Turquoise
Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has been prized as a gemstone for millennia due to its hue.
| Turquoise | |
|---|---|
| General | |
| Category | Phosphate minerals |
| Formula (repeating unit) | CuAl6(PO4)4(OH)8·4H2O |
| IMA symbol | Tqu |
| Strunz classification | 8.DD.15 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H–M symbol) |
| Identification | |
| Colour | Turquoise, blue, blue-green, green |
| Crystal habit | Massive, nodular |
| Cleavage | Perfect on {001}, good on {010}, but cleavage rarely seen |
| Fracture | Conchoidal |
| Mohs scale hardness | 5–6 |
| Luster | Waxy to subvitreous |
| Streak | Bluish white |
| Diaphaneity | Opaque |
| Specific gravity | 2.6–2.9 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.610 nβ = 1.615 nγ = 1.650 |
| Birefringence | +0.040 |
| Pleochroism | Weak |
| Fusibility | Fusible in heated HCl |
| Solubility | Soluble in HCl |
| References | |
Like most other opaque gems, turquoise has been devalued by the introduction of treatments, imitations, and synthetics into the market. The robin egg blue or sky blue color of the Persian turquoise mined near the modern city of Nishapur, Iran, has been used as a guiding reference for evaluating turquoise quality.
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