An analytical electron microscopy study of pyroxene-to-pyroxenoid reactions

Two occurrences of johannsenite partially altered to pyroxenoid minerals were studied by X-ray emission analytical electron microscopy in the context of the pyroxene-wollas-tonite polysomatic series in order to determine the relationships between composition and structure during the reactions. The complete reaction from johannsenite to rhodonite occurs in a stepwise fashion in both composition and structure. The intermediate product is a metastable mixture of rhodonite and pyroxmangite with high-Ca concentrations and with pyroxene and wollastonite polysomatic slab proportions intermediate between those of johannsenite and rhodonite. The Ca contents exceed the octahedral site occupancy limits of one-seventh and one-fifth normally observed for pyroxmangite and rhodonite, respectively. The compositions of the intermediate rhodonite and pyroxmangite are identical within analytical error, which may indicate that the pyroxene slabs in this pyroxmangite structure are more Ca rich than those of the rhodonite. This high-Ca pyroxmangite might also be viewed as a mixture of alternative slabs similar to structural slabs that occur in johannsenite and rhodonite. The M2 site of johannsenite remains rich in Ca until it reacts to the pyroxenoids, and fine-scale intergrowths of johannsenite and the pyroxenoids thus have compositions that are linear combinations of the johannsenite and pyroxenoid compositions.

The end products of two geometrically distinct solid-state reaction mechanisms, lamellar and bulk, yield pyroxenoids having the same compositions within analytical error. Dissolution of these topotactic products and reprecipitation produce unoriented, gemmy pink rhodonite with lower and presumably equilibrium Ca contents. Therefore, the polysomatic slabs change in composition during the reactions.

Late-stage alteration reactions produce several products: (1) mixed double- and triple- chain silicates that replace the pyroxene and pyroxenoids in orientations typical of amphiboles replacing pyroxenes, (2) fine-grained quartz, (3) amorphous material, and (4) carbonates. These products may be produced by either decreasing temperature or increasing X_CO2 and indicate that complex variations in metasomatic fluid chemistry may have occurred during alteration.