Occasionally I get questions from readers by email, although I haven't published or commented on these bloggishly, so far. Sometimes the questions are interesting: I suspected one, for example, of asking a particular question in order to try to flush out my home location - although I was not sure of that. Usually, I go ahead and answer questions - if possible, and if the questions aren't creepy in any way - and usually I receive nice thank-you emails back. Usually.
One thing that came up recently: a particular Googled question led a reader to this blog. My blog post that Google brought up didn't, however, answer the reader's question; as the particular post was more general than specific.
THE QUESTION: What element occurs in quartz, olivine, and calcite?
Now, really, this isn't a trick question at all, but quartz and olivine are silicates, and therefore have the element silicon in them (Si), but calcite is a carbonate, and has zero Si in it.
So, what do they have in common? Or, maybe they have nothing, elementally speaking, in common.
To figure this one out, all we have to do is write out the chemical formulas for each mineral: the mineral quartz, the mineral olivine, and the mineral calcite. I'll have to look one of them up, because it's been a while since I've practiced mineral formulas with homemade flash cards.
Olivine: Mg2SiO4 to Fe2SiO4, with olivine actually being a series of minerals between (and including) the two end-member mineral species forsterite (the magnesium end-member) and fayalite (the iron end-member) - in other words, what is called a solid-solution series in chemistry and mineralogy. Because the olivine group minerals form a solid solution series between the Mg and Fe end members, the formula can also be written: (Mg,Fe)2SiO4.
Well, so far what do we have. We have the elements Si (silicon) and O (oxygen) in quartz, and we have the elements Mg (magnesium), Fe (iron), Si (silicon), and O (oxygen) in olivine. Of our three minerals in question, then, we potentially have two elements that they might have in common: silicon and oxygen. Now for our third mineral.
Ha! Calcite has the elements Ca (calcium), C (carbon), and O (oxygen).
That means the answer has to be - - - - - Oxygen! The only element present in all three minerals! Mystery solved.
For more information on minerals and many, many more pictures than the pictures linked to above at Andrew's About.com:Geology, go to one of my favorite mineral sites: Mindat. Here are the Mindat links for quartz, olivine, and calcite.