Neosilicates: Aluminum Silicates Flashcards
Aluminium Silicates Formula
Al2SiO5
3 Polymorphs of Alluminum Silicates
Andalusite
Kyanite
Sillimanite
Polymorphs
Polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism.
Sillimanite
High-temperature form of Aluminium Silicate
Andalusite
Low temperature - Low pressure form of Aluminum Silicate
Kyanite
High pressure polymorph of aluminum silicate
Triple point of Aluminium Silicates
525 C
0.385 GPa
Invariant
Point where all 3 phases coexist
Importance of Aluminum Silicates
Their triple point is in the middle of common metamorphic conditions making them useful to determine the conditions at which the rock formed
Paragenisis of Aluminum Silicates
Metamorphic rocks formed in the conditions described be the triple point
Andalusite colour in thin section
Colourless
Kyanite colour in thin section
Colourless to pale blue
Sillimanite colour in thin section
Colourless to light yellow
Pleochroism in Aluminum Silicates
None / very weak
Relief in Aluminum Silicates
High
Aluminum Silicates colour in thin sections
Colourless / Pale
Andalusite form in thin section
Euhedral crystals with dark CROSS
Dark inclusions of carbonaceous material is frequently arranged symmetrically to form a Maltese Cross.
Andalusite Cleavage in thin section
Parallel to 110, in cross-sections intersection at 90 degrees
Andalusite birefringence in thin section
weak up to 1st order yellow
Andalusite extinction in thin section
Parallel but symmetric in cross sections
Andalusite twinning in thin section
Rare
Andalusite distinguishing features in thin section
Cross, weak birefringence
Kyanite form in thin section
Tabular often deformed
Kyanite cleavage in thin section
2 good cleavages parallel to 100 and 010 at 85 degrees
Kyanite birefringence in thin section
Weak 1st order red maximum
Kyanite extinction in thin section
Oblique at 30 degrees, parallel in cross sections
Kyanite Twinning in thin section
Frequent 100 and 001
Kyanite distinguishing features in thin section
Strong cleavage, Cross parting
Sillimanite form in thin section
acicular or fibrous, square in cross-sections
Sillimanite cleavage in thin section
1 good cleavage normal to length of crystal - poorly visible
Transverse fractures are characteristic
Sillimanite birefringence in thin section
2nd order blue maximum
Sillimanite extinction in thin section
Parallel
Sillimanite twinning in thin section
None
Sillimanite distinguishing features in thin section
Fibrous and/or acicular habit. Distinctive transverse fracture.
Structure of Aluminum Silicates
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
Crystal Systems of Aluminum Silicates
Sillimanite - Orthorhombic
Andalusite - Orthorhombic
Kyanite - Triclinic
Common rock they occur in
metapelites
