Pneumatolysis

PNEUMATOLYSIS (nu-mat-611-sis), in petrology, dis charge of vapours from igneous magmas and effects produced by them on rock masses (so called from Gr. rvEiwa, vapour, and Xbei,v, to set free). In volcanic eruptions the gases given off by 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.

Cause of Volcanic Explosions.

Most authors ascribe vol canic 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 sur face waters percolating in from above, but to the action of the steam and other gases set free as the lava crystallizes. The zeo lites 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.

Solfataric Activity.

The active gases were evidently in solu tion in the magma as it rose to the surface. Geologists now believe they are of subterranean origin like the lava itself, and an essential or original component of the magma. 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 prev alent gas is steam with sulphuretted hydrogen and carbonic acid. 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 a solfataric region may be the discharge of heated waters, which have ascended from the deep-seated magma far below the surface, and make their appear ance as groups of hot springs ; these springs persist long after the volcanoes which give rise to them have become extinct.

Role of Hot Waters.—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 quanti ties 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, Nev., hot springs, probably of magmatic origin, are forming deposits of cinnabar; at Cripple Creek, Colo., 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, and 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 conse

quently the parent sources of many valuable mineral deposits, and the agency by which they were brought into their present situ ations 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. In what condition the metallic ores are dissolved and by what reactions they are pre cipitated 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 experi ment to act also as solvents. In these ore deposits not only the heavy metals are found, but often a much larger quantity of min erals 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. Alteration of Minerals.—In their passage upwards and out wards 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; the granite or other plutonic mass from which the vapours are derived is specially liable to transformation, probably because it is at a high tempera ture, not having yet completely cooled down. Around the tin-bear ing 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 (q.v.) 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 silici fication 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 transforma tion very readily ; at the same time they are regarded as rocks very favourable to the deposition of ores—probably the great fre quency 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 "steatization," 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 (q.v.), 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.)