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Danalite is a mineral of the helvite group found mainly in granite pegmatite and in hydrothermal veins. Danalite was first discovered at Rockport, Cape Ann, Massachusetts and was confirmed as a distinct species in 1866 and was named in honour of James Dana, a mineralogist at Yale University. Danalite is an iron-rich mineral and appears in octahedra, dodecahedra and as granular masses, and is grey, yellow, red or brown in colour with a vitreous or resinous lustre.
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Danburite is a mineral and gemstone which occurs with feldspar in dolomite, granite pegmatites, and non-marine evaporates. It is commonly associated with apophyllite, cassiterite, calcite, chalcopyrite, corundum, fluorite, pyrite, sphalerite and quartz. Danburite is a borosilicate of calcium and has the formulae CaB2(Si2O4)2 and a relative hardness of 7. It was named after the place where it was discovered - Danbury in Connecticut. Danburite is often colourless, but may sometimes appear pale yellow, pale pink, brown or white and nearly always has a striking glassy or greasy sheen.
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Datolite is a mineral of secondary origin found usually in cavities in basalt lavas and similar rocks. It has the formulae CaBSiO4(OH) and a relative hardness of 6.
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Delvauxite is a mineral source of phosphate. It has a relative hardness of 2. 5.
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Denudation is a geological term for the wearing away of the earth's surface by the various agents - rain, frost, rivers, glaciers and ocean waves, each agent exhibiting a different kind of erosion.
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In geology, deposition is a term applied to the laying-down of material by the various agents, such as wind, rivers, lakes, oceans and glaciers, each deposit exhibiting distinct characteristics.
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Descloizite is a rare secondary mineral that forms in the oxidation zones of lead and zinc ore deposits and veins. It is named after the man who discovered it - Alfred Legrand des Cloizeaux, professor of mineralogy at the University of Paris, France. Descloizite is a hydrous vanadate, the main metal it contains is lead but it also features zinc and copper.
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Destinezite is a mineral. It is a source of phosphate. It has a relative hardness of 3.
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Detrital sediment refers to deposited rock and mineral fragments.
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The Devonian was the sixth geological period, 300,000,000 years ago. It marked the evolution of the insects and amphibians.
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Diabase is a dark, intrusive, basic igneous rock made up of plagioclase feldspar crystals, surrounded by smaller grains of pyroxenes, such as augite and up to ten percent quartz. Diabase is formed from magna that has cooled just below the Earth's surface, and is sometimes found as intrusions in older rock. A common place to find diabase is in the neck of old volcanoes, where it has formed a plug.
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Diamond (once known as adamant) is the hardest naturally occurring mineral and the most important of the gem stones. Diamonds occur in pale shades of several colours, deep shades are rare. Gem quality diamond is transparent, of an attractive colour, and without internal fractures or inclusions of other materials. It has a relative hardness of 10. In jewellery terms, diamonds are measured in carats, with one carat weighing 0.2 grams, so a five carat diamond weighs one gram.
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Diaspore is a mineral similar to boehite, but harder and heavier, and more suitable for use as a gemstone. It is a constituent of bauxites and a major source of aluminium. It has the formulae AlO(OH) and a relative hardness of 7.
Diaspore is typically colourless, but occurs in brown, green, grey, pink, light red, dark red, purple, white and yellow colours depending upon the impurities in the sample. Diaspore was confirmed as a distinct mineral in 1801.
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Dike refers to a wall-like body of igneous rock that cuts across layers of surrounding rocks.
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Diopside is a mineral usually found as a contact metamorphic mineral in crystalline limestones. Transparent varieties have been cut and used as gemstones. It has the formulae CaMgSi2O6 and a relative hardness of 6.
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Dioptase is a bright-green silicate of copper and is found in the oxidation zone of copper deposits and in cavities of massive copper minerals. It resembles emerald in appearance, but has the formulae CuSiO2(OH)2 and a relative hardness of 5.
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Diorites are a group of igneous rocks composed essentially of a soda-lime feldspar and hornblende, embracing a wide range of types from acid to basic. Diorites were formed by cooling far below the surface and occur in the Scottish Highlands, the Channel Islands, California and other places.
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Dolomite is a potential ore of magnesium and occurs in widely extended rock masses as dolomitic limestone. It is often intimately mixed with calcite. It is formed from ordinary limestone by the replacement of calcium by magnesium. It has the formulae CaMg(CO3)2 and a relative hardness of 4.
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Dolostone is a sedimentary rock composed of at least fifty percent dolomite and is formed from limestone deposits. It is creamy, grey or white in colour, compact and earthy, usually a fine to medium grained rock popular as a building stone.
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Dumortierite is an often bright coloured and fibrous mineral occurring in metamorphic rocks rich in aluminium and also in pegmatites and contact metamorphic rocks. It has the formulae Al7(BO3)(SiO4) 3O3 and a relative hardness of 7.
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Dysodil is a yellow or green foliated mineral found in limestone. It is a kind of natural bitumen.
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Abbreviations
Rocks & Minerals
