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Lizardite

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Lizardite

Népouite
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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 09.ED.15
Dana classification 71.1.2b.3
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
Crystal system Orthorhombic Pyramidal H-M Symbol mm2
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 USA.

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

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. It forms a solid solution series with népouite.

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

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