Trioctahedral vermiculite in a 1980 pyroclastic flow, Mt. St. Helens, Washington

Trioctahedral vermiculite, previously unreported, is an abundant phyllosilicate in a Mt. St. Helens pyroclastic flow that was emplaced during the 18 May 1980 eruption. The response of this mineral to various treatments suggests its layer charge is approximately 0.6 per O/sub 10/(OH)/sub 2/. In the pyroclastic flow, vermiculite is present in samples collected in 1981 from the surface-to-90-cm depth, but it is present only from 5 to 90 cm deep in samples collected in 1983. The apparent depletion in trioctahedral vermiculite of the 1983 surface (0-1.5 cm) is believed to be caused by weathering due to the prevailing acidic rainfall (pH = 4.0-4.6). One crucial problem in tephritic soils is to establish the origin of the 2:1 phyllosilicates. The authors findings show that these layer silicates, possessing different layer charges, were most likely present with the lithic fragments of the ejecta. Consequently trioctahedral vermiculite and the previously reported saponite present in Mt. St. Helens tephra should be considered detrital minerals.