FX4-Limestone & Pyrite
Fossil Experiment #4 is an attempt to create a fish fossil in a limestone matrix that is permineralized with Iron Disulfide or Pyrite, FeS2. I combined the limestone, Naturally Reprecipitated Calcium Powder, with fine iron powder and pure sulfur powder. I then added water to achieve a clay like consistency. Within three layers of the limestone paste I added four fish and one large dragonfly. I was pleasantly surprised to find what I thought was Pyrite beginning to form not only where iron powder and sulfur were in contact with each other but also growing on some the fish bones where there was no iron or sulfur initially. Upon further analysis using a scanning electron microscope with EDS, the particles of Pyrite were found to actually be Pyrrhotite, a slightly different variant of iron sulfide. Many thanks to Stephen Horstemeyer and Dr. Mark Atwater of Liberty University for the use of their brand new SEM.
Originally this experiment was meant only for fish. However while on a bike ride we discovered these huge dragonflies on the road. As it turned out, I think this dragonfly fossil is one the most beautiful that I’ve done so far. Every little detail in the transparent wings, down to each vein was captured in the limestone matrix. Under the microscope you can see small areas under the wings that appear to be shiny gold particles of pyrite forming. My favorite is the black picture taken from the side with my old iPhone. I am told that the 4th one from the left looks like a little fairy, you can see her face shoulder and arm.
FX4 Fossil Experiment Details, Limestone and Pyrrhotite
1. 2Sep2022
2. Prepare reaction vessel according to standard assembly procedure.
3. Limestone used is powdered “naturally reprecipitated calcium powder”, 93%CaCO3, Iron Fe 0.27%, Calcium (Ca) 37%, sold by “The Seed Supply”.
4. Iron powder, very fine 320 mesh, ArtMolds p/n SM400241R
5. Powdered Sulfur, 99% pure, CAS # 7704-34-9
6. Limestone Powder 1.50KG, Fe powder 190.5gm, Sulfur powder 109.5 gm
7. Combined sulfur and iron powder dry then added limestone and mixed dry until smooth consistent color.
8. Added water to achieve a smooth clay like consistency. Water added = .5 kg
9. Added Layer 1, dragonfly, layer 2, 4 goldfish, layer 3 per standard assembly procedure.
10. Placed vessel in oven and placed thermocouple #1 against cylinder using stainless hose clamp. T/C#2 was left hanging to measure oven air temperature.
11. 2Sep2022 15:00, Attached inlet outlet and vent lines to vessel and set hydraulic ram force, Fhyd, to 5,000lbf for 2 hours. Since the inlet and vent valves are shut at this point the inlet pressure rose to 350psi as the water was being pressed out of the sample.
12. 17:00 Increased Fhyd to 10,000lbf for 2 hr 20min. Inlet pressure went from 350 to 670 psi. (10,000lbf ~ 750psi compression on sample.)
13. 19:20, Increased Fhyd to 13,000 lbf or 1000psi compression. Cylinder temperature rose 15F from self-heating reaction in about 30 minutes.
14. 21:00 hr set oven temperature to 250F. As water in the inlet started to heat and expand the inlet pressure started to exceed 1000psi so I added 800 psi of Argon to the water tank attached to the vessel inlet and opened the inlet valve. This allows a constant pressure to be maintained at the inlet as the water expands and more water is pressed out of the sample. By 21:50 hr the cylinder temperature rose to 180F. I decided to heat the sample in steps and watch for an exothermic reaction to spike the sample temperature.
15. 22:00hr with inlet valve open I can now increase Fhyd to 20,000lbf or 1500 psi compression.
16. 22:30hr Did not notice the vessel temperature rising due to exothermic reaction as thought so increased oven temp to 350F
17. 23:00hr I closed the inlet valve and the inlet pressure rose immediately meaning that the sample is still being compressed and water is still coming out. Tcyl =275F Tair = 330F
18. 23:30hr Tcyl =312F and outlet pressure is starting rise, Pout =~50psi.
