Helping San Diego, California and beyond since 1997.
Do you need
volunteer, community service, work, military or court hours?
|
|
Helping San Diego, California and beyond since 1997.
|
|
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: SUS-TAV |
|
|
TACHYLYTES, or TACHYLITES (from Gr. Taxbs, swift, Xbew, to dissolve, meaning " easily fused," though some have erroneously interpreted it as " easily soluble in acids ") , in petrology, the vitreous forms of the basic igneous rocks; in other words, they are basaltic obsidians. They are black in colour, dark brown in the thinnest sections, with a resinous lustre and the appearance of pitch, often more or less vesicular and sometimes spherulitic. They are very brittle, and break down readily under the hammer
The fine scoria ashes or " cinders " thrown out by basaltic volcanoes are often spongy masses of tachylyte with only a few larger crystals or phenocrysts imbedded in black glass. Such tachylyte bombs and scoria are frequent in Iceland, Auvergne, Stromboli, Etna, and are very common also in the ash beds or tuffs of older date, such as occur in Skye, Midlothian and Fife, Derbyshire, and elsewhere. Basic pumices of this kind are exceedingly wide-spread on the bottom of the sea, either dispersed in the "red clay" . and other deposits or forming layers coated with oxides of manganese, precipitated on them from the sea water. These tachylyte fragments, which are usually much decomposed by the oxidation and hydration of their ferrous compounds, have taken on a dark red colour. This altered basic glass is known as "palagonite "; con-centric bands of it often surround kernels of unaltered tachylyte, and are so soft that they are easily cut with a knife. In the palagonite the minerals also are decomposed, and are represented only by pseudomorphs. The fresh tachylyte glass, however, often contains lozenge -shaped crystals of plagioclase felspar and small prisms of augite and olivine, but all these minerals very frequently occur mainly as microlites or as beautiful skeletal growths with sharply-pointed corners or ramifying processes. Palagonite tuffs are found also among the older volcanic rocks. In Iceland a broad stretch of these rocks, described as " the palagonite formation," is said to cross the island from south-west to north-east
Canary
A second mode of occurrence of tachylyte is in the form of lava flows. Basaltic rocks often contain a small amount of glassy ground-mass, and in the limburgites this becomes more important and conspicuous, but vitreous types are far less common in these than in the acid lavas. In the Hawaiian Islands, however, the volcanoes have poured out vast floods of black basalt, containing felspar, augite, olivine, and iron ores in a black glassy base. They are highly liquid when discharged, and the rapid cooling which ensues on their emergence to the air prevents crystallization taking place completely. Many of them are spongy or vesicular, and their upper surfaces are often exceedingly rough and jagged, while at other times they assume rounded wave-like forms on solidification. Great
drawn
A third mode of occurrence of tachylyte is as the margins and thin offshoots of dikes or sills of basalt, dolerite and diabase. They are sometimes only a fraction of an inch in thickness, resembling a thin layer of pitch or tar on the edge of a crystalline dolerite dike, but veins several inches thick are sometimes met with. In these situations tachylyte is rarely vesicular, but it often shows very pronounced fluxion banding accentuated by the presence of rows of spherulites which are visible as dark brown rounded spots. The spherulites have a distinct radiate structure and sometimes exhibit zones of varying colour. The non-spherulitic glassy portion is sometimes perlitic and these rocks are always brittle. The commonest crystals are olivine, augite and felspar, with swarms of minute dusty black grains of magnetite. At the extreme edges the glass is often perfectly free from crystalline products, but it merges rapidly into the ordinary crystalline dolerite, which in a very short distance may contain no vitreous base whatever. The spherulites may form the greater parr of the mass, they may be a quarter of an inch in diameter and are occasionally much larger than this. These coarsely spherulitic rocks pass over into the variolites (q.v.) by increasing coarseness in the fibres of their spherulites, which soon become recognizable as needles of felspar or feathery growths of augite. The ultimate product of decomposition in this case also is a red palagonitic substance, but owing to the absence of steam cavities the tachylyte selvages of dikes are more often found in a fresh state than the basic lapilli in ash-beds. Many occurrences of basaltic pitchstones have been reported from Skye, Mull, and the western part of Scot-land; they are found also in connexion with the intrusive dolerite sills of the north of England and the centre of Scotland. In the Saar district
Other localities for tachylytes of this group are Nassau, Silesia and Sweden. The chemical composition of some of the rocks of this group is indicated by the analyses given below:-passage of his Agricola, describing this as a "singularly blessed time," but the hideous reign of terror had stamped itself ineffaceably on his soul, and when he sat down to write his History he could see little but the darkest side of imperialism. To his friend the younger Pliny we are indebted for the little we know about his later life. He was advanced to the consulship in 97, in succession to a highly distinguished man, Verginius Rufus, on whom he delivered in the senate a funeral eulogy. In 99 he was associated with Pliny in the prosecution of a great
special
SiO2. Al203. FeO. Fe203. CaO. MgO. Na2O. K20. H2O. I. Palagonite. Seljadalr, Iceland . 38.96 11.62 ... 14'75 9.13 6.29 o68 0.72 17.85 II. Palagonite from deep-sea deposits, Pacific 44.73 16.28 I 4'57 1.88 2.2 3 4'5 0 402 9'S 6 Ocean (with 2.89 % MnO2) ' IV. Tachylyte. Ardtun, Mull, Scotland 53.03 20.09 9'53 6'05 2.63 4'52 1.27 2.64 V. Tachylyte. The Beal, Portree, Skye .. 52'59 17.33 11.14 6'47 2'62 4.24 2.40 3'27 End of Article: TACHYLYTES, or TACHYLITES (from Gr. Taxbs, swift, Xbew, to dissolve, meaning " easily fused," though some have erroneously interpreted it as " easily soluble in acids ") If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/SUS_TAV/TACHYLYTES_or_TACHYLITES_from_.html"> TACHYLYTES, or TACHYLITES (from Gr. Taxbs, swif... </a> |
|
|
(Previous) TACHIENLU |
(Next) TACITUS, CORNELIUS (c. 55-120) |
|
Sponsored Advertisements