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). Andalusite in a rock indicates relatively low pressure and temperature. Kyanite indicates high pressure and low temperature. Sillimanite indicates high temperature.

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

Formula: Al 2 SiO 5 Group: Aluminosilicates Class: Nesosilicates System: Triclinic

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

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

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

Al 2 SiO 5

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.

Crystal Habit (Triclinic) for Kyanite

Formula: Al 2 SiO 5 Group: Aluminosilicates Class: Nesosilicates System: Orthorhombic

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

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

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

Al 2 SiO 5

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).

Crystal Habit (Orthorhombic) of Andalusite

Formula: Al 2 SiO 5 Group: Aluminosilicates Class: Nesosilicates System: Orthorhombic

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 ( ).

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

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

Al 2 SiO 5

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.

Crystal Habit (Orthorhombic) of Sillimanite

Kyanite Sillimanite Andalusite