Whilst on our way out to Lark Quarry we found reference to these ‘Council Gravel Pits’ in good seasons these gravel pits often hold water, which becomes clear due to the presence of Gypsum in the soil bringing the clay and silts out of suspension.
Taking every opportunity to teach the Geo kids some natural sciences we made a point of stopping here and we though this lesson would be good for other travelling geo kids who may or may not be missing too much school! This location shows a good example of Evaporite forming. We have another Gypsum Earth Cache near Hughenden you might get the opportunity to visit. Click for Broken Glass GC6RV44 here
Gypsum is a soft sulphate mineral composed of calcium sulphate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, Byzantine empire and the Nottingham alabasters of medieval England. It is the definition of a hardness of 2 on the Mohs scale of mineral hardness. It forms as an evaporite mineral and as a hydration product of anhydrite.
Gypsum is moderately water-soluble (~2.0–2.5 g/l at 25 °C) and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures. When gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate, (bassanite, often simply called "plaster") and, if heated further, to anhydrous calcium sulfate (anhydrite). As for anhydrite, its solubility in saline solutions and in brines is also strongly dependent on NaCl concentration.
Gypsum in the Eromanga Basin is derived from two sources. Much, but not all, of it was formed in the ancient shallow seas 100 million years ago, when under conditions of high evaporation and low water circulation the combination of calcium (Ca) and sulfate (SO4) formed a relatively insoluble mineral on the sea bed. However some of the gypsum is of much more recent origin - sulfide (SO3) and sulfur (S) from the weathering Cretaceous shales (where it was formed from the decomposition of marine organisms) being oxidised to into sulfate (SO4) and combining with the ubiquitous lime (Ca) also produces the mineral.
Evaporite is a name for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes.
Evaporites are considered sedimentary rocks and are formed by chemical sediments. Although all water bodies on the surface and in aquifers contain dissolved salts, the water must evaporate into the atmosphere for the minerals to precipitate. For this to happen, the water body must enter a restricted environment where water input into this environment remains below the net rate of evaporation. This is usually an arid environment with a small basin fed by a limited input of water. When evaporation occurs, the remaining water is enriched in salts, and they precipitate when the water becomes supersaturated.
Non-marine evaporites are usually composed of minerals that are not common in marine environments, because in general the water from which non-marine evaporite precipitates have proportions of chemical elements different from those found in the marine environments. Common minerals that are found in these deposits include blödite, borax, epsomite, gaylussite, glauberite, mirabilite, thenardite and trona. Non-marine deposits may also contain halite, gypsum, and anhydrite, and may in some cases even be dominated by these minerals, although they did not come from ocean deposits. This, however, does not make non-marine deposits any less important; these deposits often help to paint a picture into past Earth climates. Some particular deposits even show important tectonic and climatic changes. These deposits also may contain important minerals that help in today's economy. Thick non-marine deposits that accumulate tend to form where evaporation rates will exceed the inflow rate, and where there is sufficient soluble supplies. The inflow also has to occur in a closed basin, or one with restricted outflow, so that the sediment has time to pool and form in a lake or other standing body of water. Primary examples of this are called "saline lake deposits". Saline lakes includes things such as perennial lakes, which are lakes that are there year-round, playa lakes, which are lakes that appear only during certain seasons, or any other terms that are used to define places that hold standing bodies of water intermittently or year-round.
Evaporite formations need not be composed entirely of halite salt. In fact, most evaporite formations do not contain more than a few percent of evaporite minerals, the remainder being composed of the more typical detrital clastic rocks and carbonates.
For a formation to be recognised as evaporitic it may simply require recognition of halite pseudomorphs, sequences composed of some proportion of evaporite minerals, and recognition of mud crack textures or other textures.
To log this Earth Cache we require you to find some Gypsum, consider the information given and perhaps if you need to do some research of your own, then message us with the following answers to the best of your ability;
1. If there is water in the pit, what colour is it, what does this colour suggest to you about the length of time since it last rained in the area? If there is no water when you visit hypothesise about what cloudy water would suggest about rain?
2. Find some Evaporite, how dense is it, what colour is it? What does it feel and look like? *this will be difficult to do if rain has been very recent, if this is the case hypothosise what you would expect.
3. Experiment time, If you can get to the waters edge, collect some water 100 - 250ml will be enough, take it home with you. Once you are home you will also need the same amount of bottled water in a separate comparably sized container, let it evaporate in the sun or use a heat source to speed the process. Once all the water has evaporated, what are you left with? Is there any difference between the two? What do the results suggest to you? * If there is no water available, you could try this experiment with water from other nearby water sources.
4. A photo of your team or GPS near GZ, upload as many photos as you like! (Optional) As a bonus we have included a link for a fun and useful experiment that can be done at home that involves Gypsum – Click here Making Street Chalk! We would love to hear about your chalk and see photo’s.
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within 24 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Source Wikipedia, sweetpaulmag.
