01 September, 2010

SIWOTI Syndrome: Gabbro Edition

Lockwood's scavenging me some lovely gabbro trim (and can possibly scavenge you some, too, if you ask nicely and pay your own shipping).  Gabbro is something I've wanted to get my hands on for some time now.  Y'see, my kitchen is white.  White cabinets, white counters, white appliances, so much white it looks like a ski slope plunked down in the middle of my house.  In the immortal (heh) words of Gina on Highlander: "I'm sick of white!"  I've slightly remedied the situation with a few brown marble tiles, but what I've really had my eye on is some gorgeous gabbro countertop tiles at Home Depot.  Yes, yes, they say they're black granite, but we all know better.

For various reasons, I've never found the opportunity to buy them.  Now, thanks to Lockwood, I don't have to.  O joyous day!

I decided to do a little piece on gabbro in honor of that occasion, but have been sidetracked by this howler:
When magma gets trapped in a single layer underneath the earth's crust, it will gradually cool and form gabbro. Now if the same material were to be pushed up from below under great pressure, it would form granite. But then again, if that magma were to come in contact with water it would turn instantly into basalt. Are you still following me here? Gabbro, granite and basalt are essentially the same material in different forms. In order to form granite, magma needs a good hard squeeze so its component materials can separate and form large crystals. If magma cools very quickly upon contact with air or water, it will form basalt. And if magma is left alone for a couple million years it will form gabbro.

Oh, Paul.  Paul, Paul, Paul.  Go back to designing kitchens, and attempt geology no more.

He did get this much right: "black granite" is actually gabbro.  And he's probably right about the finishes and so forth - I defer to his expertise there, since apartment living has never afforded me the opportunity to install decent countertops.  Closest I've been able to come is the current adventure in gluing bits of felt to the undersides of small tiles and laying them on top of the counter, and of course that's just Not Done on surfaces where one wields sharp knives, as things may skid and lead to tragedy.  Besides, no grout = maclargehuge mess as various juices and so forth trickle between the tiles.  But I digress.  We were about to set the poor man straight on the difference between gabbro and granite (and, for that matter, basalt and rhyolite).

The difference, y'see, is rather simple.  Gabbro is basalt that cooled slowly deep underground.  Granite is basically rhyolite that did the same.  Contrary to Paul (who should fire his geology source), cooling by water ain't got nothing to do with it. It's all in the chemical composition, baby, yeah!

Let's have a look at gabbro first.  According to my handy-dandy Smithsonian Rock and Gem guide, what we have here is a rock composed of calcium plagioclase feldspar and pyroxene, with a bit of olivine and magnetite mixed in.  Gabbro is low in silica, quartz is practically non-existent (duh, right?), and it's also low in water.  It forms from the same magma that gives us basalt, those nice quiet flowy eruptions that create our sea floors and our Hawaiis.  It's not common on the Earth's surface, although it's not exactly rare, either.

Now, granite.  When we look at the minerals, we know we're already in much different territory: it's composed mostly of potassium-feldspar, quartz and mica, with bits of sodium-plagioclase and hornblende.  Totally different animal, there, innit?  Quartz, of course, is our old friend SiO2, which means granite is very rich in silica.  Granite forms the bulk of the continents, and as far as intrusive igneous rocks go, if you stub your toe on one, you can guess with a pretty good probability of being right that you've just hit granite or one of its close cousins.  It's common as dirt.  In many cases, it is dirt - after plenty o' weathering, o' course.  And when it erupts, we are talking rhyolite, which because of its high gas content and sticky silica, makes a great big boom.  You can stand on a volcano and watch the basalt erupt.  You would not want to attempt the same with a volcano erupting rhyolite.  It would be painful.

One other thing you'll notice in the better class o' rock and mineral guide: granite is listed as felsic, while gabbro is listed as mafic.  Felsic basically means it's pale, light, and high in silica.  Mafic means it's dark, heavy, and rich in magnesium and iron.  There's more to it than that, o' course, but that's close enough for laymen's work.

And my advice to folks who install kitchens: consult a geologist, or at the very least Wikipedia, before you venture an opinion on the geologic origins of the rocks you work with.

Photo sources: gabbro came from here, which has a very easy-to-follow guide to yer basic igneous rocks.  Granite came from here.  For SIWOTI Syndrome, see here.

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