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?