WAVELLITA
Wavellite – Al3(PO4)2(OH,F)3.5H2O – is a relatively rare phosphate, of little importance as an ore. Aesthetic pieces fetch high prices, several hundred dollars each, in the collector’s mineral market.
It is classified in the Wavellite Group and constitutes the OH analogue of fluorine wavellite. It may contain Fe and F. There is a variety that occurs as a gel (“gelfischerite”).
There is very little information available about the characteristics of wavellite under a microscope, as it is yet another mineral that is simply not covered in mineralogy textbooks or other sources.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Wavellite can occur in many different colors. It is most commonly found in green to yellowish-green hues, but can also be yellow, greenish-white, yellowish-brown, brown, blackish-brown, blue, white, and colorless. It can also be zoned.
Habit: A very typical feature is the habit of long, almost acicular, prismatic crystals, forming radial aggregates in perfect spheres (spherulites), up to 3 cm in diameter. Massive, stalagmite-like, crusty. Rare crystals.
Cleavage: {110} perfect., {101} good, {010} distinct. Striations // to [001].
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: White.
Lustre: Vitreous to resinous.
Diaphaneity: Transparent.
Density (g/cm³): 2.36
2. Geology and Deposits
Wavellite is a secondary mineral typical of low-grade aluminous metamorphic rocks, where it is located in fractures and cavities. It occurs in high-P content Fe ores (banded iron formations), where it hinders ore beneficiation, whose maximum P content should be <0.05%. Wavellite is much more difficult to remove from Fe ore than apatite, which is the most common phosphate.
Wavellite is especially common in deposits that have undergone supergene enrichment, such as limonitic and phosphate deposits, where it probably formed from apatite.
It is found in porphyry copper deposits, associated with other phosphates.
It rarely occurs as a late mineral in hydrothermal veins.
3. Mineral Associations
It occurs associated with common minerals such as quartz, limonite (goethite), and pyrite.
It also occurs with other phosphates, such as apatite, crandallite, varsicite, senegalite, turquoise, vauxite, paravauxite, metavauxite, cacoxenite, sigloite, strengite, rockbridgeite, frondelite, gorceixite, chalcosiderite, and barrandite. More rarely with carminite (Pb-Fe arsenate).
In Fe ore (banded iron formations) with other phosphates (such as those above), hematite, magnetite, goethite, quartz, and kaolinite.
In porphyry copper deposits with Cu minerals (chalcopyrite, chalcocite, covellite, malachite, azurite, cuprite, native copper), common sulfides of this paragenesis (pyrite, molybdenite, sphalerite, galena), other phosphates (apatite, turquoise, metatorbernite, libethenite), wulfenite, quartz and sericite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.518 – 1.535 nβ: 1.524 – 1.543 nγ: 1.544 – 1.561
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless, it may exhibit weak pleochroism ranging from greenish to yellowish.
Relief: Low.
Cleavage: {110} perfect, {101} good, {010} distinct.
Habits: Long prismatic, typically radial, forming perfect spheres. Massive, stalactitic, crusty. Rare crystals.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.026, corresponding to colors up to the middle of the second order: strong, intense, very vibrant colors in orange, red, and blue.
Extinction: Probably parallel in longitudinal sections.
Elongation sign: No information available.
Twins: No.
Zoning: Possible, perhaps only in hand specimen.
CONVERGENT LIGHT
Character: B(+)
2V angle: 60 – 70º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying wavellite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with wavellite, such as pyrite, hematite, and magnetite.
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: