How to identify Black Spinel

To correctly identify Black Spinel (MgAl2O4), 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

   Opaque to translucent black. Rarely, very dark brown or purplish-black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-adamantine, often appearing somewhat greasy on fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically smooth on crystal faces, granular in massive forms.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Most commonly forms distinct octahedral crystals. Also found as dodecahedra, combinations of octahedra and dodecahedra, or as granular to massive aggregates. Twinning is common, particularly as spinel-law twins.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to indistinct octahedral cleavage {111}. Fracture is more prominent than cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (contact and regional), mafic and ultramafic igneous rocks, pegmatites, and placer deposits.

Key Facts

Hardness

8 on the Mohs scale

Specific Gravity

3.58 - 4.06 (typically 3.60 - 3.70 for Mg-rich spinel; increases with Fe content)

Crystal System

Isometric (Cubic)

Color

Opaque to translucent black

Luster

Vitreous to sub-adamantine

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

Poor to indistinct octahedral {111}

Composition

Magnesium Aluminum Oxide (MgAl2O4), with common substitutions of Fe2+, Fe3+, Cr3+, Zn2+

Physical characteristics

• Crystal Habit: Octahedral, dodecahedral, granular, massive. Spinel-law twinning is common.
• Cleavage Type: Poor to indistinct octahedral {111}
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to sub-adamantine

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