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I was visiting an elements exhibition in the Ulster Museum (Northern Ireland) and there were these native sulfur crystals on display.

Ulster Museum sulfur crystals

My question is how do sulfur crystals like these form in nature? Does the sulfur precipitate out of a solution? Are they the product of a chemical reaction? I am confused because of sulfur's negligible solubility in water and I couldn't imagine any other way such perfect crystals could form.

I am really interested in making such crystals myself but as I have said, I don't know by what process or under what conditions this crystallization of sulfur occurs.

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    $\begingroup$ Nature makes huge crystals out of many things; there is no guarantee that you can reproduce those even in a well-equipped lab. Negligible solubility over a million years may pretty well yield something worthwhile. Then again, maybe it was formed in an entirely different way. Slow cooling of molten sulfur, for example - why not? And by slow, I mean really, really slow. No, I mean even slower than that. $\endgroup$
    – Ivan Neretin
    Mar 23, 2017 at 16:02
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    $\begingroup$ Hint: by slow temperature lowering of molten sulphur, near volcanos, for example (en.wikipedia.org/wiki/Sulfur#Natural_occurrence). $\endgroup$
    – Buttonwood
    Mar 23, 2017 at 18:05
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    $\begingroup$ Crystals like the lower specimen, with large yellow crystals, couldn't come from cooling molten sulfur. Rather they would be made from an aqueous solution where there was a very low concentration of elemental sulfur depositing out. $\endgroup$
    – MaxW
    Mar 23, 2017 at 18:09
  • $\begingroup$ I used to dissolve sulfur in carbondisulfide and acetone, and some pretty cool little crystals would form if a tube of it dried out. By "pretty cool" I don't mean anything like what's in that photo though. $\endgroup$
    – airhuff
    Mar 23, 2017 at 20:17
  • $\begingroup$ If you want to grow any sort of crystals an Alum is good to thing try as they easily form very large crystals. $\endgroup$
    – porphyrin
    May 10, 2021 at 12:34

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Tl;DR: Wikipedia mentions some of the ways in which elemental sulfur is produced naturally:

  1. Through volcanic emissions, including emissions from hydrothermal vents. Some of them found near hot springs and near volcanic deposits. Also, lakes of molten sulfur up to ~200 m in diameter have been found on the sea floor due to submarine volcanoes.
  2. As minerals: sulfide minerals, such as pyrite (iron sulfide), cinnabar (mercury sulfide), galena (lead sulfide), sphalerite (zinc sulfide), and stibnite (antimony sulfide); and the sulfate minerals, such as gypsum (calcium sulfate), alunite (potassium aluminum sulfate), and barite (barium sulfate).
  3. Action of anaerobic bacteria on sulfate/sulfide minerals.

Long answer:

I found a paper1 which discussed #3 point in details. It said that although sulfide minerals can be oxidized to zero-valent sulfur by sulfate reducing bacteria in presence of molecular oxygen, it may pose the following problems:

  1. exposure to oxygen would drastically decrease growth of microbes thereby slowing down sulfide production
  2. on geologic timescales, excess supply with oxygen would convert sulfide into sulfate rather than native sulfur
  3. enormous amounts of oxygenated water would be needed to produce sulfur in large amounts

So, in order to overcome these problems, scientists proposed four possible mechanisms as a means to form native sulfur:

  1. a modified sulfate reduction process that produces sulfur compounds with an intermediate oxidation state,
  2. coupling of sulfide oxidation to methanogenesis that utilizes methylated compounds, acetate or carbon dioxide
  3. ammonium oxidation coupled to sulfate reduction
  4. sulfur comproportionation of sulfate and sulfide.

These reactions are found to be thermodynamically favorable and especially useful in environments containing dissolved sulfide. This provide evidence that microbial species functioning in such environments can produce native sulfur in absence of light and external oxidants such as $\ce{O2}$, nitrate, and metal oxides.

Reference

  1. Labrado, A. L., Brunner, B., Bernasconi, S. M., & Peckmann, J. (2019). Formation of Large Native Sulfur Deposits Does Not Require Molecular Oxygen. Frontiers in microbiology, 10, 24. https://doi.org/10.3389/fmicb.2019.00024
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