2. Controls, Textures, Naming

Question 1

What is the driving force for metamorphic changes?

Answer 1

Thermodynamics

Reactions occur in a direction that lowers the overall chemical potential energy = Gibbs free energy

Question 2

What is Gibbs free energy?

Answer 2

The amount of energy available for a system to do useful work at constant T and P

Equilibrium is achieved when Gibbs free energy, G, is at a minimum

Question 3

What is activation energy?

Answer 3

All thermally activated processes are opposed by a free energy barrier which must be overcome before the reaction proceeds, even if it is thermodynamically favourable. This barrier is the activation energy

Question 4

Define Metastable

Answer 4

The activation energy is too large for a reaction to take place

e.g. Kyanite → sillimanite

P & T can be in the sillimanite field but there’s not enough free energy for kyanite to convert to sillimanite so it is metastable

Question 5

What three factors increase the rate of chemical equilibrium?

Answer 5

Small grain size - small intra/inter-granular diffusion paths & large total surface area for each mineral

Heating - Higher temp → faster diffusion, heat supplies activation anergy

Presence of fluid - especially H₂O: acts as catalyst for the reaction

Question 6

Compare Prograde and Retrograde metamorphism

Answer 6

• Peak metamorphic paragenesis is normally formed at the max T
• Prograde path starts with small grainsize, recrystallisation → larger grain sizes
• Largest grainsize at peak metamorphism → retrograde reactions are kinetically hindered
• Prograde path starts with wet rock so reactions happen faster, water is driven off → may be no water left at peak, so retrograde reactions are kinetically hindered and cant form hydrous minerals

Question 7

Define Peak-Metamorphic Paragenesis

Answer 7

The set of minerals in equilibrium together in a rock at the thermal peak of metamorphism

Question 8

What evidence would show equilibrium has been achieved in a rock?

Answer 8

Optical evidence: no obvious reaction textures, crystals of each mineral are in direct contact with every other mineral (no gaps or inclusions)

Chemical evidence: absence of zoning - though rims could be in equilibrium in a zoned crystal, composition of each mineral is roughly constant

Phases: number of phases is as predicated by the Phase Rule - the rock will contain only a small number of minerals

Question 9

Define a SCHIST

Answer 9

a medium to coarse-grained metamorphic rock displaying a pervasive schistosity ( alignment of platy minerals) on the hand specimen scale. Individual mineral grains can be seen with the unaided eye.

Question 10

Define a PHYLLITE

Answer 10

a fine to very fine grained metamorphic rock displaying a cleavage. When split, the cleavage surfaces typically have a silky sheen

Question 11

Porphyroblast vs Porphyroclast

Answer 11

Blast: large grain of metamorphic origin, in a finer-grained matrix

Clast: A large relict grain preserved in a finer-grained matrix, generally in a rock which has been strongly deformed

Question 12

Define Poikiloblast

Answer 12

A porphyroblast containing abundant small inclusions

Question 13

Three terms to describe development of crystal faces

Answer 13

Question 14

Five terms for crystal habit

Answer 14

Question 15

Define Compositional layering

Answer 15

layering defined by differences in mineral proportions. Often relict bedding, but may be gneissic foliation

Question 16

Define Crenulation and crenulation cleavage

Answer 16

small-scale (usually mm/cm scale) folding of a pre-existing cleavage

Crenulation cleavage: a spaced cleavage resulting from small-scale crenulation of a pre-existing cleavage

Question 17

Define Mineral lineation

Answer 17

a lineation defined by a preferred orientation of prismatic or acicular crystals

a rock with a strong mineral lineation can be said to have a ‘nematoblastic’ texture

Question 18

Define Pseudomorph

Answer 18

grain altered or replaced while retaining its original shape

the secondary mineral is usually much finer grained than the primary mineral

Question 19

Describe a GNEISS

Answer 19

Question 20

Five types of foliated rock

Answer 20

Question 21

Define Granofels and Hornfels

Answer 21

Granofels: a massive, non-schistose metamorphic rock

have granoblastic, decussate or radiating textures, charateristic of ‘static crystallisation’ - crystallisation in the absence of deformation

Hornfels: a fine-grained granofels with granoblastic texture, from a thermal aureole

Question 22

Five rock names based on mineralogy

Answer 22