2 Ways to Grow Sulfur Crystals

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Over time, monoclinic needle crystals spontaneously change shape into rhomboid sulfur crystals.
Over time, monoclinic needle crystals spontaneously change shape into rhomboid sulfur crystals. (photo: Mike Beauregard)

It’s easy to grow yellow sulfur crystals. Sulfur is a shape shifter that spontaneously transitions from a needle-like crystals into rhomboid geometrical shapes, depending on temperature. While the translucent yellow crystals are beautiful enough to be gems, the pure element is too soft (1.5 to 2.0 on the Mohs scale) and brittle for use in jewelry. Here are two easy ways to grow sulfur crystals that will change shape over time.

Grow Sulfur Crystals From a Melt

Sulfur is inexpensive and readily available in pure form as a powder, so it’s easy to grow crystals from a melt, much like you would grow bismuth crystals. You can purchase sulfur powder online, although stores like Walmart and Home Depot also carry it in the gardening section as a natural fungicide and pesticide.

All you need is:

  • Pure sulfur
  • Spoon or piece of foil
  • Heat source

Sulfur melts at 115.21 °C or 239.38 °F, so you can achieve the melting point using an outdoor grill (because sulfur is smelly), burner, or stove top.

  1. Put a spoonful of sulfur powder on a spoon or piece of foil.
  2. Heat the sulfur until it melts. When sulfur melts, it turns into a blood red liquid. Don’t apply too much heat or else sulfur will combine with air and burn with a blue flame!
  3. Once the sulfur melts, remove it from heat.
  4. Either let the liquid cool on its own or else pour it into cold water or even over crushed ice. Feel free to experiment with cooling methods to observe different forms of the element.

Initially, sulfur cools to form an amorphous, rubbery brownish solid. This is sometimes called plastic sulfur because it’s a polymer made of chains of sulfur atoms. Amorphous sulfur spontaneously transitions into sulfur crystal needles. Within a few hours, the atoms again rearrange to form rhombic crystals. Sulfur actually forms over 30 solid allotropes! They include translucent and opaque forms, in colors including brown, yellow, greenish-yellow, and orange-yellow.

Grow Sulfur Crystals From Solution

While sulfur is insoluble in water, it does dissolve in various organic solvents. These include carbon disulfide, xylene, and toluene. Of these, toluene is the safest and most readily available. You can find it at home supply stores. Xylene is used as paint thinner. It works fine for this project, but only if you have a fume hood. Using sulfur and a solvent, crystals grow much like sugar crystals or borax crystals. You heat the liquid to dissolve the solid until you get a saturated solution and then crystals grow as the solution cools.

  • Sulfur
  • Toluene
  • Heat source

Toluene has a high vapor pressure, plus it’s flammable. It’s best to heat it slowly either outdoors or within a fume hood indoors. Its boiling point is 111 °C or 232 °F.

  1. Warm the xylene (but do not boil it).
  2. Stir in sulfur until it stops dissolving.
  3. Carefully pour the solution through a filter to capture the undissolved particles. For the best crystals, use a warm filter and collect the liquid in a warm container.
  4. Wrap the container with hot towels to slow the rate of cooling. A slow cooling rate produces the largest crystals.

Initially, you’ll get crystal needles. Over time, the sulfur will transition into the rhomboid shape. The crystals may change color slightly, too. For a large crystal specimen, save the nicest crystal from this batch to use as a seed crystal for a new batch. If you do a second crystallization, let the liquid cool a bit before adding the seed crystal, or else it may dissolve.

You can grow sulfur crystals by dissolving sulfur powder in toluene. Crystals form as the solution cools. (Maxim Bilovitskiy)

References

  • Meyer, Beat (1976). “Elemental sulfur”. Chemical Reviews. 76 (3): 367–388. doi:10.1021/cr60301a003
  • Shakhashiri, B. Z. (1983) Chemical Demonstrations: A Handbook for Teachers of Chemistry. vol. 1. ISBN 978-0299088903.
  • Steudel, Ralf; Eckert, Bodo (2003). Solid Sulfur Allotropes Sulfur Allotropes. Topics in Current Chemistry. 230. pp. 1–80. doi:10.1007/b12110 ISBN 978-3-540-40191-9.
  • Tebbe, Fred N.; Wasserman, E.; Peet, William G.; Vatvars, Arturs; Hayman, Alan C. (1982). “Composition of Elemental Sulfur in Solution: Equilibrium of S6, S7, and S8 at Ambient Temperatures”. Journal of the American Chemical Society. 104 (18): 4971–4972. doi:10.1021/ja00382a050