GY 111 Lecture Note Series Lab 2 Introduction to Igneous Rocks Lecture Goals A Definitions and advice B Compositions C Textures D Nomenclature AKA rock names E More advice References GY 111 Earth Materials Lab Manual Chapter 2 A Definitions There are a couple of critical definitions that pertain to igneous rocks that you need to know before we get to the rocks themselves If you can understand these and know how they apply to the igneous rocks you will succeed Failure to do this will result in well failure Here they go Composition The composition of igneous rocks deals with two components Their geochemistry this is their bulk chemistry and their mineralogy the composition and percentages of the minerals that comprise them In general the mineralogy of the igneous rocks is much easier to determine at this level than is their geochemistry All you have to do is identify the major minerals that comprise the igneous rocks and estimate their proportions Of course in order to do this you need to be able to identify the minerals This was what we did during the first phase of this course By now all of you should be able to pick up an unknown mineral and determine what it is based upon its properties In order to identify igneous rocks you will have to be able to pick up an unknown rock and identify the minerals that it contains Here is your first piece of advice There aren t that many minerals that make up igneous rocks because there aren t that many elements that make up the magma lava that will ultimately form igneous rocks The only major minerals that you have to be able to identify in igneous rocks are quartz orthoclase Ca plagioclase Na plagioclase olivine amphibole pyroxene biotite muscovite Now 9 minerals all of which comprise the Bowen s Reaction Series doesn t sound that bad does it The problem is that these minerals are going to be much smaller than in the specimens you saw in the minerals component of the course You are absolutely going to need a hand lens for the igneous part of this class And you are also going to have to be able to estimate the proportions of each of the major minerals in the rocks This will take a bit of practice The other term that you must understand is texture The texture of an igneous rock is more or less related to the size shape and density of the crystals that comprise the rocks It is related to the cooling speed of the magma and is best explained in a series of cartoons and pictures B Composition of igneous rocks In order to understand igneous compositions we need to reconsider the Bowen s Reaction Series As a magma lava cools minerals will precipitate in a specific suite The ferromagnesium minerals precipitate in the following order from very hot to moderately hot temperatures Olivine Pyroxene Amphibole Biotite Feldspars precipitate in the following order from moderately hot to cool temperatures Ca plagioclase Na plagioclase Orthoclase At the bottom 1 of the Bowen s Reaction Series is Muscovite and Quartz The igneous rocks are classified in part on the minerals they consist of Rocks dominated by olivine pyroxene and Ca plagioclase form from very hot to hot magma lava and are classified as Mafic because they are enriched in Mg Ca and Fe bearing minerals In a way this designation addresses geochemistry rather than mineralogy and it is useful because it allows for significant variations in mineral content within specific rock types A rock containing 20 olivine 30 pyroxene and 50 Ca plagioclase is essentially the same as a rock containing 5 olivine 60 pyroxene and 35 Ca plagioclase They look different they are different but they are both treated as mafic There are 3 other compositions that characterize the igneous rocks Rocks that are dominated by quartz orthoclase and muscovite are considered Felsic because of their enriched silica content Felsic rocks form from rather cool magma lava Rocks that are dominated by hornblende biotite and Na plagioclase are called Intermediate because they form at intermediate temperatures and lie mid way between mafic and felsic rocks The fourth type of igneous rocks the Ultramafics They form from very hot magma lava and contain only olivine and or pyroxene The table below comes from the lecture notes lecture 9 but has been modified to include something called the colour index Composition Formation Temperature Dominant Minerals Silica content Colour Index Ultramafic Very high Olivine pyroxene Very low 45 Light dark Mafic High Olivine pyroxene Ca plagioclase low Dark Intermediate Medium Na Plagioclase amphibole biotite moderate Medium Felsic Medium low Orthoclase quartz muscovite biotite high 65 light In earlier labs I warned against using colour alone to identify minerals But as you all know some minerals are darker in colour than others even if they are not consistent in colour Quartz is usually light in colour Pyroxene is usually dark in colour A rock that contains mostly pyroxene and Ca plagioclase mafic will therefore be rather dark in colour In contrast a rock containing quartz orthoclase and muscovite felsic will be rather light in colour The colour index is a useful means of quickly identifying the composition of an igneous rock but it is not absolute Mafic rocks tend to be darker than intermediate rocks and felsic rocks but there are exceptions Ultramafic rocks for example can be very dark or very light Intermediate rocks can be dark light or both slat and peppery Make notes of the variations during the lab show and tell session C Textures This is the second means by which igneous rocks are classified and it is the trickiest thing to try and explain As magma lava cools crystals precipitate according to the Bowen s Reaction Series The longer a crystal has to grow the larger it becomes The fewer crystals that are growing in molten rock the better crystalline they become As long as the crystals don t bump into one another they will maintain their crystal habits Well crystalline or euhedral quartz is hexagonal 2 in outline The feldspars are blocky the micas muscovite and biotite are platy etc If however the crystals have a long time to grow and they start to bump into one another they start to lose their well define crystal shapes they become anhedral The only way that you will be able to identify quartz or orthoclase or amphibole is by their other properties cleavage luster hardness etc As already mentioned it is the rate of cooling that controls the texture of igneous rocks If the rate of cooling is slow as would