content Flashcards
age of galaxy
14 billion years
composition of terrestrial planets
all have/had a core of some type (Fe, Ni)
mostly made of SiO2 and MgO
composition of gas giants
jupiter and saturn -> mostly made of hydrogen and helium
uranus and neptune -> mostly made of water and methane (icy planets)
evidence for having a rocky core
what are the 3 major groups of meteorites
- irons : iron-nickel alloy
- stony irons : both silicate minerals and Fe-Ni alloys
- stones : mostly composed of silicate materials (chondrites and anchondrites)
what are the 3 subcategories of stony meteorites
chondrites, anchondrites and carbonaceous chondrites
what are chondrites
contain millimetre sized rounded masses known as chondrules and matrix, both formed of anhydrous high temperature minerals
what are anchondrites
no chondrules and textures and minerals similar to earth and lunar like primitive igneous rocks (basalts)
what are carbonaceous chondrites
contain chondrules, hydrous low temperature minerals, hydrocarbons, water and volatile elements
the most primitive type of meteorite
what is the valency of an ion
the charge on an ion, single elements can form ions with different valency, eg Cu(I) and Cu(II)
what is the octet rule
- elements try to achieve a stable octet by transferring or sharing electrons with other atoms, resulting in the formation of chemical bonds.
- if a bond creates an outer shell of 8 electrons then the “octet rule” is obeyed
what is first ionisation energy
the energy required to remove one electron from an atom of a given element
what is the coordination number of an atom or ion
- the coordination number of an atom or ion is the number of neighbours nearest to it
- eg cubic close packing = coordination number 12
what is closest packing
- if spheres of equal size are packed together as closely as possible within a plane, each sphere is in contact with 6 others. this is called closest packing
- a second layer is places on the first layer so that each sphere in the second later is in contact with 3 spheres in the first
- the third layer can be positioned in one of two ways:
- the third later lies directly above the spheres in the first layer. this is called hexagonal close packing
- the third layer is offset from the first layer. this is called cubic close packing
what is the structure of silicate minerals
- they are built around the silica tetrahedron - four oxygens surrounding a silicon ion
- these tetrahedra combine to make the framework of the silicates
- different combinations produce different structures
what is a mineral
naturally occurring inorganic solid with an ordered internal structure made up of a regular and repeating arrangement of atoms and a definite but not fixed chemical composition
how are minerals formed
- crystallisation from a magma
- magma, high temp liquid → high kinetic energy no solid is stable
- magma cools, kinetic energy decreases until atoms slow enough for ionic bonds to persist → nucleation of crystals from a melt
- if high temps remain, magma partially molten, atoms still quite mobile and move towards the surfaces of growing crystals → larger well formed crystals
- rapid cooling → smaller less well formed crystals
how do minerals form in the solid state
- crystal growth in the solid state eg metamorphic process
- chemical reactions and replacement of pre-existing minerals by new ones
- bonds broken and atoms migrate by solid state diffusion or transported short distances by inter granular fluids to sites of new mineral growth
why is mineralogy of metamorphic rock more diverse than sedimentary or igneous
includes minerals found in both other rock types plus high pressure/temperature equivalents
how are minerals classified
grouped on basis of chemical composition based on dominant anion or anion complex
how many recognised minerals are there
4,150
what are native elements (metals)
- composed of single element
- most metals weather ie oxidise
- generally only less reactive metals eg gold, silver, platinum, copper found as native metals
- gold - nuggets, veins or wires; silver/copper - dendritic coatings
what are native elements (non-metals)
- carbon comes in different forms - polymorphism
- graphite - carbon atoms in sheets, weak bonding, soft
- diamond - formed at very high pressures (mantle) → carbon atoms densely packed into tetrahedral framework, hard and strong
what are oxides
- metal cations ionically bonded to oxide anions
- split into groups
- simple oxides
- hematite group
- rutile group
- spinel group
chemical formula of simple oxides
X2O and XO
chemical formula of hematite group oxides
X2O3
chemical formula of rutile group oxides
XO2
chemical formula of spinel group oxides
XY2O4
what are hydroxides
- metal cations combined with hydroxide anions
- typically form at lower temperatures and are softer than oxides
- often weathering or degradation products
what are sulfides
- metal cations ionically bonded to sulfur anions
- indicative of reducing conditions
what are sulfates
metal cations ionically bonded to sulphate anions
one of primary evaporite minerals formed in tropical restricted marine environments
what are native element (metals) used for
valuable for use in electronics and as precious metals
why are oxides important
economically important as metal ore
examples of hydroxides
- goethite and limonite as weathering products of iron rich deposits / pyrite
- limonite - form in fresh water and bogs
- gibbsite - important component of aluminium ore bauxite
why are sulfides important
