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Black Spinel is a magnesium aluminum oxide mineral belonging to the spinel group. It is characterized by its opaque to translucent black color, high hardness (8 on the Mohs scale), and vitreous to sub-adamantine luster. Its chemical formula is MgAl2O4, though iron (Fe2+ and Fe3+) often substitutes for magnesium and aluminum, leading to variations in color and properties. The black color is typically due to the presence of iron (Fe2+ and Fe3+) and sometimes chromium (Cr3+) within the crystal lattice. It crystallizes in the isometric system, commonly forming octahedral crystals, but also occurs as granular masses.
How to Identify
- Color
- Opaque to translucent black. Rarely, very dark brown or purplish-black.
- Luster
- Vitreous to sub-adamantine, often appearing somewhat greasy on fracture surfaces.
- Texture
- Typically smooth on crystal faces, granular in massive forms.
- Crystal Form
- 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.
- Cleavage
- Poor to indistinct octahedral cleavage {111}. Fracture is more prominent than cleavage.
- Geological Environment
- 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+
Quick Check
- Color: Black
- Luster: Vitreous to sub-adamantine
- Streak: White to grayish white
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
Formation
Black spinel forms under high-temperature and often high-pressure metamorphic conditions, typically in contact metamorphic aureoles, regionally metamorphosed limestones, and schists. It can also crystallize from mafic and ultramafic igneous rocks, such as peridotites and basalts, and is a common accessory mineral in some pegmatites. Detrital spinel is found in placer deposits due to its hardness and resistance to weathering.
Usage
Primarily used as a gemstone in jewelry due to its hardness, durability, and attractive luster. Historically, larger crystals were sometimes mistaken for black diamonds. It also has minor industrial applications as an abrasive and in some refractory materials due to its high melting point and chemical stability.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.
Where to Find
Sri Lanka
Known for producing fine quality spinel, including black varieties, often found in alluvial deposits.
Myanmar (Burma)
Mogok region is a classic locality for various spinel colors, including black, from both primary and secondary deposits.
Thailand
Gem-quality spinel, including black, is found in alluvial deposits.
Vietnam
Luc Yen and other areas yield black spinel, often associated with marble and other metamorphic rocks.
Madagascar
Various localities produce black spinel, often in association with other gem minerals.
Tanzania
Several regions, particularly in the Tanga and Morogoro areas, are known for spinel occurrences.
Afghanistan
Historically significant source of spinel, including black varieties.
Brazil
Some occurrences of black spinel have been reported.
United States
Minor occurrences in various states, often in metamorphic terrains (e.g., New York, New Jersey).
Finding Tips
Check Metamorphic Terrains
Focus on areas with contact metamorphic aureoles around igneous intrusions, especially those involving carbonate rocks (limestones, dolomites) or schists.
Inspect Mafic/Ultramafic Rocks
Look for black spinel as an accessory mineral in peridotites, serpentinites, and some basalts.
Explore Placer Deposits
Due to its hardness and resistance to weathering, black spinel often concentrates in alluvial (river) and eluvial (residual) deposits. Pan for heavy minerals in stream beds originating from spinel-bearing source rocks.
Look for Octahedral Crystals
Black spinel commonly forms distinct octahedral crystals. Identifying this crystal habit is a strong indicator.
Perform Hardness Test
With a Mohs hardness of 8, black spinel will scratch quartz (7) and feldspar (6-6.5). This helps differentiate it from softer black minerals.
Observe Luster
Its vitreous to sub-adamantine luster is characteristic and can help distinguish it from duller black minerals.
Check for Magnetism
While pure MgAl2O4 is non-magnetic, iron-rich varieties (pleonaste, hercynite) can be weakly to moderately magnetic due to the presence of Fe2+ and Fe3+. This can help differentiate it from non-magnetic black minerals like tourmaline or onyx.
Similar Rocks
Black Tourmaline
Sodium Iron Aluminum Borosilicate Hydroxide
Also known as: Schorl
Black Garnet
Calcium Iron Silicate
Also known as: Melanite (Andradite variety)
Onyx
Silicon Dioxide
Also known as: Black Chalcedony
Obsidian
Amorphous Silicon Dioxide
Also known as: Volcanic Glass
Black Diamond
Carbon
Also known as: Carbonado
Scientific Classification
- Mineral Class
- Oxides
- Group
- Spinel Group
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- MgAl2O4
- Composition
- Magnesium Aluminum Oxide
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