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In jewellery, a table diamond is a thin diamond cut with a large, square, flat upper surface encompassed by four lesser faces.
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Tachhydrite is a hydrous chloride of calcium and magnesium occurring in yellowish masses which rapidly deliquesce upon exposure. It is found in the salt mines at Stassfurt.
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Tachylite (Tachylyte) is a natural glass, formed by the rapid cooling of molten basalt. It is a black or dark- brown and greasy looking substance. It is very brittle and occurs in basaltic obsidians in dikes, veins and intrusive masses.
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Taconic designates a rock pertaining to the series of rocks forming the
Taconic mountains in Western New England. They were once supposed to be older than the Cambrian, but later proved to belong to the Lower Silurian and Cambrian.
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Talc has the formulae Mg3Si4O10(OH)2 and a relative hardness of 1. It is a mineral of secondary origin formed by the alteration of magnesium silicates. Usually found in metamorphic rocks where, in a non-crystalline form, it occurs as 'soapstone' - and can make up large rock masses. Used for laboratory table tops and for many industrial uses. It has a greasy feel.
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A talus is a sloping heap of fragments of rock lying at the foot of a precipice.
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Tantalite is a heavy mineral of an iron-black colour and sub-metallic lustre. It is essentially a tantalate of iron. Tantalite is formed in igneous rocks, especially granite pegmatite, and in placer deposits. It is associated with amblygonite, apatite, beryl, cassiterite, feldspar, quartz, spodumene and tourmaline. Tantalite is the main ore of tantalum.
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Tectonic designates rock structures and external forms resulting from the deformation of the earth's crust, such as, tectonic arches or valleys.
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Tektites are small, irregularly shaped glassy nodules that have no crystal structure and are similar to obsidian. Tektites are thought to be from meteorites, but their origin is not proven, some scientists maintaining that they are terrestrial and produced as a result of meteorite impacts, others believing them to be meteorite material.
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Tellurite is a soft secondary mineral, an oxide of tellurium. It occurs sparingly in tufts of white or yellowish crystals formed through the oxidation of pre-existent tellurium compounds or of the element itself. Tellurite is most commonly associated with paratellurite with which it is dimorphous. Tellurite was confirmed as a distinct species of mineral in 1845 and is named after its main elemental component, tellurium. Tellurite has a relative hardness of 2.
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Tenacity refers to the ability of a substance to resist being separated.
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Tennantite, named after Smithson Tennant, an English chemist is a blackish lead-grey mineral, closely related to tetrahedrite. It is essentially a sulphide of arsenic and copper.
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Tephrite is an igneous rock consisting essentially of plagioclase and either leucite or nephelite, or both.
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Tephroite is an ash-grey coloured silicate of manganese.
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In geology, a terrace is a level plain, usually with a steep front, bordering a river, a lake, or sometimes the sea. Many rivers are bordered by a series of terraces at different levels, indicating the flood plains at successive periods in their history.
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In geology, the term terrane describes a group of rocks having a common age or origin. The term is nearly equivalent to formation, but is used somewhat less comprehensively.
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In geology, the term tertiary period describes the first period of the age of mammals, or of the Cenozoic era.
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Tetradymite (telluric bismuth also known as bismuth telluride) is a telluride of bismuth. It is of a pale steel-grey colour and metallic lustre, and usually occurs in foliated masses mixed with sulphur and selenium.
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Tetragonal refers to a crystal with four rectangular(not square) sides and two square bases. A butter package is an example.
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Tetrahedrite (grey copper ore, fahlore, or panabase), so called because the crystals of the species are commonly tetrahedrons, is sulphide of antimony and copper, with small quantities of other metals. It is an ore of copper and silver. It is commonly found in hydrothermal veins formed at low to moderate temperatures and is usually associated with other silver, lead, and copper minerals. It has the formulae (Cu,Fe)12Sb4Si3 - (Cu,Fe) 12As4S13 and a relative hardness of 5.
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In geology, thalassic describes rocks pertaining to the sea. The term is sometimes applied to rocks formed from sediments deposited upon the sea bottom.
