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Silica Diagenesis (Factors Diagenetic Change Opal (Burial depth, Opal…
Silica Diagenesis
Factors Diagenetic Change Opal
Burial depth
Opal species
pH
temp
conc dissolved SiO2
Diagenetic
Opal Δ
Opal-A transforms to Opal-CT with burial
Increasingly crystalline with depth
Crystalline = more energy efficient state
Diagenesis Opal Crystallinity Δ
Increasingly crystalline
with depth
Silica reorganises its crystal lattice
Crystalline form = more efficient energy state
Diatomaceous Rock
Very porous ~ 80%
Composed of diatom tests
Diatom = unicellular siliceous algae
photosynthesising plant plankton (microalgae)
Example Porosity Δ
with depth
Bury diatomaceous rock - porosity ~80%
Opal CT forms @ 500 m porosity ~ 40%
Diatom opal dissolves & nucleates to form opal-CT
Chalcedony forms @ 1500m porosity~10%
Opal-CT unstable so dissolves & reprecipitates as quartz
Reorganises crystal structure
--> becomes fully crystalline
Chert Porosity Δ with Depth
Stepwise, not gradual
decrease porosity with burial
Reflects silica phase changes
Contrasts terrigenous sediment
gradual decrease porosit
acts like sponge slowly expelling water
Elevated Temps & Diagenesis
Diagenesis accelerated near volcanoes
Quartz occurs
sooner (diagenesis faster)
shallower
Opal-CT / Quartz found in young sediments as a result
Chert Formation
Takes a very long time (>20Myr)
Chert not present in surface sediment
Need sediment atleast 20 Myr before chert develops
Opal vs Quartz
Quartz
(chalcedony)
has fixed crystallinity
Opal-CT
has unfixed crystallinity (partially crystalline)
changes with burial
Silica diagenesis Summary
Increasing diagenetic grade
opal-A --> opal-CT --> chalcedony (quartz)
XRD
Opal-A = low hump
Opal-CT = 2 peaks, one double other
Quartz = three sharp peaks ~ 40, 30, 20 degrees 2thetea
Diagenetic Grade = f(...
Burial depth
(tho anomalies exist)
Temperature
(relates to burial depth)