Presentation on theme: "1 IGNEOUS ROCKS SELF-DIRECTED NAME: _______________________________ (PRINT CLEARLY) DIRECTIONS There are several pages to read and some diagrams to study."— Presentation transcript:
1 IGNEOUS ROCKS SELF-DIRECTED NAME: _______________________________ (PRINT CLEARLY) DIRECTIONS There are several pages to read and some diagrams to study. Once you have done this, you will be ready to work the problems and identify the different rocks (identified by their mineral composition). You will be doing similar work in your Plutonic and Volcanic Rock Labs. 2.
2 EXPLANATION OF THE ROCK NOMENCLATURE TRIANGLES The included triangles show three end-members that exit at each end of the triangle. On the Felsic diagram, the maximum amount of quartz is 100% at the top of the triangle. As you move down either side of the triangle, the amount of quartz decreases. These end members are generally referred to as Q, A and P. This is quartz or (SiO 2 ), alkali feldspar (containing potassium (K) and plagioclase feldspar containing calcium and sodium. The bottom of the triangle represents the alkali feldspar to plagioclase feldspar percentages. At the far left is 100% alkali feldspar, to the far right is 100% plagioclase feldspar. The rocks that are defined by this triangle has X% of quartz and Y% of feldspar which equals 100% of the entire rock. The amount of feldspar, Y%, can be broken down into A% for alkali feldspars and B% for plagioclase feldspars. This means that the entire rock is considered to be X% quartz and Y% feldspar (A% + B%). The combined X% and Y% = 100%. If we have a rock with 45% quartz, then the remaining amount is 65% feldspar (ignoring trace amounts of other minerals). If there is 92% alkali feldspar and 8% plagioclase feldspar, then the amounts are 45, 92, and 8. Plotting this on the triangle places this rock into area 2 on the Felsic triangle. Therefore the rock is either plutonic, and is an alkali feldspar granite, or is volcanic and is an alkali feldspar rhyolite. These categories for Felsic rocks look at the amounts of feldspar and silica (therefore the name). But other constituents may exist in a rock, you may have muscovite in the sample. If there are appreciable amounts of other minerals that are not part of the end-members, not quartz or feldspar, this mineral can become part of the name.
3 Normalization If we have a rock with 6% biotite, 44% quartz and 50% feldspar (25% alkali feldspar and 25% plagioclase feldspar), where would the rock fall on the Felsic triangle? We ignore the biotite and normalize for the end-members. That is 100% - 6% gives 94% of the constituents that are the end-members of the triangle. Then divide the 44% quartz by 94% and the 25% alkali by 94% and 25% plagioclase by 94%. This gives normalized values of 46.8% quartz, 26.5% alkali and 26.5% plagioclase. Which is 47 (rounded up) plus 53 that equals 100% of the normalized amount. This would fall in area 3 on the triangle. The rock would be a granite if it is plutonic and rhyolite if it is volcanic. Based on the nature of the crystals and whether or not there were vesicles (holes) in the rock, the we would determine if the rock is plutonic or volcanic. If the biotite is visible with other mineral crystals visible, the rock would be called a biotite granite. As the rock sample’s constituents move further and further away from the quartz end, the rocks typically become darker in color, less milky or glassy quartz. The alkali-rich rocks usually become red to pink in color. The plagioclase rocks tend to be white in color. Overall the rocks on this triangle are light in color. They vary from milky or glassy white to red to pink, pink and white, and white. We look at color to determine if it is Felsic. If it is dark in color it falls on the Mafic triangle. Then we look to see what amounts are alkali or plagioclase. If it is red, it is very alkali. If it is very white, it is plagioclase rich. Texture is when the rock is smooth or coarse. Very coarse crystals show there was slow cooling of an intrusive magma. If the texture is very smooth the rock was extrusive or volcanic. We also look for vesicles (holes) to indicate that the rock may have been volcanic. Often volcanic rocks have a smooth texture with occasional large crystals or phenocrysts (crystals visible with the naked eye). The Mafic triangle shows that alkali feldspars and plagioclase end-members are present. In lieu of quartz, we see feldspathoids as the third end-member. All of the mafic rocks are poor in quartz (little to none).
4 The following table is an example of a simplified method of igneous rock identification. In lieu of considering the mineral content of quartz, feldspars or feldspathoids (on the second triangle included), we will look at color and texture to determine a general composition, make an educated guess as to the mineral composition, and ascertain if they were formed from a magma or a lava. Felsic and Mafic Rock Identification Table FELSIC OR MAFIC FELSIC MORE WHITE THAN DARK MINERALS TEXTURES COARSE WITH CRYSTALS VISIBLE IGNEOUS COLORS WHITE AND/OR PINK WITH LITTLE BLACK MINERALS QUARTZ WHITE FELDSPAR OR PINK FELDSPAR INTRUSIVE OR EXTRUSIVE INTRUSIVE FELSIC MORE WHTE THAN DARK MINERALS GLASSY OR FEW VISIBLE CRYSTALS OCCASIONAL LARGE CRYSTALS SOME HOLES VOLCANIC WHITE AND/OR PINK WITH LITTLE BLACK A FEW FLEDSPAR CYRSTALS BUT MOST MINERALS CANNOT BE DETERMINED BY HAND SAMPLE EXTRUSIVE MAFIC MORE DARK THAN WHITE MINERALS COARSE WITH CRYSTALS VISIBLE IGNEOUS DARK GRAY TO BLACK COLOR DARK GREEN TO BLACK COLOR SOME CRYSTALS SHOWING 120 DEGREE ANGLES AND STUBBY ALMOST SQUARE CRYSTALS WITH STRIATIONS (AMPHIBOLE AND PYROXENE) INTRUSIVE MAFIC MORE DARK THAN WHITE MINERALS GLASSY OR FEW VISIBLE CRYSTALS OCCASIONAL LARGE CRYSTALS SOME HOLES VOLCANIC DARK GRAY TO BLACK COLOR DARK GREEN TO BLACK COLOR UNDETERMINABLE A FEW CRYSTALS GREEN TO BLACK IN COLOR ARE VISIBLE EXTRUSIVE
5 Using the Felsic and Mafic Rock Identification Table on the previous page, and these descriptive narratives, identify the following rocks. Circle the appropriate terms for each sample. 1.The sample is light pink with a fairly smooth texture. Some small crystals smaller than one millimeter in length are visible with the unaided eye. felsicor mafic coarse texture or fine texture intrusiveor extrusive light coloror dark color 2.This is a solid rock with a blackish to dark red color. There are no visible crystals. It is filled with vesicles (holes) that range in diameter from 1 to 3 millimeters. felsicor mafic coarse texture or fine texture intrusiveor extrusive light coloror dark color 3.Black, massive, heavy and with a fine texture. There are no vesicles. There are no crystals visible with the unaided eye. felsicor mafic coarse texture or fine texture intrusiveor extrusive light coloror dark color
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