Phyllosilicate formation in berthierine peloids and iron oolites

Abstract

Summary Electron microprobe compositions of phyllosilicates formed in berthierine pellets and iron oolites suggest two different reaction paths which result in slightly different bulk chemical compositions. In the case of berthierine peloids, for mineral aggregates formed and transformed at or very near the sediment—sea water interface, the starting materials seem to be most often dominated by kaolinite. The gradual incorporation of iron and magnesium produces the chlorite composition within the pellet. Systematic compositional zoning is apparent when pellet edges are compared to pellet centres. The compositions of the chlorite end-product have a rather large range of Fe/Fe + Mg (0.65–0.88). The general compositional range of the transformed berthierine peloids is less aluminous than kaolinite suggesting a high silica activity in the solutions, which is independent of the dissolution of kaolinite. In the case of berthierine/chamosite-bearing iron oxide oolites, the system is dominated by the presence of iron, as would be expected. The range of compositions observed using the electron microprobe on oolites which still contain iron oxides gives an Al/Si ratio of one, that of kaolinite. The formation of these chlorites is through a combination of divalent iron and kaolinite. These minerals have Fe/ Fe + Mg = 0.88–1.0. The bulk composition is Si 26 Al 26 (Fe,Mg) 48 . Multiple point analyses in the same oolite or pellet indicate that the formation of chlorite occurs at various points in the mass with no specific zoning pattern. When the iron oxide in a rock is completely reacted to form silicate and/or phosphate phases (the oolites are completely chloritized), the berthierine/chamosite mineral composition can become less aluminous due to a general chemical equilibrium established throughout the sample. In more metamorphic iron ore rocks, where the oolitic structure is absent and where new iron silicates are present (such as stilpnomelane) and iron oxides are present, the chlorite composition can be found to vary from that formed in the initial reaction, i.e. Si/Al = 1. They can be more silica-rich or more aluminous. These analytical results indicate that the composition of the chlorites formed in high iron environments reflect the chemical system in which they form and the scale of that system, a pellet or a rock.