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In geology, the Thanet beds is the name given to the lowest division of the Eogene in Britain. Also known as the Thanet Sands, they consist of yellow to greenish sands, well developed in the Isle of Thanet, and contain a large number of marine fossils.
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In geology, 'the age of acrogens' is the age of coal plants, or the carboniferous era.
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Thenardite, named after the French chemist, Louis-Jacques Thenard, is a white or brown coloured mineral with a vitreous lustre that dissolves easily in water and has a weak salty taste. It forms in sedimentary evaporate deposits in lakes and the playas of desert climates. Thenardite was confirmed as a distinct species of mineral in 1826 and is used in the glass and paper-making industries. Anhydrous sodium sulphate It has the formulae Na2SO4 and a relative hardness of 3.
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Thinolite is a calcareous tufa, in part crystalline, occurring on a large scale as a shore deposit about the Quaternary lake basins of Nevada.
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Thomsenolite, named after Dr. J. Thomsen of Copenhagen, is a fluoride of aluminium, calcium, and sodium occurring with the cryolite of Greenland.
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Thomsonite (mesole, or comptonite), named by its discoverer, the English mineralogist Brook after his friend Thomas Thomson, of Glasgow, is a zeolitic mineral, occurring generally in masses of a radiated structure. Thomsonite is found fairly abundantly in cavities of phonolites and basalt rocks, more rarely in pegmatite veins. It is a hydrous silicate of alumina, lime, and soda.
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Thorianite is a rare mineral found in Sri Lanka, and containing thorium, uranium, cerium and other rare earths. The percentage of throium oxide is as much as 70 in some specimens.
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Thorite, so called by Berzelius from the Scandinavian god Thor, is a mineral of a brown to black colour, or, as in the variety orangite, orange-yellow, and with a glassy lustre. It is essentially a silicate of thorium. Thorite is chiefly found in Norway and Sri Lanka.
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In geology, thrust is a name given to a type of fault or break in a rock formation. The term thrust is particularly applied to those faults the planes of which make a small angle with the horizontal. By thrust action masses of rock are pushed over underlying masses, sometimes for many kilometres. Examples of this type of action are apparent in Scandinavia, Scotland and the Appalachians.
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In geology, a thrust plane is the surface along which dislocation has taken place in the case of a reversed fault.
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Thulite is a variety of zoisite used as an ornamental stone. It is a basic calcium and aluminium silicate, rose-red or pink in colour, and is found in Telemarken in Norway.
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Thuringite, named after Thuringia where it is found is a mineral occurring as an aggregation of minute scales having an olive-green colour and pearly lustre. It is a hydrous silicate of alumina and iron.
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Tigers-eye is a siliceous stone of a yellow colour and chatoyant lustre, obtained in South Africa and much used for ornament. It is an altered form of the mineral crocidolite.
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Tile ore is an earthy variety of cuprite.
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A tilestone is a kind of laminated shale or sandstone belonging to some of the layers of the Upper Silurian.
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In geology, a till is a stiff unstratified deposit of clay mixed with sand, gravel and boulders without lamination, formed in a glacier valley by means of the waters derived from the melting glaciers. The term is sometimes applied to alluvium of an upper river terrace, when not laminated, and appearing as if formed in the same manner.
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A tillite is a rock composed of consolidated till.
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Tincal is a crude form of borax found in lake-deposits in parts of Asia.
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Tonalite is a group of rocks consisting chiefly of quartz, orthoclase, plagioclase, hornblende and biotite. Apatite, magnetite, zircon, iron oxides etc are also often present in the rocks of the group. Tonalite resembles dark granite in its general appearance, and is so called from its occurrence at Monte Adamello, near Tonale in the eastern Alps. The rocks are also found in Scotland, Ireland and America.
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Topaz is a mineral aluminium fluosilicate occurring in rhombic prisms, generally yellowish and pellucid, also colourless, and of greenish, bluish, or brownish shades with a glassy lustre, fine varieties of the mineral are valued as gem stones. It sometimes occurs massive and opaque. Topaz is formed by fluorine-bearing vapours given off during the last stages of the solidification of igneous rocks and is frequently associated with tin stone, tourmaline, mica. beryl, etc. The deep orange yellow varieties of the mineral are most valued, and the finest stones have been found in the Ural Mountains and Brazil. Some of the Brazilian stones become pink on heating and are used in cheap jewellery. Topaz crystals are also found in Saxony, Scotland, Ireland, the south-west of England, Africa, India, Japan, North America, etc. Many so-called topazes are really only yellow varieties of quartz.Topaz has the formulae Al2SiO4(F,OH)2 and a relative hardness of 8.
