Geographic occurrences Tachylite

1 geographic occurrences

1.1 scoria sources
1.2 lava flow sources
1.3 dike , sill sources

geographic occurrences
scoria sources

the fine scoria ashes or cinders thrown out basaltic volcanoes spongy masses of tachylite few larger crystals or phenocrysts imbedded in black glass. such tachylite volcanic bombs , scoria frequent in iceland, auvergne, stromboli , etna, , common in ash beds or tuffs of older date, such occur in skye, midlothian , fife, derbyshire, , elsewhere. basic pumices of kind exceedingly widespread on bottom of sea, either dispersed in pelagic red clay , other deposits or forming layers coated oxides of manganese precipitated on them sea water. these tachylite fragments, decomposed oxidation , hydration of ferrous compounds, have taken on dark red color. altered basic glass known palagonite ; concentric bands of surround kernels of unaltered tachylite, , soft cut knife. in palagonite minerals decomposed , represented pseudomorphs. fresh tachylite glass, however, contains lozenge-shaped crystals of plagioclase feldspar , small prisms of augite , olivine, these minerals occur microlites or beautiful skeletal growths sharply-pointed corners or ramifying processes. palagonite tuffs found among older volcanic rocks. in iceland broad stretch of these rocks, described palagonite formation, said cross island south-west north-east. of these tuffs fossiliferous; others intercalated glacial deposits. lavas occur olivine-basalts. palagonite tuffs found in sicily, eifel, hungary, canary islands, , other places.

lava flow sources

flaked stone artefacts australia, made of tachylite

a second mode of occurrence of tachylite in form of lava flows. basaltic rocks contain small amount of glassy ground-mass, , in limburgites becomes more important , conspicuous, vitreous types far less common in these in acid lavas. in hawaiian islands, however, volcanoes have poured out vast floods of black basalt, containing feldspar, augite, olivine, , iron ores in black glassy base. highly liquid when discharged, , rapid cooling ensues on emergence air prevents crystallization taking place completely. many of them spongy or vesicular, , upper surfaces exceedingly rough , jagged, while @ other times assume rounded wave-like forms on solidification. great caves found crust has solidified , liquid interior has subsequently flowed away, , stalactites , stalagmites of black tachylite adorn roofs , floors. on section these growths show central cavity enclosed walls of dark brown glass in skeletons , microliths of augite, olivine , feldspar lie imbedded. crater of mt. kilauea, thin clouds of steam rise constantly, , bubbles of vapor liberated molten rock carry air them thin fibers of basalt solidify @ once , assume form of tachylite threads. under microscope prove glassy small circular air vesicles drawn out long tubes. in hawaiian islands glassy basaltic lavas of kind @ common. small outcrop @ spring hill in victoria, australia has tachylite has been exploited material making aboriginal flaked stone implements.

dike , sill sources

a third mode of occurrence of tachylite margins , thin offshoots of dikes or sills of basalt , diabase. fraction of inch in thickness, resembling thin layer of pitch or tar on edge of crystalline diabase dike, veins several inches thick found. in these situations tachylite vesicular, shows pronounced fluxion banding accentuated presence of rows of spherulites visible dark brown rounded spots. spherulites have distinct radiate structure , exhibit zones of varying color. non-spherulitic glassy portion perlitic, , these rocks brittle. common crystals olivine, augite , feldspar, swarms of minute dusty black grains of magnetite. @ extreme edges glass free crystalline products, merges rapidly ordinary crystalline diabase, in short distance may contain no vitreous base whatever. spherulites may form greater part of mass, may quarter of inch in diameter , larger this. these coarsely spherulitic rocks pass on variolites increasing coarseness in fibers of spherulites, become recognizable needles of feldspar or feathery growths of augite. ultimate product of decomposition in case red palagonitic substance, owing absence of steam cavities tachylite selvages of dikes more found in fresh state basic lapilli in ash-beds. many occurrences of basaltic pitchstones have been reported skye, mull, , western part of scotland; found in connection intrusive diabase sills in north of england , center of scotland. in saar district of germany similar rocks occur, of have been described weisselbergites (from weisselberg).

other localities tachylites of group new providence, silesia , sweden.