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Micromorphological and Submicroscopical Features Related to Pyrite Oxidation in an Inland Marine Shale from East Central Saskatchewan¹

ABSTRACT

The micromorphology and submicroscopy of a soil mapped within the Arborfield Association in east central Saskatchewan, which is derived from Cretaceous marine shale, were investigated. Qualitative analysis of well-formed monomineralic crystals in the oxidation zone by energy dispersive analysis of x-ray (EDAX) system attached to transmission electron microscope (TEM) confirmed them to be jarosite in which most of the monovalent cation positions were occupied by sodium with minor quantities of Potassium (natrojarosite, Na(K)Fe₃(SO₄)₂(OH)₆). They occur as void natrojarositans, neonatrojarositans, and quasinatrojarositans. Persistence of natrojarosite in some aggregates within the upper Cgk and Cg horizons with high pH is attributed to the low solubility of this mineral. A comparison of the size, shape and distribution of pyrite and natrojarosite suggest that natrojarosite is essentially formed by crystallization from solution rather than in situ replacement of pyrite. Ferric glaebules in the form of nodules and concretions, found only in the upper part of the profile, were characterized as the ultimate development phase of pyrite oxidation. Stabilization of iron as glaebules leaves the sodium and sulphate in the soil solution. These are generally the major soluble ion species in saline soils in Saskatchewan. Presence of planar voids (craze and skew planes) and their orientation pattern, especially in the lower part of the profile, were ascribed to the poor drainage conditions, which is an important factor affecting the persistence of pyrite and natrojarosite in the profile studied.

¹ Contribution no. R368 from Saskatchewan Institute of Pedology, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0W0, Canada. Contribution no. LRRI 84-48 from Land Resource Research Institute, Agriculture Canada, Ottawa, Ontario, K1A 0C6. Research supported by the Natural Science & Engineering Research Council of Canada, Grant no. A0464.

² Research Scientist (Adjunct Associate Professor), Institute of Pedology, University of Saskatchewan, Canada; Post Doctoral Fellow, Massey University, Palmerston North, New Zealand; and Soil Scientist, LRRI, Agriculture Canada, Saskatoon, SK.

Received for publication December 12, 1983. Accepted for publication August 22, 1984.