If you ever wanted to inform yourself about mineral structures, composition or properties, you can always search on Google (or the search engine of your choice); this will surely lead you to the Wikipedia page on that mineral. As good as the online encyclopedia is, I’d recommend also checking out the two links you’ll have below it: mindat.org (http://www.mindat.org) and the Mineralogy Database (http://www.webmineral.org).
These two webpages offer a detailed description of each and every mineral there is, including X-Ray diffraction data, crystalline structure, chemical properties, and good bibliographical references if you want to dig up the history of said mineral. So, for example, if we wanted to know more about pyrite (fool’s gold) we could search it, and click on the first three links we come across: Wikipedia, Mindat and Webmineral.
Each offers good value in its own: I feel like Wikipedia is very good at giving a general overlook on the mineral, its distribution and uses, while Mindat and Webmineral are very good at giving detailed information and references. The two latter choices also have a nifty search engine where you can specify elements of the periodic table for searches on chemical composition of minerals.
Another awesome feature of the alternatives to Wikipedia are the 3D models detailing crystalline structure or atom distribution. You’ll need Java installed, but they are powerful tools, as they let you drag the models to rotate the view.
All in all, you’ll end up using these three among many other databases, but these three are the bread and butter of the mineralogist’s day. Go check them out! Seeing your favourite mineral in glorious 3D is something to look forward to.