Page 49 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

B. In plane polarized light, boundaries between
quartz-rich patches are defined by the concentration
of fine sericite and opaque grains (arrow).

Mount Read Volcanics, Cambrian; specimen 41340,
east Darwin area, western Tasmania.

Plate 5 — Perlite
1. Classical perlite in thin-section
The glassy groundmass of this rhyolite shows
classical perlitic fractures, comprising arcuate,
overlapping and intersecting cracks (arrow). Sparse
phenocrysts of clear plagioclase and brown biotite,
and faint flecks in the glass (feldspar microlites)
define a subtle flow foliation, which is overprinted by
the perlitic fractures. Plane polarized light.

Glassy rhyolite, Pleistocene; Cala de Gaetano,
Ponza, Italy.

2. Banded perlite in thin-section

The formerly glassy groundmass of this strongly
flow-foliated dacite contains rectilinear fractures
(arrow) typical of banded perlite. The fractures are
preserved by a thin, infilling layer of sericite and
chlorite. Most of the glass has recrystallized to a
mosaic of quartz and feldspar; some has been
replaced by sericite and chlorite. Plane polarized
light.

Mount Read Volcanics, Cambrian; specimen 401117,
Pieman Road, western Tasmania.

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