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Classification
Crystal System: Triclinic
Class: Silicate
Sub Class: Nesosilicate (single silicate)
Group: Aluminosilicate Polymorph Group
Structure and Composition
Composition: Al2(SiO4)O
Structure:
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Blue, white, light gray, green, rarely yellow, orange, pink
Luster: Vitreous, Sub-Vitreous, Greasy, Pearly
Hardness: 5½ - 7
Streak: Colorless
Specific gravity: 3.53 - 3.67
Common Habit: Bladed/columnar
Cleavage: Perfect on (100), good on (010)
Identification In Thin Section
PPL Properties
Relief: High positive
Habit/Form: Commonly forms elongate bladed or columnar crystals that may appear to be bent, rarely fibrous
Color: Colorless to pale blue
Cleavage: Perfect parallel to {100}, less perfect parallel to {010}, cross parting {001} at angles of 85° with the length of the crystals
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I white and yellow to order I red (see bar below)
Extinction angle: Inclined at 0-30°
Twins: Twinning is frequent, multiple twinning as normal or parallel twins on {100} or, lamellar twins on {001} may be the result of shear pressure.
Uniaxial/Biaxial: Biaxial (-)
Kyanite (Ky) and quartz (Qtz) in thin seciton (PPL on the left and XPL on the right).
Geologic Occurrence
Kyanite is a metamorphic mineral found in medium or high-pressure mica schists and gneisses.
Significance
Kyanite is widely used in refractory materials, ceramics, and abrasives due to its high thermal stability, expansion upon heating, and durability. It is also occasionally valued as a semi-precious gemstone (a staple of most rock shows and gem stone shops in my experience). Kyanite is a critical indicator mineral for high-pressure, low-temperature metamorphic conditions.
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