Népouite

Népouite
Népouite from Népoui Mine, North Province, New Caledonia, New Caledonia Specimen size 21 cm.
General
Category	Phyllosilicates Kaolinite-serpentine group
Formula (repeating unit)	Ni3(Si2O5)(OH)4
Strunz classification	9.ED.15
Dana classification	71.1.2b.3
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Identification
Colour	bright green (typical of nickel bearing silicates) to yellowish or brownish green, depending on nickel content
Crystal habit	generally massive, also fibrous and microscopic pseudohexagonal platy crystals
Cleavage	perfect on {001}
Mohs scale hardness	2 to 2½
Luster	earthy to waxy, also pearly
Streak	greenish white
Diaphaneity	semitranslucent
Specific gravity	3.18 to 3.24 (measured)
Optical properties	Biaxial (-)
Refractive index	nα = 1.600 - 1.630 nγ = 1.635 - 1.650
Birefringence	0.035
Pleochroism	Weak. X = green to yellow green Z = yellow-green
References	[1][2][3]

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, Northern 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 in which all the nickel is replaced by magnesium, formula Mg₃(Si₂O₅)(OH)₄.^[4] These two minerals form a series, that is to say intermediate compositions are possible, with varying proportions of nickel to magnesium.^[5]

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 United Nations building 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