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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.

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.

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.

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.

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.
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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.

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.

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.