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Frequently-Used Geological Terms Part 60 Glossary Adapted from the Dictionary of Geological Terms O Organic Rock - A sedimentary rock consisting of the remains of organisms, either plant or animal. Oriental Amethyst - The violet to purple variety of sapphire. Oriental Emerald - A green variety of corundum. The yellow variety is referred to as Oriental Topaz. Orpiment - A lemon yellow to orange monoclinic mineral. Typically occurs as a deposit from hot springs and as a sublimate from some volcanoes. Ortho - Prefix which when used with the name of a metamorphic rock indicates it was derived from an igneous rock. Orthoclase - A white, pink, or gray mineral of the alkali feldspar group. As a common rock- forming mineral, it occurs mostly in granites, acid igneous rocks, and crystalline schists. Orthogneiss - A gneiss derived from an igneous rock. Orthoquartzite - A clastic sedimentary rock composed almost entirely of silica-cemented quartz sand. The rock is characterized by scarcity of heavy minerals, lack of fossils, and prominent cross-beds and ripple marks. Osann’s Classification - A chemical system of classification of igneous rocks. Outcrop - The part of a geologic formation or structure which appears at the surface of the earth. Also refers to bedrock which is covered by surficial deposits such as alluvium. Outer Core - The outer, or upper zone, of the earth’s core extending from a depth of about 1800- 3150 miles, including the transition zone. It is presumed to be liquid because it sharply reduces compressional wave velocities and does not transmit shear waves. Outpost Well - A hole drilled for oil or gas with the thought it will probably extend (by a consider- able distance) a pool already partly developed. It is far enough from the limits of the pool to make its outcome uncertain, but not far enough to be called a wildcat well. Overburden - Loose or consolidated rock material which overlies a mineral deposit and must be removed prior to mining. Also refers to the upper part of a sedimentary deposit, compressing and consolidating the material below. Look for more terms next month! Coal is an example of an organic rock. WWDR photo of coal samples from Pennsylvania. !'+$-,(.*#'. +-))-,(. ,"%&*+ ===-039</<45=4<"0-#4 ;@?A&9<4!/57A$3<773 .,5+0,39=57"*A.'AA8@?2? 166>);?>6@26A#A18:>:;?>;@@@ %9+(A18:>:;?>@82: &77"80/,75431 80.7#362' %62-85/8+08)8!7*228,6-7835846..718 6057 854618$/+85431-( 49 WorldWide Drilling Resource ® APRIL 2017