TURQUOISE
Turquoise – CuAl6(PO4)4(OH)8 · 4H2O – is a very rare phosphate, typical of recent orogenic regions, where it is mined as a valuable gemstone and as an ornamental stone. Imitations are very common in the market for gemstones and collector minerals.
It is classified in the Turquoise Group, being isostructural with chalcosiderite and planetite. It is the copper analogue of faustite. The dominant color is blue, but upon dehydration or by replacing Al with Fe the colors turn greenish to green. It may also contain Ca. There are two varieties.
Under long-wave ultraviolet light, turquoise occasionally fluoresces green, yellow, or light blue. There is no fluorescence under shortwave UV light or X-rays.
There is very little information about the characteristics of turquoise under the microscope, as it is one of those minerals that are simply not covered in mineralogy textbooks and other sources. A very good compilation of general data is available on Wikipedia.
1. Characteristics
Crystal system: Triclinic pinacoidal.
Color: Light blue, sky blue, pale green, blue-green, turquoise blue, apple green, gray green.
Habit: Usually massive. Crust, globular, fine granular, vein filling. Crystals very rare.
Cleavage: {001} perfect, {010} good, rarely discernible.
Tenacity: Brittle.
Twinning: No.
Fracture: Smooth to sub-conchoidal.
Mohs Hardness: 5 – 6
Parting: No.
Streak: Pale blue-green to white.
Lustre: Sub-vitreous, resinous, matte, earthy.
Diaphaneity: Transparent.
Density (g/cm³): 2.6 – 2.8
2. Geology and Deposits
Turquoise is typically a secondary mineral, formed at temperatures between 90 and 195ºC and occurring mainly in the potassium alteration zone of hydrothermal porphyry copper deposits.
It is generated by the action of meteoric waters, more easily in arid regions, on aluminous, igneous or sedimentary rocks. In these occurrences it is found as vein filling in igneous (granite), metamorphic (quartzite) and sedimentary (limestone, sandstone) rocks, as well as in phosphate sediments.
It often forms pseudomorphs on orthoclase, apatite, bones and teeth. “Odontolite” is a term assigned to fossil bones or ivory replaced by turquoise and other phosphates such as vivianite.
Turquoise is commonly intergrown with other secondary minerals of Cu, SiO2 and H2O in the mineral called chrysocolla.
3. Mineral Associations
It associates with minerals characteristic of supergenic alteration, such as chalcedony, clay minerals (kaolinite, montmorillonite), allophane, alunite and limonite.
It evidently occurs with other phosphates, such as wavellite, senegalite, variscite, crandallite, libethenite, fluorapatite and planerite. Also with pyrite, quartz, varlamoffite and olivenite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.610 nβ: 1.615 nγ: 1.650
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Weak pleochroism between colorless and pale blue or pale green.
Relief: Moderate.
Cleavage: {001} perfect, {010} good, rarely discernible as a function of grain size, which is usually between submicroscopic and cryptocrystalline.
Habits: Usually massive. Crust, globular, fine granular, vein filling. Very rare crystals.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.040, corresponding to strong, vivid colors, up to the end of 2nd order.
Extinction: Probably oblique, but difficult to detect due to the small grain size.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 40º
Alterations: no information available.
May be confused with: other blue-colored secondary Cu minerals.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of turquoise. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with turquoise.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics: