W8 Metamorphism: Crystal Growth and Textures Flashcards
Metamorphic reactions occur
in the solid state
Metamorphic Reactions involve
no change in the bulk chemistry of a rock (they are isochemica
Metamorphic reactions result
in the crystallisation of new minerlas
neoblasts
Metamorphic reactions texture
result in the formation of new textures and fabrics foliation banding preferered grain orientations usually cause change in colour
Protolith Sandstone
LG: Phylite
MG_ schist
HG: gneiss
Protolith Limestone
LG MG and HG marble!
persistent
Protolith: Basalt
LG greenschist MG amphibolite HG eclogite protholiths → low grad of that stone → medium grade of that stone → high grade of that stone with pressure and temperature increasing
Protolith Granite
LG metagranite
MG metagranite
HG gneiss
Metamorphism of a Limestone
Metamorphism of a Limestone CaCO3 CaCO3 • recrystallisation means all sedimentary textures are lost • a marble is produced - harder, denser, lower porosity rock
metamorphic reactions proceed as direct result of
incerasing
heat
pressure / applied stress
this increase results from
burial
proximity to igneaous intrusions (High temp, low pressure)
Deformation
Impacts
pressure and tempth reelated to depth
pressure and temp increase with deepth
30 degrees Cper km depth
so if D= 1km T= 30 C
D= 3km T= 90C
Effects of Heat on crystal
As solids are heated …
• there is an increase in molecular motion
> ions can migrate
> defects can relocate
Effects of Pressure
Increase in pressure favours the formation of denser
crystal forms (& usually with a decrease in volume)
Eclogites form only at the highest possible pressures …
… they are the densest of all rocks (specific gravity around 3.5)
Effects of Deformation / Deviatoric Stress
changes the shape of rocks & minerals (strain)
Deformation at high pressure
at high pressures - ductile deformation
> crystals will ‘flow
Deformation at low pressure
brittle deformation
> rocks will fracture
Mylonite - an example of extreme brittle deformation, Dynamic Metamorphism associated with faults
Speed of process
Recrystallisation of minerals during metamorphic reactions is a very slow process - requires millions of years
EXCEPT for
• meteorite impact related metamorphism
• seismic fault movement dynamic metamorphism
Crystal growth in Metamorphic Rocks
atoms approach metamorphic crystal nucleii by diffusion through the solid (slow process)
• large grains grow at the expense of smaller ones
• for monomineralic rocks (marble, quartzite) - large
grains grow easily (and relatively quickly)
> atoms only have to diffuse short distances
grains with large surface areas
recrystallise more quickly
grains with many inclusions
recrystallise more quickly
Bubbles in Crystals
Bubbles in a foam as a proxy for crystal growth in metamorphic rocks
Ostwald Ripening
Large grains consuming smaller ones
A very stable structure with minimum surface energy results
creates granoblastic texture in a marble
NB granoblastic texture is common only in monomineralic rocks and M-H P&T rocks. Once formed, it is very stable.
Porphyroblastst
equivalents of phenocrysts in igneous rocks
ex. snowball garnet rotated during formation
not always equal sizes, also can be different shapes
Cleave results from
Cleavage results from alignment of platey minerals + deformation (in fine grained rocks)
i.e. slate, slatey cleavage with signs of compression and deformation
Grain size increases
with an increase in T and P
Foliation
All these rocks have a ‘foliation’ defined by elongate or platey minerals
Foliation definition
Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness
Pattern
banding in a gneiss- mineral segregations
does not follow banding in protolith
Microfolds
Crennulation in fibrous talc crystals - microfolds
Deformation can cause
twins to form in crystal
ex. calcite