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Topazolite is a topaz-yellow variety of garnet.
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Torbanite (also known as boghead coal) is a dark brown variety of cannel coal found at Torbanehill in Scotland. Torbanite contains a large proportion of algae of the Palaeozoic age. Similar coal has been found in New South Wales. Torbanite contains over 60 per cent of volatile matter and is valuable for the extraction of burning and lubricating oils and paraffin.
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Torbenite (chalcolite), named after the Swedish chemist Torbern Bergman, is a secondary mineral, a uranium ore, originating from the weathering of uranium. It contains up to 61 percent uranium. It has a relative hardness of 2.5.
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In geology the torridonian are a series of pre-Cambrian deposits found in the north-west Highlands of Scotland. They are chiefly red sandstones, and are so called from Loch Torridon, in the neighbourhood of which they are well developed.
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Touchstone, also known as Indian Stone, is a hard, black variety of quartz. Touchstone is traditionally used for making a rough approximation of the fineness of gold, the gold being rubbed across the touchstone leaves a streak which is then moistened with a mixture pf nitric acid and hydrochloric acid. A comparison of the effects of the mixture on the streak determining the purity of the gold.
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Tourmaline is the most commonly found in granite pegmatites. It is usually black in colour, but lighter coloured gem varieties are also found. It occurs usually in three-sided or six-sided prisms terminated by rhombohedral or scalenohedral planes. Black tourmaline (schorl) is the most common variety, but there are also other varieties, as the blue (indicolite), red (rubellite), also green, brown, yellow (peridote) and white. The red and green varieties when transparent are valued as jewels. Crystals of tourmaline when heated exhibit electric polarity.
Tourmaline is also used in the form of a polariscope called tourmaline tongs.
Tourmaline is a complex aluminium borosilicate containing varying proportions of iron, magnesium, flourine, chromium etc, and has the formulae (Na, Ca)(Al,Fe,Li,Mg)3A16(BO3)3(Si6O18) and a relative hardness of 8.
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Trachyte is a group of igneous volcanic rocks, grey, yellow, brown, green and red in colour consisting chiefly of alkali feldspar, and often containing crystals of glassy feldspar, mica, hornblende, or augite. Trachyte occurs in lava, intrusive sheets, and dykes from the early Tertiary period. It is sometimes used as a paving material.
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Trass is a white to grey coloured volcanic tufa, formed of decomposed trachytic cinders, sometimes used as a cement.
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Travertine is a white or light-coloured crystalline concretionary limestone, usually hard and semi crystalline, deposited from springs and rivers and used for building. Extensive deposits exist at Tivoli, near Rome.
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Tremolite, named after Tremola, a valley in the Alps, where it was discovered, is a white variety of amphibole, or hornblende, occurring in long, bladelike crystals, and coarsely fibrous masses. The fibrous variety has been used for asbestos. The compact variety is called nephrite and is used for ornamental purposes in the orient. It is most often found in impure limestones where the rock has re-crystallized during metamorphism. Tremolite has the formulae Ca2Mg5Si8O22(OH)2 and a relative hardness of 6.
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Trenton beds are beds of rocks of the Trenton Period, so named from the typical example of the beds found at Trenton Falls, New York, USA.
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In geology, the Trenton period is a subdivision in the lower Silurian system of rocks It is so named from Trenton Falls, in New York, USA, a typical example of the system. The rocks are mostly limestones and black carbonaceous shales, and the period is divided into the Trenton, Utica, and Cincinnati epochs. The rocks occur in many localities in north America and are chiefly used in building.
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The trias is the formation situated between the Permian and Lias, and so named by the Germans, because it consists of three series of strata, which are called in German the Bunter Sandstein, Muschelkalk, and Keuper.
