September 2, 2011
Mystery Rock #3
Posted by Evelyn Mervine
Jennifer & Jeff's beautiful granite countertop.
Back in May I posted about a Mystery Rock and then a Mystery Rock #2. Pictures of these two mystery rocks were sent in by blog readers. I had a great time thinking about these two mystery rocks and helping with their identification, and there was some great discussion among geologists and non-geologists in the comments.
For awhile I’ve been meaning to post Mystery Rock #3. A couple of months ago, my good friends Jennifer & Jeff, who live in Chicago, sent me some pictures of their kitchen countertop. The mystery of this rock actually isn’t the identification of the rock. The countertop is granite, probably a true granite. I have a little bit of trouble estimating the mineral proportions from the pictures (of polished rock! the horror… all the distinctive cleavage is gone!), but the countertop is definitely a true granite or a close granitoid relative.
For those of you who are not familiar with the classification of granites and closely-related rocks, igneous rocks with less than 90% mafic minerals (such as olivine and pyroxene) are classified using the QAPF Diagram. The letters in the acronym stand for Quartz, Alkali Feldspar, Plagioclase, and Feldspathoid, which are minerals or groups of minerals. For igneous rocks with large crystals that can easily be seen in hand sample (these are called plutonic rocks), the plutonic QAPF diagram is used to determine the rock name:
Plutonic QAPF Diagram. Image downloaded from Wikipedia Commons. The original diagram is from Igneous Rocks: A Classification and Glossary of Terms, 2nd Edition; 2002; R.W. Le Maitre editor; Cambridge University Press.
Granites and granitoids plot in the top triangle on the diamond-shaped diagram. A true granite contains 20-60% quartz and about equal proportions of plagioclase and K-feldspar. If a granitoid rock contains more plagioclase than K-feldspar, then it is called a “monzogranite.” Similarly, if a granite contains more K-feldspar than plagioclase, then it is called a “syenogranite.” Granitoid rocks that contain more than 65% plagioclase are technically “granodiorites” while granitoids that contain more than 90% K-feldspar are technically “Alkali feldspar granites.” In addition to quartz, plagioclase, and K-feldspar, granites also usually contain mica (biotite or muscovite or sometimes both) and hornblende. Apatite, zircon, titanite, and magnetite are commonly present in small amounts.
If you want to learn more about the use of classification diagrams for igneous rocks, here’s a great website.
Here’s a picture of Jennifer & Jeff’s countertop, with some minerals labeled:
Jennifer & Jeff's granite countertop with some minerals labeled.
An aside before I go on to the mystery: many kitchen countertops are actually not granite. For example, countertops are often made of rocks such as granodiorite, quartzite, rhyolite, travertine, marble, soapstone, or gneiss. Many countertops are manmade– they are made of pieces of rocks, often quartz-rich, that are put together in a manner designed to be pretty and also nonporous. As a geologist, I sometimes run into trouble when I visit someone’s house and compliment the rock countertops. Fortunately, many of my friends are fellow geologists or scientists (or are poor students who can’t afford nice countertops), but sometimes the conversation goes something like this:
Me: Wow! Those are some beautiful kitchen countertops you have.
Host: Thanks! I’ve always wanted granite countertops. When we built this house, we decided we just had to have them, even though it was quite expensive.
Me: Granite? These countertops aren’t granite. But they’re gneiss.
Host, slightly taken aback: What do you mean they aren’t granite? They must be granite.
Me: Oh, they’re not granite. But they are gneiss, which is…
Host: Nice? Just nice? Henry, did you hear this? She says our countertops aren’t real granite!
Host’s Husband, Henry: Well, of course they’re granite. I bloody well paid enough for them.
Me: Please, let me explain. I mean the rock type gneiss, which I actually think can be prettier for countertops than granite.
Henry: Ha ha! Oh, I’ve heard of gneiss. You geologists have gneiss schist, don’t you know?
….awkward digression into geology puns…
Me, desperately attempting to change the conversation: And you have such beautiful cabinets, too! I love the cherry color.
