Observations on the nature of unmixing in peristerite plagioclases

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

The exsolved phase of the structurally inhomogeneous peristerite plagioclases (An2-An17) has been observed to take the form of minute ( - 0.5 µ) domains scattered in apparently random array throughout the host feldspar (Ribbe and Van Cott (1962)). It is shown, however, that there must be mutual crystallographic orientation of the two phases, An2.5 and An25, to explain the schiller phenomenon. A detailed comparison of the atomic structures of albite and oligoclase (An22), while failing to indicate any geometric relationships that would explain the directional schiller (Bøggild ( 1924)) or the related (081) lamellae (Brown ( 1960 b)), indicates a el ose similarity of the structural frameworks in the two end-members.

The peristerite subsolidus is discussed in detail from two viewpoints: that of heat-treated natura! specimens and that of natural (equilibrium) plagioclase. It is postulated that the unmixing solvus is asymmetric, rising sharply in the vicinity of An2•5 and sloping off to An17 where a steep gradient marks the limit of peristerite exsolution. From An17 the solvus is flared out at very low temperatures to the An20-30 compositions, and evidence is given that a sodic peristerite will tend to unmix to An2.5 and An30 and a calcic peristerite will unmix to An2.5 and An20. Unmixing is postulated to be contiguous with the Al/Si ordering process in the peristerite range.