- The aluminosilicates (Al 2 SiO 5 ) kyanite, sillimanite, and andalusite are po

Question

- The aluminosilicates (Al2SiO5) kyanite, sillimanite, and andalusite are polymorphs that are commonly found in metamorphosed Al-rich sedimentary rocks (e.g., pelitic schists).

1. What is the structural difference between the aluminosilicates?

2. What type of polymorphic reaction occurs between each of these minerals?

3. Despite having known (i.e., equilibrium) pressures and temperatures of formation, the aluminosilicates can be found outside of this range. Given the type of polymorphic reaction that occurs between these minerals, why do you think disequilibrium (i.e., non-ideal) conditions are preserved?

Explanation / Answer

Ans: 1.)

Andalusite, kyanite, and sillimanite are naturally occurring aluminosilicate minerals that have the composition Al2SiO5. Although they are polymorphs and have the same constituent elements, owing to varying temperature and pressure zones, their structures are rearranged consequently. The structural difference between these aluminosilicates are as follows:

SILLIMANITE: Orthorhombic: Octahedral Al chains (6-fold) are crosslinked by both Si and Al tetrahedra (4-fold).

ANDALUSITE: Orthorhombic: 5-coordinated Al; Same octahedral (6-fold) chains.

KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6-fold).

Ans: 2.)

Phase transformations require rebonding of the Aluminium atom. Reconstructive polymorphism requires more energy than do displacive transformations. The changes in structure are in response to changing P and T. Result is changing in Al coordination.

The main reactions are of course solid-solid reactions between the polymorphs but there are also some dehydration reactions involved with some other aluminosilicates.

Solid-solid reactions There are three univariant reactions, each representing the phase transitions: andalusite <=> sillimanite : Al2SiO5 <=> Al2SiO5

andalusite <=> kyanite : Al2SiO5 <=> Al2SiO5

and kyanite <=> sillimanite : Al2SiO5 <=> Al2SiO5

The transition from kyanite to andalusite is known as the andalusite isograd, while the transition from andalusite to sillimanite is called the sillimanite isograd.

Dehydration reactions Hydrated aluminosilicate minerals are referred to as zeolites. For a

Al2SiO5 system with excess SiO2 and H2O, preferably in a low grade metamorphic, the reaction:

Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O
  Pyrophyllite Kyanite or Andalusite Qtz           fluid

Ans: 3.)

Disequilibrium reactions are more common in nature than the theoretical 'stable' conditions. Occurrences of metastable phases of one polymorphic mineral in the stability field of another polymorphic mineral indicate the presence of impurities like water, often as a catalyst or solvent, rather than a species in the stoichiometric reaction. In presence of excess water, metastable kyanite can dissolve and precipitate more stable sillimanite along lower temperature and pressure regimes. In real-world fieldwork, we often stumble upon andalusite or sillimanite in veins and fractures (later structures in metamorphic history) in otherwise massive kyanite bearing rocks.

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