The vitric matrix of pre-caldera acid tuff and tuff breccia of the Santorini volcano, Aegean Sea, Greece has been generally replaced by one or more of the following authigenic minerals: K-rich and (K,Ca)-rich clinoptilolite, mordenite, opal-CT, and clay minerals. Halite is also present in some samples. Initial compositional inhomogeneities between the dacitic blocks in tuff breccia and tuff seem to have controlled the type of K-rich heulandite-group zeolite that formed. Mordenite postdates the heulandite-group zeolites and opal-CT. Some mordenite has replaced the rims of glass shards. The alteration minerals are not related to vertical or lateral zonation, and the irregular distribution of their assemblages is attributed to variations in heat flow, ionic activity in interstitial waters, and permeability. The pyroclastic rocks were in a region of active heat flow during and after their emplacement. The formation of authigenic silicates may have led to the sealing of open spaces and fractures, imposing barriers to permeability and subdividing the original open system into smaller closed systems. As alteration progressed, some of the trapped water in each individual domain was consumed in hydration reactions. Salts could have been concentrated by such a process, and halite probably precipitated from solutions of appropriate composition in the individual closed systems.

Montmorillonite and K-feldspar deposits of potential economic interest occur in the Late Oligocene-Miocene tuffs of the Vizcarra Formation in the state of Durango, Mexico. The two minerals were formed separately from rhyodacitic to rhyolitic pyroclastic deposits in a closed hydrologie system and diagenetically altered following two different patterns. In material deposited on dry land the glass was completely replaced by K-feldspar, and the interstices between the replaced glass bubbles and shards were filled with chalcedony, quartz, and albite. Pyroclastic material deposited in an alkaline lacustrine environment were diagenetically altered to montmorillonite, which formed the bentonitic tuffs widely exposed beneath the K-feldspar-rich tuff. These bentonitic tuffs contain as much as 85% montmorillonite plus authigenic chalcedony and quartz. Pyrogenic sanidine, quartz, and oxybiotite, coarse glass shards, and clastic grains make up about 8% of the clay-rich tuffs. The composition of the montmorillonite corresponds to the formula (Si3.88Al0.12)(Al1.41Mg0.59)O10(OH)2(Ca0.07Mg0.11,Na0.28K0.06). The montmorillonite is dioctahedral, the surface acidity is of the Lewis type, and the clay swells to one- and two-layer complexes. The cation-exchange capacity is 64 meq/100g; base exchange is Ca2+, 15; Mg2+, 20; Na+, 4.1; and K+, 1.0 meq/100 g. Its interlamellar charge is 18.6 microcoulombs/cm2. Dacitic pyroclastics were deposited later at higher elevations along the margins of the basin. Percolating solutions apparently removed a siliceous leachate from the dacitic glass and partially altered it to clinoptilolite.

Volcanic tuffs are important in determining the stratigraphic age of sedimentary sequences, which is closely related to the tectonic, sedimentological, geomorphological, palaeoclimatic, and palaeo-ecological evolution of the sequences. However, it may be difficult practically to identify tuffs in stratigraphic sections, especially when they are altered after deposition. In this study, a series of petrographic, mineralogical, and geochemical analyses was deployed in the tuff layers cropping out in the Lunpola Basin of the central Tibetan Plateau, which is a crucial site for studying the dynamics and processes of the Cenozoic uplift of the Tibetan Plateau and its environmental impacts. In these pyroclastic layers, authigenic analcime is the main volcanic glass alteration product. The analcime-bearing samples are Na-enriched, in contrast to the K-abundant magmatic rocks in the central Tibetan Plateau. The distribution patterns of the rare earth elements (REEs) of the bulk analcime-bearing samples are similar to those of the magmatic rocks but different from those of the analcime-poor sediments and analogues of well-mixed upper continental material (e.g. the upper continental crust and post-Archaean Australian average shales). The distinct distributions of Na and REEs among analcime-bearing samples, analcime-poor sediments, and volcanic rocks reveal that analcime formed both from alteration of volcanic material and from mixing processes with non-volcanic sediments. It is proposed, therefore, that the discrimination plot of Na2O/Al2O3-(La/Yb)N may be used to distinguish the analcime-related pyroclastic rocks from the basin sedimentary sequence, and thus it may provide a means of discriminating between various volcanic material and of supporting age dating by tephrochronology in the central Tibetan Plateau.

The Late Pleistocene kamafugite–carbonatite association at San Venanzo-Pian di Celle forms part of the Umbria-Latium Ultra-alkaline District (ULUD) of central Italy and, together with Toro-Ankole, SW Uganda and Mata de Corda, Brazil, represents one of three similar occurrences so far reported worldwide.

