shells and can have crystalline calcite and iron oxide?
Shelly oolitic limestone
What is the term for a sedimentary rock with shells and mud?
Shelly mudstone
What is the term for a sedimentary rock with mud?
Mudstone
What are the non-silicate minerals you are expected to recognise in hand specimen?
Magnetite Hematite Pyrite Galena Gypsum Calcite Halite
What are the silicate minerals you are expected to recognise in hand specimen?
Quartz Alkali feldspar Plagioclase feldspar Olivine Garnet Clinopyroxene Orthopyroxene Amphibole Muscovite Biotite
How can gypsum and calcite be distinguished using hardness?
Gypsum can be scratched by a fingernail, calcite can not
How can magnetite and hematite be distinguished?
Magnetite moves a compass
Hematite produces a red colour when scratched across a streak plate
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
How can pyroxene and amphibole be distinguished?
Amphibole has 60-degree and 120-degree cleavage
Pyroxene has 90-degree cleavage
How can micas be distinguished?
Mica sheets can be peeled
Biotite is dark, Muscovite is colourless
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
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
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
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
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
What is the crystal system, colour, lustre, habit and cleavage for calcite?
Crystal system: trigonal Colour: translucent Lustre: pearly Habit: equant Cleavage: rhombohedral
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
What is the crystal system, colour, lustre, habit and cleavage for olivine?
Crystal system: orthorhombic Colour: pale green Lustre: vitreous Habit: equant Cleavage: none
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
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
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
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
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
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
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
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
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
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
What kind of twinning is there in alkali/orthoclase feldspars?
Carlsbad twinning
What kind of twinning is there in plagioclase feldspars?
Albite twinning
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
Define accessory phase
Phases which only form a few vol% of a rock
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
Which rotation effect can help you recognise carbonates in the thin section?
Twinkling
A succession of high and low relief positions
What is the pleochroic effect?
Change in colour of a mineral grain as it is rotated
What is the optical path difference equation?
Δ=tδn
where δn = ε-ω (birefringence)
For a quartz wedge, as the thickness increases, what is the effect on the light?
Optical path difference increases
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
Which minerals are seen in a basalt?
Pyroxene
Olivine
Plagioclase feldspar
How are plagioclase feldspars identified in thin section with the analyser in?
Albite/lamellar twinning
What is undulose extinction?
In waves
Doesn’t all go extinct at once
What are the four minerals seen in acid igneous rocks?
Quartz
Orthoclase
Plagioclase
Microcline (K-feldspar)
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
How could you demonstrate the presence of glass in a thin section?
Optically isotropic so will be black
Which minerals are seen in rhyolite obsidian?
Describe them
Plagioclase: acicular crystals
Biotite: brown-yellow pleochroism, straight extinction
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
What is the difference between peridotite, dunite and pyroxenite?
Dunite is mostly olivine
Peridotite is mostly olivine and pyroxene
Pyroxenite is mostly pyroxene
How can olivine often be distinguished?
Serpentine cracks
High birefringence
What are cleaved rocks?
Rocks that split along closely spaced, regular, parallel, secondary foliation surfaces
How does rock cleavage arise?
Weakness due to the alignment of many platy minerals such as fine-grained micas
What are slates?
Fine-grained, cleaved rocks
Rock cleavage cutting across the bedding
Splitting property called fissility
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
What are schists?
Medium- to coarse-grained, foliated and/or lineated rocks
Foliation called schistosity
Rock cleavage tends to irregular
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
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
What can be seen in mudstones?
Tiny quartz grains
Low birefringent background of clay minerals
Tiny mica flakes with random orientation
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
How does a slate differ from a mudstone?
Mica flakes are oriented parallel to cleavage rather than bedding
How can you tell a foliation has been kinked in the thin-section?
Lines bend around the minerals
What are porphyroblasts?
Large crystals
Grown in a rock during metamorphism
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
What produces a lineation?
What produces a foliation?
Lineation: rock is stretched
Foliation: rock is squashed
How is an amphibolite generally formed?
Basalt + water
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
Which minerals are present in eclogite?
Garnet
Pyroxene
How does an eclogite form?
Conversion of basalt directly to eclogite
Dehydration of amphibolite
Amphibole + plagioclase = garnet + pyroxene + water
