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Pneumatolysis

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(Gr. 7re€i ua, vapour, and Anew, to set free), in petrology, the discharge of vapours from igneous magmas and the effects produced by them on rock masses. In all volcanic eruptions the gases given off by the molten lavas are powerful agencies. The slaggy clots of lava thrown out from the crater are so full of gas that when they cool they resemble spongy pieces of bread. The lava streams as they flow down the slopes of the volcano are covered with white steam clouds, while over the orifice of the crater hangs a canopy of vapour which is often darkened by fine particles of ash. Most authors ascribe volcanic explosions to the liberation of steam from the magma which held it in solution, and the enormous expansive powers which free water vapour possesses at very high temperatures.

Of these gases the principal are water and carbonic acid, but by analysis of the discharges from the smaller fumaroles, for the active crater is generally too hot to be approached during an eruption, it has been ascertained that hydrogen, nitrogen, hydrochloric acid, boron, fluorine, sulphuretted hydrogen and sulphurous acid are all emitted by volcanoes. A recent lava flow has been likened to a great fumarole pouring out volatile substances at every crack in its slaggy crust. Many minerals are deposited in these fissures, and among the substances produced in this way are ammonium chloride, ferric chloride and oxide, copper oxide (tenorite and cuprite) and sulphur; by reacting on the minerals of the rock many zeolites and other secondary products are formed. These processes have been described as "juvenile" or "post eruptive," and it is believed that the amygdales which occupy the cavities of many porous lavas are not due really to weathering by surface waters percolating in from above, but to the action of the steam and other gases set free as the lava crystallizes. The zeolites are the principal group of minerals which originate in this way together with chlorite, chalcedony and calcite. The larger cavities (or geodes) are often lined with beautiful crystal groups of natrolite, scolecite, thomsonite, stilbite, and other minerals of this order.

The active gases were evidently in solution in the magma as it rose to the surface. Some geologists believe it is of subterranean origin like the lava itself, and is an essential or original component of the magma. They point to the exist ence of gases in considerable quantity in meteorites, and, comparing the earth to a great aerolite, insist that it should contain gases in solution like the smaller masses of the same kind, Others hold it more probable that the water has percolated in from the surface, or seeing that many volcanoes stand near the sea margin and by their linear disposition may be disposed along fissures or lines of weakening in the crust, they argue that the water of the sea may have filtered down even in spite of the great outward pressure exerted by the steam generated by contact with the intensely heated rock. The abundance of chlorides and hydrochloric acid is appealed to also in favour of a marine origin for the water. Against this we may place the fact that at great depths whence active magmas ascend the rocks are under so great pressures that every fissure is closed up; in fact in some of the deepest mines the quantity of water found in the workings is often exceedingly small. Probably there is some truth in both theories, but the balance of probability seems to incline in favour of the view that the water is an original and essential part of the magma and not an introduction from above.

Long after a lava has cooled down and become rigid the vapours continue to ooze out through its fissures, and around many volcanoes which are believed to be extinct there are orifices discharging gas in great quantities. This state of activity is said to be "solfataric," and a good example of it is the volcano called the Solfatara near Naples. The numerous "Soufrieres" of the West Indies are further instances. The prevalent gas is steam with sulphuretted hydrogen and carbonic acid. At the Grotto del Cane in the Phlegraean Fields. (Italy) the carbonic acid rising from fissures in the bottom of a cave covers the floor as a heavy layer, and a dog placed in the interior of the cave becomes stupefied by the narcotic gas; such gas-springs have been called "mofettes." Around them there is often a deposit of sulphur, produced by oxidation of the sulphuretted hydrogen, and the rocks are bleached, softened and decomposed. White crusts of alum, various sulphates, and sulphides such as pyrites, also carbonates of soda and other bases, are formed by the action of the acid vapours on the volcanic rocks. The final manifestation of volcanic activity in such a region may be the discharge of heated waters, which have ascended from the deep-seated magma far below the surface, and make their appearance as groups of hot springs;. these springs persist long after the volcanoes which give rise to them have become quite extinct.

