Sillimanite (Cat's-Eye) | Gemopedia

The term cat's eye, or chatoyancy, is used to describe a phenomenal optical property in gemstones, in this case, sillimanite. The effect, when present, appears as a bright, narrow slit similar to the pupils in the eyes of your favorite feline. This phenomenon is caused by parallel fibrous or needle-like inclusions that interfere with the passage of light throughout the crystal, scattering and reflecting light back to the viewer as a thin line. Best observed when gems are cut en cabochon, cat's eye sillimanite makes a phenomenal addition to any gemstone collection.
Alternate
Names
Fibrolite
Colors
Gray, Green, Brown

Sillimanite (Cat's-Eye) Classification

Common Name

Sillimanite (Cat's-Eye)

Species

Sillimanite

Variety

Cat's-Eye Sillimanite

Colors

Gray, Green, Brown

Alternate Names

Fibrolite

Gemstone Groups

Key Separations

RI, SG, fracture and fibrous appearance.

Comments

Sillimanite is a polymorph of andalusite and kyanite.

Sillimanite (Cat's-Eye) Optical Properties

Transparency

Translucent - Opaque

Refractive Index

1.659-1.68
Tolerance:(+0.004/-0.006)

Birefringence

0.015-0.021

Optic Character

Biaxial

Optic Sign

Positive

Polariscope Reaction

Aggregate (AGG)

Fluorescence

SWUV: blue:inert or weak red
LWUV: blue:inert or weak red

CCF Reaction

Pleochroism

Unobservable

Dispersion

Comments

Sillimanite (Cat's-Eye) Chemistry & Crystallography

Chemical Name

aluminum silicate

Chemical Formula

Al2SiO6

Synthesis

Crystal System

Orthorhombic

Classification

Silicate

Nature

Natural

Crystallinity

Crystalline to Aggregate

Comments

Sillimanite (Cat's-Eye) Characteristic Physical Properties

Hardness

6-7.5

Streak

White

Specific Gravity

3.14-3.27 Typical:3.25

Toughness

Poor

Inclusions

Sillimanite will appear fibrous is often vertically striated and they will contain parallel rutile needles or hypersthene's fibers.

Luster

Vitreous

Stability

Fracture

Uneven

Cleavage

Perfect, in one direction

Comments

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