Page 33 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Lithophysae cooled (or reheated) slowlv.

Lithophysae are spherulites that have a central vug Perlite (5)
(Wright, 1915; Ross and Smith, 1961) (3.7, 25.1). They
begin to grow at an early stage in the cooling history, Perlite is volcanic glass in which there are abundant,
when the hot glass is still able to deform plastically, and delicate, intersecting, arcuate and gently curved cracks
involve nucleation of spherulites on small vesicles. As that surround cores of intact glass, generally less than a
spherulitic crystallization proceeds, the vesicles are few millimetres across (2.6B, 5, 27.2D, 42.2, 42.6-7).
expanded by the exsolving volatiles. The vugs vary Perlitic cracks develop in response to hydration of the
from circular to star shaped, and may remain open or be glass. Hydration involves the diffusion of water into the
lined or filled with minerals such as agate or solid glass, accompanied by a volume increase. Strain
chalcedony. Lithophysae range up to larger diameters associated with hydration is released by means of
than spherulites, reaching a few tens of centimeters perlitic cracks. In classical perlite, the cracks are
across. As for spherulites, lithophysae are characteristic distinctly arcuate and concentrically arranged around
products of high-temperature devitrification of coherent spherical, non-hydrated cores (Ross and Smith, 1955;
silicic glass, and are found in formerly glassy lavas and Friedman et al., 1966; Allen, 1988) (5.1). In strongly
welded pyroclastic deposits. flow-banded glassy lava, perlitic fractures form a
roughly rectilinear network, comprising cracks that are
Micropoikilitic texture subparallel and strongly oblique to the banding (banded
perlite) (Allen, 1988) (5.2). Hydration occurs after
Micropoikilitic texture consists of small (<1 mm), emplacement and late in the cooling history of the glass,
commonly irregular crystals of one mineral that or else after complete cooling to surface temperatures.
completely enclose even smaller crystals of another Although perlitic fractures are not primarily the result of
mineral. Micropoikilitic texture comprising optically cooling (cf. Marshall, 1961), residual stress acquired
continuous quartz that encloses laths or spherulites of during cooling is probably partly released when they
feldspar is especially common in rhyolites (4) and has form (Allen, 1988).
been generated artificially in devitrification experiments
on rhyolitic glass (Lofgren, 1971b). The same texture Perlitic fractures can develop in any hydrated coherent
was termed snowflake texture by Anderson (1969). The glass, including that in glassy lavas, shallow intrusions
optically continuous quartz patches may include several and densely welded pyroclastic deposits. They may
spherulites or be densely crowded with feldspar crystals, occur in the glassy domains between spherulites in
so that only a few small inclusion-free areas of quartz partially devitrified obsidian. Although most commonly
are left. The result in hand specimen is a finely granular found in hydrated silicic glass, perlitic fractures also
texture (4.1). occur in mafic and intermediate composition glasses.
Perlite is usually recognizable with a hand lens but, in
An incipient stage in development of micropoikilitic some cases, may only be evident in thin-section. Under
texture in rhyolite involves poorly segregated quartz- favorable circumstances, an identical but much larger
rich patches in the groundmass (4.3-4). Under crossed texture (macro-perlite) can be recognized in outcrop
nicols these patches extinguish concurrently. Further (42.6). Yamagishi and Goto (1992) described macro-
development is reflected in the formation of more perlite with cores up to about 6 cm across in Late
pronounced boundaries, visible in both plane polarized Miocene submarine rhyolite. They concluded that the
light and with crossed nicols (4.5). Abundant feldspar macro-perlite formed before other columnar and
laths that are enclosed in the micropoikilitic quartz show polygonal joints that also occur in the rhyolite, and
no preferred orientation. In addition, feldspar (and primarily as a result of quenching rather than hydration.
sericite after feldspar) is concentrated at the margins of The rhyolite is apparently not hydrated and does not
quartz-rich patches. show micro-perlitic cracks.

In some instances, the cores of the micropoikilitic Hydration initially affects outer surfaces of glassy lava
quartz crystals are free of inclusions and very distinct flows (or shallow intrusions), margins of cracks or
(4.1-2). The cores extinguish concurrently with the joints in glassy lava flows and densely welded
remainder of the micropoikilitic quartz and commonly pyroclastic deposits, or surfaces of glassy clasts in
have highly irregular outlines. Sericite (after feldspar) is volcaniclastic aggregates. Hydration proceeds inward
concentrated in the interstices between micropoikilitic along a hydration front defined by strain birefringence,
quartz. The granular ("sugary") texture of the hand a change in the colour of the glass, and a change in the
specimen (4.1) resembles well-sorted sandstone. glass index of refraction (Ross and Smith, 1955;
Friedman et al., 1966; Lofgren, 1971b). The water
Micropoikilitic texture results from initial devitrification content of obsidian is typically less than 1 wt%, and is
of cooling glass, and develops in both coherent glassy considered to be an original component of the magma
rocks (lavas and shallow intrusions) and densely welded (Ross and Smith, 1955; Friedman and Smith, 1958).
ignimbrites (Anderson, 1969; Lofgren, 1971b). The The higher water content of perlite (up to about 5 wt%)
texture occurs mainly in rocks of silicic composition. is attributed to addition of "secondary" water from
Lofgren (1971b) considered that micropoikilitic texture external sources, such as ground water or surface water.
develops through primary devitrification, especially in Measurable changes in alkali contents and in the
glasses that have relatively high water contents or are
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