Carbonic acid when combined with a base is a weak acid, readily displaced by a stronger, as sulphuric, hydrochloric or phosphoric acid, and also by sulphur and by many of the organic acids. But the molecule of carbonic acid is never broken up, is never separated into its component elements under conditions ordinarily subsisting in the earth’s crust, at least not at the depths reached in the underground circulation of meteoric water.
Volcanic action alone, or an earth-temperature far above normal, furnishes the physical conditions in which carbon dioxide is dissociated, or the conditions that admit of its being reduced to the monoxide by carbon, or by other deoxidizing agents.
Carbonic acid is a permanent refuge of oxygen; once locked up in combination with carbon, oxygen remains inert for all time, in a condition of stable equilibrium, inactive, and chemically indifferent to all the complex changes taking place in the depths of the earth. Even in ore-deposits undergoing oxidation and re-formation, carbonic acid, once formed, is itself in-susceptible to chemical change. Neither hydrogen, carbon, sulphur, or the most powerful deoxidizing metallic sulphides, can decompose it at ordinary temperatures. This immutability, however, is maintained only under the conditions subsisting in the depths of the strata, where life does not exist; the carbonic acid in the soil and in the atmosphere readily gives up its oxygen and carbon to plant-life.
Water almost equals carbonic acid in stability. It is true that water is decomposed by electrolysis and in many chemical reactions in the laboratory; yet, at temperatures approaching the normal, water is probably not dissociated, except in a very limited way, in any of the processes incidental to ore-formation.
In the complex chemical changes that take place in the oxidation and re-formation of ores, it is possible that water may be decomposed; but, quantitatively, such dissociation must be insignificant. Practically, water may be regarded as chemically stable under ordinary conditions and temperatures. The fact that, in the presence of the deep-circulating underground waters that contain no free oxygen, the complex sulphides forming the ore-bodies have been preserved unaltered for ages, is evidence that water is chemically inert to all the elements present in the depths of the strata.
Among minerals, many oxygen compounds, as, for example, quartz, corundum, cassiterite, rutile and zircon, resist decomposition, as do also the silica, the alumina and the other acid radicals in silicates, aluminates, borates, phosphates, titanates, tantalates, tungstates and chromates. Minerals containing oxides of the alkalies and the alkaline earths, while usually permanent, may have their oxygen displaced by sulphur and by the halogens, chlorine, bromine, iodine and fluorine. In the greater number of the refractory minerals and permanent oxygen-compounds, the chemical union of the elements with oxygen may probably date back to the primal origin of the earth.
Carbon and hydrogen alone, of all the elements, unite with oxygen under conditions now subsisting in ore-deposits, to form fixed compounds, that, sealed in the rocks, can endure to the end of time.