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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?

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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: $$\rm Ca_{2}(Mg_{4.5-2.5}Fe^{2+}_{0.5-2.5})Si_{8}O_{22}(OH)_{2}$$

So, for your question:

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?

the answer is no: at best, Mg and Fe can be present in equal amounts for the mineral to still be called actinolite, but note that in that particular case you are on the "border" between actinolite and ferro-actinolite, so perhaps what you choose to call it is up to you. And as mentioned above, if magnesium is dominant, the mineral is tremolite.

In the case where iron is the dominant atom, the mineral is called ferro-actinolite with the formula: $$\rm Ca_{2}(Mg_{2.5-0.0}Fe^{2+}_{2.5-5.0})Si_{8}O_{22}(OH)_{2}$$

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    $\begingroup$ 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.) $\endgroup$
    – R.M.
    Commented Jul 13, 2015 at 15:43
  • $\begingroup$ The order of the elements does not tell you anything about the relative amounts of each, as you correctly indicate. The more specific ranges on stoichiometric ratios - indicated above by subscripts and typically seen for minerals in solid-solution series - gives the more specific description. $\endgroup$
    – Todd Minehardt
    Commented Jul 13, 2015 at 16:02
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    $\begingroup$ If the order of the elements does not tell you anything about the relative composition, then are they listed by increasing atomic number? $\endgroup$
    – Ben Norris
    Commented Jul 13, 2015 at 23:37
  • $\begingroup$ Honestly, I don't know - there are enough historical artifacts in re: naming and conventions that it could be the case. There is a reference on Wikipedia that states that in one database (mindat.org), the dominant cation goes first in the parentheses. BUT the International Mineralogical Society is the IUPAC equivalent for minerals since 1959, and if you look at their 2015 master list of mineral names, you'll see the formulae for the 3 minerals here with Mg first and Fe second. $\endgroup$
    – Todd Minehardt
    Commented Jul 14, 2015 at 0:17

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