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pneumatolysis (adj. pneumatolytic) Changes in rock mineralogy and chemistry that are initiated by the action of a hot, chemically active, gaseous solution derived from a magma during its final stages of crystallization. The gaseous solution, rich in volatile elements such as fluorine, chlorine, boron, and hydrogen, is released from the crystallizing interior of the granite body and rises through cracks and fissures in the crystalline and cooler upper carapace to initiate greisenization and tourmalinization of the roof-zone granite.