Excellent field exposure and stratigraphic control prompted a study of the kamafugite–carbonatite suite and related phase interactions to understand the nature of the distinct mineral assemblages of the pyroclasts, compared to that of the lavas, the former containing essential potassium feldspar and aluminous diopside crystals, absent in the latter.

The pyroclastic rocks represent a small amount of magma characterized by ubiquitous mantle xenocrysts and emplaced by early high-velocity eruptions. All the investigated specimens show a high Mg/(Mg+Fe2+) ratio (0.84–0.93) and high compatible elements (Ni+Cr>1000 ppm). Lavas (venanzite, i.e. leucite melilitite) and a sill (uncompahgrite, i.e. melilitolite) represent final events in the volcanic sequence. They yielded a (Na+K)/Al ratio of c. 1.1 and are larnite-bearing in the CIPW norm. Glass from the lapilli is peralkaline, i.e. (Na+K)/Al>2, and close to the lava in composition. Glass from melilitolite yielded CIPW Or and Hy and is strongly peralkaline, i.e. (Na+K)/Al = 5–6. The lapilli typically exhibit concentrically zoned structures which compound subliquidus venanzite phases, e.g. melilite, leucite, and kalsilite, with mantle xenolithic/xenocrystic debris and carbonatite phases. These lapilli represent a distinct variant of the venanzite liquid, mechanically fractionated and quenched by the diatremic process.

Mantle-normalized HFSE for both lava and lapilli show typical extrusive-carbonatite patterns. Carbonatitic beds intercalated with the pyroclastic suite are distinct and typically consist of carbonates high in Sr, Ba and REE. Primary carbonate yielded C isotope compositions ranging from –5.0 to –6.0 δ13C‰, falling within the range of mantle compositions. Distinct differentiation trends of the venanzite magma and its derivatives were recognized, hinging on the coexistence of the silicate and carbonatite fractions. Potential sanidine crystallization trends are suggested, distinct from the venanzite→melilitolite trend, reported for Oldoinyo Lengai assemblages.

Unusual aspects of the San Venanzo rock association, relative to similar rock types elsewhere, include the combination of a rare mantle source composition with a lithosphere about 80 km thick. A genetic model for the origin of the San Venanzo kamafugite–carbonatite association and related carbonate-silicate interactions is proposed and discussed. This may be relevant to the petrogenesis of similar rocks elsewhere, particularly in the light of the detailed data on the pyroclasts.

Hydrated halloysite was discovered in books, a morphology previously associated exclusively with kaolinite. From ∼1.5 to ∼1500 μm in length, the books showed significantly greater mean Fe contents (Fe2O3 = 5.2 wt.%) than tubes (Fe2O3 = 3.2 wt.%), and expanded rapidly with formamide. They occurred, along with halloysite tubes, spheroids and plates, in highly porous yet poorly permeable, silt-dominated, Si-rich, pumiceous rhyolitic tephra deposits aged ∼0.93 Ma (Te Puna tephra) and ∼0.27 Ma (Te Ranga tephra) at three sites ∼10–20 m stratigraphically below the modern landsurface in the Tauranga area, eastern North Island, New Zealand. The book-bearing tephras were at or near saturation, but have experienced intermittent partial drying, favouring the proposed changes: solubilized volcanic glass + plagioclase→halloysite spheroids→halloysite tubes→halloysite plates→ halloysite books. Unlike parallel studies elsewhere involving both halloysite and kaolinite, kaolinite has not formed in Tauranga presumably because the low permeability ensures that the sites largely remain locally wet so that the halloysite books are metastable. An implication of the discovery is that some halloysite books in similar settings may have been misidentified previously as kaolinite.

The Recent Saefell tuff-ring on Heimaey, Iceland was formed by surtseyan activity in shallow seawater. The tuff-ring has a basal diameter of about 3 km, a maximum rim height of 188 m above sea-level and a crater diameter of 1300 m. Three tuff-units are recognised, separated by unconformities on and inside the crater rim due to syndepositional slumping. The crater contains a nested rim which was constructed above slumped crater tuffs. Directional data indicate strongly directed blasts to the SW at a late stage in the activity.

Throughout the volcanic activity, base-surges formed antidunes, U-shaped channels, vesiculated tuffs, small ripples and plastering structures. One antidune reflects a decrease in surge flow power during deposition and subsequent slumping due to base-surge drag and instabilities developed during growth. On the basis of field characteristics, the structures are divided into those deposited by hot, dry, fast-moving surges and those by cooler, wet, slow-moving surges. Base-surges are compared with turbidity currents and deposition of distinct structures by the head, body and tail regions is interpreted.