Volcanic gypsum

While sulphur in the solid state is immediately recognisable by its typical yellow colour, you can often see white crystals around the fumaroles.
These are gypsum crystals, also produced by the sublimation (see Sublimates link) of the volcanic gases as they leave the surface.
In fact, gypsum is a sulphate of calcium hydrate; it is formed in equal measure by calcium and sulphur atoms held together by water.
The sulphur condensed by the fumaroles can then join the calcium found in the surrounding soil, slowly forming these small crystals.
cristalletti di zolfo