Feldspars Flashcards
polymorphism
the ability of a solid material to exist in multiple forms or crystal structures
SiO2 polymorphs
Quartz
Tridymite
Cristabolite
Al2SiO5 polymorphs
kyanite
sillimanite
andalusite
more than 2/3 of earths crust is composed of what group of silicates?
framework
Framework silicate Si:O ratio
1:2
the structure of framework minerals is based in a framework of ______?
AlSi:O4
Because Each O2 anion is shared with another tetrahedra, this results in 2 characteristics. What are they?
Composition: open structure can easily accomodate large cations
Physical: framework silicates have a low specific gravity and generally are not stable @ high Temp.
Quartz is Uni- or Bi- axial?
uniaxial
Feldspars are Uni- or Bi- axial?
biaxial
piezioelectric effect
the ability of certain minerals to generate and electric charge when mechanical stress is applied
Properties of Quartz (7)
translucent non-metallic no cleavage conchoidal fracture low density hardness of 7 piezoelectric effect [SiO2 is found in nature in an amorphous state]
What is the principle framework silicate?
The feldspar group
- it is the most important silicate in igneous rocks
Z/O ratio in feldspars
1:2
Chem formula of Alkali feldspars
K,Na [AlSi3O8]
Chem formula of Plagioclase feldspars
Na[AlSi3O8] – Ca[Al2Si2O8]
Physical Properties of Feldspars (5)
Hardness: all 6
Colour: white and pink (orthorclase generally)
Density: 2.6
Cleavage : 2 @ 90
twinning: simple = albite & orthorclase; multiple = plagioclase, microcline
What is an Alkali Feldspar?
feldspars rich in Na and K
What are the Alkali Feldspars? (5)
monoclinic = sanidine and orthoclase triclinic = anorthoclase, microcline and albite
Distinguishing Features (K-feldspar vs Plag or Qtz) (4)
- simple (carlsbad) or tartan twinning
- exsolution is common
- zoning is rare
- biaxial -ve, large 2V angle
Distinguishing Features (Orthorclase vs Microcline vs Sanidine) (3)
sanidine = small 2V angle & simple twins Microcline = tartan twinning Orthorclase = large 2V angle
Volcanic or Plutonic?
- sanidine
- orthorclase
- microcline
sanidine = volcanic orthorclase = plutonic microcline = plutonic
What form do the Alkali Feldspars Albite and K-Feldspar have?
exsolution lamellae (perthitic) form during the unmixing of homogeneous K, Na feldspar during slow cooling
Properites of Sanidine (4)
- common in volcanic rocks
- prismatic habit
- may have simple twinning
- colourless and low birefringence
Plagioclase Feldspar Distinguishing features (Plag vs K-Feld or Qtz) (4)
- polysynthetic twinning
- zoning common
- exsolution is rare
- can determine plag composition using Michel-Levy Method
types of twinning in feldspars (6)
- carlsbad (simple)
- polysynthetic(albite) = plagioclase
- Manebach = reflection on (001)
- Baveno = reflection on (021)
- Tartan = microcline
- pericline = plagioclase
Which 2 elements occupy tetrahedral sites in the ordering of feldspars structure?
Si
Al
what are the 3 K-feldspar polymorphs?
sanidine
orthoclase
microcline
what is the ordering of the 3 polymorphs?
Highest Temp & random ordering
Sanidine
Orthorclase
Microcline
Lowest Temp & most orderedat high temperatures is there (random or non-random) distribution of Si & Al?
random
random distribution of Si & Al
- disordered
- monoclinic symmetry
- Sanidine & orthoclase
non-random distribution of Si & Al
- ordered
- triclinic symmetry
- microcline