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In geology, the Triassic (named after the Trias) was the ninth geological period, 170,000,000 years ago, following the Carboniferous and preceding the Jurassic periods. The rocks are the oldest of the Mesozoic formations and, once known as the New Red Sandstone are now divided into three series: the Keuper, Muschelkalk and Bunter. The rocks of the Triassic period are chiefly red or mottled sandstones or shales and limestones, and red marls, and often reach great thickness.
The rocks of the period are widespread, though in America they grade insensibly into the Jurassic and are often designated Jura-Trias. In the Midlands of England and in Scotland and Ireland, Triassic rocks cover large areas, those at Northwich, Middlewich, Droitwich etc being important sources of rock salt. Triassic rocks cover practically all of Germany and large areas in Europe, north and south America, Africa, Asia and Australasia.
During the Triassic period cycads, conifers and gigantic equisetums flourished, amphibian labyrinthodonts were common, and ichthyosaurs, dinosaurs, crocodiles and plesiosaurs among reptiles were numerous.
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Trichroism is the quality possessed by some crystals of presenting different colours in three different directions.
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Triclinic refers to a crystal with six faces as parallelograms and three axes of unequal length all inclined to each other. An uncommon form of crystal.
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Tridymite is a form of pure silica, like quartz, but having a lower specific gravity and crystallizing in hexagonal tables. It is found in trachyte and similar rocks, such as the acidic volcanic rocks at San Cristobal in Mexico, and is also found in the Euganean Hills and at Siebengebirge on the Rhine.
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Tripestone is a variety of anhydrite composed of contorted plates thought to resemble pieces of tripe.
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Triphylite is a mineral of a greyish-green or bluish colour, consisting of the phosphates of iron, manganese, and lithia. A salmon-coloured or clove- brown variety containing very little iron is known as lithiophilite.
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Triplite is a mineral of a dark brown colour, generally with a fibrous, massive structure. It is a fluophosphate of iron and manganese.
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Triploidite is a manganese phosphate similar to triplite, but containing hydroxyl instead of fluorine.
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Troilite, named after Dominico Troili, an Italian of the 18th century, is a native iron protosulphide, FeS. It is known only in meteoric irons, and is usually found in imbedded nodular masses of a bronze colour.
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Trona is a greyish, white, pale yellow or pale brown coloured mineral typically forming massive layers, first confirmed as a distinct species of mineral in 1773. Trona is a hydrated bicarbonate of sodium commonly found in deserts. Trona is used as a source of sodium. It has a relative hardness of 2.5 to 3.
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Troostite is an alternative name for willemite. It is named after Dr. Gerard Troost.
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Trydimite is a rare mineral. It has the formulae SiO2 and a relative hardness of 7.
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Tufa is a soft white porous rock of calcium carbonate deposited from solution in spring water or percolating ground water.
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Tuff is rock debris consisting of volcanic ashes and igneous rocks of fine-grained material.
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Tungstite is the oxide of tungsten, a yellow mineral occurring in a pulverulent form. It is often associated with wolfram.
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Turnerite, named after the English chemist and mineralogist C. H. Turner, is a variety of monazite.
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Turonian is one of the geological subdivisions into which the Upper Cretaceous formation of Europe is divided.
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Turquoise (also known as calaite) is a mineral of secondary origin usually found in small veins and stringers. Turquoise is a hydrous phosphate of alumina containing a little copper. It has a blue, or bluish green, colour, and usually occurs in reniform masses with a botryoidal surface. It has a relative hardness of 6. Turquoise is susceptible of a high polish, and when of a bright blue colour is much esteemed as a gem. The finest specimens come from Persia. It is also found in New Mexico and Arizona, and is regarded as identical with the chalchihuitl of the Mexicans.
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Twin refers to a mineral specimen comprised of two or more single crystals intergrown in a systematic arrangement.
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A typolite is a stone or fossil which has on it impressions or figures of plants and animals.
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Tyrolite, named after the Tyrol, where it occurs, is a translucent mineral of a green colour and pearly or vitreous lustre. It is a hydrous arsenate of copper.
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Tysonite, named after S. T. Tyson, is a fluoride of the cerium metals occurring in hexagonal crystals of a pale yellow colour.
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Abbreviations
Rocks & Minerals
