Crystals and Crystal Growth

Question 1

deepest point rock

Answer 1

garnet peridite is the rock we have from the deepest point of the earth

Question 2

Crystals grow by

Answer 2

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.)

Question 3

TX Diagram

Answer 3

T X Diagram
T: temperature and composition
X: cooling of crystals in a melt

Question 4

crystals growing in melt procedure

Answer 4

nucelation –> crystallisation –> crystal nucei –> growth initiated

Question 5

Nucelation

Answer 5

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

Question 6

XY has to be supersaturated a d super cooled

Answer 6

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

Question 7

Crystal embyros

Answer 7

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

Question 8

if undercooling occurs

Answer 8

no crystals form, amorphous glass forms (ex obisidian)

not enough time: no crystals , kinetics , it is a rate dependet process

Question 9

Homogenous nucelation

Answer 9

first crystalisation of material in a liquid

Question 10

Heterogenous Crystallisation

Answer 10

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

Question 11

growth is initiated, requirea

Answer 11

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

Question 12

growth depenedent on

Answer 12

diffusion rate
and rate of cooling
slower the cooling rate the larger the crystal that will form

Question 13

growth changes due to

Answer 13

pressure, composition, temp, rate of cooling, diffusion rate, water

Question 14

limiting factors crystal growth

Answer 14

run out of components or space
system changes (temo etc)
growth  in unrestricted space : euhedral crystals

Question 15

Planar defects

Answer 15

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

Question 16

Pregnatite

Answer 16

giant crystals

Question 17

igneaous rocks with slow cooling rate

Answer 17

igneaous rock with slow cooling rates are coarse grained

moderate density of nucelation points and slow growth

Question 18

igneaous rocks with rapid cooling

Answer 18

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

Question 19

prophytric texture

Answer 19

bimodal grainsize distribution

from rapid cooling

Question 20

Crystallisation History

Answer 20

1. Phenocrytss grow in rapidly cooling melt just before eruption
2. melt flows toward surface, phenocrysts aligned
3. fine grained groundmass formed instantanuously when melt brought close to surface

Question 21

Zoning

Answer 21

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

Question 22

Growht Zoning

Answer 22

succesive periods of growth over geological time
represents topotactic growth in series of systems
inner zones can be mio years older than outer zones

Question 23

Twinning

Answer 23

multiple crystals growing in same place at different orientations
can never have a reentrent (internal) angle in a crystal without it being twinning