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Tiger Iron is a distinctive banded rock composed of three primary components: hematite (a dark, metallic iron oxide), jasper (a dense, opaque variety of microcrystalline quartz, typically red or brown), and tiger's eye (a chatoyant variety of quartz, typically golden-brown). The banding is often undulating or folded, showcasing the deformation the rock has undergone. The interplay of the metallic luster of hematite, the earthy colors of jasper, and the silky, chatoyant sheen of tiger's eye creates a visually striking material.
How to Identify
- Color
- Distinctive banding of metallic dark grey/black (hematite), red/brown (jasper), and golden-brown/yellow (tiger's eye).
- Luster
- Varies within the rock: metallic (hematite), dull to vitreous (jasper), silky to chatoyant (tiger's eye).
- Texture
- Fine-grained to microcrystalline, with a fibrous appearance in the tiger's eye bands. Often exhibits a smooth, polished feel when worked.
- Crystal Form
- Not applicable as a rock; individual mineral components have their own crystal forms (e.g., hematite can be massive, platy; quartz is microcrystalline).
- Cleavage
- None for the rock as a whole. Individual minerals may exhibit cleavage (e.g., hematite has no true cleavage but can show parting).
- Geological Environment
- Formed within ancient banded iron formations (BIFs) that have undergone subsequent low-grade regional metamorphism and hydrothermal alteration, leading to the silicification and replacement of original minerals by quartz varieties.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz components), though hematite is 5-6.5.
- Specific Gravity: Approximately 3.3-4.0, varying with the proportion of hematite (SG ~5.3) to quartz (SG ~2.65).
- Crystal System: Not applicable as a rock; individual components are trigonal (quartz) and hexagonal/rhombohedral (hematite).
- Color: Banded dark metallic grey/black, red/brown, and golden-brown/yellow.
- Luster: Mixed: metallic, dull/vitreous, silky/chatoyant.
- Transparency: Opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: None (as a rock).
- Composition: A composite rock primarily composed of hematite (Fe2O3), jasper (SiO2), and tiger's eye (SiO2 with fibrous inclusions).
Quick Check
- Color: Banded dark metallic grey/black, red/brown, and golden-brown/yellow.
- Luster: Mixed: metallic, dull/vitreous, silky/chatoyant.
- Streak: Reddish-brown (from hematite) to white (from quartz components).
Physical Characteristics
- Crystal Habit: Massive, banded (for the rock); individual minerals are microcrystalline (jasper, tiger's eye) or massive/platy (hematite).
- Cleavage Type: None.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Mixed: metallic (hematite), dull to vitreous (jasper), silky to chatoyant (tiger's eye).
Formation
Tiger Iron forms from banded iron formations (BIFs) that have undergone subsequent metamorphism and metasomatism. The original BIFs are sedimentary rocks composed of alternating layers of iron-rich minerals (like hematite and magnetite) and chert (microcrystalline quartz). The formation of Tiger Iron specifically involves the introduction of fibrous quartz (tiger's eye) and microcrystalline quartz (jasper) into these banded structures, often through hydrothermal alteration and silicification processes. The characteristic chatoyancy of the tiger's eye component develops from the parallel alignment of crocidolite fibers (a blue asbestos mineral) that have been replaced by quartz while retaining their fibrous structure, and subsequently oxidized to a golden-brown color.
Usage
Primarily used as an ornamental stone, for lapidary purposes (cabochons, beads, carvings), and in jewelry. It is also collected by mineral and rock enthusiasts.
Age Distribution
Predominantly Precambrian (Archaean to Proterozoic Eons), typically 2.5 to 3.8 billion years old for the banded iron formation component.
Where to Find
Pilbara region, Western Australia
The primary and most significant source of high-quality Tiger Iron, particularly from the Ord Range and other areas rich in ancient banded iron formations.
South Africa
Some occurrences are noted, often associated with the extensive banded iron formations of the Kaapvaal Craton, which also produce significant tiger's eye.
United States
Minor occurrences of similar banded iron formation material, though not typically marketed as 'Tiger Iron' with the same distinct composition.
Finding Tips
Look for Banded Iron Formations
Tiger Iron is a derivative of BIFs. Prospecting in areas known for ancient BIFs, especially those that have experienced metamorphism and hydrothermal activity, increases the chances of finding it.
Identify Component Minerals
Familiarize yourself with the appearance of hematite (metallic luster, dark streak), jasper (opaque, earthy colors, conchoidal fracture), and tiger's eye (silky luster, chatoyancy). Tiger Iron will exhibit all three.
Examine for Chatoyancy
The presence of the golden-brown, fibrous tiger's eye with its characteristic shimmering effect is a key diagnostic feature that distinguishes Tiger Iron from other banded iron formations.
Consider Metamorphic Zones
Tiger Iron's formation involves metamorphic processes. Look in geological settings where ancient sedimentary rocks have undergone regional or contact metamorphism.
Similar Rocks
Banded Iron Formation (BIF)
Banded Iron Formation
Also known as: Ironstone, Taconite
Tiger's Eye
Quartz (variety Tiger's Eye)
Also known as: Crocodilite Asbestos (precursor)
Jasper
Quartz (variety Jasper)
Also known as: Chert (opaque variety)
Hematite
Hematite (Fe2O3)
Also known as: Specularite, Kidney Ore
Scientific Classification
- Mineral Class
- Not a single mineral; components are Oxides (hematite) and Silicates (quartz varieties).
- Group
- Not applicable as a rock.
- Crystal System
- Not applicable as a rock; components are Trigonal (quartz) and Hexagonal/Rhombohedral (hematite).
- Chemical Formula
- Not applicable as a rock; components are Fe2O3 (hematite) and SiO2 (jasper, tiger's eye).
- Composition
- A composite rock of hematite, jasper, and tiger's eye.
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