93 0 obj <>stream 78 0 obj <> endobj Diabase Formation. If there is more sodium in the plagioclase, then the rock type is called diorite. It is a medium grained igneous rock, and can be dark grey or black with greenish shades in colour. Synonym: A synonym of Dolerite In the United States, an intrusive rock whose main components are labradorite and pyroxene and that is characterized by ophitic texture. 2010-05-11T21:28:49+10:00 Plagioclase is sodium-calcium feldspar. It is a calcium magnesium carbonate with a chemical composition of CaMg(CO3)2. Diabase is the preferred name in North America, while dolerite is the preferred name in the rest of English-speaking world, where sometimes the name diabase is applied to altered dolerites and basalts. Description of constituent minerals 37 Mineral composition 38 . 3c). Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Discussion 44 References 45 . The wide range of modal composition is illu-Mcp 0, 0 o o 00.'!. Introduction: The intrusive equivalent of gabbro as it has the same mineral and chemical composition.Dolerite is found in either sills, sheet-like compositions that intrude horizontally, and dykes, which are vertical or inclined features. 1. Diorite, gabbro, anorthosite — the three root names in this field are separated according to the colour index and the average composition of their plagioclase – anortho… Analyses were per- formed with a 15 KV accelerating voltage, 15× 10−10A beam current, 5 μm spot size and 50 s live time for each element. 0000006679 00000 n Diorite is defined as follows (p.198): a group of plutonis rocks intermediate in composition between acidic and basic rocks, characteristically uuid:6965ee2a-ca18-4027-9cb3-535df64e55ad “Ultramafic” means that mafic minerals form more than 90% on the rocks composition. Diabase (dolerite) is a dark-colored igneous rock. 78 16 noun a dark basic intrusive igneous rock consisting of plagioclase feldspar and a pyroxene, such as augite; often emplaced in dykes any dark igneous rock whose composition cannot be determined with … phyric dolerite, small lenses of coarse-grained pegrna-toid, and bands of granophyre with a composition roughly equivalent to that of a granite (62 to 67% Si02). Etymol: Greek doleros, 'deceitful,' in reference to the fine-grained character of the rock that makes it difficult to identify megascopically. Diabase typically has a mineral composition that is dominated by the plagioclase feldspar known as labradorite (approximately 40% to 70% of the rock). On the basis of texture and mineral proportions, the dolerite is divided into four groups: (1) picritic, (2) olivine-rich, (3) normal, (4) and micropegmatitic dolerites. Introduction . The mineral composition of the dolerites was determined by X-ray diffraction. 0 The rocks in the earth’s crust continuously undergo changes in their composition which leads to formation of other rocks. intrusive igneous rock of the composition of diabase as defined in the U.S. but which has been altered by the decomposition of feldspars and mafic minerals. 3d). Limestone that contains some dolomite is known as dolomitic limestone. 0000000841 00000 n All five of the analysed dykes in- 0000000016 00000 n Diabase formation took place millions of years ago. The accessory minerals are quartz, orthoclase, chlorite and magnetite. It is a medium grained dark and heavy rock, harder than granite. The term dolerite, synonymous with diabase and microgabbro, is used to describe an igneous hypabyssal rock of dark color composed of plagioclase (labradorite in composition) and clinopyroxene (normally augite or titanoaugite), with opaques as the main accessory minerals (magnetite, titanomagnetite or ilmenite). These larger occurrences of dolerite mostly contain pyroxene as the dominant ferromagnesian mineral and are largely the rock called "diabase" by petrographers, while cases where hornblende is dominant are mostly confined to dikes and smaller intrusions, especially in the older rocks. 2000-07-20T23:26:09+11:00 Most common mafic minerals in ultramafic rocks are definitely pyroxenes and olivine (if hornblende is present it is added to pyroxenes). startxref The Amata Dolerite has a medium grey to greenish grey colour on freshly broken surfaces and typically consists of interlocking white plagioclase crystals and dark coloured mafic minerals (Fig. dolerite (diabase, microgabbro) A dark-coloured, medium-grained igneous rock which contains plagioclase feldspar of labradorite composition and pyroxene of augite or titanaugite composition as essential minerals, and magnetite, titano-magnetite, or ilmenite as accessory minerals.