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[bugrock]

Some of the iron and manganese oxide/hydroxides have chemical
formulas preceded by Greek letters. Have yet to run across an explanation for this notation. Can someone explain?

Goethite: α-Fe3+ O(OH)

Akaganeite: β-Fe3+ O(OH,Cl)

Lepidocrocite: γ-Fe3+ O(OH)

Akhtenskite: ε-Mn4+ O2



Further, many Fe/Mn species with similar formulas lack the prefix. For instance Groutite (Mn3+ O(OH)), which based on the formula would appear to be an analog of Goethite.

Thanks

[alfredo]

The greek letters are related to the crystal structures, not the chemistry - They merely indicate that there are other species with the same formula but different structures. The greek letters are often omitted when writing the chemical formula - they aren't really necessary unless you are talking about two related species and need to differentiate between them.