Thursday, April 22, 2010

More on Granite

I've been thinking about this "granite" controversy (me arguing with myself, as usual), and what a person might actually get when they purchase a so-called "granite" counter top. Last night I took a look back at one of my beloved geology textbooks for enlightenment, and what did I find?

I know you are just perched on the edge of your seats for this one.

GRANITE DOES EXIST in the IUGS classification scheme.

The what scheme???

The International Union of Geological Sciences has a LOT of diagrams (probably several million, would be my guess) for figuring out how to classify all sorts of rocks. In the case of our granite, the classification covers coarse-grained rocks that cooled underground (as opposed to fine-grained rocks that cooled above ground, such as basalt).

All igneous rocks are classified according to how much of this mineral or how much of that mineral is present. How much alkali feldspar? How much quartz? How much plagioclase? And all these proportions have to be "normalized", and you have to determine the "mode" at which point you determine the "volume percent" of all this plus do some more complicated stuff.

So... Inside one of these multitudinous classification diagrams sits a little area called "granite."

Be that as it may, what a person gets when buying a counter top still most likely will NOT be granite. If it's black it will probably be a gabbro. If it has a lot of light-colored quartz or feldspar it may be a granodiorite or a quartz monzodiorite. OR... it may be something altogether different such as a metamorphic rock!!!

Aren't you glad you asked about granite?

Ya just gotta love geologists. We think this stuff is F-U-N.



    The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

    Granitoid is used as a descriptive field term for general, light colored, coarse-grained igneous rocks for which a more specific name requires petrographic examination.

    Granite is a common and widely occurring type of intrusive, felsic, igneous rock.
    Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with porphyritic texture is sometimes known as porphyry.
    The average density of granite is 2.75 g/cm3 and its viscosity at standard temperature / pressure is ~4.5 * 10/19 Pa*s .

    Granite is classified according to the QAPF
    diagram for coarse-grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half or the diagram.

    True granite according to modern petrologic convention contains both plagioclase and alkali feldspars.

    When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called tonalite with pyroxene and amphibole common. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. The volcanic equivalent of plutonic granite is rhyolite.


  2. An interesting addition follows from your comment that "The average density of granite is 2.75 g/cm3." "Granite" is the primary (and less dense) composition of the continental crust (as opposed to the denser 3.3g/cm3 oceanic crust).


    This is why oceanic crust subducts under continental crust - oceanic crust is denser.

    ACK!!! Physics lives!!!

    We now have this information at our fingertips and are ready with some scintillating cocktail party conversation.


    First you are talking about checking out the microbreweries in Anchorage to correctly name the granitoid bar tops and then you are ready for some "Scintillating Cocktail Party Conversation".
    You are going to ALASKA, not L.A. or Miami or New York. Just make sure you don't order a drink that has a paper umbrella or that requires you to stick your pinkie out while drinking.

  4. I stand corrected! I'll be ready for some scintillating microbrewery conversation. Probably not a wine bar or cocktail lounge (complete with lifted pinkies)in sight. Hey - this is Alaska!

    I've been thinking again. The bar will probably be made of wood or metal, after all.

    I will report back post haste. Or at least as soon as I get there.