These beautiful crystals exhibit "hopper" faces, a complex form visible to the naked eye, but shown more clearly here under a binocular microscope, illuminated in a blend of incident and transmitted light. The long-axis field of view is approximately 8 mm, nominal magnification 16X. Under the microscope, the crystals assume fantastic forms due to variability in what crystallographers refer to as crystal habit: kites, pagodas, Mayan temples. The taste, of course, is a staple of culinary desire, sought by humans and many other creatures.

Hopper crystals are known in a number of natural minerals. They are perhaps best-described in terms of rapidly-cooled (quenched) crystals of the silicate olivine, in rocks, meteorites and their synthetic analogues. Hopper forms are found in minerals which, like halite, have cubic symmetry: pyrite, native gold and chromite, and in orthorhombic minerals such as olivine and native sulphur.

"Rock of the Month # 110, posted August 2010" ---

Halite, common salt, has for millennia been one of the best-known and most desirable of minerals, as well as one of the truly essential ones. Halite (NaCl) is a cubic, water-soluble mineral vital to life. Young (1977) reviews the human dimension to common salt, including the colourful salt trade in the Danakil Depression. Kurlansky (2002) looks at salt in great detail, from a range of historical and geographic perspectives. It turns out that the USA is the world's largest salt producer, ahead of China, Germany and Canada. Just 8% of US salt is used in food, the bulk is used on winter roads to combat dangerous ice build-up.

On an industrial scale, salt is usually acquired by mining, in mines both on and beneath the Earth's surface. Salt, or a mixture of NaCl and other water-soluble salts, can of course be extracted from sea water, which averages about 3.7 weight percent NaCl (Li and Schoonmaker, 2005), unless diluted by influx of freshwater from nearby estuaries (the Dead Sea is of course denser yet, roughly 31% total salts).

Common salt, properly known as halite, occurs as a water-soluble mineral of cubic crystal form. This natural sodium chloride (NaCl) is often found with other minerals such as sylvite (a bitter-tasting potassium chloride, KCl). The crystals exemplify hopper faces: in an ideal example, each of the six faces of the cube are not flat but stepped downward to a central apical depression, like a tiny square-sided amphitheatre. Hopper crystals form when crystal growth occurs at differing rates across a crystal face (Phillips, 1971; Bishop, 1972). Halite is typically colourless, but may acquire other hues in the presence of impurities such as iron oxides.

Over geological time, mineable concentrations of salts accumulate through evaporation in shallow saline lakes or coastal bays, and are preserved between layers of less-permeable strata, often rich in clay. Because salt will behave in a plastic way under pressure it can deform within and eventually diapirically penetrate its host strata to form salt domes, which are economically important as possible traps for petroleum (Martinez, 1991), and have even been suggested as safe long-term repositories for radioactive waste materials.

Essex sea salt is one form of this mundane yet essential commodity. The taste of brine on your lips along an ocean shore is a reliable indicator of the sea's saltiness: evaporate a tonne of sea water (roughly 1 cubic metre) and some 37 kilograms (81 pounds) of salt would be left behind.

The sea salt featured here is prepared by a small, independent industry which continues since Roman times. It is located beside the low cliffs and extensive dunes, marshes and mud flats that characterize the North Sea margin of the southeast corner of England, located east of London, opposite the coast of the Netherlands (Holland) which is famously lower yet. A sequence of eastward-draining estuaries score the gentle coastlines of the counties of Essex and Suffolk. The Essex coast is composed of geologically recent sediments: sand, silt and mud, and units of fossiliferous, poorly-consolidated sands known locally as crags (Greensmith et al.. 1973; Lucy, 1999). These strata are underlain by the famous Cretaceous Chalk, intersected in boreholes, and a complex Tertiary sedimentary succession: the coastal features are largely of Pliocene, Pleistocene and Recent date.

The county of Essex is a busy mixture of human endeavour, from the more traditional rural and coastal regions west and south to the bustling East End of London. Historical salt pans are known around the coast (e.g., Jarvis, 1989, pp.22,142).

The material shown here is a famous commercial product, named for the town of Maldon on the Blackwater estuary, and made by the Maldon Crystal Salt Company. In this area, attractive nowadays to boaters and other tourists, a confluence of local rivers such as the Chelmer, Wid, Ter, Blackwater and Brain flows east into the North Sea to form the Blackwater estuary at Maldon, an ancient port between the Roman garrison towns known today as Chelmsford and Colchester. Maldon Sea Salt is a long-established local business: the company web pages provide an interesting history of Essex salt making, as well as details of the process by which pure sea salt is prepared from salty waters of the Blackwater estuary.

