Plancheite

Plancheite is a hydrated copper silicate mineral with the formula Cu8Si8O22(OH)4•(H2O). It is closely related to shattuckite in structure and appearance, and the two minerals are often confused.

Plancheite
Plancheite and Malachite, Milpillas mine, Sonora, Mexico
General
CategoryInosilicate
Formula
(repeating unit)		Cu8Si8O22(OH)4•(H2O)
Strunz classification9.DB.35 (10 ed)
8/F.05-10 (8 ed)
Dana classification66.2.1.1
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupOrthorhombic
Space group: Pcnb
Identification
Formula mass1,171 g/mol
ColorPale blue, light blue, dark green
Crystal habitAcicular, Fibrous, or Radial
CleavageNone observed
Mohs scale hardness5.5 to 6
LusterVitreous to silky
StreakLight blue
DiaphaneityTranslucent to opaque
Specific gravity3.6 to 3.8
Optical propertiesBiaxial (+)
Refractive indexNx = 1.645, Ny = 1.660 Nz = 1.715[1][2][3]
Nx = 1.697, Ny = 1.718, Nz = 1.741[4][5]
PleochroismX colorless or pale blue, Y colorless or blue, Z blue or dark blue
Other characteristicsNonfluorescent, nonmagnetic, not radioactive
References[3][2][1][4]

Structure

Plancheite is a chain silicate (inosilicate), with double chains of silica tetrahedra parallel to the c crystal axis.[5] It occurs as sprays of acicular or fibrous radial clusters, with fibers extended parallel to the chains, i.e. along the c crystal axis;[4] it can also form tiny tabular or platy crystals. It is a member of the orthorhombic crystal class m m m (2/m 2/m 2/m), which is the most symmetrical class in the orthorhombic system.

Properties

Usually a pale turquoise-blue, with a pale blue streak and an adamantine to silky luster. It is quite hard, with hardness 5.5 to 6, close to that of feldspar, and specific gravity 3.6 to 3.8. Optically it is biaxial (+), with refractive indices between 1.64 and 1.74, and pleochroic.[1][2][3][4]

Environment

Plancheite is a secondary mineral formed in the oxidized zone of copper deposits, associated with other copper minerals chrysocolla, dioptase, malachite, conichalcite and tenorite.[4] It occurs with primary malachite at the Milpillas Mine in Mexico.[6] The type locality is the Sanda Mine, Mindouli, Pool Region, Republic of Congo.[1]

Plancheite, Kolwezi, Katanga (Shaba), Democratic Republic of Congo. 5.9 x 3.2 x 2.6 cm.

References
  1. Mindat
  2. Webmineral
  3. Gaines et al (1997) Dana's New Mineralaogy, Wiley
  4. Handbook of Mineralogy
  5. Evans and Mrose (1977) American Mineralogist 62:491-502
  6. Moore and Origlieri (2008) Mineralogical Record 39-6:25-34

JMol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?id=00578


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