It should have been, as we say in Glasgow, a doddle. Create and design a commemorative plaque that could get sent out into space to convey the essential aspects of Rough Science to any passing alien. What could be simpler? So why am I still sitting here at dusk scratching my head about what to put on it and, more importantly, where on Earth to get the materials to make it. As a geologist, the second bit shouldn’t be a problem. But I was on my own with this one. Kathy and Jonathan were making a light-communication device, and Mike and Ellen were to knock up an upside-down pen.
Things had started OK. It was clear right away that it would take more than three days to make the plaque out of metal or rock. For me, the obvious choice was something very familiar to us all - plaster of Paris. Plaster of Paris is that white powder that forms a paste when it is mixed with water and then hardens into a solid, used in making casts, moulds, and sculpture. Chemically it is calcium sulphate, but geologically it is found naturally as the rock ‘gypsum’. And the reason I was so confident about making the plaque out of this was that in our desert environment there was likely to be tons of gypsum around. That’s because most gypsum ‘grows’ in the dried-out interiors of salt lakes, or playas, and there were plenty of them close by. In fact, we’d gone to the closest, Owens Lake, in our search for water in programme one.
What happens is that these playas are vast evaporating dishes that crust over with a collection of salt minerals. Gradual evaporation produces an orderly precipitation of salt crystals out of the shrinking pool of water. First to grow out are the most readily soluble – carbonates of calcium and magnesium which are deposited around the edges of the lake. As evaporation continues, sulphates of calcium and sodium are deposited, and if there is enough calcium left over from the first carbonate stage then we get calcium sulphate, or gypsum. If evaporation keeps going, then the water (or brine) becomes incredibly salty and ordinary table salt (sodium chloride) gets left behind.
The trouble is – that’s the theory. The reality is that playas have their own unique chemical environments, since the surrounding geology determines the chemical mixture being washed in. So different playas often have distinctive assemblages of salts. Owens Lake, for example, is mainly covered by a crust of halite (sodium chloride). A hot, sweaty walk across the dry lake bed failed to find anything that looked much like gypsum (if only I had chips to go with all that salt). That’s because there was very little calcium in the surrounding rocks – what we really needed was some limestone hills, since these would be made from calcium carbonate. As for the other dry lakes where limestone rocks were nearby, well they were off limits to us. They were either fenced off as US airforce bombing ranges or they were protected scientific sites. At Searles Lake, for example, where there ought to have been really thick deposits of almost pure gypsum, collection of rock samples is only permitted to authorised geologists during one week of the year. It was amazing to think that these barren, inhospitable and even poisonous deserts were so protected (if at least from hammers, not missiles).
With no chance getting to big deposits of gypsum, the alternative was to find places where it might crop up in smaller quantities. A common way that gypsum forms naturally is where water has moved along cracks in the rock, allowing calcium sulphate in the water to form crystals on the walls of the crack, sealing it into what is known as a vein. So the hunt was on to find rocks that had loads of veining in them, to see if some of those veins were gypsum.
With the other four scientists busy pushing on with things, Kate was more than happy to escape out into the wild. With her old Top Gear camera and sound crew in the back, out came Off-road Kate - driving down dry river beds in a 4-wheel landrover while I nervously peered up at the ravine walls for my white gypsum. Around 5 o’clock we found it – a network of white stripes cutting dark-grey limestone rocks, sometimes injected along the rock layers, other times cutting right across them. But as we hammered out what seemed like tons of the rock, I had a nagging doubt; the gypsum I knew from the UK was often soft enough to dig out by hand, whereas this stuff was hard. With the sun setting quickly, we grabbed what we could and headed for home, the dusk concealing the worry on my face that, despite 20 years of geological experience, we’d just collected a pile of .
Calcite. That’s what I think most of it is. I’ve pulverised what I thought was the gypsum and although it breaks into nice crystals that you can crush with your teeth (so it’s not quartz), I still don’t think it’s soft enough. Any geology textbook will tell you that the hardness of minerals is given by Mohs scale, ranging from the softest, talc, to the hardest, diamond. Now gypsum is the next softest up from talc, and typically lets you scratch the crystal faces with a fingernail. Next in the scale is calcite, or calcium carbonate, which you can scratch with a knife. The crystals I had didn’t seem to scratch easily and a quick dash of hydrochloric acid confirmed my fears. The frenzied fizzing of the crystals in the acid indicated that there was calcium carbonate in there. It wasn’t that surprising, since the veins were in limestone. What it looked like was we’d collected a mixture of gypsum (calcium sulphate) and calcite (calcium carbonate), and stupidly I’d lumped everything together and bashed it all to a powder. Kate looked about as crushed as the powder – not a good day for geology on Rough Science 4.
I moped around a lot.
Brainwave - Mike suggested converting the calcium carbonate back into calcium sulphate by using sulphuric acid from the numerous car batteries littering the yard. Great idea.
Started draining batteries, still moping.
Pretended to be a chemist (even got the white coat on) to mix the sulphuric acid with the gypsum/calcite mix. Hopefully what is gypsum will stay there while what is calcite will convert. By early evening I had a tablespoon of white powder. Not drugs to keep me going – gypsum residue. It was going to be a late night.
Moped a bit more.
A few late beers had put the moping to bed and an early start finds a reasonable volume of gypsum powder for me to start making my plaster of Paris plaque (yes, I’d forgotten what it was all for too). Anyway, all the trouble thinking about how to get the gypsum hasn’t left much time for creative thinking about what to put on the blasted thing. Some brainstorming with the others brings out lots of clever ideas about what are the essential elements of planet Earth and even some philosophical debates about whether we should be communicating the ingenuity of our planet or the ingenuity of us humans. In the end, desperate times meant desperate measures.
I decided that Kate and I were in this together. After all, a few years ago she’d co-presented The Essential Guide To Rocks – BBC Two’s last geology series. As an honorary geologist, she should have recognised it was calcite veins as well as gypsum. She should share the humiliation. What better then than a plaster of Paris mould of her face? The only trouble is that when calcium sulphate mixes with water the chemical reaction gives off heat, so people using it wear gloves to avoid burning the skin. Clearly Kate didn’t deserve to lose her skin for what was a relatively minor geological mistake. A safer way to make the mould was to have her make the impression into wet sand, and then pour the liquid plaster of Paris into it to set in the mould.
But would our lovely Kate really agree to bury her face in wet sand? Of course she did – what a girl! With her delicate features (squashed nose and double chin) preserved in stone for eternity, my bit was done. Of course – the others had their challenges to complete. Having been a bit too preoccupied with moping I hadn’t had much chance to check up on how theirs were going, so it was brilliant to watch as first the magical ‘sound-on-a-sunbeam’ light-communication device and then the topsy-turvy space pen worked. Magnificent. Team hugs. End of moping.