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(a)
(b)
(c)
Figure 10.12.
(a) Clastic fragment of Opal-A. (b) Opal-A coats the porous texture near the tip of speleothem
from Traba Mountains (northern Galicia, Spain). (c) Longitudinal section of a speleothem from
a sandstone shelter on the Tindal Plain (Northern Territory).
Figure 10.13.
Anomoeoneis (diatom) from Tcharkuldu Hill, Eyre Peninsula, South Australia buried by opal-A
precipitation.
massive opal gel precipitation (Fig. 10.13). This gel acts, during the third phase, as porous sub-
stratum allowing the growth of low energy crystallisation whiskers of pure minerals as phos-
phates, calcium carbonate, but habitually gypsum with an origin in bacterial activity. The
morphology of these crystals is determined by the space characteristics to grow (normally the drop
size and shape) where they develop. The crystals show different habits: crest and rosettes, needles,
prismes, etc. ( Fig. 10.14) . They are most frequently developed in the end of the speleothem
 
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