Abstract

In Fig. 4 of the paper, “Reaction Series for Subalkaline Igneous Rocks Based on Different Oxygen Pressure Conditions” (Osborn 1962), I indicated by dashed lines some simple, inferred phase relations for the right face, CaSiO3-Fe304-SiO2, of the tetrahedron, CaSiO3-Mg2SiO4-Fe304-SiO2 in air. The precise relations on this face are not important as far as the argument of the paper is concerned. There was no intention however of omitting available data. The data published by Phillips and Muan (1959) and plotted as the diagram for CaO-Fe2O3-SiO2 were obtained in an air atmosphere and hence are directly applicable. On their diagram wollas-tonite does not have a field. This phase appears only at subsolidus temperatures at this oxygen pressure. A field for hematite however is present, as well as one for magnetite. The absence of wollastonite at liquidus temperatures and presence of hematite thus change the configuration of the right face of the tetrahedron as sketched in Fig. 4. The general relations within the tetrahedron in the neighborhood of points R, D and E in Fig. 4, however, remain as shown in this figure; for as MgO is added to the system represented by the right face, liquidus temperatures drop and at the same time the temperature of stability of wollastonite rises as Mg2+ enters the structure. Hence wollastonite is the phase in equilibrium with liquid, tridymite and crystalline iron oxide in the vicinity of the point E in Fig. 4. Similarly, Mg2+ entering the magnetite structure stabilizes this form relative to hematite, with the result that magnetite may be expected to displace hematite in equilibrium with liquids in the central part of the tetrahedron even though temperature decreases. This latter has been found to be the case by Roeder (1960) in his study of a similar tetrahedron, CaAl2Si2O8-Mg2SiO4-Fe3O4-SiO2. Whether magnetite or hematite is the iron oxide phase in equilibrium with liquids in the neighborhood of points R and D of Fig. 4 is however only a minor detail from the standpoint of the use made of Fig. 4 in the paper.

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