Potassium Zinc Sulfate
Formula: K2Zn(SO4)2ยท6H2O
Potassium twin of ammonium zinc sulfate, visually indistinguishable from it. I suspect that precise measurement could reveal some subtle differences in angles between faces, but for the naked eye these compounds look the same. Similarly, baking soda can be used to improve crystal quality (see "Growing" section).
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