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Igneous Petrology > Textures (Chapter 3) > Flashcards

Textures (Chapter 3) Flashcards

1
Q

What is the difference between “textures” and “structures”?

A

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)

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2
Q

Primary textures

A
  • occur during igneous crystallization
  • result from interactions between minerals and melt
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3
Q

Secondary textures

A

Alterations that take place after the rock is completely solid

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4
Q

What are the three steps in the formation and growth of crystals?

A

1) nucleation
2) subsequent crystal growth
3) diffusion of chemical species through the surface of the growing crystal and its surroundings

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5
Q

Which process is important to maintain the equilibrium between the crystal and the melt?

A

Diffusion

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6
Q

Why is the cooling rate important?

A

It is the driving force of crystallization and it determines the texture

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7
Q

Slow cooling: small or large crystals?

A

Large crystals

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8
Q

Fast cooling: small or large crystals?

A

Small crystals

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9
Q

What is the cooling rate of a porphyritic texture?

A

Slow cooling followed by rapid cooling

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10
Q

What is the cooling rate of a seriate texture?

A

Gradually increasing cooling rate

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11
Q

At what depths does slow cooling occur?

A

Great depths (very deep)

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12
Q

At what depths does fast cooling occur?

A

Shallow depths

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13
Q

Vitrophyric

A

Phenocrysts set in a glassy groundmass

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14
Q

Poikilitic

A

Phenocrysts contain numerous inclusions of another mineral that was enveloped during growth

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15
Q

Cumulophyric texture

A

Multiple-grain clusters of phenocrysts stuck together

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16
Q

Glomeroporphyritic texture

A

Grain clusters of phenocrysts of a single mineral

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17
Q

Dendritic texture

A

Radiating / tree-like form
- when rate of diffusion is slower than growth

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18
Q

Spinifex texture

A

Ultramafic lavas that develop elongate olivine crystals (up to 1m long)
- when there is rapid growth in the olivine structure in low-viscosity magma

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19
Q

Skeletal crystal texture

A

crystal corners/edges have a larger volume than the crystal faces
- when the edges grow more rapidly due to more unsatisfied bonds

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20
Q

Swallow-tail texture

A

Much like skeletal structure where the edges/corners grow faster than the crystal face. Except plagioclase grows straighter which makes the elongate swallow tail.

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21
Q

Epitaxis

A

Preferred nucleation of one mineral on another preexisting mineral

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22
Q

Rapakivi texture

A

Plagioclase overgrowths on orthoclase in some granites

23
Q

Compositional zoning

A

Equilibrium is not maintained between the crystal and the melt. SO, a rim of new composition is added around the old rim

24
Q

Oscillatory zoning

A

Repeated pattern of zoning
- produced by injection of a hotter magma in a magma chamber and crystallization rates of T in plag

25
Q

Are euhedral minerals crystallized early or late?

A

Early

26
Q

Are subhedral and anhedral minerals crystallized early or late?

A

Late

27
Q

Ophitic texture

A

Envelopment of plagioclase laths by larger clinopyroxenes

28
Q

Interlocking texture

A

Contemporaneous crystallization of minerals with differences in surface energy.

29
Q

Granophyric texture

A

Simultaneous crystallization of qtz + k-felds showing intergrowth of skeletal shapes

30
Q

Graphic texture

A

Granophyric texture that can be seen in hand samples

31
Q

Reaction rim

A

A rim wrapping a mineral, formed by reaction between the mineral and the melt

32
Q

Resorption

A

Re-fusion or dissolution of a mineral back into a melt. Ex. sieve texture

33
Q

Trachytic texture

A

lath-shaped microlites in a volcanic rock are strongly aligned

34
Q

Pilotaxitic / Felty texture

A

Random / non-aligned microlites (poor flow)

35
Q

Cumulate textures

A

textures of crystals as they cumulate in magma
(Adcumulate, Orthocumulate, and Poikilitic)

36
Q

Primary (growth) twin

A

An intergrowth of two or more orientations of the same mineral with a special crystallographic relationship between them

37
Q

Why does a primary twin form?

A

It forms because of mistakes during crystallization from a melt

38
Q

Subophitic texture

A

Like ophitic texture but with smaller pyroxenes

39
Q

Intergranular texture (basalts)

A

crystal subequal in size, and glass is minor

40
Q

Holohyaline texture

A

If glass is 80% glass (obsidian)

41
Q

Hemihyaline texture

A

Partly glassy texture

42
Q

Vesicles

A

Trapped bubbles of escaping gas creates subspherical voids in volcanics

43
Q

True or False: Bubbles tend to rise in less viscous magma

A

True

44
Q

Scoria

A

Highly vesiculated basalt

45
Q

Vesiculation increases at what depths?

A

Decreasing depths

46
Q

Glass shards (pyroclastics)

A

Fragments of volcanic glass

47
Q

Eutaxitic textures

A

Textures caused by compression and deformation resulting from settling in hot ash accumulations

48
Q

Autometamorphic processes

A

Solid-state processes that occur as a result of igneous heat

49
Q

Transformation twins

A

Tartan twins
- when a high-T crystal structure inverts to a low-T polymorph

50
Q

Deformation twins

A

deformed and lack the straight lamellar form

51
Q

Exsolution texture

A

Formation of two mineral phases from an originally solid-solution mineral in cooling

52
Q

Deuteric alterations

A

Alterations of primary magmatic minerals by magmatic fluids

53
Q

Saussuritization

A

The alteration of plagioclase by epidote

Igneous Petrology (7 decks)

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