Page 46 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Spherulites in thin-section
An isolated spherulite (S) consists of intergrown
radiating quartz and feldspar crystal fibres, and
displays radial extinction. Concentric zoning in
spherulites is caused by variation in the packing of
fibres. Partial recrystallisation (R) to interlocking
anhedral quartz and feldspar has destroyed some
microstructures in several other spherulites. A mosaic
of anhedral quartz and feldspar (possibly
recrystallised microspherulites), with subordinate
sericite occurs between the spherulites. Crossed
nicols.
Mount Read Volcanics, Cambrian; specimen 74573,
Silverfalls Road, western Tasmania.
5. Altered spherulitic rhyolite
This texture is interpreted to result from alteration
and recrystallisation of spherulitically devitrified
rhyolite. Former spherulites are now pink-grey
bulbous aggregates of anhedral quartz and feldspar.
Some relict spherulites retain concentric zonation
(arrow). Cuspate patches of chlorite and calcite (C)
that occur between closely packed spherulites are
probably the result of alteration of former obsidian
(cf. 3.3A).

Mount Read Volcanics, Cambrian; DDH MAC22
(410 m), Hellyer mine, western Tasmania.

6. Nodular devitrification structures in flow-banded
rhyolite
Nodular- to cauliflower-shaped, aggregates of
anhedral quartz (Q) are similar to coalesced
spherulites in less altered rhyolites (cf. 3.2, 3.3A).
The quartz nodules are surrounded by smaller,
recrystallised spherulites which impart a granular
texture. Flow bands (F) can be traced through some
quartz nodules, although elsewhere flow bands wrap
around them.

Berserker Beds, Early Permian; Mount Chalmers,
Queensland.

7. Lithophysae in densely welded ignimbrite
The lithophysae consist of star- and shell-shaped
cavities surrounded by a border of pale, fine crystal
fibres. Pale purple, devitrified welded ignimbrite
occurs between the lithophysae. In this case, growth
of the lithophysae clearly postdates welding
compaction and predates final solidification of the
welded pyroclastic deposit. One lithophysal cavity is
partly filled with a lobe of the welded matrix (arrow)
squeezed in while the mass was still ductile.

Tuff of Blue Creek, 6 Ma; Snake River Plain, Idaho,
USA.

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