PLANCHEITE
Plancheite – Cu8(Si8O22)(OH)4.H2O – is an inosilicate belonging to the Amphibole Group. It is a very rare secondary copper mineral and does not constitute an ore.
Macroscopically, it closely resembles shattuckite – which is also a blue inosilicate but belongs to the Pyroxene Group. It also resembles aurichalcite. Dense aggregates exhibit a structure similar to that of chalcedony.
1. Characteristics
Crystal system: Orthorhombic dipyramidal.
Color: Blue to greenish-blue.
Habit: Fibrous, occurring as delicate crystals; forms radial aggregates, as well as separate or compact crystals.
Cleavage: No. Lengthwise striations.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 6
Parting: No.
Streak: Pale blue.
Lustre: Silky to adamantine.
Diaphaneity: Transparent.
Density (g/cm³): 3.65 – 3.8
2. Geology and Deposits
Plancheite is a very rare secondary Cu mineral that occurs in oxidized zones of Cu deposits.
3. Mineral Associations
It occurs with common gangue minerals such as quartz, barite, and carbonates (calcite).
It occurs alongside many other secondary Cu minerals, such as malachite, azurite, cuprite, tenorite, dioptase, chrysocolla, shattuckite, conichalcite, bisbeeite, and spangolite.
It also occurs with secondary Zn minerals (willemite) and secondary Pb minerals (wulfenite and mimetite).
It is associated with other very rare minerals, such as heterogenite and surite.
4. Transmitted Light Microscopy
Data on the characteristics of the plancheite under transmitted light are scarce.
Refraction indices: nα = 1.645, nβ = 1.660 e nγ = 1.715
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Blue, pleochroism between blue and pale blue.
Relief: Very high.
Cleavage: No.
Habits: Fibrous, may form radial aggregates.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: The maximum birefringence is 0.044, corresponding to intense interference colors of up to the fourth order, but its strong own color masks these colors.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: No.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 88.5°
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying plancheite. However, preparing a polished section is important for identifying the opaque minerals associated with plancheite, such as native copper.
Sample preparation: The specimen does not take a polish due to its acicular/fibrous habit. Since its typical color and characteristic habit are easily recognized under crossed nicols, this lack of polish is not a problem.
PLANE POLARIZED LIGHT – PPL
Reflection color: Very dark, almost black.
Pleochroism: No.
Reflectivity: Almost zero.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not perceived.
Internal reflections: Widespread, in sky-blue colors, revealing the habit of plancheite as radial aggregates.
May be confused with: other secondary blue copper minerals.
