Lab 4: Igneous Rocks - Igneous Compositions


Importance of Silica


Silica, SiO 2 , is the most common constituent of igneous rocks. Silica combines with the following elements to form 98% of igneous rocks:


  • aluminum (Al)
  • calcium (Ca)
  • sodium (Na)
  • potassium (K)
  • magnesium (Mg)
  • iron (Fe)


Overall Color


Color can be tricky when used for identifying many minerals, but it can be generally useful for identifying igneous rocks. We can divide them on the basis of overall color, generally into dark-colored and light-colored compositions:


Dark Color


The dark color results from the effects of iron (Fe) and magnesium (Mg) in the crystal structure. We use these element names to construct a label for dark-colored igneous rocks, ferromagnesian (ends with an "n", not an "m"). Because iron and magnesium are fairly large atoms, and because there is less silica present, these rocks tend to be more dense.


Light Color


The light color results from there being less iron and magnesium, and more of other elements like aluminum, calcium, and sodium. These rocks are also less dense, because there is less iron and magnesium, and more silica.


Magmas, Minerals, and Igneous Rocks



This diagram is very important for learning about igneous rocks. It shows the different compositions of magma, felsic, intermediate, mafic, and ultramafic, along with the minerals that are found in igneous rocks crystallizing from magmas of those compositions. Each mineral has an area on the plot that shows its occurrence in different igneous rocks. Across the top you see the names of the igneous rocks, arranged with intrusive on the bottom and extrusive on the top. The igneous rocks may be thought of as compositional pairs: granite/rhyolite (felsic), diorite/andesite (intermediate), gabbro/basalt (mafic), and periodotite/komatiite (ultramafic). The names of these compositional categories are as follows:




Fel, for abundant feldspar. si, for abundant silica. Used interchangeably with "granitic."




Intermediate between felsic and mafic. Used interchangeably with "andesitic."




Ma, for magnesium. f, for iron. Used interchangeably with "basaltic."




Ultra, for really high amount of magnesium and iron. Peridotite is mostly olivine, and makes up much of Earth's mantle, and isn't terribly common at the surface. Komatiite, the volcanic counterpart of peridotite, is likewise uncommon.


Important trends are listed at the bottom of the diagram:


Lighter in color toward the left


Granite and rhyolite are usually in the tan to pink to white range, while the rocks in the middle, diorite and andesite, tend to be some shade of gray, overall, and the rocks on the right, gabbro/basalt and peridotite/komatiite, tend to be dark gray, dark greenish black to just plain black. This overall color simply reflects the sum total of all the minerals in the rocks.


Silica content increases toward the left


All of these are minerals are silicate minerals, but the overall amount of silica (SiO 2 ) is greater in the felsic rocks. This is the primary reason for the greater viscosity in felsic magmas.


Viscosity increases toward the left


As a result of greater silica content, felsic magmas are more viscous, owing to the way that silicon-oxygen tetrahedra will be attracted to one another. Volcanoes associated with felsic to intermediate magmas tend to be more explosive, because more of the gas pressure is held in by the more viscous magma. These volcanoes are capable of tremendous explosions and eruptions of pyroclastic material. On the other hand, mafic and ultramafic magmas, with more iron and magnesium and less silica, tend to be highly fluid ("runny") magmas that do not hold back pressure. Volcanoes associated with mafic magmas, therefore, tend to be of the quieter variety, without tremendous explosions, and tend to extrude large quantities of fluid magma, instead of much pyroclastic material.


Melting point increases toward the right


Minerals are chemical compounds. Different chemical compounds have different melting points. The melting point of a rock is determined by the minerals it contains. Some minerals will begin to melt before others as the temperature goes up. Igneous rocks toward the left tend to melt sooner (at lower temperatures) than those toward the right.


Determining Mineralogies from the Diagram



It is easy to see which minerals are found in the different igneous rocks. All you do is draw a vertical line at the given composition, then see what minerals are on the line, and in what proportions. You judge the proportions of the minerals by the breadth of the mineral plot areas crossed by the line. Take, for example, a typical felsic composition, which would fit both granite and rhyolite (the difference between granite and rhyolite is crystal size, not mineralogy):




The line goes right through the fat part of the quartz plot area, and it looks like about 22% (estimate by looking at the percentages posted at left).




