MINERAL & SOIL ANALYSIS

Mineralogy is the science dealing with minerals, their crystallography, physical and chemical properties, classification, and the ways of distinguishing them. Fortunately for the forensic scientist many crime scenes involve soils which can be carried away as dust particles or trapped in the culprit’s shoes or clothing. Dust from a suspect’s clothing, ears, nose or fingernails or from the soil from shoes is analyzed and compared with soil, dust and glass particles from the scene of the crime. This can be one of the most useful pieces of evidence because it can place the suspect at the scene of the crime.

When comparing soil, dust or glass samples, it is important to find as many similarities as possible between them. Although some tests are quite simple, such as the determination of general appearance, acidity and density, other test are more complicated and require more sophisticated instruments. Petrography microscopes, x-ray diffraction, arc spectrography and thermal analysis are all used to analyze solids for mineral content. Neutron activation is used to distinguish between trace metals and between various batches of glass used in the manufacture of headlights.

Purpose: In this activity you will identify soils sample using, general appearance, density gradient, settling rate, pH and chemical properties. There are six stations for you to use in collecting data and drawing conclusion. You may visit the stations in any order.

Procedure:

Part I -- Physical Properties

Station #1

1. Write a description of the sample labeled “A”. Note the color, texture, and general appearance. Record your observations on the Data Sheet.

2. Using a magnifying glass, note the presence of any vegetation or any unusual materials. Record your observations on the Data Sheet.

3. Repeat the above steps for soil samples, “B”, “C” and “unknown”.

Station #2

4. Place each of the four samples in the “UV Box” and describe your observations.

Station #3

5. Mix 2 grams of the soil sample in 50 mL of distilled water.

6. Stir the mixture for 1.0 minute.

7. Using the CBL, record the pH of the solution.

Station #4

8. Mix 2 grams of the soil sample in 50 mL of distilled water.

9. Stir the mixture for 1.0 minute.

10. Using the CBL, record the conductivity of the solution.

Station #5

11. Observe the soil profiles in the density gradient tubes. The tube was made by layering four liquids having different densities.

a. Sketch the samples profile for each of the four samples, “A”,”B”,”C”, “unknown”.

b. Estimate the percentage of the total sample at each density.

Station #6

12. In a well plate add a small sample of soil (about the size of an aspirin). Add 10 drops of HCl to the sample. Carefully look for the presence of any gas bubbles and record your observations in the Data Sheet.

13. If gas bubbles are produced it indicates presence of a carbonate or the presence of the metals, zinc, iron, or magnesium. In soil the most likely would be carbonate but at a crime scene you might find the metal fragments mixed in with the soil.

Zn_(s) + 2HCl_(aq) Þ ZnCl_2(aq) + H_2(gas)

HCl_(aq) + CaCO_3(s) Þ CaCl_2(aq) + CO_2(g) + H₂O_(l)

Station #7

14. Often the forensics scientist must send the evidence away to another lab for analyses. At this station you will find graphs return to you for comparison to the graph of the soil taken from the crime scene. Your instructor will demonstrate the technique used to collect the data for the soil settling rate. The data you collect in class will be for the sample taken from the crime scene.

15. Compare the crime scene graph to the four reference graphs. Record your observations on the Data Table.

Name_________________________

Soil Testing - Data Table

Station #1: Description of samples

sample 1 _____________________________________________________________________

sample 2 _____________________________________________________________________

sample 3 _____________________________________________________________________

sample 4 _____________________________________________________________________

unknown ________________________________________most nearly like sample # ________

Station #2: UV Effects

sample 1 _____________________________________________________________________

sample 2 _____________________________________________________________________

sample 3 _____________________________________________________________________

sample 4 _____________________________________________________________________

unknown ________________________________________most nearly like sample # _________

Station #3: pH

1.______ 2._____ 3._____ 4. _____ unknown ______ most nearly like sample # ____

Station #4: Conductivity

1.______ 2._____ 3._____ 4. _____ unknown ______ most nearly like sample # ____

Station #5: Density Gradient

sketch what you see. Mark the sketch into

sample 1 sample 2 sample 3 sample 4 unknown most nearly like sample # ____

Station #6: Test for carbonates or metal chips

Write “B” if there is evidence of bubbles and “NB” if not bubbling is seen

1.______ 2._____ 3._____ 4. _____ unknown ________ most nearly like sample # ____

Station #7: Colorimeter graph of soils

Sketch the chart and label the first and last data point with the value from the graph

sample 1 sample 2 sample 3 sample 4 unknown most nearly

like sample # ____

The unknown is most like sample # ___________ because _______________________________________

Teacher Notes:

Collect a zip lock bag of 4 different types of soil. If you do not have soils containing carbonates in your area, you may want to add CaCO₃ to one of the samples in order to get a positive carbonate test in one sample. You may also elect to spice one of the samples with metal fragments. If you do this you should do it near the time of the testing to avoid the oxidation of the filings to a state which will not react.

I added crushed up florescent chalk to one same with good results. I got carbonate positive and UV positive results

Station #3

Set up a CBL, calculator and pH probe. Calibrate probe and set to monitor input.

TO save time you may elect to mix the soil and water and have 5 beakers already labeled (sample 1-4 and unknown) and mixed. Students will still need to stir before taking reading. Probe should be rinsed between each test.

Station #4

Set up a CBL, calculator and conductivity probe. Calibrate probe and set to monitor input.

TO save time you may elect to mix the soil and water and have 5 beakers already labeled (sample 1-4 and unknown) and mixed. Students will still need to stir before taking reading. Probe should be rinsed between each test.

Station #5

Use 5 100mL graduated cylinders. Pour 20 mL of each liquid in order into the graduated cylinders. Label each cylinder with soil number or unknown. Add about 1 teaspoon of soil sample in the corresponding cylinder and let set about 30 minutes.

Station #6

Use 6 M HCl. It may be difficult to see bubbles. Instruct students to repeat if in doubt. A hand lens may be helpful.

Station #7

This is written to have you do the graph of the 4 known soils and the unknown. I did the 4 sample a head of time and printed the graph, imported it into Graphical Analysis and set the axis to all be the same. In class I did the unknown as a demonstration and added the printout to station #7 before students began to work in groups.

Set up a CBL, calculator and colorimeter. Calibrate probe and set to red. Select Collect Data from ChemBio, then time graph. collect samples every 6 seconds and take 50 samples. This should take 5 minutes. Add a sample of the soil, about the size of an aspirin, to a vial 3/4 full of water. Shake and put into colorimeter. Press Enter to start the program. Import data to Graphical Analysis, set and label axis, and name the graph, then print.