The composition of nanoparticulate nickel sulfide

The initial nickel sulfide precipitated in normal Earth surface aqueous environments is nanoparticulate hydrated NiS with the general formula Ni1.1S·1.5H2O. The water content was determined by thermogravimetric mass spectroscopic analysis, and the chemical composition was determined by inductively coupled plasma-optical emission spectrometer (ICP-OES), back titration of CuCl2, ion chromatography (IC) and energy dispersive X-ray (EDX) spectrometry. Together with the structural characterization of the material, the results suggest that the NiS nanoparticles may be modelled as 4 nm spheres with a 1 nm anhydrous crystalline NiS core and a hydrated disordered outer shell layer containing 24 ± 3 H2O molecules per nm3. Thermogravimetric results suggest that the water is an integral part of the shell layer configuration analogous to the intralayer H2O of some clays. The results suggest that this phase determines the aqueous solubility of nickel in low temperature sulfidic systems. It also provides a novel route for the nucleation and growth of nickel sulfides in aqueous solutions.

Research Highlights
► This paper is one of a series of papers which characterise the chemical and structural properties of the NiS that is precipitated by the reaction between dissolved Ni(II) salts and S(-II) at ambient temperatures. One paper on the structure of nanoparticulate NiS is published in Inorganic Chemistry (2009). This paper, which includes some detailed analytical techniques, is submitted to Chemical Geology because Chemical Geology notes on its inside cover that the journals welcomes papers describing analytical advances of wide general interest.
► The problem of the chemical analysis of NiS has been a long-standing problem in geochemistry and we show what the causes of the problems are and how they may be overcome. We also demonstrate for the first time that NiS is hydrated and not nonstoichiometric NiS or an NiS hydroxide as shown in the textbooks.
► The report also contributes to further understanding of the chemistries of simple binary transition metal sulfides, the first of which (on FeS) was published in Chem Geol. in 2006. Although these simple compounds are familiar to most geochemists and figure in many discussions, total analyses have been lacking in modern times. This is partly due to difficulties in synthesising reproducible nanoparticulate materials and partly due to the analytical difficulties. In this report we report protocols for overcoming both these problems with respect to NiS.
► The results have wide-ranging implications in current geochemistry. The composition of NiS and its analysis are fundamental to understanding the global biogeochemical cycle of Ni as well as its availability to key biochemical systems involved in the origin and early evolution of life.