Textures (Chapter 3)

Question 1

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

Answer 1

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)

Question 2

Primary textures

Answer 2

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

Question 3

Secondary textures

Answer 3

Alterations that take place after the rock is completely solid

Question 4

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

Answer 4

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

Question 5

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

Answer 5

Diffusion

Question 6

Why is the cooling rate important?

Answer 6

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

Question 7

Slow cooling: small or large crystals?

Answer 7

Large crystals

Question 8

Fast cooling: small or large crystals?

Answer 8

Small crystals

Question 9

What is the cooling rate of a porphyritic texture?

Answer 9

Slow cooling followed by rapid cooling

Question 10

What is the cooling rate of a seriate texture?

Answer 10

Gradually increasing cooling rate

Question 11

At what depths does slow cooling occur?

Answer 11

Great depths (very deep)

Question 12

At what depths does fast cooling occur?

Answer 12

Shallow depths

Question 13

Vitrophyric

Answer 13

Phenocrysts set in a glassy groundmass

Question 14

Poikilitic

Answer 14

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

Question 15

Cumulophyric texture

Answer 15

Multiple-grain clusters of phenocrysts stuck together

Question 16

Glomeroporphyritic texture

Answer 16

Grain clusters of phenocrysts of a single mineral

Question 17

Dendritic texture

Answer 17

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

Question 18

Spinifex texture

Answer 18

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

Question 19

Skeletal crystal texture

Answer 19

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

Question 20

Swallow-tail texture

Answer 20

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

Question 21

Epitaxis

Answer 21

Preferred nucleation of one mineral on another preexisting mineral

Question 22

Rapakivi texture

Answer 22

Plagioclase overgrowths on orthoclase in some granites

Question 23

Compositional zoning

Answer 23

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

Question 24

Oscillatory zoning

Answer 24

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

Question 25

Are euhedral minerals crystallized early or late?

Answer 25

Early

Question 26

Are subhedral and anhedral minerals crystallized early or late?

Answer 26

Late

Question 27

Ophitic texture

Answer 27

Envelopment of plagioclase laths by larger clinopyroxenes

Question 28

Interlocking texture

Answer 28

Contemporaneous crystallization of minerals with differences in surface energy.

Question 29

Granophyric texture

Answer 29

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

Question 30

Graphic texture

Answer 30

Granophyric texture that can be seen in hand samples

Question 31

Reaction rim

Answer 31

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

Question 32

Resorption

Answer 32

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

Question 33

Trachytic texture

Answer 33

lath-shaped microlites in a volcanic rock are strongly aligned

Question 34

Pilotaxitic / Felty texture

Answer 34

Random / non-aligned microlites (poor flow)

Question 35

Cumulate textures

Answer 35

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

Question 36

Primary (growth) twin

Answer 36

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

Question 37

Why does a primary twin form?

Answer 37

It forms because of mistakes during crystallization from a melt

Question 38

Subophitic texture

Answer 38

Like ophitic texture but with smaller pyroxenes

Question 39

Intergranular texture (basalts)

Answer 39

crystal subequal in size, and glass is minor

Question 40

Holohyaline texture

Answer 40

If glass is 80% glass (obsidian)

Question 41

Hemihyaline texture

Answer 41

Partly glassy texture

Question 42

Vesicles

Answer 42

Trapped bubbles of escaping gas creates subspherical voids in volcanics

Question 43

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

Answer 43

True

Question 44

Scoria

Answer 44

Highly vesiculated basalt

Question 45

Vesiculation increases at what depths?

Answer 45

Decreasing depths

Question 46

Glass shards (pyroclastics)

Answer 46

Fragments of volcanic glass

Question 47

Eutaxitic textures

Answer 47

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

Question 48

Autometamorphic processes

Answer 48

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

Question 49

Transformation twins

Answer 49

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

Question 50

Deformation twins

Answer 50

deformed and lack the straight lamellar form

Question 51

Exsolution texture

Answer 51

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

Question 52

Deuteric alterations

Answer 52

Alterations of primary magmatic minerals by magmatic fluids

Question 53

Saussuritization

Answer 53

The alteration of plagioclase by epidote