Helictites
Many wonderful features known as speleothems form in the cave environment as calcium carbonate precipitates from saturated waters, including the more common stalactites and stalagmites, but my favourites are these fairly uncommon twisty to angular growths that seem to defy gravity. They are the most fragile of cave formations, made of calcite or its polymorph (same composition, different crystal structure) aragonite and they occur in many unusual shapes, usually with radial symmetry. Several locations have stunning specimens, amongst them Timpanogos cave national monument, Jenolan caves in Australia and Pozalagua in Spain.
How they form is still uncertain, though capillary force sucking up small droplets through the thin carbonate tube like a straw is the leading contender amongst a plethora of suggestions. Most helictites are not hollow though, and it remains hard to explain their twists and turns in any conventional deposition method.
Other suggestions include wind action within the cavern blowing drops to one side of a straw, and the growth direction changes as the wind direction fluctuates. The problem with this hypothesis is reconciling the timescales, winds vary with the outside weather and air pressure, while speleothems grow very slowly, and many caves displaying these ffeatures have no opening to the winds. A third idea is the activation of piezoelectric forces by changes in tectonic pressure influencing the zone of precipitation. Impurities in the calcite and the growth of uneven wedge shaped crystals atop one another have also been mooted.
They start life as tiny stalactitic straws and wander around in a variety of directions or with smaller outgrowths forming on their sides. Sometimes clusters form bushes up to a metre and a half tall, and they can also occur on cave floors (becoming heligmites).
Loz
Image credit: Mac Baker
http://www.goodearthgraphics.com/virtcave/helictit/helictit.html http://www.showcaves.com/english/explain/Speleothem/Helictite.html http://www.goodearthgraphics.com/virtcave/helictit/chasm.html
