It is a Crystallization of abundant microlites occurs in response to high degrees of undercooling and supersaturation. Swanson et al. At least in principle, primary, high-temperature devitrification is distinct from crystallization in response to metamorphism, hydrothermal alteration or weathering.
Spherulites, lithophysae, orb texture and micropoikilitic texture composed of fine-grained quartz and feldspar are characteristic products of high-temperature devitrification of silicic glass Lofgren, la, b. Subsequent recrystallization to a mosaic of quartz and feldspar can destroy or modify the original devitrification textures. Lofgren b predicted that glass subject to prolonged heat, pressure and solutions would be granophyric, consisting of fine, equigranular quartz and feldspar, and lacking in textural evidence of the former presence of glass. Central parts of very thick several tens to hundreds of metres , densely welded ignimbrites commonly display granophyric texture due to slow cooling and crystallization of the formerly glassy components welded shards and pumice Spherulites Lofgren a, b; artificially generated many of the devitrification textures found in natural rhyolitic glasses and identified important controls on the rate and products of devitrification.
The rate of devitrification is dependent on temperature and on the presence and composition of aqueous solutions Marshall, ; Lofgren, The presence of alkali-rich solutions increases devitrification rates by four to five orders of magnitude Lofgren, OH in these solutions helps to transform polymeric chains of SiO, into separate SiO4 tetrahedra and allows more rapid diffusion of Na and K; both changes promote crystallization of quartz and feldspar.
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In addition to the textural changes noted above, devitrification results in significant changes in the bulk rock chemistry, particularly affecting SiO2, H2O, Na2O, K2O and Al2O3 contents Lipman, ; Lofgren, and, in some cases, trace and rare earth element abundances Weaver et al. Spherulites consist of radiating arrays of crystal fibres 3.
Each fibre is a single crystal that has only slightly different crystallographic orientation from adjacent crystals. Spherulites are a characteristic product of the high-temperature devitrification of natural glass. Spherulites are not spherical throughout their growth history Lofgren, a; Lofgren a demonstrated that the morphology of spherulites in rhyolitic glasses varied according to the temperature of formation Fig.
At low temperatures Lofgren b distinguished two textural associations among the devitrification products of silicic glasses.
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Glassy-stage texture consists of glass that contains isolated spherulites spherulitic obsidian Perlitic fractures and a variety of quench crystallites may be present in the glassy portions. Relatively slow cooling and Spherulites typically have diameters of 0. Isolated spherulites are commonly spherical.
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Adjacent spherulites may impinge on each other and produce elongate trains of spherulites, often aligned along flow layering. Outlines of spherical spherulites are often irregular due to impingement on adjacent spherulites. Bow-tie spherulites consist of two conical bundles of fibres joined at their apices. Plumose spherulites are open, coarse and commonly fan shaped.
Fibres in axiolitic spherulites radiate from a line. Modified from Lofgren Lithophysae are spherulites that have a central vug Wright, ; Ross and Smith, 3. They begin to grow at an early stage in the cooling history, when the hot glass is still able to deform plastically, and involve nucleation of spherulites on small vesicles. As spherulitic crystallization proceeds, the vesicles are expanded by the exsolving volatiles. The vugs vary from circular to star shaped, and may remain open or be lined or filled with minerals such as agate or chalcedony.
Lithophysae range up to larger diameters than spherulites, reaching a few tens of centimeters across. As for spherulites, lithophysae are characteristic products of high-temperature devitrification of coherent silicic glass, and are found in formerly glassy lavas and welded pyroclastic deposits. Perlite 5 Perlite is volcanic glass in which there are abundant, delicate, intersecting, arcuate and gently curved cracks that surround cores of intact glass, generally less than a few millimetres across 2. Perlitic cracks develop in response to hydration of the glass.
Hydration involves the diffusion of water into the solid glass, accompanied by a volume increase. Strain associated with hydration is released by means of perlitic cracks.
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In classical perlite, the cracks are distinctly arcuate and concentrically arranged around spherical, non-hydrated cores Ross and Smith, ; Friedman et al. In strongly flow-banded glassy lava, perlitic fractures form a roughly rectilinear network, comprising cracks that are subparallel and strongly oblique to the banding banded perlite Allen, 5. Hydration occurs after emplacement and late in the cooling history of the glass, or else after complete cooling to surface temperatures. Although perlitic fractures are not primarily the result of cooling cf. Marshall, , residual stress acquired during cooling is probably partly released when they form Allen, Micropoikilitic texture Micropoikilitic texture consists of small Perlitic fractures can develop in any hydrated coherent glass, including that in glassy lavas, shallow intrusions and densely welded pyroclastic deposits.
They may occur in the glassy domains between spherulites in partially devitrified obsidian. Although most commonly found in hydrated silicic glass, perlitic fractures also occur in mafic and intermediate composition glasses.
Distinguishing between primary and secondary volcaniclastic deposits
Perlite is usually recognizable with a hand lens but, in some cases, may only be evident in thin-section. Under favorable circumstances, an identical but much larger texture macro-perlite can be recognized in outcrop Yamagishi and Goto described macroperlite with cores up to about 6 cm across in Late Miocene submarine rhyolite.
They concluded that the macro-perlite formed before other columnar and polygonal joints that also occur in the rhyolite, and primarily as a result of quenching rather than hydration. The rhyolite is apparently not hydrated and does not show micro-perlitic cracks. An incipient stage in development of micropoikilitic texture in rhyolite involves poorly segregated quartzrich patches in the groundmass 4. Under crossed nicols these patches extinguish concurrently. Further development is reflected in the formation of more pronounced boundaries, visible in both plane polarized light and with crossed nicols 4.
