Geol 333 Lab 6 (Mafic Igneous Rocks in Hand Sample and Thin Section) - Student Notes Why characterize rocks using Petrographic Microscope and Binocular Microscope? Igneous Rocks Rock Classification based on Igneous Rock Classification (Fig. 5.28) based on  Slow cooling rate (i.e., intrusive environment, deep in Earth) =  Fast cooling rate (i.e., extrusive environment, at Earth’s surface) = Composition 1. 2. 3. 4. Mafic Igneous Rock Characteristics  <50 wt % SiO2, silica  Abundant Mg and Fe; mafic silicate minerals  Dark color from mafic silicate minerals Minerals in Mafic Igneous Rocks - Characteristics in Thin Section  Olivine (p. 33 in MacKenzie & Adams, 1994) = PPL: High relief, irregular fractures, no cleavage; XPL: 2nd order birefringence  Pyroxene (p. 39 + 35 in M & A) = PPL: Medium/High relief, 2 directions cleavage at ~90°; XPL: 2nd order birefringence  Plagioclase Feldspar (Ca-rich) (p. 58 + 57 in M & A) = PPL: Low relief; XPL: Polysynthetic twinning, low birefringence - white or gray  Volcanic Glass (amorphous) (p. 105 in M & A) = PPL: variable (clear - dark) color, Low relief; XPL: Completely black - isotropic (NOTE: Vesicles can resemble clear volcanic glass.) Mafic Igneous Rock Names  Basalt (p. 73 - 75 in M & A) = fine-grained  Gabbro (p. 79, 81 in M & A) = coarse-grained  Scoria = basalt with abundant holes (vesicles) from escaping gas  Obsidian = volcanic glass (can be any composition) Class Web site has links to excellent photos of igneous rocks and minerals in thin section for later review outside of Lab. <http://classes.geology.illinois.edu/13SprgClass/geo333/333-Lab_Optical_Microscopy.html> Reading Assignment for Next Week Silicic Igneous rocks: MacKenzie and Adams p. 88 - 95, 104 - 105; Klein and Philpotts p. 166-168, 174-178 Next week’s Lab will begin with a Quiz, which will be based on key information from this week’s Lab as well as next week’s reading related to silicic igneous rocks! Classification of Igneous Rocks