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Terms in this set (21)

Primary Textures
Processes during mineral formation
Crystal nucleation
Pressure change not usually a significant factor
May alternatively have "seed crystal"
Nucleation of minerals with simple structures is easier
Crystal growth
Affected by high- and low-energy positions for ions
Affected by degree of undercooling
Chemical species
Cooling rate of the melt
Affects degree of undercooling
Critical for crystallization
Reflected in development of texture
Affects equilibrium/disequilibrium conditions
Affects diffusion and crystal growth
Limited undercooling increases both rates
Further undercooling decreases kinetic energy,
increases viscosity
Variable cooling rate affects grain sizes
e.g., two-stage cooling typically produces bimodal grain
size distribution
Other possible mechanisms for bimodal distribution exist
Bimodal grain size distributions
Affected by degree of undercooling
Porphyritic rocks classified as intrusive or extrusive based on grain size of the groundmass (matrix)
Other factors affecting crystal growth
Effect of chemistry on crystal growth
Highly-polymerized melts have higher viscosity
Wet (H2O-rich) melts have lower viscosity
Effect of crystal structure on crystal growth
Minerals with simple crystal structures tend to nucleate early and grow quickly
Crystal growth rate vs. ion diffusion rate
Elongate crystals have larger volume of surrounding melt per surface area
Dendritic texture
Spinifex texture
Preferential nucleation
Rapakivi texture
Parallel growths/twinning
Spherulitic texture
Variolitic texture
Comb structure (crescumulate texture)
Compositional zoning
Change in mineral composition during growth
Observable as a change in color or extinction
Occurs during non-equilibrium conditions
Normal zoning
Reverse zoning
Oscillatory zoning
Crystallization sequence
Early crystals tend to be relatively euhedral
Later crystals tend to be subhedral to anhedral
Cannot always infer crystallization sequence based on whether the crystals are euhedral
Inclusions in minerals formed prior to mineral in which it is included
Ophitic texture
Texture may reflect simultaneous crystallization
Granophyric and graphic textures
Chemical reactions
Crystallization may be affected by the evolving chemistry of the melt
Mineral may become partially or completely resorbed back into the melt
Sieve texture and mineral embayment
Differential movement of crystals and melt
Flow within melt can align crystals
Foliated texture
Lineated texture
Trachytic texture
Flow banding
Cumulate textures - form through accumulation of
mineral crystals concentrated in layers
Floating or sinking of crystals can segregate them from
the rest of the melt
Crystals may preferentially form in layers
Textures associated with volcanic rocks
Typically contain some quenched melt
Ophitic texture
Sub-ophitic texture
Intergranular texture
May also contain trapped gases
Glass tends to deform during accumulation and flow
Eutaxitic texture
Pele's tears
Pele's hair
Pisolitic tuffs