Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Tiger Iron is a visually striking composite material characterized by its alternating bands of metallic gray hematite, rich red jasper, and golden-brown to yellow chatoyant tiger's eye. The interplay of these distinct components creates a unique aesthetic. It is a form of banded iron formation, a significant geological indicator of early Earth's atmospheric and oceanic conditions. The tiger's eye component is a pseudomorph of quartz after crocidolite, giving it its characteristic silky luster and optical effect.
How to Identify
- Color
- Distinctive alternating bands of metallic gray (hematite), deep red (jasper), and golden-brown to yellow (tiger's eye).
- Luster
- Bands of hematite exhibit a metallic luster; jasper is dull to vitreous; tiger's eye displays a silky, chatoyant luster.
- Texture
- Fine-grained to microcrystalline, with a smooth feel when polished. The tiger's eye component may show a fibrous texture.
- Crystal Form
- Not applicable as a composite rock. Individual mineral components are cryptocrystalline (jasper, tiger's eye) or massive (hematite).
- Cleavage
- None observed in the composite rock. Hematite has no cleavage, jasper and tiger's eye (quartz) have no cleavage.
- Geological Environment
- Formed in ancient marine basins as chemical sediments, subsequently metamorphosed. Found in regions with extensive Precambrian banded iron formations.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz/jasper components), but can vary slightly with hematite content.
- Specific Gravity: Approximately 3.3-4.0, varying with the proportion of hematite (higher density) to quartz (lower density).
- Crystal System: Not applicable as a composite rock. Quartz is trigonal, Hematite is trigonal.
- Color: Alternating bands of red (jasper), metallic gray (hematite), and golden-brown/yellow (tiger's eye).
- Luster: Metallic (hematite), dull to vitreous (jasper), silky/chatoyant (tiger's eye).
- Transparency: Opaque.
- Fracture: Conchoidal to uneven (due to quartz/jasper components).
- Cleavage: None.
- Composition: A composite of microcrystalline quartz (jasper), fibrous quartz pseudomorphs after crocidolite (tiger's eye), and iron oxide (hematite).
Quick Check
- Color: Banded red, metallic gray, and golden-brown/yellow
- Luster: Metallic, dull, and silky (chatoyant)
- Streak: Reddish-brown (from hematite component)
Physical Characteristics
- Crystal Habit: Massive, banded.
- Cleavage Type: None.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Metallic, dull, vitreous, silky.
Formation
Tiger Iron is a variety of banded iron formation (BIF) that has undergone subsequent metamorphism and silicification. It forms through the deposition of alternating layers of iron-rich minerals (primarily hematite and magnetite) and silica (chert or jasper) in ancient marine environments. The distinctive chatoyancy of the tiger's eye component develops when crocidolite (a fibrous amphibole) is replaced by quartz, retaining the fibrous structure. Subsequent deformation and metamorphism can enhance the banding and create the characteristic patterns.
Usage
Primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in jewelry. It is also collected by mineral enthusiasts.
Age Distribution
Predominantly Precambrian (Archaean to Proterozoic Eons), typically 2.5 to 3.8 billion years old.
Where to Find
Pilbara region, Western Australia
The primary and most significant source of high-quality Tiger Iron, particularly from the Hamersley Basin, known for its vast banded iron formations.
South Africa
Some occurrences of similar banded iron formations with tiger's eye components are found, though less prominent than Australian sources.
United States (Michigan, Minnesota)
Banded iron formations are present in the Lake Superior region, but the specific combination and quality to be classified as 'Tiger Iron' are rare.
Finding Tips
Look for Banded Iron Formations
Tiger Iron is a specific type of BIF. Search geological maps and literature for areas known to have Precambrian banded iron formations.
Identify Component Minerals
Familiarize yourself with the appearance of hematite (metallic gray, red streak), jasper (red, opaque, microcrystalline quartz), and tiger's eye (golden-brown, chatoyant). Tiger Iron will have all three.
Observe Chatoyancy
The presence of the silky, moving band of light (chatoyancy) from the tiger's eye component is a key diagnostic feature that distinguishes it from other BIFs.
Check for Metamorphic Features
While primarily sedimentary in origin, the tiger's eye component and the overall consolidation suggest some degree of metamorphism.
Similar Rocks
Banded Iron Formation (BIF)
Banded Iron Formation
Also known as: Ironstone
Tiger's Eye
Quartz pseudomorph after Crocidolite
Also known as: Crocodilite Asbestos (precursor)
Jasper
Cryptocrystalline Quartz
Also known as: Chert (red variety)
Hematite
Iron Oxide (Fe2O3)
Also known as: Specularite, Kidney Ore
Scientific Classification
- Mineral Class
- Not a single mineral; a rock composed of oxides and silicates.
- Group
- Composite rock (specifically a variety of banded iron formation).
- Crystal System
- Not applicable for the composite rock. Individual components: Quartz (Trigonal), Hematite (Trigonal).
- Chemical Formula
- Variable, primarily SiO2 (quartz/jasper/tiger's eye) and Fe2O3 (hematite).
- Composition
- A natural aggregate of Jasper (cryptocrystalline SiO2), Hematite (Fe2O3), and Tiger's Eye (fibrous SiO2 pseudomorph after crocidolite).
Explore Tiger Iron
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.