# Different formulas for copper pyrites and bauxite

In my book in the chapter on General Principles and Processes of Isolation of Elements, I found that the formula for copper pyrites was stated as $$\ce{Cu2S.Fe2S3}$$ at one place, $$\ce{CuFeS2}$$ at another place and $$\ce{Cu2FeS2}$$ at another place.

Are all these taken to be the formula for copper pyrites or are these printing mistakes. I have searched the internet but could not find any useful information regarding this.

Both $$\ce{Cu2S.Fe2S3}$$ and $$\ce{CuFeS2}$$ are the equivalent means to denote chalcopyrite. The first notation, $$\ce{Cu2S.Fe2S3}$$, commonly used a few decades ago, shows that two sulfides are not just a mechanical mix, but form a chemical compound (same as for crystallohydrates, e.g. $$\ce{CuSO4 * 5 H2O}$$). The second one, $$\ce{CuFeS2}$$, is a formula unit, a more universal and modern representation. Chalcopyrite is a mineral of $$\ce{ABX2}$$ type, crystallizes in $$I\bar{4}2d$$ space group.

Figure 1. Unit cell of chalcopyrite $$\ce{CuFeS2}$$. Color code: $$\color{#FFFF30}{\Large\bullet}~\ce{S}$$; $$\color{#E06633}{\Large\bullet}~\ce{Fe}$$; $$\color{#C88033}{\Large\bullet}~\ce{Cu}$$.

On the other hand, $$\ce{Cu2FeS2}$$ is a reduced formula of $$\ce{Cu8Fe4S8}$$, a superstructured bornite [1]. A compound of $$\ce{AB2X2}$$ type, crystallizes in $$F\bar{4}3m$$ space group.

Figure 2. Unit cell of superstructured bornite $$\ce{Cu8Fe4S8}$$. Color code: $$\color{#FFFF30}{\Large\bullet}~\ce{S}$$; $$\color{#E06633}{\Large\bullet}~\ce{Fe}$$; $$\color{#C88033}{\Large\bullet}~\ce{Cu}$$.

Structurally, chalcopyrite and superstructured bornite have very little in common. Unless there is a specific context given, I'd rather say that $$\ce{Cu2FeS2}$$ is an outlier among the three and is probably a typographic issue. Also, it's not a good practice to mix dot-notated formulas with formula unit representations unless one wants to underline some structural aspects (e.g. molecular assemblies/coordination polyhedra/domains etc.)

### References

1. Ding, Y.; Veblen, D. R.; Prewitt, C. T. Possible $$\ce{Fe/Cu}$$ Ordering Schemes in the 2a Superstructure of Bornite ($$\ce{Cu5FeS4}$$). American Mineralogist 2005, 90 (8–9), 1265–1269. https://doi.org/10.2138/am.2005.1518.

I know of five copper iron sulfide minerals:

Bornite - $$\ce{Cu5FeS4}$$

Chalcopyrite - $$\ce{CuFeS2}$$

Cubanite - $$\ce{CuFe2S3}$$

Idaite - $$\ce{Cu5FeS6}$$

Isocubanite - $$\ce{CuFe2S3}$$

• Here is some more: talnakhite $\ce{Cu18Fe16S32}$, valleriite $\ce{Cu2Fe4S7}$, fukuchilite $\ce{Cu3FeS8}$, nukundamite $\ce{Cu_{3.39}Fe_{0.61}S4}$, haycockite $\ce{Cu4Fe5S8}$, mooihoekite $\ce{Cu9Fe9S16}$. – andselisk Jan 26 at 20:05