Growth mechanisms of plagioclase in a basaltic melt

Abstract

Growth mechanism of plagioclase crystals from a basaltic melt of tholeiite type prepared by chemical reagents was studied by using the isothermal-dropping technique with iron and platinum capsules. Produced phases are plagioclase, olivine, pigeonite and augite. Crystal morphology of plagioclase varies from prismatic to spherulitic with increasing degree of undercooling, ΔT.
Crystal length along the elongated direction of plagioclase increases with the square root of time, and the normal growth rates are in the range of about 0.2−13.0×10⁻³ (mm/min) at ΔT of 9–95°C. At ΔT less than 30°C, the spiral or two dimensional nucleation mechanism is dominant. At ΔT between 30 and 70°C, two-dimensional nucleation, and at ΔT more than 70°C, diffusion controlled continuous or non-faceted growth mechanisms are dominant.