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Granitoids

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granite

Alkali feldspar granite

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Granite rock has become so common in homes and buildings that anyone these days can name it when they see it in the field. But what most people would call granite, geologists prefer to call "granitoid" until they can get it into the laboratory. That's because relatively few "granite rocks" out there are truly petrologically granite. How does a geologist make sense of granitoids? Here's a simplified explanation.

The Granitoid Criterion

A granitoid meets two criteria: (1) it is a plutonic rock that (2) has between 20 percent and 60 percent quartz.

  • Plutonic rocks cooled at depth very slowly from a hot, fluid state. A sure sign is well-developed, visible grains of various minerals mixed in a random pattern, as if they had been baked in a pan in the oven. They look clean, and they don't have strong layers or strings of minerals like those in sedimentary and metamorphic rocks.
  • As for the quartz, a rock with less quartz than 20 percent is called something else, and a rock with more than 60 percent quartz is called quartz-rich granitoid (a remarkably simple answer in igneous petrology).

Geologists can assess both of these criteria (plutonic, abundant quartz) with a moment's inspection.

The Feldspar Continuum

OK, we have abundant quartz. Next the geologist evaluates the feldspar minerals. Feldspar is always present in plutonic rocks whenever there's quartz. That's because feldspar always forms before quartz. Feldspar is mainly silica (silicon oxide), but it also includes aluminum, calcium, sodium and potassium. Quartz—pure silica—won't start forming until one of those feldspar ingredients runs out. There are two types of feldspar: alkali feldspar and plagioclase.

The balance of the two feldspars is the key to sorting out the granitoids into five named classes:

True granite corresponds to the first three classes. Petrologists call them by their long names, but they also call them all "granite."

The other two granitoid classes aren't granites, although granodiorite and tonalite in certain cases can be called a name very much like granite (see the next section).

If you have followed all this, then you will readily understand the QAP diagram that shows it graphically. And you can study the gallery of granite pictures and assign at least some of them exact names.

The Felsic Dimension

OK, we've dealt with the quartz and the feldspars. But granitoids also have dark minerals, sometimes quite a lot and sometimes hardly any. Usually feldspar-plus-quartz dominates, and geologists call granitoids felsic rocks in recognition of this. A true granite can be rather dark, but if you ignore the dark minerals and assess only the felsic component, it can still be properly classified.

Granites may be especially light-colored and nearly pure feldspar-plus-quartz—that is, they may be very highly felsic. That qualifies them for the prefix "leuco," meaning light-colored. Leucogranites may also be given the special name aplite, and leuco alkali feldspar granite is called alaskite. Leuco granodiorite and leuco tonalite are called plagiogranite (making them honorary granites).

The Mafic Correlative

Dark minerals in granitoids are rich in magnesium and iron, which don't fit in felsic minerals and are called the mafic ("MAY-fic" or "MAFF-ic") component. An especially mafic granitoid may have the prefix "mela," meaning dark-colored.

The most common dark minerals in granitoids are hornblende and biotite. But in some rocks pyroxene, which is even more mafic, appears instead. This is unusual enough that some pyroxene granitoids have their own names: Pyroxene granites are called charnockite, and pyroxene monzogranite is mangerite.

Still more mafic a mineral is olivine. Normally olivine and quartz never appear together, but in exceptionally sodium-rich granite the iron-bearing variety of olivine, fayalite, is compatible. The granite of Pikes Peak in Colorado is an example of such a fayalite granite.

A granite can never be too light, but it can be too dark. What stone dealers call "black granite" is not a granite at all, because it has little or no quartz in it. It's not even a granitoid (although it is a true commercial granite). It's usually gabbro, but that's a subject for another day.

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  5. More on Igneous Rocks
  6. Granitoids - Granite and the Related Rocks Granodiorite, Diorite and Tonalite

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