TACHYLYTES, or Tachylites (from Gr. ταχύς, swift,
λύειν, 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. Small crystals of felspar or
of olivine are sometimes visible in them with the unaided eye.
All tachylytes weather rather easily, and by oxidation of their
iron become dark brown or red. Three modes of occurrence
characterize this rock. In all cases they are found under
conditions which imply rapid cooling, but they are much less
common than acid volcanic glasses (or obsidians), the reason being
apparently that the basic rocks have a stronger tendency to
crystallize, partly because they are more liquid and the molecules
have more freedom to arrange themselves in crystalline order.
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 widespread
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”; concentric
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. Some of these tuffs
are fossiliferous; others are intercalated with glacial deposits.
The lavas with which they occur are mostly olivine-basalts.
Palagonite tuffs are found in Sicily, the Eifel, Hungary, Canary
Islands, &c.
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 caves are found where the crust has solidified and the liquid
interior has subsequently flowed away, and stalactites and stalagmites
of black tachylyte adorn the roofs and floors. On section
these growths show usually a central cavity enclosed by walls of dark
brown glass in which skeletons and microliths of augite, olivine
and felspar lie imbedded. From the crater of Kilauea thin
clouds of steam rise constantly, and as the bubbles of vapour are
liberated from the molten rock they carry into the air with them
thin fibres of basalt which solidify at once and assume the form of
tachylyte threads. Under the microscope they prove to be nearly
completely glassy with small circular air vesicles sometimes drawn
out to long tubes. Only in the Hawaiian Islands are glassy basaltic
lavas of this kind at all common.
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 part 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 Scotland;
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 of Germany similar rocks occur, some of which have
been described as weisselbergites (from Weisselberg).
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:—
SiO2
Al2O3
FeO
Fe2O3
CaO
MgO
Na2O
K2O
H2O
I. Palagonite. Seljadalr, Iceland......................
II. Palagonite from deep-sea deposits, Pacific
Ocean (with 2.89% MnO2)
III. Franz Joseph Land.....................................
IV. Tachylyte. Ardtun, Mull, Scotland................
V. Tachylyte. The Beal, Portree, Skye.............
