Rates and processes of crystal growth in the system anorthite‐albite

Abstract

Rates of plagioclase crystal growth at constant temperature have been measured for a series of compositions in the system CaAl2Si2O8(An)—NaAlSi3O8 (Ab). A microscope heating stage was used at temperatures near the liquidus to observe and photograph the growing crystals, and a resistance furnace quenching technique was used at larger degrees of undercooling with observations and measurements made on quenched samples of heat‐treated glasses. Results from the two techniques agree within the temperature range of overlap. Growth rates are independent of time and increase (at either constant temperature or constant undercooling) with increasing An content. The crystal morphologies for all compositions are faceted near the liquidus and become progressively skeletal, dendritic, and fibrillar with increasing undercooling. Even the spherulitic or fibrillar crystals may be faceted at the growing interface. Maximum growth rates vary from about 5×10−1cm/min at an undercooling of 150°C (T = 1400°C) for pure CaAl2Si2O8 (An100) to about 2 × 10−5 cm/min at an undercooling of 380°C (T = 960°C) for An20.