Does crystal splitting play a part in the curvature of helictites?


George W. Moore
Journal of Cave and Karst Studies, v. 62, p. 37 (2000).
At straight segments of calcite helictites, the crystal axis of fastest growth maintains a constant orientation, whereas at curved segments it follows the curvature. Three mechanisms for rotated crystals are (1) the long axes of the crystals in conical stalactites always point toward their curved outer surface, because they crowd out slower growing seed crystals of other orientations; (2) Russian investigators explain radiating crystals by crystal splitting, the insertion of molecular wedges to divide an initial crystal; and (3) microorganisms cause a nontypical diagonal orientation of calcite moonmilk grains. Helictites are nourished by a capillary at the tip. The growth increments consist of nested cones, but thin sections show that the crystal units are wedges that diagonally cross the whole helictite. The youngest wedge points toward the intersection between two faces of the helictite's 3-sided pyramidal tip. The wedges grow toward that intersection (the trace of a scalenohedral tip) and by crystal overgrowth at the edges of older wedges. Deposition follows the crystal lattice of the wedges but is greatest near the capillary, leading to an increased angle. When water flow stops periodically, subsequent growth is controlled by crystal crowding. It is perpendicular to the outside of the curve next to the capillary, and its crystal orientation differs slightly from that of the previous wedge. We cannot with certainty distinguish crystal crowding from crystal splitting near the orifice of the capillary, but crystal splitting ought to cause fanning from the orifice, whereas the wedges are planar across the entire helictite.