Structural variations in anorthites

Variations of structure and optical properties in anorthites (An 93–97%) of different origin are analyzed with the petrographic microscope, U-stage methods, X-ray single crystal analysis and high voltage electron microscopy. No significant variation has been found in the orientation of the indicatrix and of the lattice constants. But c-type reflections (h + k even, odd) are strong and sharp in anorthites from slowly cooled rocks and diffuse in anorthites of identical chemical composition from quenched igneous rocks. Large type c-antiphase domains (5000–10000 Å) are found in the slowly cooled rocks, c-domains in volcanic rocks are small (100 Å) or could not be imaged. The presence of only b-domains in lunar basalt 14310 indicates quenching of this rock. Large c-domains in the Apollo 15 genesis rock (15415, Lally et al., 1972) indicate slow cooling similar to terrestrial metamorphic rocks.