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

The structure of a volcanic transitional anorthite with diffuse type-"c" reflections has been refined three-dimensionally. It is observed to have similar lattice dimensions to low (primitive) anorthite. Its atomic coordinates are insignificantly different. There may be some importance in the variation of its Ca thermal parameters from those of the low-temperature structure, although at this stage in the work all that can be said with certainty is that transitional anorthite is made up of quenched-in domains of primitive anorthite occurring in an anti-phasc relationship such that the perfectly ordcred Al/Si alternation is not disturbed. There is no evidence of other disorder in the structure.