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Turquoise is an opaque, blue-to-green hydrous phosphate of copper and aluminum, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is a relatively soft mineral, typically found in cryptocrystalline masses, nodules, or veinlets. Its color is highly variable, ranging from sky-blue to greenish-blue to apple-green, influenced by the presence of copper (blue) and iron (green). It often exhibits a spiderweb-like matrix of host rock, which can be black, brown, or yellowish-brown.
How to Identify
- Color
- Sky-blue, robin's egg blue, greenish-blue, apple-green. The most prized color is an intense, uniform sky-blue.
- Luster
- Waxy to subvitreous, dull in massive forms.
- Texture
- Typically cryptocrystalline, massive, reniform, botryoidal, or stalactitic. Individual crystals are rare and microscopic.
- Crystal Form
- Usually massive, nodular, or as vein fillings. Microscopic triclinic crystals are extremely rare.
- Cleavage
- None observed in massive forms due to its cryptocrystalline nature. Microscopic crystals show perfect {001} cleavage.
- Geological Environment
- Found in arid regions, typically in highly weathered, altered volcanic or sedimentary rocks (e.g., porphyries, sandstones, shales) that contain copper, aluminum, and phosphorus. Often associated with limonite, kaolinite, chalcedony, and quartz.
Key Facts
- Hardness: 5-6 (Mohs scale)
- Specific Gravity: 2.60-2.90 g/cm³
- Crystal System: Triclinic
- Color: Sky-blue, robin's egg blue, greenish-blue, apple-green
- Luster: Waxy to subvitreous, dull
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect {001} (rarely observed due to cryptocrystalline nature)
- Composition: Hydrous copper aluminum phosphate
Quick Check
- Color: Blue to green
- Luster: Waxy to dull
- Streak: White to pale bluish-green
Physical Characteristics
- Crystal Habit: Cryptocrystalline, massive, reniform, botryoidal, stalactitic, vein fillings, crusts. Individual crystals are microscopic and extremely rare.
- Cleavage Type: Perfect on {001} (only visible in rare, microscopic crystals). Typically appears to have no cleavage in hand specimens.
- Fracture Type: Conchoidal to uneven, splintery.
- Tenacity: Brittle
- Luster Type: Waxy to subvitreous, dull
Formation
Turquoise is a secondary mineral formed by the percolation of acidic, copper-rich groundwater through aluminous rocks. It precipitates in fractures, veins, and cavities in highly weathered volcanic rocks, often in association with limonite, kaolinite, and other secondary minerals. The copper is typically derived from primary copper sulfides, the aluminum from feldspars, and the phosphorus from apatite or other phosphate minerals. The presence of iron oxides often gives it a brownish or black matrix.
Usage
Primarily used as a gemstone for jewelry and ornamental objects due to its attractive blue-green color. Historically, it has been used for carvings, inlays, and decorative items by various cultures worldwide. It has also been used as a pigment in some ancient cultures, though this is less common today.
Age Distribution
Formed in arid regions, typically associated with secondary mineralization in weathered igneous or sedimentary rocks, often Cenozoic in age, but can be found in older deposits.
Where to Find
Southwestern United States (Arizona, Nevada, New Mexico, Colorado)
Historically significant and currently productive regions, particularly Arizona (Sleeping Beauty, Kingman mines) and Nevada (Lander Blue, Lone Mountain, Carico Lake mines). These deposits are typically found in porphyry copper deposits or associated with weathered granitic rocks.
Iran (Persia)
Nishapur region, known for producing some of the finest 'Persian blue' turquoise, often found in weathered trachyte and porphyry.
China (Hubei province)
Significant producer of turquoise, often with a greener hue, found in altered sedimentary rocks.
Egypt (Sinai Peninsula)
Ancient mines at Serabit el-Khadim and Wadi Maghareh, where turquoise occurs in sandstone.
Mexico
Various deposits, often associated with copper mineralization.
Finding Tips
Geological Setting
Look for turquoise in arid or semi-arid regions, particularly in areas with past or present copper mining activity. It forms in the oxidized zones of copper deposits.
Host Rocks
Search in highly altered and weathered igneous rocks (e.g., porphyries, rhyolites) or sedimentary rocks (e.g., sandstones, shales) that are rich in aluminum and phosphorus.
Associated Minerals
Turquoise is often found with limonite (iron oxides), kaolinite, chalcedony, quartz, and other secondary copper minerals.
Veins and Nodules
It typically occurs as vein fillings, crusts, or irregular nodules within the host rock. Look for blue-green streaks or masses in rock fractures.
Hardness Test
Turquoise has a Mohs hardness of 5-6. It can be scratched by a steel file but will scratch glass. This helps differentiate it from softer imitations.
Similar Rocks
Chrysocolla
CuSiO3·nH2O
Also known as: Gem Silica
Variscite
AlPO4·2H2O
Also known as: Utahlite
Howlite
Ca2B5SiO9(OH)5
Also known as: White Turquoise (imitation)
Magnesite
MgCO3
Also known as: White Turquoise (imitation)
Scientific Classification
- Mineral Class
- Phosphate mineral
- Group
- Turquoise group
- Crystal System
- Triclinic
- Chemical Formula
- CuAl6(PO4)4(OH)8·4H2O
- Composition
- Copper aluminum phosphate hydroxide hydrate
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