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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 distinctive color, ranging from sky-blue to apple-green, is highly prized. Often, it contains dark matrix inclusions, typically limonite or other iron oxides, which can create attractive patterns known as 'spiderweb' turquoise.
How to Identify
- Color
- Ranges from sky-blue to greenish-blue, apple-green, and yellowish-green. The most valued color is an intense, uniform sky-blue.
- Luster
- Waxy to subvitreous, dull in massive forms.
- Texture
- Typically massive, cryptocrystalline, or reniform (kidney-shaped). Can be found as veinlets or crusts. Often has a fine-grained, porcelain-like texture.
- Crystal Form
- Rarely forms distinct crystals; when it does, they are small, triclinic prisms. Usually found as cryptocrystalline aggregates, nodules, or encrustations.
- Cleavage
- Perfect on {001} and good on {010}, but rarely observed due to its cryptocrystalline nature. Fracture is typically conchoidal to uneven.
- Geological Environment
- Found in arid regions, typically in weathered zones of aluminum-rich igneous or sedimentary rocks that contain secondary copper mineralization. It forms in veins, nodules, and concretions within limonite, sandstone, or altered volcanic rocks.
Key Facts
- Hardness: 5-6 (Mohs scale)
- Specific Gravity: 2.60-2.90
- Crystal System: Triclinic
- Color: Blue, greenish-blue, green
- Luster: Waxy, subvitreous, dull
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect on {001}, good on {010} (rarely observed)
- Composition: Hydrous copper aluminum phosphate
Quick Check
- Color: Sky-blue to greenish-blue, apple-green
- Luster: Waxy to subvitreous, dull
- Streak: White to pale bluish-green
Physical Characteristics
- Crystal Habit: Cryptocrystalline masses, nodules, reniform, botryoidal, stalactitic, veinlets, encrustations. Rarely as small, tabular triclinic crystals.
- Cleavage Type: Perfect on {001}, good on {010}. Due to its cryptocrystalline nature, cleavage is seldom seen in hand specimens.
- Fracture Type: Conchoidal to uneven, splintery.
- Tenacity: Brittle
- Luster Type: Waxy to subvitreous, dull in massive forms.
Formation
Turquoise is a secondary mineral formed by the percolation of acidic, copper-rich groundwater through aluminum-rich rocks. It precipitates in fractures, cavities, and porous zones, often replacing feldspar, apatite, or other minerals. The presence of copper provides the blue color, while iron impurities can lead to green hues. It forms at relatively low temperatures and pressures.
Usage
Primarily used as a gemstone for jewelry and ornamental objects. Historically, it has been used for decorative purposes, amulets, and inlays by various cultures worldwide. Lower quality material can be used for carvings or as a pigment.
Age Distribution
Formed in arid regions, typically in secondary enrichment zones of copper deposits, often associated with Cenozoic volcanic rocks, but can be found in older formations where conditions are met.
Where to Find
Southwestern United States (Arizona, Nevada, New Mexico, Colorado)
Historically significant and currently major producers, particularly Arizona (e.g., Sleeping Beauty, Kingman mines) and Nevada (e.g., Lander Blue, Lone Mountain, Carico Lake mines). Occurs in porphyry copper deposits and associated altered rocks.
Iran (Persia)
Nishapur district has been a source of high-quality turquoise for millennia, often considered the benchmark for fine blue color. Occurs in altered trachyte and other volcanic rocks.
China (Hubei province)
A significant producer, with material often exhibiting a range of blue to green colors and matrix patterns. Found in altered sedimentary and metamorphic rocks.
Egypt (Sinai Peninsula)
Ancient mines, particularly Serabit el-Khadim and Wadi Maghareh, were among the earliest known sources of turquoise, dating back thousands of years. Occurs in sandstone.
Mexico
Various deposits, particularly in Sonora, producing material often with a greenish hue or distinct matrix.
Afghanistan
Known for producing good quality turquoise, often with a distinct blue color.
Finding Tips
Look for Arid Environments
Turquoise forms in dry, desert-like conditions where copper-rich waters can percolate through host rocks. Focus exploration in regions with a history of copper mineralization.
Identify Host Rocks
Search in areas with altered igneous rocks (e.g., porphyries, volcanic tuffs), sandstones, or shales that are rich in aluminum and show signs of copper staining (malachite, azurite).
Examine Fractures and Veins
Turquoise typically forms as secondary infillings in fractures, seams, and cavities within the host rock. Look for blue or green streaks or nodules.
Check for Associated Minerals
Turquoise is often found with limonite, kaolinite, chalcedony, and other secondary copper minerals like malachite and azurite. These can be indicators of its presence.
Use a Hardness Test (Carefully)
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 or harder minerals.
Similar Rocks
Chrysocolla
Chrysocolla
Also known as: Gem Silica (when pure)
Variscite
Variscite
Also known as: Utahlite
Howlite
Howlite
Also known as: White Turquoise (when dyed)
Magnesite
Magnesite
Also known as: White Turquoise (when dyed)
Scientific Classification
- Mineral Class
- Phosphates
- Group
- Turquoise Group
- Crystal System
- Triclinic
- Chemical Formula
- CuAl6(PO4)4(OH)8·4H2O
- Composition
- Copper aluminum phosphate hydroxide hydrate
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