19. Still no sign of exothermic temperature rise so Oven temperature was raised to 480F.
20. 3Sept2022, 01:30hr, Tcyl =453F,,,,,, 12:45hr Tcyl=478.7
21. 4Sep2022, 09:30hr Tcyl=479.9F, Pin = 860psi, Pout = 700psi. Fhyd = 20,500lbf. NO LEAKS!
22. 15:15hr Tcyl=479.9F, Pin=900psi. Pout=600 psi
23. Theory: Mineral content at a specific temperature will saturate. As the temperature slowly goes down, the mineral content super-saturates and begins to crystalize or precipitate out. So it was decided to drop the temperature at 100F/day. Would like to confirm this with a Geologist if possible.
24. 15:30hr dropped oven temperature to 400F. Did not vent the outlet. Slowly vented water tank until inlet pressure stopped dropping meaning that saturated steam conditions have been met at approx. 650psi. all valves closed. So Inlet and outlet pressures will be allowed to fall as the temperature drops.
25. 5Sep2022, 11:20hr, Tcyl=397.7, Pin=390psi, Pout=370psi
26. 16:00hr reduced oven temp to 300F,
27. 6Sep2022, 15:12hr, Vented inlet and outlet to allow sample to dry out. Vented gasses went into a stainless-steel coil submersed in water bath and then into a bottle. The bottle was vented underneath a fume exhaust hood.
28. 17:30 hr reduced Fhyd to 10,000lbf
29. 7Sep2022, 09:30hr. Turned oven and hydraulics off to allow to cool slowly.
30. 15:00hr took sample out of oven and pressed the sample out of the cylinder. NOTE: at 1500psi compression the residual hoop stress was not enough to fracture or crush the sample.
Results and Observations
1. Sample was slowly compressed to 20,000lbf or 1500psi over 7 hr. Heated at 480F for 41 hr. Slowly cooled to room temperature over 46 hr. Total process time 94hr.
2. The sample was cracked open with a sharp knife and hammer, tapping the back of the blade, going slowly 3-4 times around the circumference of the sample approximately 1 inch from the end, exposing the dragonfly. The same method was used approximately 1 inch from the other end to expose the fish.
3. The limestone was very hard and much stronger than the sample from FX3. Under a microscope, small crystals of what first appeared to be Pyrite can be viewed growing on parts of the fish and the dragonfly. After further examination using SEM EDS, Dr Mark Atwater determined that the crystals were actually Pyrrhotite, a slightly different variation of iron sulfide than Pyrite.
4. Pieces of the limestone were soaked in water and rubbed together. Most of the pieces dissolved into a smooth almost soapy like or clay like consistency. However, there were several pieces of hard material found that could not be easily broken apart. One piece was viewed under a microscope and found to contain small golden particles that appear to be Pyrite. These particles appear to be iron powder that was transformed into Pyrite directly. On the other hand, on one of the fish bones, there appears to be many small rectangular crystals of Pyrite where there were no iron particles to start with. Is this an example of “Dissolution and Reprecipitation”? I don’t know yet. The crystals are 0.0005” to 0.0007” in size using a stereo microscope with a 2 axis micrometer stage.
5. Not sure yet if crystal growth is just a matter of time at temperature, 500F, or a certain time at temperature followed by a slow ramp down. I suggest repeating the experiment and holding at 480F for longer periods to evaluate or measure the crystal growth rate.
6. Need to better understand the chemistry or cementation rate or conditions under which the limestone powder binds together into a waterproof or weather resistant rock. We are assuming or hoping this process would be the same for this “reprecipitated“ limestone as well as the “naturally precipitated” limestone from seawater. Finding small pieces of hard stone in the dissolved matrix suggests that this process had started but not sure if this would have continued at the initial 480F, or on the ramp down, or is there some other mechanism or chemical that promotes this cementation.
7. Need better photos. Bright LEDs produce washed out monochromatic bluish pictures while incandescent bulbs produce yellow pictures. Need to take more care to adjust the white balance. Also need higher magnification to see and photograph the Pyrrhotite crystals.
