Zeolite

It's been a while since I did something really sciency.  So, what is a zeolite, and why do they seem to appear in every other scientific article I've been reading lately?


Zeolites, according to a popular dictionary, are "any of various hydrous silicates that are analogous in composition to the feldspars, occur as secondary minerals in cavities of lavas, and can act as ion-exchangers; also : any of various natural or synthesized silicates of similar structure used especially in water softening and as adsorbents and catalysts."


That, as usual, is entirely accurate, but not necessarily very helpful unless you're already a geochemist.  Let's break it down.  


A hydrous silicate is a chemical compound, generally a mineral, that contains water (hydro-) and a silicate group, a combination of silicon and oxygen.  Note that "contains water" doesn't mean that it's wet-- the water in a hydrous compound is locked up chemically, and won't come out unless something unusual happens to the compound.  Feldspar is a classic example of such a mineral, being mostly aluminum silicate (with some traces of other stuff), and it's dirt common, so it's a good reference point as such things go.  A "cavity of lava" is fairly straightforward-- as lava rises to the surface of the earth and cools, it often leaves pockets or cavities behind that contain useful minerals.  And finally, an ion-exchanger is a material that swaps ions-- extra unbound electrons that allow one chemical to react with another.  This is a useful chemical process, which is why I'm seeing it in scientific articles, but it's also a useful industrial process-- for example, in the further mentioned softening of water, in which sodium ions from the softener are swapped out with calcium ions in the water, that would otherwise form scale on your pipes.  The calcium is instead bound into the zeolitic material, and the sodium just washes away.  Jumping ahead a bit, moving ions around can also cause certain chemical reactions to happen more readily, which is what a catalyst does, so that solves that bit.


Finally, adsorption is sort of like absorption, only instead of pulling something into the interior of a substance (as a sponge absorbs water), something becomes adhered to the outside-- an adsorbent will coat itself with a thin layer of something, holding on to it tightly.  This property means that many synthetic cat litters contain zeolite crystals--it's especially good at grabbing onto ammonia.