Textures (Chapter 3) Flashcards
What is the difference between “textures” and “structures”?
Textures are the characteristics of individual cyrstals or grains.
Ex. (grain size/shape)
Structures are the characteristics of assemblages of crystals or grains.
Ex. (layering/contacts)
Primary textures
- occur during igneous crystallization
- result from interactions between minerals and melt
Secondary textures
Alterations that take place after the rock is completely solid
What are the three steps in the formation and growth of crystals?
1) nucleation
2) subsequent crystal growth
3) diffusion of chemical species through the surface of the growing crystal and its surroundings
Which process is important to maintain the equilibrium between the crystal and the melt?
Diffusion
Why is the cooling rate important?
It is the driving force of crystallization and it determines the texture
Slow cooling: small or large crystals?
Large crystals
Fast cooling: small or large crystals?
Small crystals
What is the cooling rate of a porphyritic texture?
Slow cooling followed by rapid cooling
What is the cooling rate of a seriate texture?
Gradually increasing cooling rate
At what depths does slow cooling occur?
Great depths (very deep)
At what depths does fast cooling occur?
Shallow depths
Vitrophyric
Phenocrysts set in a glassy groundmass
Poikilitic
Phenocrysts contain numerous inclusions of another mineral that was enveloped during growth
Cumulophyric texture
Multiple-grain clusters of phenocrysts stuck together
Glomeroporphyritic texture
Grain clusters of phenocrysts of a single mineral
Dendritic texture
Radiating / tree-like form
- when rate of diffusion is slower than growth
Spinifex texture
Ultramafic lavas that develop elongate olivine crystals (up to 1m long)
- when there is rapid growth in the olivine structure in low-viscosity magma
Skeletal crystal texture
crystal corners/edges have a larger volume than the crystal faces
- when the edges grow more rapidly due to more unsatisfied bonds
Swallow-tail texture
Much like skeletal structure where the edges/corners grow faster than the crystal face. Except plagioclase grows straighter which makes the elongate swallow tail.
Epitaxis
Preferred nucleation of one mineral on another preexisting mineral
Rapakivi texture
Plagioclase overgrowths on orthoclase in some granites
Compositional zoning
Equilibrium is not maintained between the crystal and the melt. SO, a rim of new composition is added around the old rim
Oscillatory zoning
Repeated pattern of zoning
- produced by injection of a hotter magma in a magma chamber and crystallization rates of T in plag
Are euhedral minerals crystallized early or late?
Early
Are subhedral and anhedral minerals crystallized early or late?
Late
Ophitic texture
Envelopment of plagioclase laths by larger clinopyroxenes
Interlocking texture
Contemporaneous crystallization of minerals with differences in surface energy.
Granophyric texture
Simultaneous crystallization of qtz + k-felds showing intergrowth of skeletal shapes
Graphic texture
Granophyric texture that can be seen in hand samples
Reaction rim
A rim wrapping a mineral, formed by reaction between the mineral and the melt
Resorption
Re-fusion or dissolution of a mineral back into a melt. Ex. sieve texture
Trachytic texture
lath-shaped microlites in a volcanic rock are strongly aligned
Pilotaxitic / Felty texture
Random / non-aligned microlites (poor flow)
Cumulate textures
textures of crystals as they cumulate in magma
(Adcumulate, Orthocumulate, and Poikilitic)
Primary (growth) twin
An intergrowth of two or more orientations of the same mineral with a special crystallographic relationship between them
Why does a primary twin form?
It forms because of mistakes during crystallization from a melt
Subophitic texture
Like ophitic texture but with smaller pyroxenes
Intergranular texture (basalts)
crystal subequal in size, and glass is minor
Holohyaline texture
If glass is 80% glass (obsidian)
Hemihyaline texture
Partly glassy texture
Vesicles
Trapped bubbles of escaping gas creates subspherical voids in volcanics
True or False: Bubbles tend to rise in less viscous magma
True
Scoria
Highly vesiculated basalt
Vesiculation increases at what depths?
Decreasing depths
Glass shards (pyroclastics)
Fragments of volcanic glass
Eutaxitic textures
Textures caused by compression and deformation resulting from settling in hot ash accumulations
Autometamorphic processes
Solid-state processes that occur as a result of igneous heat
Transformation twins
Tartan twins
- when a high-T crystal structure inverts to a low-T polymorph
Deformation twins
deformed and lack the straight lamellar form
Exsolution texture
Formation of two mineral phases from an originally solid-solution mineral in cooling
Deuteric alterations
Alterations of primary magmatic minerals by magmatic fluids
Saussuritization
The alteration of plagioclase by epidote
