WorldWide Drilling Resource

Frequently-Used Geological Terms Part 50 Glossary Adapted from the Dictionary of Geological Terms M Metasomatic - Pertaining to the process of metasomatism and its results. The term is especially used in connection with the origin of ore deposits. Metasomatism - The process of practically simultaneous capillary solution and deposition by which a new mineral may grow in the body of an old min- eral or mineral aggregate. The presence of interstitial chemically active pore liquids or gases contained within the rock body or introduced from external sources are essential for the process, which often, though not necessarily, occurs at constant volume with little disturbance of textural or structural fea- tures. Metasomatite - A rock which has undergone metasomatism. Metastasy - Lateral adjustments of the earth’s crust as opposed to vertical move- ments (isostasy). Methane - A colorless, odorless, flamma- ble gas and is the simplest of the alkanes. It is the main component of natural gas and is also found with crude oil. mgd - Acronym for millions of gallons per day. This abbreviation is commonly used when referring to rate of flow. Miarolitic - A term applied to small irreg- ular cavities in igneous rocks, mainly gran- ites, in which crystals of the rock-forming minerals protrude. Also refers to such cavities or a rock containing them. Mica - A group of monoclinic minerals consisting of complex phyllosilicates with perfect basal cleavage which split into thin elastic laminae, ranging from colorless to black. Any mineral of the mica group, in- cluding muscovite, biotite, lepidolite, phl- ogopite, and sericite. Mica Book - Acrystal of mica, usually large and irregular. Its name comes from how the cleavage plates resemble the leaves of a book. Microcline - A white, gray, brick red, or green mineral of the alkali feldspar group. Microcline is a common rock-forming min- eral of granitic rocks and pegmatites. It is often secondary after orthoclase and is characterized by crosshatch twinning. Microcrystalline - Said of the texture of a rock consisting of crystals visibly only under the microscope. Look for more terms next month! Bazzite located in a miarolitic cavity courtesy of Rob Lavinsky, iRocks.com 17 WorldWide Drilling Resource ® MAY 2016