Mackinawite: Properties and Occurrences

Mackinawite is an iron-nickel sulfide mineral with formula (Fe, Ni)1+xS (where x = 0 to 0.11). It is a tetragonal iron-nickel sulfide mineral that occurs as opaque bronze to grey-white tabular crystals and anhedral masses. This corrosion compound is typically found in river sediment reducing environments and is possibly generated by the action of sulfate-reducing microorganisms.

Mackinawite is also known as tetragonal iron sulfide due to its tetragonal cell units. It was first described in 1962 for an occurrence in the Mackinaw mine, Snohomish County, Washington for which it was named.

General Information

  • Category: Sulfide mineral
  • Formula: (Fe,Ni)1+xS (where x = 0 to 0.11)
  • Strunz classification: 2.CC.25
  • Crystal system: Tetragonal
  • Crystal class: Ditetragonal dipyramidal (4/mmm)

Properties

Mackinawite is a type of nickel sulfide iron mineral. The mineral crystallizes in the tetragonal crystal system and has been described as a distorted, close-packed, cubic array of S atoms with some of the gaps filled with Fe. It appears as gray or opaque bronze anhedral masses and crystals. It has a specific gravity equivalent to 4.17 and 2.5 Mohs hardness.

  • Formula mass: 85.42 g/mol
  • Color: Bronze to white grey
  • Crystal habit: As well-formed thin tabular crystals; massive, fine-feathery
  • Cleavage: Perfect on {001}
  • Mohs scale hardness: 2.5
  • Luster: Metallic
  • Streak: Black
  • Diaphaneity: Opaque
  • Specific gravity: 4.17

Occurrence

Mackinawite occurs as opaque bronze to grey-white tabular crystals and anhedral masses. It occurs in serpentinized peridotites as a hydrothermal alteration product, in meteorites, and in association with chalcopyrite, cubanite, pentlandite, pyrrhotite, greigite, maucherite, and troilite. Mackinawite also occurs in reducing environments such as freshwater and marine sediments as a result of the metabolism of iron and sulfate-reducing bacteria.

Laboratories have also produced synthetic mackinawite to study its formation using several different methods such as reacting sulfide with metallic iron or a solution of ferrous iron, growing sulfide reducing bacteria using Fe2+, and electrochemically.

Association: Chalcopyrite, cubanite, pentlandite, pyrrhotite, greigite, maucherite, troilite.

 

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