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MY TRIPLET POLYMORPHS: Kyanite Andalusite Sillimanite

MY TRIPLET POLYMORPHS: Kyanite Andalusite Sillimanite. Al 2 SiO 5 Polymorphs The presence of different minerals and polymorphs (different minerals with the same chemical composition) may indicate temperature and pressures under which rocks were formed (e.g. in mountain chains).

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MY TRIPLET POLYMORPHS: Kyanite Andalusite Sillimanite

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  1. MY TRIPLET POLYMORPHS: Kyanite Andalusite Sillimanite

  2. Al2SiO5 Polymorphs The presence of different minerals and polymorphs (different minerals with the same chemical composition) may indicate temperature and pressures under which rocks were formed (e.g. in mountain chains). Andalusite in a rockindicates relatively low pressure and temperature. Kyaniteindicates high pressure and low temperature. Sillimaniteindicates high temperature.

  3. Different pressure (P) and temperature (T) conditions exist in mountains, therefore different minerals form in different P and T zones.

  4. Kyanite

  5. Formula: Al2SiO5 Group: Aluminosilicates Class: Nesosilicates System: Triclinic

  6. Occurrences: Brazil, North Carolina and Georgia, Switzerland, Russia, Serbia, India and Kenya. Also found in metamorphic rocks (pelite, schist, gneiss) and hydrothermal areas (pegmatite) Uses: to manufacture spark plugs Name Origin: From the Greek word kyanos, which means BLUE

  7. Physical Properties Color:blue white gray green Streak: white Luster: vitreous Hardness: 4-7 Specific Gravity: 3.53 – 3.65 Fracture: Splintery Cleavage: perfect, prismatic Diaphaneity: transparent to translucent Habits & Appearances: long bladed crystals, radiating aggregates

  8. Optical Properties: Relief: high Birefringence: 1st Order Pleochroism: yes – violet to blue Elongation: yes Extinction: inclined Plane Polarized Light Cross Polarized Light

  9. Al2SiO5 Atomic Structure

  10. Crystal Chemistry • In kyanite, the octahedra in the Al octahedral chains are more ordered and more compact than in sillimanite or andalusite. • Thus kyanite is the high pressure polymorph.  • Kyanite crystals tend to break along a specific plane.

  11. Crystallography Crystal Habit (Triclinic) for Kyanite

  12. Andalusite

  13. Formula: Al2SiO5 Group: Aluminosilicates Class: Nesosilicates System: Orthorhombic

  14. Occurrences: Andalusia, Spain; Austria, California and China.In low to medium grade metamorphic rocks, and at contact, regional and hydrothermal areas (pegmatite). Uses: to manufacture spark plugs Name Origin: named after where it was found, Andalusia, Spain

  15. Physical Properties Color:pink reddish brown rose red white Streak: white gray green or colorless Luster: vitreous Hardness: 6.5 – 7.5 Specific Gravity: 3.13 – 3.21 Fracture: uneven, subconchoidal, brittle Cleavage: good, prismatic Diaphaneity: transparent to translucent to opaque Habits & Appearances: prismatic crystals, sometimes stubby and nearly square in cross section also fibrous, compact or massive

  16. Optical Properties: Relief: medium to high Birefringence: 1st to 2nd Order Pleochroism: yes, pink, green and yellow Elongation: yes Extinction: parallel, symmetrical Plane Polarized Light Cross Polarized Light

  17. Al2SiO5 Atomic Structure

  18. Crystal Chemistry • In andalusite, the Al octahedral chains are also linked by alternating Al octahedra and Si tetrahedra in a 3-D lattice. • This lattice is oriented parallel to diagonals of the unit cube (when viewed in the initial orientation). 

  19. Crystallography Crystal Habit (Orthorhombic) of Andalusite

  20. Sillimanite

  21. Formula: Al2SiO5 Group: Aluminosilicates Class: Nesosilicates System: Orthorhombic

  22. Occurrences: Brazil, New England and Europe. In igneous rocks as an accessory, metamorphic as medium to high grade and hydrothermal areas. Uses: to manufacture spark plugs Name Origin: Named after the American chemist and mineralogist, B. Silliman (1779-1824).

  23. Physical Properties Color:white colorless gray brown yellow blue Streak: white Luster: satiny, vitreous, conchoidal Hardness: 6.5 – 7.5 Specific Gravity: 3.23 – 3.27 Fracture: uneven, splintery, conchoidal Cleavage: perfect, parallel Diaphaneity: transparent to translucent Habits & Appearances: long prismatic crystals nearly square in cross sections, fibrous masses

  24. Optical Properties Relief: high Birefringence: 1st Order Pleochroism: yes, yellow, blue and brown Elongation: yes Extinction: parallel Plane Polarized Light Cross Polarized Light

  25. Al2SiO5 Atomic Structure

  26. Crystal Chemistry • In sillimanite, the Al octahedral chains alternate with Si tetrahedra / Al octahedra chains also oriented parallel to the c-axis. • Consequently, sillimanite has a prismatic form.  • In their chains, the Si tetrahedra and the Al octahedra alternate with each other and are perfectly aligned along one of the crystal axes.  • You can easily see this with the Al atoms highlighted by tilting the structure slightly from the initial view. • The alignment of the tetrahedra and octahedra contrasts with the arrangements found in the polymorphs kyanite and andalusite. • Sillimanite is the stable polymorph at high temperatures. 

  27. Crystallography Crystal Habit (Orthorhombic) of Sillimanite

  28. My babies all polished UP! Andalusite Kyanite Sillimanite

  29. http://mineral.galleries.com/minerals/silicate/kyanite/kyanite.htmhttp://mineral.galleries.com/minerals/silicate/kyanite/kyanite.htm http://minerals.er.usgs.gov/minerals/pubs/commodity/kyanite/ http://www.webmineral.com/data/Kyanite.shtml http://www.webref.org/geology/a/andalusite.htm

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