The crystal growth of gypsum has normally a needle form and can be described by spiral
growth theory. This theory is bases on a particular mode of dislocation of the crystalline, the
screw dislocation. A screw dislocation forms when the atoms are displaced along a dislocation
line, rather that at right angles to it as in the case of the edge dislocation. The attachment
of growth units to the face of the dislocation results in the development of a spiral growth pattern
over the crystal face [10]. This growth looks like it is growing up a spiral staircase’.