This special issue of Geoderma is the result of the work of members of the “International Working-Group on Submicroscopy of Undisturbed Soil Materials (IWGSUSM)”. The papers are from the second workshop of IWGSUSM, organized in 1981 by J. Ducloux at the University of Poitiers, France; the symposium on “Submicroscopy of Undisturbed Soil Materials” organized in 1981 by M.L. Thompson in Atlanta, Georgia, U.S.A. during the annual meeting of the Soil Science Society of America, and work done in 1982. The papers mainly concern in situ electron microscopic studies of materials in thin sections of soils and of unimpregnated soil constituents in peds. Ion microscopy is discussed in one paper.

VAN RENSBERGEN, P., HILLIS, R.R., MALTMAN, AJ. & MORLEY, C.K. Subsurface sediment mobilization: introduction
MALTMAN, AJ. & BOLTON, A. How sediments become mobilized Shallow subsurface sediment mobilization

OWEN, G. Load structures: gravity-driven sediment mobilization in the shallow subsurface
HARRISON, P. & MALTMAN, A. J. Numerical modelling of reverse-density structures in soft non-Newtonian sediments
PARIZE, O. & FRIES, G. The Vocontian clastic dykes and skills: a geometric model
BANKWITZ, P., BANKWITZ, E., BRAUER, K., KAMPF, H. & STORR, M. Deformation structures in Plioand Pleistocene sediments (NW Bohemia, Central Europe)

Sulfur is the fifteenth most abundant element in the continental crust of the Earth (260 ppm), and the sixth most abundant element in seawater (885 ppm). Sulfur (atomic number 16) has the ground-state electronic structure [Ne]3/V, and is the first of the group VIB elements in the periodic table (S, Se, Te, Po). In minerals, sulfur can occur in the formal valence states S S°, S4+, and S6+, corresponding to the sulfide minerals, native sulfur, the sulfite minerals, and the sulfate minerals. In the sulfide minerals, S2~ functions as a simple anion (e.g. CuFeS2, chalcopyrite) and as a compound S2 anion (e.g. FeS2, pyrite). In the sulfosalts, S2~ functions as a component of a complex anion (e.g. ASS3 in tennantite, CU12AS4S13).

The metal sulfides are the raw materials for most of the world supplies of non-ferrous metals and, therefore, can be considered the most important group of ore minerals. Although there are several hundred known sulfide minerals, only a half dozen of them are sufficiently abundant as to be regarded as "rock-forming minerals" (pyrite, pyrrhotite, galena, sphalerite, chalcopyrite and chalcocite; see Bowles and Vaughan 2006). These mostly occur as accessory minerals in certain major rock types, with pyrite being by far the most important volumetrically.