Rapid formation of rock varnish and other rock coatings on slag deposits near Fontana, California

Manganiferous rock varnish, silica glaze and iron skins have formed on 20- to 40-year-old slag piles. Rapid rock-varnish formation is associated with an unidentified cocci bacterium combined with the likelihood that Mn-rich solutions flow over slag surfaces. A new model is proposed for the formation of silica glaze, involving soluble Al-Si complexes and the nature of wetting films. Iron skins occur where runoff is concentrated, and where iron bacteria grow. Observations indicate: i) substrate can be important in supplying Mn, as well as maintaining a stable surface and channelling runoff to microspots where varnish grows; ii) ignorance of boundary layer pH/Eh conditions makes chemical-physical models of varnish formation speculative; iii) the balance between cementation of recently nucleated varnishes and shear stresses is poorly understood, but is probably an important limiting factor on varnish growth rates; and iv) there must be at least two steps in biotic varnishing - accretion of Mn-Fe in casts, followed by cementation of clays by either in situ cast residue or Mn-Fe remobilized from casts. -from Authors