2002 Volume 6 Issue 2 Pages 53-59
Non-magmatic minerals are abundant in volcanic ash, especially in volcanic ash of phreatic and phreatomagmatic eruptions. Accidentally incorporated minerals from country rocks (rock into which magma or water intrude) are common in all types of eruptions. Any kind of rock-forming minerals in the crust and mantle may be incorporated into volcanic ash by mechanical stripping from conduits or magma chambers. Volcanic ash often contains hydrothermal minerals, such as smectite, kaolinite, cristobalite and gypsum, that are formed by rock-fluid or magma-fluid interactions. Hydrothermal minerals are common in the volcanic ash of phreatomagmatic and phreatic eruptions. Some kinds of minerals in volcanic ash must have precipitated from volcanic fluids that involve hydrothermal water in subvolcanic aquifers, crater lakes, fumarolic gases, and volcanic gases. Sulfate, halide, and native sulfur may be the major minerals precipitating from volcanic fluid, but they have not been well documented.
Volcanic ash commonly contains hydrothermal clay minerals which are similar to soil clay formed by weathering of magmatic fragments. Sulfide and halide minerals in volcanic ash have an influence on human environments such as through soil properties and climate.
