Salt is one of the most important resources we have, and evidence has shown it was first mined c. 3500 BC in Azerbaijan. We have used it to preserve food since Roman times, when it was also used in place of currency, and it was one of the earliest forms of chemical warfare, in the Ancient Near East (4th Millennium BC to c.600 BC) “salting the earth” of a fallen city was a common practise of the conquering side. Interestingly, it also refers to a positive character trait being described as the “salt of the earth” dates to biblical times (the Sermon on the Mount (Matthew 5:13)) and implies a good and honest person.

In our increasingly extreme winters, salt has also become essential for the maintenance off our roads, a recent National Geographic Article highlights the benefits of salting roads “a study carried out by Marquette University in Milwaukee, Wisconsin, found that road salt reduced crashes by 88 percent, injuries by 85 percent, and accident costs by 85 percent.”. In the UK, this is a costly process, with over 2 million tonnes spread each winter. The British Geological Survey has a useful summary here.

Salt deposits can be found all over the world, commonly a result of a desiccated ocean. In the past, entire oceans have been cutoff and evaporated, leaving behind vast volumes of salt (halite) and its related deposits. The closest example is the the extensive salt deposits in southern europe, the result of a period of time known as the Messinian Salinity Crisis. Fluctuating sea levels and plate tectonics conspired to isolate the mediterranean for periods of time, allowing entire basins to dry up between 5.96 and 5.3 million years ago. The Mediterranean-Atlantic gateway closed, allowing desiccation, and then re-opened, allowing basin recharge. The effect of this is to produce thick sequences of minerals, in some parts of the basins, salt is over 1500m thick, with 150 m of gypsum (hydrated calcium sulphate). You can read an excellent summary in this blog post.

When seawater evaporates, it deposits a series of minerals that can be found in most deposits formed by the same process. They are collectively refered to as “evaporites” (we have some in the UK, more details here), and are formed when the level of energy in water is very low, allowing the very fine particles to fall out of suspension and settle out. These muds are often called marls, and are followed by gypsum, then halite and polyhalite. Each of these minerals are widely used. Gypsum can be found in a huge variety of beauty and cosmetic products. It will be in your toothpaste, makeup, talcum powder, fertilisers and importantly, a key ingredient in plaster.

Gypsum is a soft mineral, you can scratch it with your fingernail, but it takes on a wonderful variety of crystal forms and habits, and easily alters its colour in the presence of other elements. here are a few select examples of gypsum crystals:

Sometimes, gypsum grows as a flower form in very arid environments, and is referred to as a “desert rose”:

Once gypsum has been deposited in the evaporite sequence, the salt (halite) can be deposited. Salt is similar to gypsum in that it can take on a range of colours, but it often takes a perfect, cubic form as can be seen below.

Sometimes, these crystals can be preserved as perfect moulds, forming a particularly intriguing geological feature known as a “halite pseudomorph”. These are rarely found perfectly preserved, as the perfect halite crystal grows in the layer of salt, is covered by a fine sediment, is dissolved and this infills the mould. Once the rock has formed, you can sometimes find these perfect structures preserved:

The salt that we use on our food is often refined salt, effectively a pure sodium chloride with added stabilisers. However, there are variations on this, the easiest to find is sea salt, sea water is evaporated slowly to leave behind larger crystals, you may well have some at home!

Some salts are mined, and hence tend to be quite old. The most famous example of this is the Himalayan Pink Salt, marketed as a health food, this is a salt from the Tethys ocean, that used to exist between India and the Asian continent (a forerunner of the mediterranean). The pink colour is a result of staining from iron oxides, and according to various websites, the salt is a gourmet ingredient, a cosmetic product, or a nice table light.

Salt comes in other colours, depending on what else has contaminated it, this example is from Germany and is an amazingly bright blue:

This spectacular purple halite is part of a fascinating online collection you can view here.

In some parts of the world, gypsum and halite find just the right conditions and can grow to spectacular sizes. One such location was featured on an Iain Stewart documentary “How earth made us”, which is well worth a watch when available online. These crystals have been reported to reach 40m in length, and there is an excellent article here.

Sometimes, salt makes its way to the surface in other ways. These are a few images from one of the best geology websites around, and a real treasure trove of everything volcano logical. Dallol, in East Africa, is a salt lake formed by the extrusion of salt rich geothermal heated waters. The Afar spreading zone provides enough heat to dissolve the salt from deep underground and bring it to the surface. Once it reaches the surface, the liquid cools, and salt precipitates out. Because of the abundant impurities, the waters take on extraordinary colours, such as these:

There are more of these incredible images in several galleries at Stromboli on Line (Dallol 2008 , Dallol 2011, Salt Caravan 2008, Dallol salt lakes, Dallol Springs).

Despite its many benefits, and fascinating links to our development and health, salt is also posing a growing problem worldwide. In regions that are very hot, water is drawn up to the surface with the groundwater, and deposited on the surface. This is most significantly a problem in areas which used to have water either as rainfall or dried up lakes, and worst of all, in areas where significant irrigation takes place in arid/semi-arid environments. Salinisation leaves behind a large volume of salt that renders the land useless as it is toxic to plants.

You can read about the lake and the problems of salivation here.

Feel free to ask questions or share links to other images or stories about salt!

@GeoMillie

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