The elegant crystals are a source of fascination to children of all ages! Since the crystals are prepared by human industry, they can be said to be a synthetic form of this naturally-occurring mineral.

Salt in the human diet. A brief, non-technical review of recent nutritional information from Canada, the U.K. and Australia suggests the following:

• Human beings need salt on a daily basis, in quantity depending on their size, age and activity level and the ambient climate (e.g., it's hot as I write this, and I'm sweating --- losing salt --- I'll sweat more after more vigourous activity).
• It has been estimated that an ideal adult salt intake should lie between 2.2 and 5.6 grams per day (0.9-2.3 g Na).
• Average salt intake is commonly somewhat higher in the industrialised countries, e.g., 8.5 g/day in Canada and 10 g/day in the U.K. Athletes and others engaged in hard physical work presumably sweat out and thus need to replenish salt at higher rates than less-active persons.

Another world of salt can be found in the east African rift valley in Ethiopia. Salt, being essential to life and for millennia an item of trade, has its own history around the world. One ancient historical source of salt is the Danakil desert of east Africa, shared between northern Ethiopia, Eritrea and Djibouti (Englebert, 1970). The ancient salt trade continues today, with camel- or donkey-powered salt caravans which make their way from the Dalol (Dallol) region around Lake Asele up the Saba river, westwards to Berahile and the market centre of Mekelle (Mekele, Makale) in the more temperate Ethiopian highlands in Tigrai province. Muslim Afar and Christian Tigrayan people alike engage in the salt trade (Morell and Peter, 2005). Dainelli (1943) provided a brief description of the salt deposits (pp.400-405). Garland (1980) describes the region, by way of contrast with the more temperature highlands beyond the Rift, a short distance to the west. The Danakil is a hot plain filled by evaporites, recent volcanics and alluvium, its centre largely below sea level (the elevation at Dalol is -100 m).

References

"The Maldon Crystal Salt Company Limited" --- history and manufacture of Essex sea salt at Maldon

"The Blackwater river and estuary"

Bishop,AC (1972) An Outline of Crystal Morphology. Hutchinson and Co., London, 2nd edition, 314pp., pp.259-260.

Dainelli,G (1943) Geologia Dell'Africa Orientale, Vol. I. Il Progresso delle Conoscenze. Reale Accademia d'Italia, Rome, 464pp. (in Italian).

Englebert,V (1970) The Danakil: nomads of Ethiopia's wasteland. National Geographic 137 no.2, 186-211, February.

Fawn,AJ, Evans,KA, McMaster,I and Davies,DMR (1990) The Red Hills of Essex: salt-making in antiquity. Colchester Archaeological Group, 96pp. plus 7 plates.

Garland,CR (1980) Geology of the Adigrat area. Geological Survey of Ethiopia Memoir 1, 51pp.

Greensmith,JT, Blezard,RG, Bristow,CR, Markham,R and Tucker,EV (1973) The Estuarine Region of Suffolk and Essex. Geologists' Association Guide 12, 41pp.

Jarvis,S (1989) Hidden Essex. Countryside Books, Newbury, Berkshire, England, 175pp.

Li,Y-H and Schoonmaker,JE (2005) Chemical composition and mineralogy of marine sediments. In `Sediments, Diagenesis, and Sedimentary Rocks' (Mackenzie,FT editor). Treatise on Geochemistry volume 7 (Holland,HD and Turekian,KK editors), Elsevier-Pergamon, Oxford, 425pp., 1-35, pp.3-4.

Kurlansky,M (2002) Salt: A World History. Knopf Canada / Walker and Company, 484pp.

Lucy,G (1999) Essex Rock, a Look Beneath the Essex Landscape. Essex Rock and Mineral Society, 128pp.

Martinez,JD (1991) Salt domes. American Scientist 79 no.5, 420-431, September.

Morell,V and Peter,C (2005) Cruelest place on Earth: Africa's Danakil desert. National Geographic 208 no.4, 32-53, October.

Phillips,FC (1971) An Introduction to Crystallography. Oliver & Boyd, Edinburgh, 4th edition, 351pp., pp.173-175.

Young,G (1977) Salt: the essence of life. National Geographic 152 no.3, 380-401, September.