The last couple of ANDY-SEZs have dealt with carbon dioxide, water (which is really dihydrogen oxide!) and the mechanisms of solution and deposition of the mineral calcite (calcium carbonate - CaCO3). It is time for a change and although we will come back to the earlier subjects I have decided (in absence of any feedback - still and again) to diversify a bit.

Matter, that is the gases, liquids and solids that we all encounter from day to day, is made up of atoms of the 100 or so elements. Those of you who are theoretical and particle physicists amongst cave managers will immediately point out that atoms are collections of sub-atomic particles of various wondrous sorts - I am quite happy to discuss all of these if your all-seeing editor is happy to a) get his proof reading right as there will be many more subscripts and superscripts plus Greek alphanumeric than we have had before and b) if he is willing, or able, to address Heisenberg's Uncertainty Principle and Quantum Mechanics (I am game if you are Kent - OK, he has given in). Matter is made of atoms.

Atoms arrange themselves in set, regular ways in solids but in liquids and gases they are far less disciplined - that is why they are gases (very unorganised) and liquids (not much better structured). The elemental atoms make up solids like diamond (carbon atoms in tetrahedral arrangement) or graphite (carbon atoms in a series of plates lying above one another so that they slide - this is why it is a lubricant) or of gold or silver or lead. These atoms arrange themselves in fixed three dimensional arrays of atoms called lattices. The arrangement of atoms is fixed by the electrostatic attractions between the atoms. Liquids and gases may be made up of atoms or compounds of atoms but they are not in fixed arrays - which is why they slosh around or escape everywhere (bit like cave managers!). However, solids and liquids and gases may be made up of combinations of atoms.

There are around 100 elements (pure substances). However there are millions of compounds made up of two or more atoms (most of the millions are organic compounds - made up of endless combinations of carbon, hydrogen and oxygen plus elements like sulphur and iron amongst others). In caves we are chiefly interested in inorganic compounds - those made up of metals (like calcium, sodium, magnesium and so on) and various non-metals like chlorides, sulphates, silicates and, most importantly, carbonates.

In this ANDYSEZ we are going to talk about calcium carbonate which is a compound made up of three elements. These are (at normal temperatures) the solid metal calcium, the solid carbon and the gas oxygen. However, before we talk about how these combine I have to introduce the concept of ions. Ions are of two kinds. Firstly, they may be atoms missing one or more electrons (small negatively charged sub-atomic particle) and are thus positively charged. These are called cations. Secondly, anions are atoms with one or more additional electrons and are thus negatively charged. In the case of calcium carbonate, which is the major constituent of the impure rock called limestone and of the pure mineral compound called calcite, the compound is made up of a calcium cation (with a double positive charge) plus an anion made up of a combination of one atom of carbon and three of oxygen. This anion is called the carbonate ion and has a double negative charge. Thus when a calcium cation meets with a carbonate anion they meet in electrically stable wedlock. In chemical shorthand this looks like this:

Ca2+ + CO3- = CaCO3 (one calcium ion and one carbonate ion together produces the electrically neutral compound calcium carbonate)

However, the magical and important thing to understand here is that the ions are arranged in a regular geometric array (the lattice) which extends from the individual ions infinitely in all directions. Consider at compound of the solid metal sodium (cation) with the gas chlorine (anion). These together make the white crystal we know as salt. (Chemists use the term salt to describe any combination of cation and anion so I had better be more specific - this is table salt or sea salt - the proper name of this salt is halite).

The sodium ion and the chlorine ion (called a chloride) are of similar size and make a very cubic array like this:

This extends indefinitely. The chemical shorthand for this is: Na+ + Cl- =NaCl (one positive ion (cation) of sodium plus one negative ion (anion) of chlorine (chloride) produces one sodium chloride atom).

Note that every sodium ion has a chloride ion as its neighbour (and vice versa). The combination of ions is known as a molecule. In a calcium carbonate molecule each calcium ion has a carbonate ion as its neighbour (and vice versa). But, because the calcium and carbonate ions are of different sizes they do not pack together in exactly the same way as the nicely cubic table salt molecule does. Differences in the way ions are packed together produce different shaped crystals. The shape produced by calcium carbonate lattice looks like this:

Even in the one lattice array a number of different crystal forms can result. This is the case with the calcium carbonate we know and love so well. One form of packing results in a mineral known as calcite which has a number of crystal forms such as these:

Another form of packing produces the mineral known as aragonite which has exactly the same chemical composition but has the crystal form like this:

All the five pictures above are of the chemical compound calcium carbonate - but of two different packing arrays which produce two different minerals. Note that there are other calcium carbonate minerals with differing packing!

The difference in packing means that the molecules of calcite have different physical properties to those of aragonite. The densities and hardness and indeed solubilities and stabilities of calcite and aragonite differ. More on this later.

This ANDYSEZ has probably introduced more jargon than we have been used to. This however is inescapable if we want to communicate relatively concisely and precisely (I realise that many of you work for government and are thus precluded from being allowed to communicate concisely - if at all).

I could say each time that I needed to talk about an ion that I was referring to a "positively or negatively charged atom" or that when two ions joined together that it was "an electrically neutral combination of two lumps of matter one of which was negatively charged and the other positive". The shorthand (= jargon) is easier.

The next ANDYSEZ will be go for something completely different and will be produced by Dr Ruth Lyons who is visiting the University of NSW at the Australian Defence Force Academy campus in Canberra. She will, I hope, introduce the subject of dating of cave contents.