What is the structure of powdered sulfur? Quoting this paper:


  A variety of allotropic states of elementary sulfur have been identified.
  Solid crystalline sulfur exists either as rings of 6-12 sulfur atoms (cyclohexasulfur, $\ce{S6}$, cyclooctasulfur $\ce{S8}$, etc. or as chains of sulfur atoms (catenasulfur $\ce{S_{\infty}}$).


As said on the same paper, Alpha-sulfur (orthorombic - $\ce{S_{\alpha}}$) is the thermodynamically stable form under ordinary conditions.

What happens with powdered sulfur (also called "dusting sulfur", normally used as fungicide)? Are sulfur atoms still organized in rings or chains? 

Can we associate a precise crystalline phase to powdered sulfur?
Powdered sulphur is produced by grinding raw sulphur as it is mined. According to Wikipedia, mining is done as follows:


  On Earth, elemental sulfur can be found near hot springs and volcanic regions in many parts of the world, especially along the Pacific Ring of Fire; such volcanic deposits are currently mined in Indonesia, Chile, and Japan. Such deposits are polycrystalline, with the largest documented single crystal measuring 22×16×11 cm.[21] Historically, Sicily was a large source of sulfur in the Industrial Revolution.[22]


Raw sulphur is polycrystalline (meaning many crystals grown together). So basically, powdered sulphur is the same as crystalline sulphur just as very small crystals. Consequently, what you found in that paper for crystalline sulphur also applies to powdered sulphur.