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Turquoise is an opaque, blue-to-green hydrous phosphate mineral of copper and aluminum, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is a secondary mineral, meaning it forms through the alteration of pre-existing minerals. Its characteristic color, ranging from sky-blue to greenish-blue and apple-green, is due to the presence of copper (blue) and iron (green). It often exhibits a waxy to subvitreous luster and can be found in massive, botryoidal, or reniform habits, rarely forming distinct crystals. Turquoise is relatively soft, with a Mohs hardness of 5-6, making it suitable for carving and polishing. It is often found associated with limonite, quartz, and other secondary copper minerals.
How to Identify
- Color
- Sky-blue, robin's egg blue, greenish-blue, apple-green. The blue color is due to copper, while green hues are often caused by iron impurities or dehydration.
- Luster
- Waxy to subvitreous, sometimes dull or earthy.
- Texture
- Typically massive, cryptocrystalline, or microcrystalline. Can be botryoidal, reniform, or stalactitic. Often found as veinlets, nodules, or crusts.
- Crystal Form
- Rarely forms distinct crystals; when it does, they are typically small, tabular to prismatic triclinic crystals. More commonly found as cryptocrystalline aggregates.
- Cleavage
- Perfect on {001} and good on {010}, but rarely observed due to its cryptocrystalline nature. Fracture is typically conchoidal to uneven.
- Geological Environment
- Forms in arid and semi-arid regions as a secondary mineral in the oxidized zones of copper deposits. It is found in fractures and cavities within highly altered igneous, metamorphic, or sedimentary rocks, particularly those rich in aluminum and phosphate. Common host rocks include limonite, sandstone, shale, and altered volcanic tuffs.
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, sometimes dull or earthy
- Transparency: Opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect on {001} and good on {010}, but rarely observed due to cryptocrystalline nature
- Composition: Hydrated copper aluminum phosphate
Quick Check
- Color: Sky-blue to greenish-blue
- Luster: Waxy to subvitreous
- Streak: White to pale bluish-green
Physical Characteristics
- Crystal Habit: Typically cryptocrystalline to microcrystalline massive aggregates; botryoidal, reniform, stalactitic, encrusting, or as veinlets and nodules. Rarely as small, tabular to prismatic triclinic crystals.
- Cleavage Type: Perfect on {001} and good on {010}, but usually not visible in massive material.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Waxy to subvitreous, sometimes dull or earthy.
Formation
Turquoise forms as a secondary mineral in arid and semi-arid regions. It precipitates from circulating groundwater that has percolated through copper-rich igneous rocks (e.g., porphyry copper deposits) and aluminum-rich sedimentary or volcanic rocks (e.g., shales, tuffs, or highly altered granites). The presence of phosphate is often derived from the alteration of apatite or other phosphate-bearing minerals in the host rocks. The process involves the oxidation of copper sulfides, releasing copper ions, which then react with aluminum and phosphate in acidic solutions. As these solutions move through fractures and cavities, the pH increases, leading to the precipitation of turquoise. It typically forms in veins, nodules, and crusts within limonite, sandstone, or other altered host rocks.
Usage
Turquoise has been highly prized as a gemstone and ornamental stone for thousands of years. It is used extensively in jewelry, carvings, and decorative objects. Historically, it was used by ancient Egyptians, Persians, and Native American cultures for its aesthetic appeal and perceived spiritual properties. In modern times, it remains a popular choice for cabochons, beads, and inlay work. Lower-grade material may be used for decorative aggregates or as a pigment, though this is less common now.
Age Distribution
Turquoise deposits are typically found in arid regions and are associated with secondary mineralization processes, often forming in relatively young geological settings (Cenozoic to Mesozoic) where hydrothermal alteration has occurred.
Where to Find
Southwestern United States (Arizona, Nevada, New Mexico, Colorado, California)
Historically significant and currently active mining regions, particularly Arizona (e.g., Sleeping Beauty, Kingman, Morenci mines) and Nevada (e.g., Lander Blue, Lone Mountain, Carico Lake mines). These deposits are typically associated with porphyry copper mineralization.
Iran (Persia)
Nishapur region, known for producing some of the finest 'Persian Blue' turquoise for millennia. These deposits are found in altered trachyte and andesite.
Egypt (Sinai Peninsula)
Ancient mines at Serabit el-Khadim and Wadi Maghara, exploited for over 6,000 years. Deposits occur in sandstone.
China (Hubei province)
Significant producer of turquoise, often with a greener hue. Deposits are found in altered sedimentary rocks.
Mexico (Sonora)
Various deposits, often associated with copper mineralization.
Chile
Deposits found in association with large copper porphyry systems.
Australia (Victoria, Queensland)
Smaller deposits, often with a greener color.
Finding Tips
Look for Copper Deposits
Turquoise is a secondary copper mineral, so search in areas known for copper mineralization, especially in arid or semi-arid environments. Look for signs of oxidized copper minerals like malachite (green) and azurite (blue).
Examine Altered Rocks
Focus on areas with highly altered igneous, metamorphic, or sedimentary rocks, particularly those with evidence of hydrothermal activity. Turquoise often forms in veins, seams, and nodules within limonite, sandstone, or shale.
Check Fractures and Cavities
Turquoise typically precipitates in open spaces. Inspect fractures, fissures, and vugs within the host rock for blue to green mineral coatings or fillings.
Observe Color and Luster
The distinctive sky-blue to greenish-blue color and waxy to subvitreous luster are key indicators. Be aware that other minerals can have similar colors, so further tests may be needed.
Test Hardness
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 or harder look-alikes.
Consider Associated Minerals
Turquoise is often found with limonite (iron oxides), quartz, chalcedony, kaolinite, and other secondary copper minerals like chrysocolla, malachite, and azurite.
Similar Rocks
Chrysocolla
CuSiO3·nH2O (hydrated copper silicate)
Also known as: Gem Silica (when pure blue)
Variscite
AlPO4·2H2O (hydrated aluminum phosphate)
Also known as: Utahlite
Howlite
Ca2B5SiO9(OH)5 (calcium borosilicate hydroxide)
Also known as: White Turquoise (when dyed)
Magnesite
MgCO3 (magnesium carbonate)
Also known as: White Turquoise (when dyed)
Odontolite
Fossilized bone or ivory colored by vivianite or other phosphate minerals
Also known as: Bone Turquoise
Scientific Classification
- Mineral Class
- Phosphate mineral
- Group
- Turquoise group
- Crystal System
- Triclinic
- Chemical Formula
- CuAl6(PO4)4(OH)8·4H2O
- Composition
- Copper (Cu), Aluminum (Al), Phosphorus (P), Oxygen (O), Hydrogen (H). Specifically, it is a hydrous phosphate of copper and aluminum.
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