How to identify Chiastolite

To correctly identify Chiastolite (Andalusite variety chiastolite), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Main crystal: white, gray, pink, reddish-brown. Inclusions: black, dark gray.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous, sometimes dull on weathered surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically prismatic crystals, often with a rough or striated surface. The most defining feature is the internal cruciform pattern visible when cut perpendicular to the c-axis.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic, typically forming stout to elongated prismatic crystals, often nearly square in cross-section. The cross pattern is best observed in transverse sections.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Good to distinct on {110} and {100}, forming angles of approximately 90 degrees. This can be observed as parallel lines or steps on broken surfaces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks, particularly schists, slates, and hornfels, derived from aluminum-rich sedimentary protoliths. It is a characteristic mineral of medium-grade regional metamorphism and contact metamorphism.

Key Facts

Hardness

6.5-7.5 on the Mohs scale (anisotropic hardness, meaning it varies with direction, often harder parallel to the c-axis).

Specific Gravity

3.13-3.16 g/cm³.

Crystal System

Orthorhombic.

Color

White, gray, pink, reddish-brown (main crystal); black, dark gray (inclusions).

Luster

Vitreous to sub-vitreous.

Transparency

Translucent to opaque.

Fracture

Uneven to subconchoidal.

Cleavage

Good to distinct on {110} and {100}.

Composition

Aluminum nesosilicate, Al2SiO5, with carbonaceous or argillaceous inclusions.

Physical characteristics

• Crystal Habit: Prismatic, often stout to elongated, with a nearly square cross-section. The distinctive cruciform pattern is due to systematic inclusions.
• Cleavage Type: Good to distinct in two directions at approximately 90 degrees ({110} and {100}).
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous.

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