Népouite

Népouite is a rare nickel silicate mineral which has the apple green colour typical of such compounds. It was named by E Glasser in 1907 after the place where it was first described (the type locality), the Népoui Mine, Népoui, Nouméa Commune, North Province, New Caledonia. The ideal formula is Ni3(Si2O5)(OH)4, but most specimens contain some magnesium, and (Ni,Mg)3(Si2O5)(OH)4 is more realistic. There is a similar mineral called lizardite (named after the Lizard Complex in Cornwall, England) in which all of the nickel is replaced by magnesium, formula Mg3(Si2O5)(OH)4.[4] These two minerals form a series; intermediate compositions are possible, with varying proportions of nickel to magnesium.[5]

Népouite
Népouite from Népoui Mine, North Province, New Caledonia Specimen size 21 cm.
General
CategoryPhyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)		Ni3(Si2O5)(OH)4
Strunz classification9.ED.15
Dana classification71.1.2b.3
Crystal systemOrthorhombic
Crystal classPyramidal (mm2)
(same H-M symbol)
Space groupCcm21 (no. 36)
Identification
Colourbright green (typical of nickel bearing silicates) to yellowish or brownish green, depending on nickel content
Crystal habitgenerally massive, also fibrous and microscopic pseudohexagonal platy crystals
Cleavageperfect on {001}
Mohs scale hardness2 to ​2 1⁄2
Lusterearthy to waxy, also pearly
Streakgreenish white
Diaphaneitysemitranslucent
Specific gravity3.18 to 3.24 (measured)
Optical propertiesBiaxial (-)
Refractive indexnα = 1.600 - 1.630 nγ = 1.635 - 1.650
Birefringence0.035
PleochroismWeak. X = green to yellow green Z = yellow-green
References[1][2][3]

Pecoraite is another rare mineral with the same chemical formula as népouite, but a different structure; such minerals are said to be dimorphs of each other, in the same way as graphite is a dimorph of diamond. Népouite, lizardite and pecoraite are all members of the kaolinite-serpentine group.[6]

Garnierite is a green nickel ore that formed as a result of weathering of ultramafic rocks, and that occurs in many nickel deposits worldwide. It is a mixture of various nickel and magnesium phyllosilicates (sheet silicates), including népouite. Associated minerals include calcite, chlorite, goethite, halloysite, nontronite, pimelite, quartz, sepiolite, serpentine, talc and willemseite.

As well as the type locality in New Caledonia, it has been found in Australia, Austria, the Czech Republic, the Democratic Republic of Congo, Germany, Greece, Italy, Japan, Morocco,[7] Poland, Russia, South Africa and the US.

Structure

Space Group Ccm21. Unit Cell: a = 5.31 Å, b = 9.19 Å, c = 14.50 Å

X-Ray Powder Diffraction
d spacing 7.31 4.55 3.63 2.89 2.50 2.31 2.20 1.53
relative intensity 10 5 9 6 7 4 4 6

Lizardite
Polished serpentine, from the Oberlystal, Val d'Aosta, sold as Gressoney. Used at the headquarters of the United Nations in New York.

Lizardite, which is much more common, forms a solid solution series with népouite. Extremely fine-grained, scaly lizardite (also called orthoantigorite) comprises much of the serpentine present in serpentine marbles. It is triclinic, has one direction of perfect cleavage, and may be white, yellow or green. Lizardite is translucent, soft (hardness 2.5) and has an average specific gravity of 2.57. It can be pseudomorphous after enstatite, olivine or pyroxene, in which case the name bastite is sometimes applied. Bastite may have a silky lustre.

Lizardite is named after its type locality on the Lizard Peninsula, Cornwall, UK.[8] It is worked by local artisans into various trinkets which are sold to tourists.

References
  1. http://rruff.geo.arizona.edu/doclib/hom/nepouite.pdf Handbook of Mineralogy
  2. http://www.mindat.org/min-2882.html Mindat.org
  3. http://www.webmineral.com/data/Nepouite.shtml Webmineral data
  4. http://www.mindat.org/min-8771.html
  5. American Mineralogist (1975): 60: 863-871
  6. Dana’s New Mineralogy, Eighth Edition, 1997, Gaines et al., Wiley.
  7. Mineralogical Record 38-5, page 384
  8. Lizardite: Lizardite mineral information and data
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