Summary: The optical properties and specific gravity, and the variations in these properties, of analysed plagioclases separated from the meteorite (eucrite) from Juvinas, France, are compared with the corresponding values for a terrestrial plagioclase of almost identical composition from an allivalite from Rhum. The range in properties for the Juvinas plagioclase indicates a compositional range from about Ab₃₅An₆₅ to Ab₁₅An₈₅, but the great majority of grains are bytownite with approximately 83 % (molecular) anorthite.

The frequency of occurrence of the twin-laws of the Juvinas plagioclase resembles that of plagioclase from basalts and allied rocks.

The variable changes and, in some grains, absence of change in the optical properties shown by the Juvinas plagioclase crystals, after heating to 1100°C. for 48 hours followed by rapid cooling, contrasts with the small but constant optical changes shown by the allivalite plagioclase (for which geological evidence indicates slow cooling) after identical heat treatment. It is inferred that the meteoritic plagioclase grains have had differing thermal histories, and that some grains have been subjected to previous natural quenching.