calcite aragonite vaterite. solid phases calcite - most common and stable aragonite - often...
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Calcite Aragonite
Vaterite
Solid Phases
• Calcite - most common and stable
• Aragonite - often kinetically favored
• Vaterite - rare in nature
Potential Causes of Whitings
• Increased temperature
• Increased Ca2+ and/or CO32-
• phytoplankton blooms
• combination of all of above
Background: Nucleation
• Homogeneous nucleation - from solution
• Heterogeneous Nucleation - on a substrate
• Induction time
Background: Saturation State
• Often denoted by
• = (aCa2+aCO32-)/Karagonite
• If for a mineral in solution, precipitation is favored
We know increasing temperature and nutrient concentrations enhances
aragonite growth.
However, it is much less clear whether phytoplankton act as nucleation "seeds".
Nodularia spumigena 10m
Simple Nucleation Experiments
• Add excess Ca2+ to Pyramid Lake
water until whiting is observed.
• Quantify how much Ca2+ is required
for given conditions
Equipment Required
• Collection bottle• Filtration apparatus• Clock or watch• Stir plate• Burette
• Glass flask
• 0.1 M CaCl2 solution
• Thermometer• Notebook and pen• pH meter (optional)
Variables to Keep Constant
• Temperature
• Stir rate
• Interval of Ca2+ additions
• Particulates in solution
Procedure
• Add drops of 0.1M CaCl2 solution in predetermined time intervals
• Record time and pH after each addition
• Stop when whiting is observed
Data
• The main variable of interest is the amount of titrant required to produce the whiting event.
pH
8.60
8.65
8.70
8.75
8.80
8.85
8.90
8.95
9.00
9.05
9.10
9:21:36 9:36:00 9:50:24 10:04:48 10:19:12
mL
0.00
0.50
1.00
1.50
2.00
9:21:36 9:36:00 9:50:24 10:04:48 10:19:12
Time
mL
tit
ran
t
whiting observed
whiting observed
aragonitegrowth
Results: Nucleation with and without phytoplankton
EXP ID description drop rate mL titrant Final CCPL-4 none 1/15s 2.2 99CCPL-5 w/phyptn 1/15s 2.0 92CCPL-6 none 1/15s 2.4 107CCPL-7 w/phyptn 1/15s 2.2 92
Aragonite nucleates in waters with phytoplankton more readily
Results: Nucleation with and without filtering
EXP ID description drop rate mL titrant final MMR-2 unfiltered 1/7s 1.6 77MMR-3 unfiltered 1/7s 1.5 73MMR-4 filtered 1/7s 1.8 85
EXP ID description drop rate mL titrant final CCPL-2 unfiltered 1/15s 2.0 92CCPL-1 filtered 1/15s 2.1 95
Aragonite nucleates in unfiltered waters more readily
SEM Imaging
• Aragonite crystals look similar in experiments performed with and without added phytoplankton.
control experimentnucleated in the
presenceof nodularia
Discussion/Conclusions
• The amount of added Ca2+ required for nucleation was reproducible in control exps.
• Filtering appears to have an effect on nucleation
• Nucleation occurs more readily in the presence of phytoplankton; suggesting that algal blooms may enhance whiting events.
General implications for natural systems
• Natural waters are commonly supersaturated with respect to CaCO3 minerals, but the concentration and chemical nature of natural inhibitors will determine whether, and at what rate crystal growth actually occurs.
• Substances that enhance nucleation of crystals do not necessarily affect mechanisms of growth, as these are two very different processes.