Grain Growth Flashcards
What is a granoblastic texture?
Ideal texture
Grains have straight boundaries with dihedral growth at 120 degrees minimum surface area
Most common in monomineralized rocks e.g. amphibolite, marble, quartzite
Growth easy and fast
How does mica effect grain growth?
Inhibits growth as micas keep their shape, pins grains so they cannot grow around grain boundaries
What effect does an increase in temperature have on mica?
Increase in temperature leads to small micas (chlorite) disappearing when it hits the staurolite isograd.
What happens during diffusion in low grade rocks?
In low grade rapid growth occurs with many inclusions e.g. garnet has many inclusions as it grows before quartz can escape.
What is diffusion controlled by?
temperature (fast when high) and mechanism (fast along grain boundaries and in fluids, slow inside grain)
What four things control grain growth?
Temperature
Grain size - smaller leads to more grain boundaries
Deformation - creates small grain size, creates gaps accelerating diffusion, lets fluids in, moves grains
Interconnected fluid network
How are fluid inclusions measured?
Experiments measure dihedral angle between mineral and fluid
If 60, individual fluid inclusion. (more common)
How do porphyroblasts grow?
First stage is nucleation (initial growth) favoured during deformation. Then rapid growth occurs. Rapid growth along grain boundaries allows many inclusions. Not liked so inclusions removed through grain transport but a very slow process.
Syntectonic, not rotated. Grow in active low strain parts of rock.
Where can reaction textures be seen in?
Retrograde reactions (cooling) with limited water supply. Cooling temps cause fine grained minerals to be rimmed by coarse grained reactants. Granulites (dry rocks), at high temps symplectites form which shows importance of fluids Difficult mineral transformations, e.g. kyanite to sillimanite. Kyanite replaced by muscovite before being replaced by sillimanite.