Abundant feldspar laths that are enclosed in the micropoikilitic quartz show no preferred orientation.
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In addition, feldspar and sericite after feldspar is concentrated at the margins of quartz-rich patches. In some instances, the cores of the micropoikilitic quartz crystals are free of inclusions and very distinct 4. The cores extinguish concurrently with the remainder of the micropoikilitic quartz and commonly have highly irregular outlines. Sericite after feldspar is concentrated in the interstices between micropoikilitic quartz. The granular "sugary" texture of the hand specimen 4. Hydration initially affects outer surfaces of glassy lava flows or shallow intrusions , margins of cracks or joints in glassy lava flows and densely welded pyroclastic deposits, or surfaces of glassy clasts in volcaniclastic aggregates.
Hydration proceeds inward along a hydration front defined by strain birefringence, a change in the colour of the glass, and a change in the glass index of refraction Ross and Smith, ; Friedman et al. Measurable changes in alkali contents and in the Micropoikilitic texture results from initial devitrification of cooling glass, and develops in both coherent glassy rocks lavas and shallow intrusions and densely welded ignimbrites Anderson, ; Lofgren, b. The texture occurs mainly in rocks of silicic composition.
The rate of hydration is higher at higher temperatures and in the presence of alkali-rich solutions Lofgren, , and is also dependent on the glass composition, especially the water content Friedman and Long, Vesicles in pumice and scoria vary widely in both size and shape, even in the products from one eruption.
Tube pumice is characterized by vesicles with extremely elongate cylindrical shapes that have subparallel alignment, imparting a silky or fibrous or woody texture to the pumice 6. Tube pumice forms when vesicles are stretched during flow of vesiculating magma and usually involves silicic compositions, because these typically have appreciable yield strength Heiken and Wohletz, Mixed or "streaky" pumice consists of clots, bands, or layers of two or more magma compositions e.
Phenocrysts in pumice and scoria have the same textural characteristics as phenocrysts in non-vesicular or sparsely vesicular lavas, being euhedral, evenly distributed and ranging up to about 3 cm in size. A small proportion of phenocrysts within pumice or scoria can be fragmented in situ. Relict perlitic fractures are commonly present in ancient, altered, formerly glassy volcanic rocks. The texture is accentuated by crystallization of secondary minerals in the cracks and by narrow zones of devitrification in the adjacent glass Marshall, Alteration of glassy perlitized volcanic rocks can also be focused along the perlitic fractures In strongly altered rocks, relict perlitic fractures are difficult to recognize and easily overlooked or misinterpreted.
Allen described false pyroclastic textures in altered perlitized lavas from Benambra, Victoria. In these rocks, cuspate shard-like shapes are defined by phyllosilicate alteration of parts of the original arcuate perlitic fracture network, or else by the siliceous segments remaining between the altered perlitic fractures Fig. Correct identification as coherent, formerly glassy lava is favored where there is a gradation from the apparent shard texture to less altered perlite and an association with euhedral, evenly distributed phenocrysts Allen, Pumice and scoria pyroclasts are formed by explosive disruption of vesiculating magma.
Subaerial coherent lava flows are, in most cases, partly pumiceous or scoriaceous, and are associated with pumiceous or scoriaceous autoclastic deposits 2. Parts of subaqueous silicic lava flows, domes, cryptodomes and associated hyaloclastite can also be pumiceous Kato, ; Pumice and scoria 6 Pumice is highly vesicular volcanic glass with or without crystals 6. The term scoria is usually used for pumice of mafic to intermediate composition 6.
Reticulite thread-lace scoria is an Fig.
B False vitriclastic texture; apparent shards are phyllosilicate-altered sections of the perlitic fractures. C False vitriclastic texture; apparent shards are defined by interconnected phyllosilicate alteration along the perlitic fractures. Modified from Allen Pumice and scoria lapilli in pyroclastic flow and surge deposits can be appreciably rounded due to abrasion during transport Autoclastic pumice fragments are blocky or prismatic with planar to curviplanar surfaces.
Quench-fragmented tube pumice commonly breaks along surfaces normal to the elongation of the tube vesicles woody pumice. Autoclastic scoria fragments associated with a'a lava have ragged, twisted, spinose shapes. Transport and reworking of pyroclastic or autoclastic pumice and scoria by wind or water result in well-rounded shapes. Achneliths, bombs and blocky juvenile clasts 6 In explosive eruptions of low viscosity magmas, some pyroclasts are ejected in a molten condition and drawn out into elongate ribbons or aerodynamically-shaped achneliths and bombs Macdonald, ; Walker and Croasdale, ; Williams and McBirney, 6.
These may solidify before deposition and retain their distinctive shapes, or else be flattened into irregular rounded disks on impact.
Explosive magmatic and phreatomagmatic eruptions that accompany extrusion of silicic lava domes and flows generate non- to moderately vesicular, angular, blocky pyroclasts, some of which may be flow banded. In some cases, the interiors of bombs and juvenile blocks continue to vesiculate after deposition, causing the chilled outer surfaces to crack in a breadcrust pattern 6. Pumice and scoria fragments commonly have densities less than that of water 1.
If pumice fragments from subaerial eruptions are transported to shorelines or deposited on water, they can be transported by flotation in surface currents for thousands of kilometers prior to becoming waterlogged and sinking.