Zircon experiment 1

For this experiment I used a granodiorite gneiss (WD-507D), a sill of the Hardwick pluton in west-central Massachusetts, U.S.A. This rock has nearly 500 ppm Zr, and zircons up to 125 μm long and 50 μm across. Its age is Devonian, though it may have been substantially heated in the late Paleozoic. The sample was crushed using a Rocklabs tungsten carbide ring mill, and 200 mg of sample was added to each of 16 vessels.

  1. To half of the vessels, 3 ml of HNO3 (70%) and 3 ml of HF (50%) were added, and the samples run with a program that heated them to 175°C over 2 hours, and were held at this temperature for 24 hours. The samples were then evaporated to dryness. The purpose of this pretreatment step was to see if removing most silica first would hasten zircon dissolution. It did not.
  2. To all 16 vessels 6 ml of HF (50%) was added and the samples heated according to the same program as in Step 1.
  3. Two vessels were removed and set aside, and the rest put through another heating cycle. This was repeated for the next 7 cycles, so in the end there was a set of pre-treated and untreated samples spanning HF only reaction times of 1 through 8 days.
  4. All samples were evaporated to dryness.
  5. All samples had added 15 ml of water and 4 ml of HNO3 (70%) to put the solids into solution.
  6. Each resulting sample was decanted into a conical-bottom test tube, gently rinsed several times to remove acid.
  7. Residue was transferred to a watch glass and inspected under a microscope.

Bottom line: The pre-treatment step (1) had no significant effect in the amount of residual zircon after 1 day. No zircons were found in samples heated longer than three days. This procedure therefore works for this sample and these zircons, though to be safe I would recommend heating the samples for 4 days (96 hours).

 

 

Zircon dissolution experiment

Zircon experiment 2

In this experiment I tried dissolving pure zircon.

  1. I took two zircon crystals 0.8 cm across. One was annealed at 1000°C for 3.5 hours to remove radiation damage. The annealed zircon therefore mimics very young zircons. These were hand crushed in a Plattner mortar and sieved, and the 70-250 μm fraction retained.
  2. 10.0 mg of the zircons were weighed into sample vessels: 8 for the annealed zircon and 8 for the unannealed.
  3. 6 ml of HF (50%) was added to each sample, and the vessels heated to 175°C over 2 hours and held at that temperature for 21 hours. Thus there was no pretreatment step.
  4. After one heating cycle, one annealed and one unannealed sample were removed. Most of the HF was removed from these by pipette, and the remaining HF serially diluted by adding DI water and pipetting away most of the mixture. 15 ml of water and 4 ml of HNO3 (70%) was added to these samples and then they were put back in the heating block for a heating cycle with the remainder of the samples which still had HF. This step was to remove any tiny amount of insoluble fluorides that might precipitate from minor components in the crushed zircon samples.
  5. The HNO3 solutions from the previous cycle were decanted and washed into conical-bottom test tubes and diluted, and later examined under a microscope and photographed. In addition, two more HF-bearing samples were removed and subjected to the treatment described in Steps 4 and 5. The end result is that annealed and unannealed sample pairs were heated for between 1 and 8 cycles.

The figure shows the results. This experiment was a stringent test of zircon dissolution because the zircon fraction used was quite large, probably larger than most zircons in igneous rocks after ring mill crushing, and because half of the zircon samples had their radiation damage annealed. The top row in the image shows 10 mg of raw zircon prior to any treatment with HF. Subsequent rows show the zircon quantity remaining after each of the eight dissolution runs. For the unannealed zircon, roughly 5% remained after 63 hours and perhaps 1 or 2% after 168 hours. For the annealed zircon dissolution was considerably slower, with perhaps 50% of the zircon left after 63 hours and perhaps 20% left after 168 hours. The annealed zircon remaining after 168 hours is somewhat more than that left of the annealed zircon after 21 hours. Clearly, pure HF is not capable of decomposing all of the large radiation damaged zircons even after 168 hours (7 days), though perhaps 98%+ is enough for most applications. Recall that experiment 1, above, had no discernable zircon remaining after three days, though those zircons were from a different source and were smaller.


