248 MINERALOGY 



Pneumatolysis. Most mineralizers form gases and volatile 

 compounds which penetrate the cracks, cavities, and pores of adja- 

 cent rock formations, where they may be condensed, or by decom- 

 position deposit compounds ; or by the reduction of temperature 

 and pressure deposit compounds carried in solution; or by the 

 direct interaction with the rock mass form minerals, all of which 

 are concentrated near the margin of large intruded igneous masses 

 and extending out as impregnations in the sedimentary formations 

 of the immediate vicinity. Numerous ore deposits are of this na- 

 ture, especially those of tin or cassiterite, where the tin has been 

 concentrated by a squeezing out of the volatile tin fluoride to be 

 decomposed by contact with steam ; it deposits SnO 2 in the open 

 veins and cracks of the formation; this decomposition produces 

 hydrofluoric acid to further react with the walls of the formation 

 to produce such minerals as fluorite and topaz. Minerals formed 

 by the action of gases or volatile compounds are said to be pneu- 

 matolytic or formed by pneumatolysis. The gases most active in 

 pneumatolysis are fluorine, water or steam, hydrogen sulphide, 

 boron, chlorine, and their volatile compounds. 



Typical minerals formed by this process are tourmaline, topaz, 

 cassiterite, rutile, oxides of iron, micas, fluorite, quartz, sulphides 

 of copper, lead, arsenic, also titanite and axinite, and numerous 

 rare minerals as wolframite, scheelite, uraninite, and allanite. 



Intimately connected, possibly with the last stages of cooling of 

 large intruded igneous rock masses, is the formation of the so- 

 called contact minerals ; produced by the interaction of and the 

 impregnation of the formation by steam and associated with the 

 heat of contact and pressure. This stage directly follows that of 

 the pneumatolytic action of gases and is usually termed thermal 

 metamorphism, though heat is connected with all metamorphic 

 changes; in this case the temperature is comparatively high, 

 especially near the contact of the igneous rock intruded. The min- 

 erals formed under such conditions, when the volatile gases are pre- 

 vented from escape and are held under pressure, are of a complex 

 nature when contrasted with those formed where there is a ready 

 escape of gases. 



The materials at hand for the formation of new minerals are not 

 only those contained in the sedimentary formation under the pro- 

 cess of alteration, but also those elements carried in solution, by 

 the interaction of which at high temperatures and pressure and con- 

 tinued through long periods of time large and most beautifully de- 



