Danalite is a mineral of the helvite group found mainly in granitepegmatite 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 resinouslustre. Research Danalite
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. Research Danburite
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. Research Datolite
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. Research Denudation
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. Such matter layed down is known as a deposit. Research Deposition
Descloizite is a rare secondary mineral that forms in the oxidation zones of lead and zincore 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. Research Descloizite
Diabase is a dark, intrusive, basic igneous rock made up of plagioclasefeldspar 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. Research Diabase
Diamond (once known as adamant) is the hardest naturally occurring mineral, the purest form in which the element carbon is found, and the most important of the gem stones. Carbon crystallizes to form diamonds in forms belonging to the regular or cubic system, the most common being the regular octahedron and rhombic dodecahedron (twelve faces). The finest diamonds are colourless, perfectly clear, and pellucid. Such are said to be of the finest water. But diamonds are often blue pink, green, or yellow, and such are highly prized if of a decided and equal tint throughout. The hardness of the diamond is such that nothing will scratch it, nor can it be cut but by itself. The value of a diamond is much enhanced by cutting facets upon it inclined at certain angles to each other so as to produce the greatest possible play of colour and lustre. What is called the brilliant cut best brings out the beauty of the stone. Its upper or principal face is octagonal, surrounded by many facets. But this form of cutting requires an originally well-shaped stone. For other diamonds the rose cut is used. In this form six triangles are cut on the top so that their apices meet in a point called the summit. Round this are disposed other facets. Stones which are too thin to cut as rose-diamonds are cut as table-diamonds, which have a very slight play of colour. Diamonds have 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.
The art of cutting and polishing the diamond was unknown in Europe until the 15th century, and the stone itself was not nearly so highly valued in the middle ages as the ruby. Diamonds are valuable for many purposes. Their powder is the best for the lapidary, and they are used for jewelling watches, as lenses for microscopes, and in the cutting of window and plate glass. When used as a glazier's tool the diamond must be uncut. Inferior kinds of diamonds are also extensively used by engineers in rock-boring and by copperplate engravers as etching-points. Diamonds are obtained from deposits of various kinds, mostly alluvial (sands, clays, etc), being separated by washing. They have been found in India, Borneo, and other parts of the East; sometimes in North America and Australia; Brazil has produced large numbers; but the chief diamond-field since 1900 has been in South Africa, the centre being Kimberley. Diamonds were discovered here in 1867, and since then the output has been immense. The diamonds were originally obtained by mere surface workings, but from the late 19th century the excavations have been carried down to considerable depths. 'River diggings' are also carried on the banks of some of the rivers. Some of the South African diamonds are very large. One of them, the Cullinan diamond, discovered in 1905, is a monster of 3025 carats, of very good colour, being by far the largest diamond known. A celebrated diamond is the Koh-i-noor (Mountain of Light), an Indian stone belonging to the British crown. Its history extends over five or six centuries. It weighed at one time 280 carats, but by cutting has been reduced to about 106. The Orloff diamond, formerly belonging to the Emperor of Russia, weighs 193 carats; the Pitt diamond, among the French crown jewels, 136.5 carats. Research Diamond
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. Research Diaspore
Diatomite is a diatomaceous earth generally found underlying peat in various districts of Scotland. In Skye, at Loch Quire, where large supplies of diatomite were discovered in the 19th century, it is found about 45 cm below the surface, and extends downward for about two meters, and in some places to a much greater depth. Diatomite is principally used for the manufacture of dynamite, its value as an absorbent being fully double that of the ordinary German Kieselguhr. It is described also as extremely well adapted for the manufacture of silicate and ultramarine paints, siliceous glazings, porcelain, boiler-coatings, isolating felt, etc. Research Diatomite
Dichroic crystals are those that have the property of exhibiting different colours when polarized light is passed through them in different directions. Thus Iolite (dichroite), a mineral observed by Hauy, appears deep-blue in the direction of the principal axis, and yellowish-brown in a direction at right angles to it, even when viewed with ordinary light. Research Dichroic Crystals
In geology, a dike or dyke is a term applied to intrusions of igneous rock, such as basalt, greenstone, etc, which fill up veins and fissures in the stratified systems, and sometimes project on the surface like walls. Research Dike
In crystallography, dimorphism is the crystallization of a body in forms belonging to two different systems, or in incompatible forms of the same system, a peculiarity exhibited by sulphur, carbon, etc. Research Dimorphism
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. Research Diopside
Dioptase is a bright-green silicate of copper and is found in the oxidationzone 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. Research Dioptase
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. Research Diorites
In geology a dip is the inclination or angle at which strata slope or dip downwards into the earth. The degree of inclination or amount of the dip, which is easily measured by a special instrument, is determined by the angle which a line drawn perpendicular to the direction of the stratum makes with the horizon. The line in which such strata cut the surface is called the strike, and is always at right angles to the dip. Research Dip
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. Research Dolomite
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. Research Dolostone
In lapidary work, a doublet is a counterfeit stone composed of two pieces of crystal, with a colour between them, so that they have the same appearance as if the whole substance of the crystal were coloured. Research Doublet
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. Research Dumortierite
Dysodile is a yellowish or greenish foliated mineral found in limestone, with remains of fish and of plants, which, when ignited, burns and emits a very bad smell. Research Dysodile
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Rocks & Minerals
Research Danalite
