1.1 Rocks and minerals Flashcards
Define the term: mineral
A naturally occurring chemical substance having a definite composition and crystalline structure.
Define the term: rock
An aggregate or mixture of one or more minerals.
Define the term: crystal
A solid with plane faces formed when atoms are arranged in a structurally ordered pattern.
What is the chemical formula for quartz?
SiO₂
What is the chemical formula for calcite?
CaCO₃
What is the chemical formula for pyrite?
FeS₂
What is the chemical formula for galena?
PbS
What is the chemical formula for native sulphur?
S
What is the chemical formula for native copper?
Cu
What is the name of the mineral with the chemical formula FeS₂?
pyrite
What is the name of the mineral with the chemical formula CaCO₃?
calcite
What is the name of the mineral with the chemical formula SiO₂?
quartz
What is the name of the mineral with the chemical formula S?
native sulfur
What is the mineral with the chemical formula PbS?
galena
What is the mineral with the chemical formula Cu?
native copper
Define the term: lustre
The surface appearance of a mineral, as it interacts with light
What is sublimation? How does it relate to crystal formation?
Sublimation is the transition of a substance directly from the gas phase to the solid phase, or the reverse, without passing through the intermediate liquid phase.
Native sulphur crystals sometimes form by sublimation around volcanic vents.
Define the term: glass
An amorphous solid with no crystalline structure.
Define the term: grain boundary
The line of contact between mineral crystals in a rock.
What is the most common element in the Earth’s crust?
oxygen
What is the second most common element in the Earth’s crust?
silicon
How do natural glasses form?
Very rapid cooling of a liquid - the disorganised arrangement of the liquid is ‘frozen’.
What is obsidian?
A natural glass formed of silicon and oxygen atoms.
Why is glass not a rock or mineral?
It is not a mineral because there is no organised crystalline structure.
It is not a rock because it is not composed of one or more minerals.
How do mineral crystals grow?
Mineral crystals grow outwards from the centre by adding to their faces.
Why are the angles between the faces of a given mineral constant?
The angle between the faces is a result of the regular arrangement of the atoms in their structure.
Why do larger crystals form when a liquid is cooled more slowly?
Fewer crystals begin to form if a liquid is cooled more slowly. This smaller number of crystals have more space into which they can grow.
Why are the grain boundaries between crystals in a rock often irregular?
The crystals will grow until they meet each other, so they fill all available space.
Crystals that grow into a cavity will have good crystal shape.
Why is colour not always a useful diagnostic characteristic of a mineral?
Some minerals can occur in several different colours.
Which minerals can be distinguished by colour?
The two micas can be distinguished by colour - muscovite is pale grey and biotite is black
How is a streak test performed?
A mineral specimen is scraped across the surface of a piece of unglazed porcelain known as a streak plate.
What is the streak of a mineral?
Streak is the colour of the powder left by the mineral when a streak test is performed.
Why is the streak test not useful for distinguishing between silicate minerals?
Most silicates have a white streak, which does not help to distinguish between them.
What colour is galena’s streak?
lead-grey
What colour is the streak for iron pyrite?
black (although pyrite is brassy yellow)
Why do some minerals not leave a streak?
If the mineral is harder than the streak plate, it will not make a powder.
What is meant by the term earthy lustre?
The mineral has a dull, rough surface.
What is meant by the term adamantine lustre?
The mineral shines like a diamond.
What is the technical word for a ‘glassy’ lustre?
vitreous
What does the term crystal habit mean?
the shape of the crystal: cubic, hexagonal, etc.
What shape are quartz crystals?
hexagonal
Which mineral is characterised by a twinning crystal habit?
feldspars (particularly K-feldspar/plagioclase feldspar)
Define the term: cleavage
Cleavage refers to the planes of weakness in the atomic structure of crystals.
Which mineral group have perfect cleavage in one direction (planar)?
micas (sheet silicates)
Describe the cleavage of calcite.
Calcite has three cleavage directions which are not at right angles.
What is meant by the term fracture when applied to minerals?
Minerals that break along irregular surfaces tend to fracture. They do not have cleavage because the bonds between the atoms are strong.
What is meant by the term conchoidal fracture?
Conchoidal fracture refers to the fracture of a mineral in a series of concentric curved cracks - like a broken glass bottle.
Which scale is used to measure mineral hardness?
Mohs scale of hardness
What is the maximum value on Mohs scale of hardness?
10
State the reference minerals for Mohs scale, in order from soft to hard.
Talc, Gypsum, Calcite, Fluorite, Apatite, Feldspar, Quartz, Topaz, Corundum, Diamond
What is the hardness of quartz?
7
What is the hardness of diamond?
10
What is the hardness of calcite?
3
How could you distinguish between quartz and calcite, if you did not have any HCl acid?
quartz (7) is harder than calcite (3)
What are the units for density?
grams per cubic centimetre ( g cm -3 )
Define the term: specific gravity
Specific gravity is the ratio of the mass of a mineral compared with the mass of an equal volume of water.
It provides the same figures as density expressed in g cm-3 but as it is a ratio there are no units.
What is the specific gravity of quartz if it has a density of 2.65 g cm-3?
2.65
What is the diagnostic test for carbonate minerals such as calcite (CaCO3)?
carbonate minerals react strongly with dilute HCl acid - they effervesce (produce bubbles of CO2)
What is the density of water?
1 g cm-3
How can you calculate the specific gravity of a mineral sample, if given the volume of water displaced by the sample?
Divide the mass of the specimen by the mass of the water it displaces.
20 cm-3 of water has a mass of 20 g
What are the diagnostic characteristics of quartz?
hardness 7
no streak
no cleavage / irregular fracture
What are the diagnostic characteristics of feldspar?
cleavage in two directions
white streak
approximately rectangular habit
hardness 6
What are the diagnostic characteristics of mica?
good cleavage in one direction
hardness 2.5-3
What are the diagnostic characteristics of augite (most common pyroxene)?
square or eight-sided habit in cross-section
two directions of cleavage (93°)
dark green to black
What are the diagnostic characteristics of hornblende (most common amphibole)?
diamond shaped in cross-section
two directions of cleavage (c. 60°)
green to black
What are the diagnostic characteristics of olivine?
rounded shape
poor cleavage / conchoidal fracture
hardness 6.5-7
What are the diagnostic characteristics of calcite?
reacts with dilute HCl acid
rhombic crystal habit with three cleavage planes (or irregular)
hardness 3
What are the diagnostic characteristics of garnet?
rounded or six-sided in cross-section
hardness 6 - 7.5
no cleavage / poor conchoidal fracture
deep red to brown colour
What are the diagnostic characteristics of kyanite?
flat or blade crystal habit
two directions of cleavage
If you observe pink crystals in an igneous rock, which mineral are they most likely to be?
feldspar (particularly orthoclase feldspar)
If you observe white crystals in a dark-coloured igneous rock, what mineral is it most likely to be?
feldspar (usually plagioclase/K-feldspar)