economically important as metal ores eg galena (Pb) and sphalerite (Zn)
example of sulfides
pyrite - “fools gold” - common component of sedimentary systems under reducing / low oxygen conditions (high organic matter content)
why are sulfates important
economically important as component of plaster
example of sulfates
most important and common in gypsum
3 ‘forms’ of gypsum: satin spar (fibrous), selenite (transparent), alabaster (fine grained, massive form)
what are halides
metal cations and halogen anions
examples of halides
- only halite and sylvite are common in sedimentary rocks
- halite typically found as rock salt eg as an evaporite from salt lakes of seawater
- sylvite also an evaporite but much less common than halite
- fluorite is a hydrothermal mineral associated with lead, zinc and other metal ores or in the fractures in limestone or dolomite
what are carbonates
- metal cations ionically bonded to carbonate anions
- 3 important and common forms (mostly sedimentary and metamorphic rocks)
- calcite - dominant form of calcium carbonate
- aragonite - polymorph of calcite - low temp and metastable relative to calcite - often biogenic
- dolomite - often associated with diagenesis in reducing conditions
example of a silicate mineral
olivine
chemical formula of olivine
Mg2SiO4
structure of olivine
- isolates silicate tetrahedra (not touching each other)
- 4- charge on silica tetrahedra needs to be balanced for the mineral to be electrostatically neutral
- positive cations between tetrahedra balance negative charge
what is a unit cell
- can define a crystal structure in terms of its smallest repeating unit - unit cell - which can be defined in terms of:
- 3 axes, a, b and c, with specific lengths (longest is c axis)
- 3 angles between these axes
what is an orthorhombic structure
- a, b and c axes of the unit cell are different lengths but they are all at 90º
example of a mineral with an orthorhombic structure
orthopyroxene is also orthorhombic, given the structure of the unit cell but note that the unit cell is larger and more complex
how many crystal systems are there
6
how are crystal systems defines
- each system defined by a combination of three factors
- how many axes it has
- lengths of the axes
- angles at which the axes meet
what are the 6 crystal systems
simple cubic
tetragonal
orthorhombic
monoclinic
triclinic
hexagonal (split into hexagonal and rhombohedral/trigonal)
what is the cubic system
- three axes all the same length (a=b=c)
- the three axes all intersect at 90º
- a cubic unit cell is not necessarily simple
- but has high symmetry and the structure is consistent in all directions
- defines an isotropic mineral structure
what is isotropic
properties are identical in all directions
examples of minerals with the cubic system
garnets, diamond, fluorite, gold, pyrite, silver, spinel…
what is the tetragonal system
- three axes, c is longer than a and b (a=b≠c)
- the three axes all intersect at 90º to each other
- unit cell is a rectangular prism
examples of minerals with the tetragonal system
rutile and zircon
what is the orthorhombic system
- three axes all different lengths (a≠b≠c)
- the three axes all intersect at 90º to each other
examples of minerals with the orthorhombic system
andalusite, enstatite and olivine…
what is the monoclinic system
- three axes all have different lengths (a≠b≠c)
- two of the axes, a and c, meet at 90º but axis b does not
examples of minerals with the monoclinic system
orthoclase feldspar, diopside, staurolite…
what is the triclinic system
- three axes all different lengths (a≠b≠c)
- no axes meet at 90°
examples of minerals with the triclinic system
plagioclase feldspars and microcline feldspar…
what is the hexagonal system
- four axes, three of which are equal length (a1=a2=a3≠c)
- vertical c-axis intersects shorter axes at 90º; short axes intersect at 120º
- previous crystal systems represent every variation of four-sided figures with three axes
- in hexagonal system there is an additional axis, giving the unit cell six sides
examples of minerals with the hexagonal system
apatite and beryl (emerald)…
what is the trigonal subsystem
- four axes, three of which are equal in length (a1=a2=a3≠c)
- vertical c axis intersects shorter axes at 90º; short axes intersect at 120º
- trigonal system differs from the hexagonal in having a 3-fold axis of rotational symmetry or 3-fold axis of rotatory inversion as its principal axis (rather than 6-fold)
examples of minerals with the trigonal system
quartz and calcite
what is the crystal habit
the characteristic external shape of an individual crystal or crystal group
what is the crystal habit determined by
the shape of actual crystals are not identical to the shape of unit cells, but will be controlled by the fundamental symmetry of the underlying unit cell
what is crystal twinning
- during crystal growth, a phenomenon can occur where the inter-growth of two parts of a crystal are related by a symmetry element referred to as a twin
- this can take the form of:
- a mirror plane
- a rotation axis
- an inversion about a point
what is polymorphism
- individual minerals groups can span more than one crystal system, but in these cases they also have distinct chemistry
- some minerals (with exactly the same chemistry) can also exist in different structural organisations, with the stability of these dependent on pressure and temperature (P, T) conditions, eg silica (SiO2)
what features can be used to describe and identify minerals
colour
lustre
streak
hardness
cleavage
fractures
habit
