What does a comma signify in inorganic chemistry?
Quoting [1]:


  Comma. In accord with general usage, the comma "," should be used to
  indicate continuous substitution, partial as well as complete, if
  there is no change of CN. Vacancies (symbol $\square$) are treated here just
  like atoms. This usage is restricted to chemical formulas for atoms on
  a specific site. The comma is not appropriate for structure site
  formulas because different symbols indicate a change in the
  coordination number. Examples for proper use of the comma include
  forsterite-fayalite $\ce{[(Mg{,}Fe)2(SiO4)]}$ and hornblende $\ce{\{(Na{,}\square)Ca2(Mg{,}Fe{,}Al)_5[(Si{,}Al)4O11]2(OH)2\}}$. In structure type formulas, the
  comma is used to mean "or" where substitution may occur. An example is
  in the zeolites where H2O and M units may both occur.


Comma representation are often used in writing chemical compositions of minerals as the elements and its concentration vary to a certain extent. See minerals.net:


  A number of minerals contain a varying amount of two or more elements like Aurichalcite, which has a chemical formula of
  $\ce{(Zn{,}Cu)5(CO3)2(OH)6}$ contains an unspecific varying amount of
  zinc (Zn) and copper (Cu). This is indicated by comma separating the
  Zn from the Cu. If a chemical formula with two elements in
  parenthesis is separated by a comma, the number of those elements
  vary. Aurichalcite has a variable amount of zinc of copper where the
  combination of both these elements totals five. The more dominant
  element is usually listed first. (Emphasis added.)


For more information, see this previous Chemistry.SE question and see here to find steps regarding how to write such type of formulae using chemical markups.

References:


Smith, D. K.; Roberts, A. C.; Bayliss, P.; Liebau, F. A systematic approach to general and structure-type formulas for minerals and other inorganic phases. Am. Mineral. 1998, 83 (1-2), 126–132. DOI: 10.2138/am-1998-1-212. Online version: http://database.iem.ac.ru/mincryst/fst.html