Crystals and Crystal Growth Flashcards
deepest point rock
garnet peridite is the rock we have from the deepest point of the earth
Crystals grow by
evaporation (common salt, halite)
from vapour (sulphur)
solid state (garnet in metamorphic rock)
as precipitates from fluids (ore minerals)
solidification from a liquid (ice, igneaous minerals)
as secretion from biological organisms (bone, shell etc.)
TX Diagram
T X Diagram
T: temperature and composition
X: cooling of crystals in a melt
crystals growing in melt procedure
nucelation –> crystallisation –> crystal nucei –> growth initiated
Nucelation
liquid must become saturated in that particular phase
melt approaches the crystallisation temp of that phase
during this time ions(chargeed) in melt are in constant motion
crystallisation cannot occur
XY has to be supersaturated a d super cooled
Crystallisation of mineralisation will not occur until XY is supersaturated, further cooled
exact supersaturated point melt will be metastable, ions X Y in consatnt motion, unable to crystallise
stability increases as magma cools further, crystallisation is initiated
that means: crystals from at temp below at which they would normally melt
magma is supercooled with respect to crystal phase
magma is further cooled → crystallisation occurs → embryo of XY
Crystal embyros
are metastable
XY ions from XY then diperse etc
magma becomes supercooled and embryos stabilize and increase radius → nuclei
nucel have a CRTICLE RADIUS (rcrit) at which crystallisation is stabilized
rcrit attained :mineral XY crystallizes
if undercooling occurs
no crystals form, amorphous glass forms (ex obisidian)
not enough time: no crystals , kinetics , it is a rate dependet process
Homogenous nucelation
first crystalisation of material in a liquid
Heterogenous Crystallisation
seed crystals added to melt, perhaps due to erosion
crystallisation is induced on these seeds at the correct temp: Heterogeneous Crystallisation
ex. Zircons as markers in rocks can be used to date
growth is initiated, requirea
requires diffusion : atoms move through melt toward growing crystal, diffusion rate
generates heat so you have to remove heat from crystal to allow it to solidifie, remove latent heat
boundary layer with feedback system that influences temp
growth depenedent on
diffusion rate
and rate of cooling
slower the cooling rate the larger the crystal that will form
growth changes due to
pressure, composition, temp, rate of cooling, diffusion rate, water
limiting factors crystal growth
run out of components or space system changes (temo etc) growth in unrestricted space : euhedral crystals
Planar defects
natural defects can occur
ex. row added or taken away, face in spiraling or so
crystals are never perfect and characterisd by defects and dislocations
defects facilitate crystal growth
ex spiraling effect
or screw dislocation
Pregnatite
giant crystals
igneaous rocks with slow cooling rate
igneaous rock with slow cooling rates are coarse grained
moderate density of nucelation points and slow growth
igneaous rocks with rapid cooling
igrneaous rocks with fast cooling rates are fine grained
many nucelation points and rapid growth
prophyritic texture bimodal grainsize distribution
fast rates of diffsuin from long crystals
prophytric texture
bimodal grainsize distribution
from rapid cooling
Crystallisation History
- Phenocrytss grow in rapidly cooling melt just before eruption
- melt flows toward surface, phenocrysts aligned
- fine grained groundmass formed instantanuously when melt brought close to surface
Zoning
evidence for succesive crystal growth
when crystals regrow color might change etc
because crystals grow episodically
non ewuilibrium crystallisation: system is open; compositon or Pressure and Temp may change
Growht Zoning
succesive periods of growth over geological time
represents topotactic growth in series of systems
inner zones can be mio years older than outer zones
Twinning
multiple crystals growing in same place at different orientations
can never have a reentrent (internal) angle in a crystal without it being twinning
