The recognition of volcanic clays and the significance of heavy minerals

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Abstract
The explosive ejection of material from a volcanic vent produces pyroclastics. These are stratigraphically important because of their isochronous nature, lateral extent and potential for absolute age determinations. The pyroclastics also provide information on the nature of the volcanic activity, which is possibly the only record because of non-preservation in other areas. The recognition of pyroclastics is therefore important, but often difficult,because the high-temperature igneous material is susceptible to alteration in the diagenetic environment. In Quaternary sediments the composition of glass shards is an important aspect of tephrochronology (Westgate & Briggs, 1980) but in older sediments glass shards are usually altered to clay minerals. Exceptionally, shards are preserved unaltered, as in the case of early carbonate and silica cementation (Francis et al., 1968; Jeans et al., 1977). According to Ross (1928), features that are diagnostic for the recognition of a volcanic clay (bentonite is the term used by Ross) are, in order of apparent reliability: (i) presence of glass relict structure; (ii) presence of minerals and crystal forms characteristic of volcanic rocks, including biotite, feldspar, pyroxenes and hornblende; (iii) absence of minerals that do not have a volcanic origin, including microcline, garnet, muscovite, kyanite and andalusite; (iv) waxy lustre; (v) reaction to water; (vi) optical properties; (vii) chemical properties.

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