Comparison · Minerals vs Silicate Minerals
Andalusite vs Sillimanite
Andalusite and Sillimanite are easy to confuse. Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Andalusite and Sillimanite
- Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Hold a thin edge to the light: Andalusite is Transparent to translucent, Sillimanite is transparent.
- They overlap in brown, green, and gray, but Andalusite also comes in red and pink and Sillimanite also comes in white and yellow.
- Well-formed crystals give it away: Andalusite is Orthorhombic, Sillimanite is orthorhombic.
Andalusite vs Sillimanite properties
| Property | Andalusite | Sillimanite |
|---|---|---|
| Type | Minerals | Silicate Minerals |
| Mohs hardness | 6.5–7.5 | 7 |
| Luster | Vitreous | vitreous |
| Streak | White | White |
| Transparency | Transparent to translucent | transparent |
| Specific gravity | 3.13–3.21 | 3.24 |
| Crystal system | Orthorhombic | orthorhombic |
| Common colors | Reddish brown, Olive green, Gray, Pink | White, Gray, Brown, Pale green, Yellow |
| Rarity | common | uncommon |
| Typical specimen value | $2–$150+ | $10–$450+ |
Which is more valuable?
Sillimanite is usually the more valuable of the two: a typical collector specimen runs about $70 against $20 for Andalusite, and premium Sillimanite can reach $450 or more.
About Andalusite
Andalusite is an aluminum silicate, Al2SiO5, and one of three natural polymorphs sharing that exact formula along with kyanite and sillimanite. It crystallizes in the orthorhombic system as stout, nearly square prisms formed under low-pressure, moderate-temperature metamorphism. Its variety chiastolite, which displays a dark cross pattern in cross-section, makes it one of the more recognizable metamorphic minerals to casual collectors.
About Sillimanite
Sillimanite is an aluminum silicate mineral that forms slender prismatic crystals or fibrous masses, the fibrous variety being called fibrolite. It is one of three aluminosilicate minerals that share the same composition but differ in structure.
Andalusite vs Sillimanite FAQ
Is Andalusite the same as Sillimanite?
No. Andalusite is a mineral while Sillimanite is a silicate mineral. Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Andalusite from Sillimanite?
Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Hold a thin edge to the light: Andalusite is Transparent to translucent, Sillimanite is transparent. They overlap in brown, green, and gray, but Andalusite also comes in red and pink and Sillimanite also comes in white and yellow.
Which is harder, Andalusite or Sillimanite?
They are about equally hard, both near Mohs 6.5–7.5.
Which is more valuable, Andalusite or Sillimanite?
Sillimanite is usually the more valuable of the two: a typical collector specimen runs about $70 against $20 for Andalusite, and premium Sillimanite can reach $450 or more.
