# If a chemical formula for a mineral contains a comma, are there limits on the amounts of each element?

I have a chemical formula for the mineral actinolite which is: $$\rm{Ca}_2({Mg,Fe})_5{Si}_8{O}_{22}({OH})_2$$

The formula contains a "," which I understand means that the magnesium and the iron can have different combinations (up to 5 total).

The question is, if magnesium is the dominant atom, is it possible for the amount of magnesium to be less than iron and still be actinolite, or must magnesium always have the greatest amount?

Actinolite is a mineral that is part of a so-called solid-solution series, which means that you get different but related minerals depending on the amounts of (in this case) iron and magnesium that are present. The magnesium-rich end member is tremolite; the iron-rich end member is ferro-actinolite; and what lies between is actinolite.

According to Wikipedia's article on actinolite, the formula is more precisely: $$\rm Ca_{2}(Mg_{4.5-2.5}Fe^{2+}_{0.5-2.5})Si_{8}O_{22}(OH)_{2}$$

In the case where iron is the dominant atom, the mineral is called ferro-actinolite with the formula: $$\rm Ca_{2}(Mg_{2.5-0.0}Fe^{2+}_{2.5-5.0})Si_{8}O_{22}(OH)_{2}$$
• That answers the specific issue with actinolite, but does not address the more general notational issue. If you have chemical formula that is not more precisely specified, is there any difference between ...$\ce{(Mg,Fe)5}$... and ...$\ce{(Fe,Mg)5}$...? - that is, does the order of elements in the parenthesis tell you anything about the relative amounts of the element in the compound? (The formula given for ferro-actinolite indicates "no", but it would be nice to have it explicit.)