Calcium (Ca) availability to ecosystems: Weathering of limestone, apatite, and Ca-plagioclase (anorthite)
Sequence of laboratory activities to develop familiarity with geochemistry bench laboratory experimental methods; demonstration of different weathering rates of different Ca-bearing minerals; connection between minerals present in a system and nutrient (Ca) availability
200-level geology class focusing on environmental geology or introduction to geochemistry (lower division)
Skills and concepts that students must have mastered
It would be ideal if the students had either taken general chemistry or else had a very strong high school background in chemistry. In addition they should have some physical geology to understand the mineralogy of different rocks and how they can differ in their susceptibility to weathering processes. Given that this laboratory focuses on mineral weathering and nutrient availability, I had students read the Blum et al. 2002 study (Mycorrhizal weathering of apatite as an important Ca source in base-poor forest ecosystems) referenced on page 1. Students should be familiar with standard curves if they will be analyzing their own data.
How the activity is situated in the course
Sequence of lab sessions that spread over several days to allow for time needed in dissolution experiments and sample analysis (measurement of Ca concentrations). It is a bridge in topics between between minerals present in a system and potential nutrient availability.
Content/concepts goals for this activity
Weathering rates of carbonate vs. silicate (as well as phosphate) minerals, substituting acid strength for time (in laboratory experiments studying ecosystem processes), connecting minerals present in soils/bedrock to time scales over which weathering occurs and nutrients become available to ecosystems
Higher order thinking skills goals for this activity
Reading primary literature (the Blum et al. study) and applying their understanding from that field setting to this lab setting; Formulation of hypotheses to contrast relative rates of mineral weathering; Data analysis to quantify the amount of each mineral that dissolved during the experiment; Interpretations are required to understand how the limestone dissolution experiment was limited by the amount/concentration of nitric acid used
Other skills goals for this activity
Introduction to bench-scale geochemistry studies, develop familiarity with chemistry/geochemistry lab and general lab skills, working in groups, develop skills/facility with in Excel to quantify % mineral dissolution (for each mineral)
Description and Teaching Materials
With this lab handout, or prior to lab, the students could read a study from the primary literature discussing mineral weathering and nutrient availability (I suggest the Blum et al. study I referenced on page 1).
Student handout #1, laboratory handout for Calcium (Ca) availability to ecosystems: Weathering of limestone, apatite, and Ca-plagioclase (anorthite) (Microsoft Word 2007 (.docx) 140kB Jun10 13)
Student handout #2, data analysis handout for Calcium (Ca) availability to ecosystems: Weathering of limestone, apatite, and Ca-plagioclase (anorthite) → Data Analysis (Microsoft Word 2007 (.docx) 132kB Jun10 13)
Day 1- students weigh out each of the minerals (previously ground by faculty/staff) into vials and weigh out/add 5 ml of acid (the concentrations could be modified to address different substitutions of "acid concentration for time"). Students could also use the pre-lab questions to brush up on their chemistry background and the calculations that underlie this study. Students can also discuss the findings of the Blum et al. paper and how that study informs this laboratory experiment. Students prepare and leave mineral dissolution vials (on agitator or rotator) to proceed overnight for 20-24 hrs.
Day 2- Students remove samples from agitator and filter to remove the leach solution. Students can dilute samples as appropriate for whichever analytical instrument (this lab describes a methodology for analysis by atomic absorbance spectroscopy). Students can develop their own standard curves (in this case we had our lab staff member help students with their analyses and everyone used the same standard curve to save time and prevent replication)
Day 3- Data analysis in a computer lab to see differences in mineral dissolution aka "weathering" and differences between replicates (generally indicative of human error or perhaps sample heterogeneity)
Teaching Notes and Tips
It may be important to allow the dissolution of the limestone vials to degas the CO2 before it is left to agitate overnight. This provides a nice reminder of how limestone reacts with acid and it allows them to make the connection of why we add acid to identify limestone in the field.
I assessed student lab notebooks, particularly focusing on their results and graphs (printed from Excel and pasted into their notebooks) and interpretations connecting their findings to the Blum paper and nutrient cycling in forest ecosystems.
References and Resources
In order to give them a real-world setting connecting geology and nutrient cycling, I had the students read: Blum, J.D., Klaue, A., *Nezat, C.A., Driscoll, C.T., Johnson, C.E., Siccama, T.G. Eagar, C., Fahey, T.J. and Likens, G.E. (2002) Mycorrhizal weathering of apatite as an important Ca source in base-poor forest ecosystems. Nature 417, 729-731. (The Blum paper's methods support/explain the reasoning behind the laboratory procedures used here and help students make the connection between field experiments and lab experiments seeking to explain phenomena observed in the field).