Question about the chemical composition of a particular mineral?

Question

The mineral apophyllite has the chemical formula:
KCa4Si8O20(F,OH)8H2O
My question is concerning the (F,OH) as whenever there is a comma like that in a chem formula there is supposed to be a "solid-solution". yet some mineralogy books say it is not a solid solution as such and in principle represents two distinct minerals: Fluoroapophyllite & Hyroxyapophyllite. So I am confused! ANy help would be appreciated.

Answer 1

It means there may be some fluorine and/or hydroxyl groups associated with it. Similar to what happens in your tooth enamel (calcium apatite) when you do a fluoride treatment, some of the OH groups get replaced by fluorine and the apatite crystals get harder.