LIBETHENITE
Libethenite – Cu2(PO4)(OH) – is a rare phosphate, typically a secondary mineral in Cu occurrences. It is part of Cu ores, but is just one of many sporadically occurring secondary minerals in the deposit. Pieces with idiomorphic crystals fetch high prices, several hundred US dollars, in the collector’s mineral market.
It is classified in the Olivenite Group, in the Libethenite-Olivenite Series. It is isostructural with adamite and eveite and may contain As. There are two varieties (with more H2O, with Zn).
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Light to dark green, green tending towards black, olive green.
Habit: Short prismatic, may be somewhat elongated. Equidimensional, pseudo-octahedral, wedge-shaped terminations.
Crystals up to 3 cm.
Cleavage: {100} poor, {010} poor. Several striations.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, conchoidal.
Mohs Hardness: 4
Parting: No.
Streak: Light green.
Lustre: Subvitreous, resinous, greasy.
Diaphaneity: Transparent.
Density (g/cm³): 3.97
2. Geology and Deposits
Libethenite is one of the many secondary Cu minerals that form in oxidized portions of Cu deposits, which may be porphyritic Cu deposits, Cu sulfides, and others. The phosphorus for its formation often derives from the weathering of rock-forming phosphates such as apatite, monazite, and xenotime.
There is a rare case of primary libethenite in a greisen deposit, but this is an exception.
3. Mineral Associations
It is associated with common gangue minerals such as quartz and with primary Cu minerals such as sulvanite.
It also occurs with other secondary Cu minerals such as malachite, pseudomalachite, azurite, atacamite, chrysocolla, turquoise, cornetite, rashleighite, zapatalite, olivenite, brochantite and chalcosiderite.
Also with secondary minerals such as goethite (limonite), pyromorphite, adamite and halloysite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.701 – 1.704 nβ: 1.743 – 1.747 nγ: 1.787 – 1.790
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Bluish-green to light green, may exhibit weak pleochroism:
X = pale yellowish-blue
Z = pale greenish-blue.
Relief: High.
Cleavage: {100} poor, {010} poor.
Habits: Short, somewhat elongated, equidimensional prismatic structure with wedge-shaped terminations. Pseudo-octahedral crystals are very typical, providing square to rhomboid cross-sections.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence of 0.080, very high, corresponding to colors in the 4th order, generally masked by the intense color of the mineral itself.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: 80 – 90º
Alterations: No information available.
May be confused with: Other secondary copper minerals, ranging in color from green to greenish, include malachite, pseudomalachite, dioptase, antlerite, atacamite, paratacamite, chalcanthite, brochantite, and chrysocolla, to name just the most common green minerals.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying libethenite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with libethenite.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray with a greenish tint.
Pleochroism: No.
Reflectivity: ~12%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not noticed.
Internal reflections: Intense olive green, widespread. The green color tends towards yellow, with citrus tones.
May be confused with: Other associated green secondary copper minerals include malachite, pseudomalachite, dioptase, antlerite, atacamite, paratacamite, chalcanthite, brochantite, and chrysocolla, to name just the most common.