Spiral growths on large crystal surfaces

Abstract

Means have been developed for maintaining observation of the individual faces of a large crystal while it is in process of growing or dissolving under controlled conditions. This has made possible an investigation into the cause of twinning occurring in lithium sulphate monohydrate crystals. Growth spirals have been detected on the two pairs of [110] wedge faces at opposite ends of the polar axis. The spirals have been shown to have characteristic shapes which are similar to the etch figures which distinguish the respective ends of the polar axis. The spirals and etch figures on the adjacent faces of a wedge pair are oppositely oriented and their approximate angular relations to the crystallographic axes have been ascertained. An estimate of the step height of the spirals indicates them to be about 3000Å. The width of the twinned growth striae which are liable to occur during crystal growth is of almost the same magnitude. It has been established in this work that the incidence of twinning is associated with a condition of high supersaturation in the nutrient solution and is frequently accompanied by spontaneous microcrystallization. The present observations therefore are considered to provide evidence that the growth mechanism is by the addition to an active edge of large unit blocks which have been preformed in solution as ordered clusters.