Subject: Weird chemical formulas

I have a mineralogy book that uses many weird ways to write formulas. Sometimes 
they write a superscript number in front of a symbol; Sometimes they enclose  a 
part of a chemical's elements in parenthes "( )" and put commas between them; 
For olivine, they used 
the formula "Mg5SiO4-Fe2SiO4", where I don't fully understand the dash. I think 
it would mean that both parts of the formula can be correct at various times, 
but commas in parenthes could mean the same thing; There are three titanium 
oxite brands given (and could you explain how titanium got oxidised, which 
means rusted?), and the difference between them was shown in the form "(greek 
letter)-TiO2";
And finally, some minerals have a forrmula of, say, CoAs(subscript 2-3)
(scutterudite) or even Fe( subscript 1-x)S (pyrhotite). What do those x-y mean? 
Could you tell me anything else you know about chemical formulas?

Re: Weird chemical formulas