Host: Yes, but you scientists are probably going to tell me they’re not real wood…
I bet many a geologist has had a variation of the above conversation. Anyway, Jennifer & Jeff do have a real granite countertop– quite a pretty one as well!
Here’s a few more pictures of the countertop:
Countertop viewed from above.
Another picture of the countertop from above.
Countertop viewed from below.
And here’s a picture illustrating the rock mystery:
A small magnet is attracted to the edge of the countertop!
Jennifer & Jeff noticed that a small magnet was attracted to the edge of their countertop– but only to the dark portions of the rock. The dark portions are likely hornblende and/or biotite (it’s a little difficult for me to tell from the photographs of the polished surface). These two minerals are generally not magnetic, so my guess is that these dark minerals contain inclusions of a mangetic minerals such as magnetite, which is commonly found in granites in trace amounts.
Do any other geologists have insight into this countertop mystery? Does anyone else have a magnetic granite countertop?
Any chance that that could be a meteorite inclusion in the granite?
I told Jen that we’d need to whack a fresh piece off of the counter for proper ID, but she stopped me from getting the hammer.
I discovered today that my 10cm square black polished ‘granite’ drink mats attract magnets, so there’s no need to dissect your counter tops! :o)
Love it when I tell someone their counters are gneiss or gabbro and they insist that it’s granite. Because that’s what they paid for at the counter store and that’s what the salesperson told them it was, damnit. No, no, I’m just the geologist!
OK, one of you fine geologists will come with me when I DO get around to ordering a stone countertop.
Just so I know WHAT the stone really is. 🙂
Then, I can be correct when I identify the counter to guests.
I went with “Engineered Stone” (a better term for the geologically challenged) being somewhat lazy- lower maintenance than real granite and generally more resistant to staining. But trust me- every bit as expensive as the real thing….unless I’d gone down the road and “liberated” a granite slab from a local quarry.
Good choice. Knowing MY luck, I’d go that route, only to find later that I got engineered stone that was made in China out of melamine dust and depleted uranium… 😉
Seriously though, I’m partial to granite or any of the other tougher stone surfaces. Marble is a bit soft to risk a pot or cast iron skilled scratching up. Not to mention SUPER easily stained and far from acid resistant.
I’ll probably get a nice marble slab for rolling out dough though, just set it on the table when I need it.
Is there any chance the countertop has hidden steel clips or brackets of some sort that are used to attach it to the cabinet? Could the magnet be “feeling” the steel clips through the granite?
Interesting idea… I would think that the granite would act as a pretty good shield, but it’s worth investigating!
Is the magnet attracted to other dark minerals in the slab, or only to the one large blob in the picture?
Good question… I think the magnet is attracted to several dark minerals, but I’ll confirm with my friends.
Jeff says that the magnet is attracted to all of the dark minerals. So, the magnet is probably not attracted to something behind the granite. I think the magnet must be attracted to the minerals themselves.
If a metal blade was used to cut the stone, could that have magnetized some of the iron rich minerals?
[…] Mystery Rock #3 […]
We just did a kitchen remodel, and I noticed that Nd magnets stick to our new “granite” countertop. As I’m not a geologist (nor was the guy at the counter store), I have no idea what the rock is. I think the quarry was in India; the color is called “Steel Grey”, which I now find hilarious. In the composite there are some crystalline-looking smooth lighter grey areas (scale 1″ or so) which do not attract the magnet – in fact, the little puck shoots over to the nearest edge to stick to the darker grey “binder” rock. (see, I told you I wasn’t a geologist!) Endless fun! I’m trying to think of some way this power can be harnessed for good, and can’t think of anything. Even with a really strong magnet, it barely holds up a sheet of paper. The main joy seems to be in forgetting all about it until there are for some reason strong magnets in the kitchen, then staggering in confusion as the tool sticks to the counter.
We accidentally came across this property in our countertops this evening on accident. We took a little video of it but I don’t think I can post it here. Email me and I can send it along for you to post.
We have kitchen counter top made from stone (Australian) which looks very much like the stone in the photo. It will hold a 10mm diameter neodymium magnet against gravity on its under surface. On close inspection it seems that it is the dark parts which are most magnetic.