Transformation Trillings in Cordierite

Abstract

Cordierite sector trillings, which are found well developed in some A1-rich volcanic rocks and pelitic buchites, are thought to have formed initially as hexagonal high cordierite crystals. Subsequent ordering of the (Al, Si) configuration in the crystal structure would have produced the present orthorhombic modification and simultaneously the sector trilling, preserving in the process the hexagonal habit of form. The predominance of the regular sixfold sector pattern in the composition plane configuration of the resulting trilling is best explained by assuming that its formation is kinetically more advantageous than that of other configurations because it mimics the hexagonal trend set by the initial crystal. The three orthorhombic orientations in the cordierite trillings have parallel c-axes and each can be transformed into either of the other two by operation of a threefold axis (rotation through 120°) parallel to the common c-axis and coincident with the main axis of the original hexagonal form. The composition planes in the sector trilling pattern are shown to be essentially irregular in the present orthorhombic form; they do not belong to {110} or {130}. The traditional explanation that the trillings are multiple twins with repeated twin and composition planes of {110} or {130} is rejected since elementary geometrical considerations show that this model is only feasible if (110) Λ (110) of orthorhombic cordierite equals 120°. X-ray work shows conclusively that this is not the case. In fact (110) Λ (110) = 120° would define a hexagonal symmetry.