Nickel sulfide
Nickel sulfide is an inorganic compound with the formula NiS. It is a black solid that is produced by treating nickel(II) salts with hydrogen sulfide. Many nickel sulfides are known, including the mineral millerite, which also has the formula NiS. Aside from being useful ores, nickel sulfides are the products of desulfurization reactions, and are sometimes used as catalysts. Nonstoichiometric forms of nickel sulfide are known, e.g., NiS and NiS. Nickel can also combine with more than one sulfur atom per nickel atom, as in the mineral Vaesite (NiS). Structure. Like many related materials, nickel sulfide adopts the  nickel arsenide motif. In this structure, nickel is octahedral and the sulfide centers are in trigonal prismatic sites. Nickel sulfide has two polymorphs. The α-phase has a hexagonal unit cell, while the β-phase has a rhombohedral cell. The α-phase is stable at temperatures above , and converts into the β-phase at lower temperatures. That phase transition causes an increase in volume by 2-4%. Synthesis. The precipitation of solid black nickel sulfide is a mainstay of traditional qualitative inorganic analysis schemes, which begins with the separation of metals on the basis of the solubility of their sulfides . Such reactions are written: Many other more controlled methods have been developed, including solid state metathesis reactions (from NiCl and NaS) and high temperature reactions of the elements. Occurrence. Natural. The mineral millerite is also a nickel sulfide with the molecular formula NiS, although its structure differs from synthetic stoichiometric NiS due to the conditions under which it forms. It occurs naturally in low temperature hydrothermal systems, in cavities of carbonate rocks, and as a byproduct of other nickel minerals. In glass manufacturing. Float glass contains a small amount of nickel sulfide, formed from the sulfur in the fining agent  and the nickel contained in metallic alloy contaminants. Nickel sulfide inclusions are a problem for tempered glass applications. After the tempering process, nickel sulfide inclusions are in the metastable alpha phase. The inclusions eventually convert to the beta phase (stable at low temperature), increasing in volume and causing cracks in the glass. In the middle of tempered glass, the material is under tension, which causes the cracks to propagate and leads to spontaneous glass fracture. That spontaneous fracture occurs years or decades after glass manufacturing.