Minerals, Lectures 4 & 5 Flashcards
Minerals
Naturally occurring, inorganic crystalline solids.
Are minerals elements, atoms or compounds
elements or compounds
Crystalline
regular internal structure
Rocks make up…
rocks,
they are the building blocks of rocks
Rocks
naturally occurring solid aggregates of minerals
Main rock forming minerals
silicates
What do sillicates contain?
Si and O, combined with metallic elements
Example of a silicate mineral?
quartz
Two most abundant elements in the Earth’s crusts
silicon and oxygen
How do minerals form?
by crystallisation
Non-silicate minerals
6
- magnetite
- haematite
- pyrite
- gypsum
- calcite
- halite
Building block of silicate minerals
silica tetrahedron - SiO4
Third most abundant element in the Earth’s crust
Aluminium
How do silica tetrahedron made up minerals?
arranged in various ways with cations in the interstices
Four constituent minerals of granite
quartz
biotite (mica)
plagioclase feldspar
orthoclase feldspar
Crystal faces
The natural, flat (planar) boundaries of crystals.
If plenty of space how do crystal faces form?
slowly
If space is limited, how do crystals grow?
grow over and coalesce to become a solid mass of crystalline particles/grains - results in few or no grains showing crystal faces
Mineral solid solutions
Cations of similar sizes substitute for each other and make mixed compounds.
Example of a solid solutions
olivine
Solid solution: olivine, what is it made up of?
Fe and Mg
Pure Fe olivine
pure Fe olivine: Fe2SiO4
Fayalite
Pure Mg olivine?
pure Mg olivine: Mg2SiO4
Forsterite
Natural olivines are …
intermediate between foresterite and fayalite
What do all major silicate groups (except quartz) show?
some degree of solid solution
Natural glass
obsidian, volcanic glass
How does natural glass form?
Forms when lava cools too quickly to form crystals.
Structure of natural glass
Atoms are arranged in silica tetrahedra but, instead of being linked in a regular manner, they are randomly arranged as they would be in a liquid.
What are the layers in obsedian?
gas bubbles
Characteristics of obsedian
colour
3
jet-black
glassy (vitreous) lustre
shiney
Characteristics of obsedian shape crystals? rock or mineral or neither? 4
conchoidal (smoothly curved) fracture
lack, or have very small crystals
may have vesicles (bubbles) and flow bands
not a rock or mineral
Polymorphism
When chemical substances form more than one kind of crystal structure: different structures are called polymorphs
Example of polymorphism
CaCO3
which conditions for one of the polymorphs?
calcite
and
aragonite (higher density, stable at higher pressures)
What can calcite and aragonite make up?
shelly, invertebrate organisms such as foraminifera, clams and corals
Example of polymorphism
carbon
naturally occurring polymorphs
graphite and diamond
Example of polymorphism
carbon
not naturally occurring
fullerenes and graphene
Which naturally occurring carbon polymorph is more stable at higher pressures and has a higher density?
diamond
A mineral has…
a narrowly defined chemical composition and characteristics physical properties
Example of a characteristics physical property of a mineral?
hardness
Microscopy
the study of samples under the microscope
Petrography
the detailed description of rocks
What is used to see small minerals within rocks?
a polarising (petrographic) microscope and thin section (rock slices 0.03mm thick)
How are the minerals in rocks viewed under a polarising/petrographic microscope?
- rocks and mienrals become transparent if sliced thinly
- the microscope passes polarised light through
Polarised light
light vibrating in only one plane
Polariser
optical filter converts beam of light of mixed polarisation into beam with well defined polarisation
Two main ways of examining thin sections under a microscope
PPL and XPL
XPL
cross polarised light
PPL
plane polarised light
Pleochroism
An optical phenomenon in which a mineral appears to be a different colour when observed at different angles. different angle = different colour
