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Geology 211 - Class Final > Feldspars > Flashcards

Feldspars Flashcards

1
Q

polymorphism

A

the ability of a solid material to exist in multiple forms or crystal structures

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2
Q

SiO2 polymorphs

A

Quartz
Tridymite
Cristabolite

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3
Q

Al2SiO5 polymorphs

A

kyanite
sillimanite
andalusite

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4
Q

more than 2/3 of earths crust is composed of what group of silicates?

A

framework

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5
Q

Framework silicate Si:O ratio

A

1:2

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6
Q

the structure of framework minerals is based in a framework of ______?

A

AlSi:O4

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7
Q

Because Each O2 anion is shared with another tetrahedra, this results in 2 characteristics. What are they?

A

Composition: open structure can easily accomodate large cations

Physical: framework silicates have a low specific gravity and generally are not stable @ high Temp.

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8
Q

Quartz is Uni- or Bi- axial?

A

uniaxial

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9
Q

Feldspars are Uni- or Bi- axial?

A

biaxial

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10
Q

piezioelectric effect

A

the ability of certain minerals to generate and electric charge when mechanical stress is applied

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11
Q

Properties of Quartz (7)

A 
translucent
non-metallic
no cleavage 
conchoidal fracture
low density
hardness of 7
piezoelectric effect
[SiO2 is found in nature in an amorphous state]
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12
Q

What is the principle framework silicate?

A

The feldspar group

- it is the most important silicate in igneous rocks

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13
Q

Z/O ratio in feldspars

A

1:2

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14
Q

Chem formula of Alkali feldspars

A

K,Na [AlSi3O8]

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15
Q

Chem formula of Plagioclase feldspars

A

Na[AlSi3O8] – Ca[Al2Si2O8]

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16
Q

Physical Properties of Feldspars (5)

A

Hardness: all 6
Colour: white and pink (orthorclase generally)
Density: 2.6
Cleavage : 2 @ 90
twinning: simple = albite & orthorclase; multiple = plagioclase, microcline

17
Q

What is an Alkali Feldspar?

A

feldspars rich in Na and K

18
Q

What are the Alkali Feldspars? (5)

A 
monoclinic = sanidine and orthoclase
triclinic = anorthoclase, microcline and albite
19
Q

Distinguishing Features (K-feldspar vs Plag or Qtz) (4)

A
  • simple (carlsbad) or tartan twinning
  • exsolution is common
  • zoning is rare
  • biaxial -ve, large 2V angle
20
Q

Distinguishing Features (Orthorclase vs Microcline vs Sanidine) (3)

A 
sanidine = small 2V angle & simple twins
Microcline = tartan twinning
Orthorclase = large 2V angle
21
Q

Volcanic or Plutonic?

  • sanidine
  • orthorclase
  • microcline
A 
sanidine = volcanic
orthorclase = plutonic
microcline = plutonic
22
Q

What form do the Alkali Feldspars Albite and K-Feldspar have?

A

exsolution lamellae (perthitic) form during the unmixing of homogeneous K, Na feldspar during slow cooling

23
Q

Properites of Sanidine (4)

A
  • common in volcanic rocks
  • prismatic habit
  • may have simple twinning
  • colourless and low birefringence
24
Q

Plagioclase Feldspar Distinguishing features (Plag vs K-Feld or Qtz) (4)

A
  • polysynthetic twinning
  • zoning common
  • exsolution is rare
  • can determine plag composition using Michel-Levy Method
25
Q

types of twinning in feldspars (6)

A
  • carlsbad (simple)
  • polysynthetic(albite) = plagioclase
  • Manebach = reflection on (001)
  • Baveno = reflection on (021)
  • Tartan = microcline
  • pericline = plagioclase
26
Q

Which 2 elements occupy tetrahedral sites in the ordering of feldspars structure?

A

Si

Al

27
Q

what are the 3 K-feldspar polymorphs?

A

sanidine
orthoclase
microcline

28
Q

what is the ordering of the 3 polymorphs?

A 
Highest Temp & random ordering
Sanidine         
Orthorclase
Microcline
                         Lowest Temp & most ordered
29
Q

at high temperatures is there (random or non-random) distribution of Si & Al?

A

random

30
Q

random distribution of Si & Al

A
  • disordered
  • monoclinic symmetry
  • Sanidine & orthoclase
31
Q

non-random distribution of Si & Al

A
  • ordered
  • triclinic symmetry
  • microcline

Geology 211 - Class Final (12 decks)

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