X-ray and electron-optical investigation of synthetic high-temperature plagioclases

Abstract

High-temperature plagioclases NaAlSi3O8(Ab)-CaAl2Si2O8(An) have been prepared by dry devitrification of glasses. X-ray powder photographs were taken to follow the time-temperature dependence of indicators of structural state and to determine the lattice parameters of the plagioclases as well as their K-exchanged equivalents. Transmission electron microscopy (TEM) has been used to study the anorthite-type antiphase domain textures. The results are as follows: (1) TEM and X-ray powder data suggest to subdivide the high-temperature plagioclase series into three structural regions: (a) An0 to An30–40: monalbite/analbite to high albite-type structures, (b) An60–70 to An100: anorthite-type structures, (c) An30–40 to An60–70: transitional structures assumed to represent the early stages in the development of the structural complexities found in chemically intermediate low temperature plagioclases. (2) b-Antiphase domains typical for the anorthite-type structure could be imaged in samples ranging between An100 and An67.5, whereas c-antiphase domains have only been found in An90 and An100. The b-domains developed during isothermal crystallization, rather than cooling. The transition from the anorthite-type structure (S.G.I\(\bar 1\)) to an intermediate C\(\bar 1\)-type structure at An60–70 appears to be a gradual one. (3) The relative contribution of various structural and chemical influences on the cell edges and angles is discussed in detail. It is shown that the nonlinear variation of γ with An content is due to incomplete Al,Si disorder in the high series.