 

Unannealed zircon after 63 hours
This is a photo of the annealed zircons after 63 hours, 200X. The surfaces are covered by a series of steps, suggesting that dissolution is fastest along step edges. Small rectangular pits are also common, though only a few are visible here, on the grain to the upper right.

Annealed zircon after 63 hours
This is a photo of the unannealed zircon after 63 hours, 200x. The zircon surfaces are spongy, with dissolution preferentially following radiation damage tracks. Note that the grain size is generally smaller than in the photo above.

 

Zircon Experiment 3

The Picotrace literature has the following procedure for dissolving zircons:

  Picotrace-suggested zircon dissolution procedure for felsic rocks   The procedure I followed
1) Weigh 100-200 mg of rock powder into sample digestion vessels.

Weigh 200 mg of WD-507D rock powder (see experiment 1 above) into 3 vessels.

Weigh 10 mg of annealed zircon (see experiment 2 above) into 3 vessels.

Weigh 10 mg of unannealed zircon (see experiment 2 above) into 3 vessels.
2) Add to vessels 3 ml of HF and 3 ml of H2SO4.

Add to 1 vessel of each sample type 3 ml of HF and 3 ml of H2SO4.

Add to 1 vessel of each sample type 6 ml of HF.

Add to 1 vessel of each sample type 3 ml of H2SO4.
3) Heat to 180°C over 3 hours and maintain for 15 hours. Heat to 175°C over 2 hours and maintain for 24 hours. Note, at that distant time I could not attain 180°C.
4) Evaporate the acid at 180°C for 100 hours. Evaporate the acid at 185°C for 96 hours. Note that higher temperature was attained with the evaporation lid, presumably because it insulates the digestion block better than the high-pressure lids. Note that at least 2 ml of H2SO4 remained in each of the 6 vessels that contained it! This means that this evaporation step was insufficient to remove 3 ml of H2SO4! It may be that our air flow during evaporation was too low, because of collapsed aspiration bottles (see fix here).
5) Add 2 ml of HNO3 and 2 ml of 6 N HCl. Add 4 ml of HNO3 and 15 ml of water.
6) ? Heat at 155°C for 21 hours to facilitate dissolution. Minor fluorides remained in the HF-only vessel with the WD-507D sample. A more concentrated acid solution is needed, or a second evaporation step with a few ml of HNO3 to drive off more of the initial fluorides. This is necessary for our low-pressure dissolution procedure.

 

The residue from dissolution was transferred to 50 ml conical-bottom test tubes, washed with DI water, concentrated in a watch glass, and photographed in the same way as in Experiment 2, above.


This shows the results for the three sample types (annealed and unannealed zircon, WD-507D granodiorite gneiss), for three sets of acids: H2SO4 only, HF + H2SO4 as recommended by Picotrace, and HF only. In the first row it is clear that zircon is unimpressed with H2SO4, as are the other silicates in WD-507D. In the second row, representing dissolution conditions very close to that recommended by Picotrace, it is clear that the dissolution of zircon is incomplete in all three samples. In the third row, dissolution is also incomplete, as was found in Experiment 1 for sample WD-507D and in Experiment 2 for annealed and unannealed zircon after heating for only 21 hours. Note that the H2SO4-only zircon samples here look much like the zero-hour samples in Experiment 2, and that the HF-bearing zircon samples look very much like the 21 hour samples in Experiment 2. This suggests that the H2SO4 is doing nothing special.


Here are closeups of the samples above, arranged in exactly the same order. In the first row with H2SO4 only, zircons appear fresh and unaffected, as do quartz, feldspar, and zircon in WD-507D. In the latter sample, Fe-Ti oxides have been largely attacked, as have all biotites. In the second, mixed acid row, etching is evident in all samples, though relatively minor in the annealed zircon. In the third, HF-only row, etching is somewhat stronger. Note the residual brown insoluble fluorides that were not decomposed during the last HNO3-water dissolution step. As mentioned elsewhere, this problem has been solved.

 

Bottom line: Although there were some minor differences between the procedure I followed here and that recommended by Picotrace, I doubt any of the differences are significant. The HF-H2SO4 procedure Picotrace presents simply does not seem capable of dissolving zircon in these samples. My recommendation is to follow our procedure. The End.

 

Kurt Hollocher

Geology Department

Union College

Schenectady, NY 12308

U.S.A.