I began thinking about the issue of defining new mineral species about 25 years ago, and soon realised that a straightforward way to approach this complicated issue is (1) to consider all possible end-member arrangements of integer charges over the sites in a crystal structure, subject to the constraint of electroneutrality, and (2) then consider all possible arrangements of ions over the sites in that crystal structure for each of those end-member charge arrangements. The result is a list of all possible end-member formulae with that general crystal-structure arrangement (Hawthorne, 2002), and I gave various examples in that paper....

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