Genesis and evolution of the square soda straws of Dry Cave, West Virginia, USA

A completely new (sub)type of calcite stalactite, similar to a soda straw but showing an external square shape, has been recently observed within Dry Cave, West Virginia, USA. Though rare speleothems with one or more planar sides (triangular to hexagonal cross sections) have been described in the past, this is the first reported example of a subaerial stalactite-like speleothem with a rhombic parallelepiped structure. More than a dozen examples were observed in the cave. The suggested genetic mechanisms allowing the atypical growth of a parallelepiped seem to be controlled by several boundary conditions that normally preclude their development. Constrained by the specifics of calcite crystallization in descending vadose waters and the morphology of collected (already broken) samples, we hypothesize that an initial very low supersaturation within the feeding tube, together with relatively fast dripping, causes the growth of a normal monocrystalline calcite soda straw with the C axis coincident with the tubular axis and circular cross sections. Calcite precipitation on the outside of the soda straw is possible only if there is a water film flowing on the straw’s external surface. Normally, this external flow will cause the rapid transformation of a tubular straw into a typical, polycrystalline conical stalactite. But in this case, the external feeding film is minimally supersaturated or even absent, which slows epitaxial growth over the pre-existing monocrystalline structure of the soda straw and suppresses radial (polycrystalline) growth of calcite crystals. This induces, in stationary conditions, transformation of the straw into parallel, twinned calcite crystals stacked to form a pyramidal stalactite with rhombic cross sections and an overall tapering angle of <2°. In the most extreme cases, the soda straws are gradually transformed into a seemingly square monocrystalline parallelepiped. The necessary boundary conditions involve a non-stationary distribution of always scarce supersaturated/undersaturated flow over the soda straw, alternating with periods of simple saturation. The controlling factor for the development of conical, pyramidal, and parallelepiped stalactites seems to be the tapering angle. This just outlined genetic mechanism is speculative and should be experimentally tested. Finally, in the few collected samples of the square straws, this shape is often masked by another development step, which probably became active when they were already broken. This final stage was characterized by a sudden increase in supersaturation of epitaxial water, which induced the deposition of a thin layer of polycrystalline calcite that masks, at least partially, the monocrystalline structure of the speleothem.