Lizardite
| Népouite | |
|---|---|
Népouite from Népoui Mine, North Province, New Caledonia, New Caledonia Specimen size 21 cm.
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| General | |
| Category |
Phyllosilicates Kaolinite-serpentine group |
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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) (same H-M symbol) |
| 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 | |
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. These two minerals form a series, that is to say intermediate compositions are possible, with varying proportions of nickel to magnesium.
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.
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.
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