thumb|Garnierite
Garnierite is a general name for a green nickel ore which is found in pockets and veins within weathered and serpentinized ultramafic rocks. It forms through lateritic weathering of ultramafic rocks and occurs in many nickel laterite deposits worldwide. It is an important nickel ore, having a large weight percent NiO. As garnierite is not a valid mineral name according to the Commission on New Minerals, Nomenclature and Classification (CNMNC), no definite composition or formula has been universally adopted. Some of the proposed compositions are all hydrous Ni-Mg silicates, a general name for the Ni-Mg hydrosilicates which usually occur as an intimate mixture and commonly includes two or more of the following minerals: serpentine, talc, sepiolite, smectite, or chlorite, and Ni-Mg silicates, with or without alumina, that have X-ray diffraction patterns typical of serpentine, talc, sepiolite, chlorite, vermiculite or some mixture of them all.
Composition
Various studies have examined the composition of garnierite. In 1964, a study was done on the composition of a talc-like garnierite and found the composition to be close to the compositions of stevensite and sepiolite, but with partial replacement of the Mg content by Ni. In 1973, another study found that chemical analysis of garnierite samples yields non-stoichiometric formulae that can be reduced to formulas like those of talc and serpentine. The authors suggested a talc monohydrate formula of for the talc-like garnierite. In 2011, the most recent study performed used Extended X-ray Absorption Fine Structure (EXAFS) analysis to determine the composition of their garnierite samples. It found that garnierite has an almost complete solid solution between Ni-sepiolite and falcondoite, with samples analyzed showing between 3 and 77 percent falcondoite composition.
Structure
Garnierite is generally a fine grained mineral with poor crystalline structure. The fact that Ni readily substitutes for Mg in garnierite explains why as NiO content goes up, MgO content goes down. The nickel in garnierite is not evenly distributed throughout the structure, but is concentrated in small zones of nickel surrounded by magnesium zones. The main difference between the serpentine-like and talc-like variants of garnierite is the spacing between layers in the structure, seen in X-ray powder diffraction studies. The serpentine-like variants have 7 Å basal spacings while the talc-like variants have a basal spacing of 10 Å. Some species of garnierite stick to the tongue and dissolve readily in water or even on the tongue. X-ray diffraction of samples from that mine show that garnierite veins include sepiolite-falcondoite and quartz (chrysoprase, a green variety of quartz with a nickel content of less than 2 weight %).