The line goes through a big patch of the orthoclase plot area, for a percentage of about 26%.




First, note that the plagioclase over toward the left is more sodium-rich. The line crosses the plagioclase plot area for about 10% Na-rich plagioclase.




The muscovite plot area is never too large, and the line crosses an estimated 2% for muscovite.




Same as for muscovite, it looks like about 2%.




The line catches the edge of the hornblende plot area, for an estimated 3%.


We can do the math on our estimates to see how much of a total we have. We need the sum total of our estimates to add up to 100%. We have:



So, our estimates are a little off, but you get the idea of how to "eyeball" percentages, and how this diagram is really useful. If you pick up a specimen of granite, you can look at this chart to see what minerals you would expect (generally speaking) to see in the rock.


Try doing that for diorite/andesite (gray line) and gabbro/basalt (green line):


Diorite/Andesite minerals:

_______________________________ ____%


_______________________________ ____%


_______________________________ ____%


Total: ____%


Gabbro/Basalt minerals:

_______________________________ ____%


_______________________________ ____%


_______________________________ ____%


Total: ____%


Now that you have the hang of reading the mineralogies in the igneous rocks, take a look at the top part of the diagram:



These are the compositional pairs:


  • granite/rhyolite
  • diorite/andesite
  • gabbro/basalt
  • peridotite/komatiite


You can keep a lot about igneous rocks straight, if you just learn the main six: granite/rhyolite, diorite/andesite, and gabbro/basalt. Print out the following chart. The chart is large enough for you to place actual specimens in the boxes. When you do this, you should see the color trend described above, with lighter color to the left and darker color to the right. You should also see the larger crystals in the intrusive igneous rocks (granite, diorite, gabbro) and the smaller (can't even see them) crystals in the extrusive igneous rocks (rhyolite, andesite, basalt).



A few things to observe about the igneous rocks, placed on a printout of the chart above:




After you get the six igneous rocks placed, step back a few feet to appreciate the overall color of the rocks. It is easier to see how the colors of the individual minerals blend to give the overall rock color, if you step back a bit.


Crystal size


It is easy to see the large crystals in granite, and it isn't too hard in diorite (or granodiorite), but it can be difficult to see the large crystals in gabbro, because all of the minerals are dark colored. As you move the rock specimen around in the light, however, you should see cleavage faces on various mineral crystals shining back the light. The important thing is that you can see the crystals with your naked eye (phaneritic texture for the intrusive igneous rocks). If you see any flecks or specks in one of the extrusive rocks (aphanitic texture, generally), they these can be called phenocrysts, and the texture can be called porphyritic. Otherwise, the texture for the extrusive igneous rocks is just aphanitic.


Now, let's add a couple of more igneous rock types, volcanic glass (obsidian) and pumice. Where might these plot by composition: felsic, intermediate, or mafic? First, you might take one look at the dark color of obsidian and think it to be mafic, or even ultramafic. But, no, the dark color is not the result of abundant iron and magnesium. It is the result of the amorphous structure of volcanic glass. Without a crystalline structure, light goes into the specimen and gets chaotically scattered, and absorbed, so that it appears dark. Obsidian is usually a highly siliceous material that is associated with felsic magmas. A similar story could be told about typical pumice, which is glassy material embedded with many air bubbles (typically 80-90% of pumice is just air bubbles, so it floats). Pumice can be associated with felsic to intermediate magmas. So, aping these two to the mix, here's a final chart:



Other Igneous Rocks


You might have a few other igneous rocks to study. Here are a few typical ones encountered in introductory geology labs:




This rock is felsic in composition, so it can be considered granitic. Its main distinguishing feature from typical granite is the very large crystal size.




Some igneous rocks are fall in-between the most well known types, and are given combination names to reflect the mineralogy. On the charts and diagrams above, granodiorite would plot somewhere on the right side of the felsic column, or left side of the intermediate column. Thus, you can read off the expected mineralogy from the diagram above, just by drawing a line in this position to see what mineral plot areas are crossed.




If you have a specimen of scoria, you'll see gas bubble holes, as in pumice, but the rock itself is not as glassy, and looks like typical basalt. Scoria is heavier also, and doesn't have as many bubble holes, which are technically called vesicles. In fact, basalt with some vesicles is called vesicular basalt, and if there are very many vesicles, it could be called scoria.