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Blue Tiger's Eye, also known as Hawk's Eye or Falcon's Eye, is a captivating variety of Tiger's Eye characterized by its striking blue to bluish-gray color and prominent chatoyancy (a silky, luminous streak resembling a cat's eye). This optical effect is caused by the parallel intergrowth of fibrous crocidolite (a blue asbestos mineral) that has been replaced by quartz. It represents an earlier stage in the formation of classic golden-brown Tiger's Eye, where the iron in the crocidolite has not yet fully oxidized.
How to Identify
- Color
- Ranges from dark blue to bluish-gray, sometimes with hints of green or brown. The blue color is often streaky or banded.
- Luster
- Silky to vitreous, exhibiting a strong chatoyancy when polished, appearing as a moving band of light.
- Texture
- Fine-grained, fibrous structure visible under magnification, often appearing as parallel fibers.
- Crystal Form
- Massive, fibrous aggregates, pseudomorphing the original crocidolite fibers. Does not form distinct euhedral crystals.
- Cleavage
- None (as quartz), but the fibrous structure of the original crocidolite can create a pseudo-cleavage or parting along the fiber direction.
- Geological Environment
- Typically found in highly metamorphosed iron-rich sedimentary rocks, particularly banded iron formations, where crocidolite has formed and subsequently been replaced by quartz.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz).
- Specific Gravity: 2.64-2.71 g/cm³.
- Crystal System: Trigonal (for quartz), but the original crocidolite was monoclinic.
- Color: Blue, bluish-gray, sometimes with greenish or brownish tints.
- Luster: Silky to vitreous, chatoyant.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal to splintery (due to fibrous structure).
- Cleavage: None (quartz), but pseudo-cleavage along fiber direction.
- Composition: SiO2 (quartz) with inclusions of fibrous crocidolite (Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2).
Quick Check
- Color: Blue to bluish-gray, often with chatoyant bands.
- Luster: Silky to vitreous, chatoyant.
- Streak: White (due to quartz).
Physical Characteristics
- Crystal Habit: Massive, fibrous aggregates, pseudomorphing original crocidolite.
- Cleavage Type: None (quartz), but parting along fiber direction.
- Fracture Type: Conchoidal to splintery.
- Tenacity: Brittle.
- Luster Type: Silky to vitreous.
Formation
Blue Tiger's Eye forms when quartz pseudomorphically replaces crocidolite, a fibrous blue amphibole mineral. The original crocidolite fibers are replaced by silica (quartz) while retaining their fibrous structure. The blue color is due to the unoxidized or partially oxidized crocidolite fibers. Over time, if exposed to oxidizing conditions, the iron in the crocidolite oxidizes, leading to the golden-brown color of classic Tiger's Eye.
Usage
Primarily used as an ornamental stone, for cabochons, beads, carvings, and other lapidary applications due to its attractive chatoyancy. Historically, it was also used for protective amulets.
Age Distribution
Typically found in Precambrian to Paleozoic age metamorphic rocks, often associated with banded iron formations.
Where to Find
Northern Cape Province, South Africa
The primary and most significant source of high-quality Blue Tiger's Eye, particularly in the Griqualand West region, associated with the Asbestos Hills and Kuruman Iron Formation.
Western Australia, Australia
Deposits are found in the Hamersley Basin, often associated with banded iron formations, similar to the South African occurrences.
Brazil
Some occurrences have been reported, though generally less significant than South African deposits.
Myanmar (Burma)
Minor occurrences have been noted.
Finding Tips
Look for Metamorphic Terrains
Focus on regions known for ancient metamorphic rocks, especially those with banded iron formations or areas historically mined for asbestos (crocidolite).
Identify Chatoyancy
The most distinctive feature is the chatoyancy. Look for a silky, moving band of light when rotating the specimen under a light source. This is best observed on polished or tumbled pieces.
Observe Color and Fibrous Structure
The characteristic blue to bluish-gray color, combined with a visible fibrous texture (even if replaced by quartz), is a strong indicator. Unpolished specimens may appear duller but will still show the fibrous nature.
Check for Associated Minerals
Blue Tiger's Eye often occurs with other metamorphic minerals and iron oxides. Its presence can indicate proximity to the source rock.
Safety Precautions
While the crocidolite in Blue Tiger's Eye is encapsulated in quartz and generally considered safe in its polished form, raw or unpolished specimens, especially those with loose fibers, should be handled with caution. Crocidolite is a form of asbestos. Avoid cutting, grinding, or sanding raw material without proper respiratory protection (P100 respirator) and ventilation to prevent inhalation of potentially harmful fibers. Wet grinding is recommended if processing raw material. Always wash hands after handling raw specimens.
Similar Rocks
Tiger's Eye
Oxidized Crocidolite in Quartz
Also known as: Golden Tiger's Eye
Pietersite
Brecciated Crocidolite and Riebeckite in Chalcedony
Also known as: Tempest Stone
Serpentine
Mg3Si2O5(OH)4
Also known as: Bowenite, Antigorite
Scientific Classification
- Mineral Class
- Silicate (specifically Tectosilicate for quartz, Amphibole for crocidolite)
- Group
- Quartz group (as a pseudomorph), Amphibole group (for original crocidolite)
- Crystal System
- Trigonal (for quartz component)
- Chemical Formula
- SiO2 (with inclusions of Na2(Fe2+,Mg)3Fe3+2Si8O22(OH)2)
- Composition
- Silicon dioxide with fibrous inclusions of sodium iron magnesium silicate hydroxide.
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