earth science

Mackinawite

Mackinawite is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mackinawite rather than just read about it. In short: Mackinawite ( MAK-i-naw-eyet) is an iron sulfide mineral, which can contain nickel substituting for iron(II), with as chemical formula (Fe,Ni)1+xS (where x = 0 to 0.11). As indicated by its chemical composition, Fe(1+×), Fe is present with an excess of × over S, and it is thus an iron-rich, or a sulfur-deficient, iron monosulfide mineral.

Key takeaways

Mackinawite belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mackinawite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mackinawite from memory before moving on to harder problems.

Reference excerpt

Mackinawite ( MAK-i-naw-eyet) is an iron sulfide mineral, which can contain nickel substituting for iron(II), with as chemical formula (Fe,Ni)1+xS (where x = 0 to 0.11). As indicated by its chemical composition, Fe(1+×), Fe is present with an excess of × over S, and it is thus an iron-rich, or a sulfur-deficient, iron monosulfide 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. Mackinawite occurs as opaque bronze to grey-white tabular crystals and anhedral masses. It has a Mohs hardness of 2.5 and a specific gravity of 4.17. It was first described in 1962 for an occurrence in the Mackinaw mine, Snohomish County, Washington for which it was named.

Occurrence Mackinawite 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. In anoxic environments, mackinawite is formed by the reaction of HS− with either Fe2+ ions or with Fe metal. Mackinawite is a metastable mineral that occurs predominantly as a poorly crystalline precipitate. After the initiation of precipitation, mackinawite can take up to 2 years to form at 25 °C. It has been reported that mackinawite can be stable for up to 16 weeks at temperatures up to 100 °C at pH values from 3–12. 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.

Transformations in the environment Depending on the redox conditions mackinawite can form more stable phases such as greigite and ultimately pyrite, an important mineral in anoxic aqueous settings that is preserved in sedimentary deposits, especially black shale. While it has been determined that mackinawite is a necessary precursor to pyrite, the pathway of iron sulfide mineral formation from aqueous species to solid mineral is still nebulous. Many iron sulfide minerals may exist in the transition between poorly ordered mackinawite and crystalline pyrite such as greigite, smithite, and pyrrhotite; however, studies have also indicated that pyrite formation from mackinawite can occur where oxidation has commenced and the sulfur present is in intermediate oxidation states (−1 to +6) and intermediate sulfur species such as elemental sulfur or polysulfides and surface oxidized monosulfide species, such as oxidized mackinawite or greigite are present.

Illustrations

Worked examples

Example 1 — a first encounter with Mackinawite

Start with the simplest possible case. Write down what Mackinawite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mackinawite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mackinawite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mackinawite

In research: Mackinawite appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mackinawite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mackinawite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Iron(II) minerals, Meteorite minerals, Minerals in space group 129, so understanding it makes those chapters shorter.
In everyday life: Look for Mackinawite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mackinawite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mackinawite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mackinawite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mackinawite in simple terms?

Why does Mackinawite matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mackinawite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mackinawite.