As many of you will know the subject of helictites is one that I dread. However, I promised you an earful on the subject in the very first ANDYSEZ and again on the last occasion I wrote something for you. So I guess I must do something on these dreadful things. I am going to restrict this discussion, for this ANDYSEZ at least, to carbonate minerals.

The first thing to remember is that there is infinite variety in nature - even for things made up of regular arrays of atoms. These regular arrays are called crystals. The second thing to remember is that more than one mechanism may be operating to produce the forms we ooh-ah about in caves. These mechanisms may act sequentially, episodically or simultaneously. We shouldn't be surprised to find things that are different. I do not know if anyone has utilised chaos theory and fractals in the analysis of helictite growth and form. I am sure that there are published papers on this somewhere - I just haven't looked (nor do I have the resources in sunny Grafton - especially when I am up against a Henderson deadline). However, the mathematics of chaos will be a fine tool in understanding helictites.

Hill and Forti (1986) sum this up as follows:

The number of common carbonate minerals occurring in caves is small, but because of the variety of controlling forces on the mode of deposition, there exists a large variety of morphological speleothem types (page 22; emphasis mine).

One of the problems with helictites is, as usual, definition. Often in Australia and New Zealand we use the term "helictite" to encompass a variety of things that are not helictites. Anthodites and frostwork are often included, and as these form by fundamentally different mechanisms, putting them in the helictite group confuses the issue.

Helictites are commonly associated with crusts or coatings on bedrock surfaces or are found growing on other speleothems, notably straws.

At the end of the last ANDYSEZ I asked that people bone up on the discussions of crystal development in earlier issues of the Journal. I imagine that no one has done this so we will avoid the detail at this stage. Many, many explanations have been, and still are, advanced to explain the wide variety of forms. Biological and meteorological phenomena are often invoked and may be involved in some cases. However, the basic influences on the modes of deposition seem to be firmly rooted in physical phenomena.

Hill and Forti make it all sound so easy when they state:

Helictites twist in every direction because they grow by capillary water seeping through tiny, internal canals (page 22).

This, however, doesn't explain anything. All helictites have these canals running through them feeding their extremities; sometimes there are secondary canals that feed other surfaces. These were pointed out as long ago as 1655 (Worm, cited in Hill and Forti). The canals range in size from about half a millimetre down to around one hundredth of a millimetre.

Again according to Hill and Forti:

The theory of helictite origin most popular at present [1986] involves an integration of Huff's (1940) hydrostatic pressure-capillarity theory with Moore's (1954) curvature theory, combined with other factors such as evaporation, air flow, impurities in solution, water supply, and intracrystalline seepage. A porous rock face (usually covered with a thin, carbonate coating or crust) is required for the beginning of helictite growth. Hydrostatic pressure forces a small amount of solution out of a pore in the wall or coating, and carbon dioxide loss results in a thin carbonate film being deposited around the pore. As water continues to seep through the opening, the pore is perpetuated as a central canal; thus the helictite grows longer as deposition continues at its tip. Because all the solvent is evaporated (the flow rate is not enough for drops to form), impurities are co-deposited along with the carbonate material. These, as well as rotation of the calcite crystallographic axis and the stacking of wedge-shaped crystals, causes the helictite to spiral or to bifurcate (or both) (page 44).

Clear?

We will examine in the next ANDYSEZ just what impurities do to the crystal structure and how crystallographic processes produce bends and thus helictites. Having raised the issue we will also need to discuss related forms such as anthodites further down the track.