Geochemistry of sulphur formation in nature with special reference to the deposites near kona

Abstract

A brief introductory review of the occurrence of sulphur in nature, and its distribution in space and time, is given.

The various theories proposed for the formation of primary and secondary sulphur have been reviewed. The formation of hydrogen sulphide from organic and inorganic sources has been examined. The role of bacteria in the biochemical reduction of sulphates is discussed. Theories for the oxidation of hydrogen sulphide to sulphur, involving chemical and bacterial agencies, are stated.

The climatic conditions, the geographical and geological features of the Kistna delta in general, and with particular reference to the Kona areas, have been described. A general description of the spots visited, and a detailed description (with the help of soil profiles) of the characteristics of the sulphur?bearing and non?sulphur?bearing spots near Kona, are given. It is indicated that the occurrence of free hydrogen sulphide and black iron sulphide is localised.

The methods employed in the laboratory investigation of the samples from Kona, i.e., physical and chemical methods, are described.

Laboratory studies have conclusively proved that hydrogen sulphide production at Kona is due to the activity of sulphate?reducing bacteria. Studies of the physical and chemical conditions required for the growth and activity of the sulphate?reducing organism (isolated from the sulphur spot near Kona) have been correlated with the biogeochemical environment present in the spot, as revealed by physical and chemical analyses. The environment is found to be highly favourable for the activity of the organism. An alkaline pH, optimum sulphate and chloride concentrations, and the presence of phosphates and organic matter favour its growth and activity. The source of the sulphates is discussed. Supplies from sea?water invasion, oxidation of iron sulphide minerals present in the bedrock, and a local cycle due to oxidation of iron and hydrogen sulphides to ferric sulphate and sulphuric acid are indicated as probable sources maintaining the supply.

The role of iron in geochemical processes in general is indicated, with special reference to the Kona area. The source of the iron present in the clays is discussed and attributed to the iron sulphide mineral present in the bedrock. Iron acts as a fixative for hydrogen sulphide, forming hydrous ferrous sulphide. Mineralogical studies show the presence of secondary iron disulphides, marcasite and pyrite, and probably melnikovite. The conditions necessary for their formation are examined with reference to those existing in the Kona sulphur area.

The origin of the red and yellow masses that constantly occur in the surface layers of the spots where hydrogen sulphide is found is discussed. It is probably ferric hydroxide, precipitated during the oxidation of an aqueous solution of ferrous sulphate to ferric sulphate.

The occurrence of non?crystalline and crystalline forms of sulphur is indicated. Theories proposed to explain the precipitation of free sulphur are reviewed. It has been suggested that in the formation of sulphur in the sulphur spots, ferric iron plays an important role in the oxidation of sulphides and free hydrogen sulphide, resulting in the precipitation of colloidal sulphur. Studies-both in the field and the laboratory-are indicated in support of this theory. The modes of crystallisation of sulphur in nature are described. The probable conditions that favoured such crystallisation of non?crystalline sulphur at Kona have been discussed. The colour and texture of the clay of the sulphur spot at Kona, its capacity to absorb and retain the sun’s radiation-especially during the hot summer months-together with a prolonged time factor, probably result in the crystallisation of white powdery non?crystalline sulphur into yellow orthorhombic crystalline sulphur.