It is now believed by a large number of geologists and mining engineers that these ascending hot waters are of paramount importance in the genesis of some of the most important types of ore deposits. Analyses have proved that the igneous rocks often contain distinct though very small quantities of the heavy metals; it is also established beyond doubt that veins of gold, silver, lead, tin and mercury most commonly occur in the vicinity of intrusive igneous masses. At Steamboat in Nevada, hot springs, probably of magmatic origin, are forming deposits of cinnabar. At Cripple Creek, Colorado, and in many other places gold-bearing veins occur in and around intrusive plugs of igneous rock. Tin ores in all parts of the world are found in association with tourmaline granites. In all cases the veins bear evidence of having been filled from below by hot waters set free during the cooling of the igneous intrusions. Volcanic rocks are consequently the parent sources of many valuable mineral deposits, and the agency by which they were brought into their present situations is the volatile products discharged as the magma crystallized. The process was no doubt a long one and it is most probable that both steam and water took part in it. Above 365° C. water is a gas under all pressures and the action is strictly pneumatolytic; below that temperature steam is changed to water by pressure and the action may be described as hydatogenetic. The distinction is unessential, and in our ignorance of the temperatures and pressures prevailing at considerable depths we lack the means of classification. In what condition the metallic ores are dissolved and by what reactions they are precipitated depends on many factors only partly understood. The tin ores are so often associated with minerals containing boron and fluorine that it is quite probable that they were combined with these elements in some way, but they were deposited in nearly all cases as oxides. Other gaseous substances, such as sulphuretted hydrogen, carbonic acid and hydrochloric acid, probably have an important part in dissolving certain metals; and the alkaline carbonates, sulphides and chlorides have been shown by experiment to act also as solvents. In these ore deposits not only the heavy metals are found, but often a much larger quantity of minerals such as calcite, barytes, fluorspar, quartz and tourmaline which serve as a matrix or gangue, and have been deposited by the same agencies, and often at the same time as the valuable minerals.

In their passage upwards and outwards through the rocks of the earth's crust, these gases and liquids not only deposit minerals in the fissures along which they ascend, but attack the surrounding rocks and alter them in many ways. The granite or other plutonic mass from which the vapours are derived is especially liable to these transformations, probably because it is at a high temperature, not having yet completely cooled down. Around the tin-bearing veins in granite there is extensive replacement of felspar and biotite by quartz, tourmaline and white micas (the last-named often rich in lithia). In this way certain types of altered granite are produced, such as greisen and schorl rock (see Schorl). In the slates adjacent to the tin veins tourmalinization also goes on, converting them into schorl-schists. The alteration of felspar into kaolin or china clay is also a pneumatolytic process, and is often found along with tin veins or other types of mineral deposit; probably both fluorine and carbonic acid operated in this instance along with water. Equally common and important is the silicification of rocks near mineral veins which carry gold, copper, lead and other metals. Granites and felsites may be converted into hard cherty masses of silica. Limestones undergo this transformation very readily; at the same time they are regarded as rocks very favourable to the deposition of ores. Probably the great frequency 'with which they undergo silicification and other types of metasomatic replacement is one of the main causes of the abundance of valuable deposits in them. The process known as "propylitization," which has extensively affected the andesites of the Hungarian goldfields, is believed to be also a consequence of the action of pneumatolytic gases. The andesites change to dull, soft, greenish masses, and their original minerals are to a large extent replaced by quartz, epidote, chlorite, sericite and kaolin. Around granites intrusive into serpentine and other rocks containing much magnesia, there is often extensive "steatisation," or the deposit of talc and steatite in place of the original minerals of the rock. Some of the apatite veins of Canada and Norway accompany basic rocks of the gabbro group; it has been argued that the apatite (which contains phosphorus and chlorine) was laid down by vapours or solutions containing those gases, which may play a similar part in the basic rocks to that taken by fluorine and boron in the pneumatolytic veins around granites. In the country rock around the veins scapolite, a lime alumina silicate, containing chlorine, often is substituted for lime-felspar.

These extensive changes attending the formation of mineral veins are by no means common phenomena, but in many plutonic masses pneumatolytic action has contributed to the formation of pegmatites (q.v.). (J. S .F.)