taenite

▪ mineral

      nickel-iron mineral having a face-centred cubic structure and playing a major role in the crystallization and structure of iron meteorites and stony iron meteorites. It is sometimes referred to as γ iron, after one of the three temperature-dependent forms (allotropes) of pure iron, because the taenite is stabilized in the same face-centred cubic structure as γ iron. In the system of nickel-iron metal solutions, taenite is the only stable mineral at temperatures above 900 °C (1,650 °F). Below 900 °C the meteoritic mineral kamacite, whose nickel content is less than 7 percent by weight, separates from taenite. If the bulk composition of the nickel-iron system contains less than about 7 percent nickel by weight and the system maintains equilibrium down to low temperatures, all the taenite transforms to kamacite. If the system contains between 7 and 40 percent nickel (as is the case for all but one iron or stony iron meteorite), taenite remains stable down to a temperature of about 400 °C (750 °F), although its nickel content increases with falling temperature as the low-nickel kamacite separates from it. The separation of kamacite from taenite, aided by small amounts of phosphorus, is the process that produces the Widmanstätten pattern in iron and stony iron meteorites. Continued separation of kamacite below 400 °C results in taenite with a nickel content of as much as 52 percent by weight. At such high nickel content and temperatures below 300 °C (570 °F), the distribution of iron and nickel in the crystal structure can be highly ordered, in which case the mineral is called tetrataenite. Almost all taenite in meteorites has broken down, albeit often on a microscopic scale, to kamacite and tetrataenite.

Conel M.O'D. Alexander

 

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