Re: Weird chemical formulas

You asked:

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?

Let's look at the formula for olivine. It's usually written as:

(Mg, Fe)₂SiO₄

What does this mean? Well, technically, olivine is not a mineral by itself, rather it's rather a group of minerals called a solid-solution series. One of the olivine minerals is fayalite. Fayalite has the formula:

Fe₂ SiO₄

Another olivine mineral is forsterite. Forsterite has the formula:

Mg₂ SiO₄

In between fayalite and forsterite are a series of olivines with varying amounts of iron (Fe) and magnesium (Mg). Some might have 75% Fe and 25% Mg. Others might have 50% Fe and 50% Mg. You get the idea. So, the overall formula for all the olivines is therefore written showing that you can have either Fe or Mg in the mineral.

Now let's look at the formula for the three titanium oxide minerals rutile, brookite, and anatase. Oxides are just minerals which are formed by elements bonding with oxygen (this is distinguished from "rusting" which is a process by which oxygen combines with iron to form rusty-red iron oxide minerals). Anyway, these titanium minerals all have the formula:

TiO₂

They are also considered different minerals. Why is that? Well, that's because these three minerals are polymorphs of each other. They have the same chemical formula but the atoms which make up the crystalline lattice within the mineral are arranged differently in each. If you took brookite, for example, and heated it up to 750 degrees Celsius, the atoms in the crystalline structure of brookite would rearrange themselves into the crystalline form for rutile.

Polymorphs are just different forms of the same mineral which are stable at different temperatures and pressures. They're considered separate minerals, however, since they look different and have different physical properties. Back to the chemical formula for these minerals, different polymorphs are sometimes distinguished by writing greek letters like alpha (a), beta (b), and gamma (g) in front of the formula.

Finally, pyrrhotite has the unusual formula:

Fe_1-xS (x=0 to x=0.2)

This just means that the formula for pyrrhotite varies from:

FeS (when x=0) to Fe_0.8S (when x=0.2)

This just means that the amount of iron in pyrrhotite naturally varies by as much as 20%.

Generally, all the weird formulas you've seen simply reflect the fact that not all minerals can be represented by simple exact formulas. Sometimes the amounts of various elements within minerals varies by a small amount and the formulas have to reflect this fact.

Visit the Mineral Gallery for more information about mineral groups and formulas.