Geology Reference
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analyses carried out on the body of the speleothems indicate a constant composition in Si for all
the samples studied independently from their host rock or environment of formation. Moreover,
other chemical elements appear in very low proportion such as Al, P, K, Fe, etc., though caolinite
speleothems have been mentioned, it is usual that mineralogically the speleothems are of opal-A
(Vidal RomanĂ­, Twidale, Bourne, 2003 and see Figs 10.11b and c). The observations under the
SEM show an identical morphology for all the samples independently from the climate under
which they were developed and from the kind of rock with which they are associated. The starting
situation is the dissolution in the water contained in the fissural system of the granitic rocky mas-
sif of the Si mainly, and of the other chemical elements forming the rock. This process is carried
out by the attack of different bacterial organisms. After this, it takes place the formation of the
speleothems by precipitation of the biogenic opal formed by this process. Three phases have been
distinguished in the speleothems development. In the first phase (biogenic phase), the texture of
the speleothems is porous, brecciate (opal-A clasts) or conglomeratic (opal-A oolites) the first
ones ( Fig. 10.12a) formed by mechanical flaking when the silica gel dehydrates, and the second
ones caused by direct precipitation of amorphous silica by minor organisms (bacteria and fungi).
In the second phase (re-solution phase), the high solubility of the biogenic opal may produce the
progressive internal silting of the porous system of the speleothem that finally is totally infilled
with the eventual formation of external coatings that covers externally the speleothem (patina)
(Fig. 10.12b), transforming the final end of it into a solid body with rhytmical texture of accretion
equivalent to that of calcareous speleothems (Fig. 10.12c). This wet and rich in silica environment
allows a brief existence of different microorganisms (fungi, diatoms, bacteria, etc.), whose life is
directly controlled by the water reserves (derived from rain, percolation, etc.). The water absence
interrupts the biological activity causing the burial and fossilization of living organisms by
(c)
Figure 10.11.
(c) SEM image of siliceous speleothem showing the typical cracks of the dessicated opal A,
Monte Louro, Galicia, northwestern Spain.
 
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