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Black spinel is a naturally occurring mineral belonging to the spinel group, characterized by its isometric crystal system and typically octahedral habit. Its black color is primarily due to the presence of iron (Fe2+ and Fe3+) substituting for magnesium and aluminum in its crystal lattice. It is a durable and relatively hard mineral, making it suitable for various applications, including jewelry. While pure magnesium aluminum spinel (MgAl2O4) is colorless, the presence of transition metals like iron, chromium, and vanadium gives rise to the wide range of colors seen in spinel, with iron being responsible for the black coloration.
How to Identify
- Color
- Typically opaque black, sometimes with a very dark brown or purplish tint when viewed under strong light.
- Luster
- Vitreous (glassy) to sub-adamantine.
- Texture
- Smooth to slightly conchoidal fracture surfaces.
- Crystal Form
- Crystals are typically octahedral, sometimes dodecahedral, or a combination. Often found as well-formed euhedral crystals. Can also occur as granular masses.
- Cleavage
- Poor to indistinct octahedral cleavage, often appearing as parting rather than true cleavage. Fracture is more prominent.
- Geological Environment
- Metamorphic rocks (e.g., marbles, gneisses, schists), mafic and ultramafic igneous rocks (e.g., peridotites, basalts), and associated placer deposits.
Key Facts
- Hardness: 8 on Mohs scale
- Specific Gravity: 3.58 - 4.10 (varies with iron content)
- Crystal System: Isometric (Cubic)
- Color: Opaque black, sometimes with very dark brown or purplish undertones
- Luster: Vitreous to sub-adamantine
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct octahedral, often parting
- Composition: Magnesium Aluminum Oxide (MgAl2O4) with significant iron (Fe) substitution, often forming a solid solution towards Hercynite (FeAl2O4)
Quick Check
- Color: Opaque black
- Luster: Vitreous to sub-adamantine
- Streak: Colorless to grey (on an unglazed porcelain streak plate)
Physical Characteristics
- Crystal Habit: Octahedral, dodecahedral, or granular masses. Often twinned (spinel law).
- Cleavage Type: Poor to indistinct octahedral {111} parting, not true cleavage.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to sub-adamantine.
Formation
Black spinel typically forms in high-temperature, low-pressure metamorphic environments, such as contact metamorphic aureoles around igneous intrusions, or in regionally metamorphosed limestones and dolomites. It can also crystallize directly from mafic and ultramafic magmas, and is found in some pegmatites. Due to its hardness and chemical stability, it is also found in placer deposits.
Usage
Black spinel is primarily used as a gemstone, particularly as a substitute for black diamond due to its similar appearance and durability. It is also used in industrial applications as an abrasive due to its hardness, and in some refractory materials. Historically, larger crystals were sometimes mistaken for other black gemstones.
Age Distribution
Spinel forms in a variety of geological settings, often associated with metamorphic and igneous rocks, which can range from Precambrian to Cenozoic in age.
Where to Find
Sri Lanka
Known for producing high-quality spinel, including black varieties, often found in alluvial deposits.
Myanmar (Burma)
A historically significant source of various spinel colors, including black, from both primary and secondary deposits.
Thailand
Produces black spinel, particularly from gem gravels.
Vietnam
Emerging source of various gem-quality spinels, including black.
Madagascar
Known for a wide range of minerals, including black spinel, found in metamorphic and alluvial settings.
Tanzania
Produces black spinel, often associated with other gem minerals.
Afghanistan
Historical source of spinel, including darker varieties.
Brazil
Some occurrences of black spinel have been reported.
Finding Tips
Check for Crystal Habit
Look for well-formed octahedral crystals, which are characteristic of spinel. These can be found in host rock or as loose crystals in alluvial deposits.
Hardness Test (Caution)
Spinel is hard (8 on Mohs scale). It will scratch glass and most common minerals. Use a known mineral of similar hardness for comparison, or a streak plate (though black spinel will leave no streak).
Specific Gravity
Black spinel has a relatively high specific gravity (3.5-4.1). In placer deposits, it will concentrate with other heavy minerals.
Associated Minerals
In metamorphic environments, look for black spinel alongside minerals like calcite, dolomite, phlogopite, chondrodite, and graphite. In igneous settings, it may be found with olivine, pyroxene, and plagioclase.
Placer Deposits
In areas known for gem production, search riverbeds and alluvial fans where heavy minerals like spinel accumulate due to weathering and erosion of primary sources.
Similar Rocks
Black Tourmaline
Schorl (NaFe3Al6(BO3)3Si6O18(OH)4)
Also known as: Schorl
Black Diamond
Diamond (C)
Also known as: Carbonado
Onyx
Cryptocrystalline Quartz (SiO2)
Also known as: Chalcedony variety
Obsidian
Amorphous Silicate Glass
Also known as: Volcanic Glass
Jet
Lignite (a form of coal)
Also known as: Black Amber
Scientific Classification
- Mineral Class
- Oxides
- Group
- Spinel Group
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- (Mg,Fe2+)Al2O4 (general formula for black spinel, indicating Mg and Fe substitution)
- Composition
- Magnesium aluminum oxide, with iron (Fe2+ and Fe3+) substituting for magnesium and aluminum, leading to the black coloration. The iron-rich end-member is Hercynite (FeAl2O4).
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