Neosilicates: Aluminum Silicates

Question 1

Aluminium Silicates Formula

Answer 1

Al2SiO5

Question 2

3 Polymorphs of Alluminum Silicates

Answer 2

Andalusite
Kyanite
Sillimanite

Question 3

Polymorphs

Answer 3

Polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism.

Question 4

Sillimanite

Answer 4

High-temperature form of Aluminium Silicate

Question 5

Andalusite

Answer 5

Low temperature - Low pressure form of Aluminum Silicate

Question 6

Kyanite

Answer 6

High pressure polymorph of aluminum silicate

Question 7

Triple point of Aluminium Silicates

Answer 7

525 C

0.385 GPa

Question 8

Invariant

Answer 8

Point where all 3 phases coexist

Question 9

Importance of Aluminum Silicates

Answer 9

Their triple point is in the middle of common metamorphic conditions making them useful to determine the conditions at which the rock formed

Question 10

Paragenisis of Aluminum Silicates

Answer 10

Metamorphic rocks formed in the conditions described be the triple point

Question 11

Andalusite colour in thin section

Answer 11

Colourless

Question 12

Kyanite colour in thin section

Answer 12

Colourless to pale blue

Question 13

Sillimanite colour in thin section

Answer 13

Colourless to light yellow

Question 14

Pleochroism in Aluminum Silicates

Answer 14

None / very weak

Question 15

Relief in Aluminum Silicates

Answer 15

High

Question 16

Aluminum Silicates colour in thin sections

Answer 16

Colourless / Pale

Question 17

Andalusite form in thin section

Answer 17

Euhedral crystals with dark CROSS

Dark inclusions of carbonaceous material is frequently arranged symmetrically to form a Maltese Cross.

Question 18

Andalusite Cleavage in thin section

Answer 18

Parallel to 110, in cross-sections intersection at 90 degrees

Question 19

Andalusite birefringence in thin section

Answer 19

weak up to 1st order yellow

Question 20

Andalusite extinction in thin section

Answer 20

Parallel but symmetric in cross sections

Question 21

Andalusite twinning in thin section

Answer 21

Rare

Question 22

Andalusite distinguishing features in thin section

Answer 22

Cross, weak birefringence

Question 23

Kyanite form in thin section

Answer 23

Tabular often deformed

Question 24

Kyanite cleavage in thin section

Answer 24

2 good cleavages parallel to 100 and 010 at 85 degrees

Question 25

Kyanite birefringence in thin section

Answer 25

Weak 1st order red maximum

Question 26

Kyanite extinction in thin section

Answer 26

Oblique at 30 degrees, parallel in cross sections

Question 27

Kyanite Twinning in thin section

Answer 27

Frequent 100 and 001

Question 28

Kyanite distinguishing features in thin section

Answer 28

Strong cleavage, Cross parting

Question 29

Sillimanite form in thin section

Answer 29

acicular or fibrous, square in cross-sections

Question 30

Sillimanite cleavage in thin section

Answer 30

1 good cleavage normal to length of crystal - poorly visible

Transverse fractures are characteristic

Question 31

Sillimanite birefringence in thin section

Answer 31

2nd order blue maximum

Question 32

Sillimanite extinction in thin section

Answer 32

Parallel

Question 33

Sillimanite twinning in thin section

Answer 33

None

Question 34

Sillimanite distinguishing features in thin section

Answer 34

Fibrous and/or acicular habit. Distinctive transverse fracture.

Question 35

Structure of Aluminum Silicates

Answer 35

Half the Al is arranged in Straight chains of AlO6 are arranged parallel to the c axis
The rest is arranged differently for each polymorph
Sillimanite in 4 fold site, andalusite in 5 fold sites, kyanite in 6 fold sites
These are linked along the c axis by SiO4 tetrahedra

Question 36

Crystal Systems of Aluminum Silicates

Answer 36

Sillimanite - Orthorhombic
Andalusite - Orthorhombic
Kyanite - Triclinic

Question 37

Common rock they occur in

Answer 37

metapelites