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| concretion
of opal |
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| volcanic
rocks |
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| minute
platy crystals |
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| 'lepispheres' |
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FORMATION OF
OPAL |
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Common
Opal |
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The glassy or semi-glassy form of opal
with conchoidal fracture, which is so
commonly encountered in nature, for example,,
is usually found by XRD analysis to be opal-CT.
It occurs under a wide variety of conditions,
ranging from nodules formed in cavities
in volcanic
rocks, probably at a later stage in
the cooling process, to concretions
in unconsolidated sedimentary beds clearly
formed close to the surface. In addition,
friable opal-CT is formed in large quantities
by diagenesis, especially in deep sea deposits
of biogenic silica.
As has been indicated earlier, opal-CT is
a crystalline material, albeit with a highly
disordered structure. This was first described
last century by Mallard (R1589)
using optical microscopy. He noted that
certain types of what were apparently opal
showed a birefringent
fibrous structure when viewed in thin
section between crossed polarisers; he termed
this material 'lussatite', a term which
is again being recognised as useful. This
phenomenon has been noted frequently in
various forms since then.
More recently actual crystals of what appears
to be the same material were encountered
in cavities in solid common opal. These
can just be seen by optical microscopy,
but scanning electron microscopy shows that
they take the form of minute
platelets coating the surfaces of the
cavities.
Similar coatings of crystallites were even
encountered in the lumina of silicified
woods (Scurfield and the author, R0658).
The same types of crystals were identified
later as a common constituent of deep sea
siliceous sediments, where they developed
more or less spherical accumulations of
these platelets; these have been termed
'lepispheres'.
Lepispheres are well illustrated by Weaver
and Wise (R1368),
who referred to them as cristobalite. Oehler
(R0919)
was able to reproduce these lepispheres
in the laboratory, but partly in view of
their habit, favoured a structural interpretation
similar to that proposed by Jones and the
author (R0242),
i.e., disordered interlayering of cristobalite
and tridymite structures (opal-CT).
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The widespread
marine deposits of opaline silica
have been extensively studied in recent
years by means of deep sea drilling
from ocean-going vessels. Silica deposits
ranging from diatoms and radiolarian
oozes composed largely of opal-A,
through opal-CT to porcelanites and
cherts have been encountered.
The primary deposition of these deposits
is largely due to biogenic processes.
Minute marine organisms such as those
mentioned, as well as sponges and
other creatures, have siliceous skeletons,
which, when the animal dies, can settle
to the bottom of the ocean. The actual
situation, however, is more complex.
Numerous studies have been made on
the deposition of silica on the ocean
floors, based in part on the origin
of the silica itself. These studies
are summarised by Oehler (R0919).
Diatoms alone are estimated to extract
some 2.5 x 1016 grams of silica annually
from near surface waters, but only
some 3% - 5% of this reaches the ocean
floor by settling of the skeletons
of the organisms. Most of the silica
is redissolved and rendered available
in a virtually closed cycle for these
organisms.
Larger particles, such as those formed
in sponges, are more likely to be
preserved, but will ultimately be
subjected to the diagenetic processes
taking place in the sea floor deposits.
Oehler considers, on the basis of
experimental and observational evidence
on the formation of opal lepispheres,
that:
A further factor which appears to
have an influence on the genesis of
the opal-CT is the chemical environment
under which diagenesis takes place.
The pore water in the sediments is
essentially saturated with silica,
and thus plays an important intermediate
part in the development of new phases.
It is suggested that the inhibition
of pore water movement in clay-rich
sediments may favour the formation
of opal-CT rather than quartz.
Lancelot (R1639)
and Greenwood (R0205)
found that silica tended to form opal-CT
in clayey sediments, but in carbonate
sediments, where, presumably pore
water is saturated in calcium carbonate,
quartz appears to be the favoured
form.
In general, the transformations found
in deep sea siliceous sediments tend
to follow the diagenetic sequence:
opal-A --> opal-CT --> quartz
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