Halite

Probably no other mineral has played as great a role in the development of human societies as halite has. Although many people in industrialized countries only think of halite as the salt used in seasoning food or melting ice off winter roads, halite’s role in our world runs far deeper. Without halite, agricultural societies and domesticated livestock would not have been possible. Since the advent of civilization up to the last few decades, halite was one of the most sought after, fought over, and traded commodities in the world. In China, it was the basis of the first state industry and monopoly, and later drove the establishment of trade routes along which flowed the exchange of goods and ideas that largely built the modern world. Wars have been fought over it and lost because of a lack of it. In short, halite directly or indirectly created our present society.

Despite its importance, halite has a remarkably simple origin. It forms from evaporation of seawater or brine lakes and may be found wherever ancient lakes or restricted seas were cut off and evaporated. The human history of this remarkable mineral is brilliantly laid out in the book Salt by Mark Kurlansky.

Description and Identifying Characteristics

Halite’s most distinctive characteristic is its salty taste. For practical and sanitary reasons, you probably should not test the taste of a mineral sample by directly licking the sample. Instead, moisten your finger, rub the sample and then taste the residue left on your finger. This will limit the amount of the material that you take in, an important consideration since there are some rare mineral salts that are poisonous in large amounts.

If you do not like the idea of ’tasting’ mineral samples, halite’s other characteristics can be used to distinguish it. Halite forms clear transparent crystals that exhibit perfect cubic cleavage. If you look closely at table salt, the salt is an array of small cubic fragments, the result of halite’s crystal structure and perfect cubic cleavage. Cubic cleavage will be evident in most samples of halite or can be produced by breaking the sample. Halite is also much softer than glass and can be easily scratched using a nail or metal surface. If any doubt remains, halite is one of the few common minerals that, when broken into small fragments, will readily dissolve in water.

Halite primarily forms from the evaporation of seawater. Roughly 90% of the original volume of seawater must be evaporated before the concentration of the remaining brine is high enough to precipitate halite. So, significant halite production is usually restricted to arid environments in which the influx of seawater is relatively low compared to the loss by evaporation. Since halite dissolves easily in water, halite outcrops typically are only found in very arid environments. However, subsurface deposits of halite occur in many areas and are far more abundant than people previously recognized until the advent of widespread oil drilling.

Presently, halite is forming along the shores of the Great Salt Lake in Utah, as well as on salt flats in Death Valley, western Texas, and other arid areas. Some of the most famous North American deposits are the salt mines beneath the mid-west cities of Cleveland and Detroit, but thick halite deposits also occur in Ontario, Saskatchewan, New York, New Mexico, and along the Gulf coast. Worldwide, one of the more remarkable halite deposits occurs beneath the Mediterranean Sea. These beds formed about eight million years ago, when the Mediterranean Sea was a land-locked salt lake and evaporation formed evaporite beds that are over 2000 meters thick. This period of evaporite deposition ended roughly five and a half million years ago, when the Straits of Gibraltar opened to once again flood the basin.

Since it forms primarily from evaporation, halite normally is found with other evaporation-associated minerals such as calcite, gypsum, anhydrite, and sylvite. Sylvite, although rare, is poisonous - so it is a good idea to use your finger to test an unknown sample’s taste, rather than directly licking the sample.

Halite gets its name from ‘hals’, the Greek name for salt. It is difficult to overstate the past importance of salt in both human and domesticated animal consumption. Without salt, widespread animal domestication (and hence modern civilization) could not have arisen. The sodium from salt is necessary for cells to function, so people need salt to survive. Although hunting and gathering societies may have been able to get the salt they needed from prey animals, agricultural societies that rely more on crops need salt as a source of sodium. Domestic animals also need salt in order to survive. Consequently control of salt resources and the salt trade influenced the politics of human society well into the early 20th century. In ancient times, salt often equaled wealth and in some societies was even used as a form of money. Historically, one of the most important uses of halite was in the preservation of meat. In times and areas where refrigeration was not an option, salting meat was often the best way to preserve it, and many societies either mined rock salt or developed ways to use natural evaporation to produce salt for this purpose. Salt was also widely used in medicine as a disinfectant for wounds.

Presently, halite is still obviously important in the preparation of food, but in North America the largest use of salt is to produce chlorine and sodium hydroxide, two of the more widely used compounds in the chemical industry. Sodium hydroxide is also widely used in the manufacture of pulp, paper, textiles and soaps. In addition, vast amounts of halite are also used as road salt in the northern areas, to melt snow and ice during winter months. Together the chemical and road salt uses account for three-fourths of salt consumption. In contrast, the use of salt for human consumption accounts for only a few percent of the total salt consumption. Large amounts of halite are also used in water softeners, as animal licks, and for medicinal purposes. This salt is either produced in evaporative salt ponds or from rock salt mining. Rock salt can be directly quarried or recovered by pumping water down into the deposit and back to the surface where it is evaporated to recover the salt.

Historically, the indirect benefits of salt were almost as important to our society as its direct benefits. Salt is heavy and difficult to move long distances, so its transport led to many transportation innovations from canals to steamships. One of the most common ways to acquire salt in the past was to drill down into the Earth in search of salty brines that were then pumped to the surface and evaporated to produce salt. Drilling techniques, originally developed to acquire salt, allowed for the development of our modern oil and natural gas industry.

The oil and gas industry benefits from salt in other ways as well. Halite deposits are relatively soft rocks that are easily deformed. Under pressure, these deposits flow upwards to form large salt domes that distort the surrounding beds, often forming traps for oil and natural gas. Much of the offshore production of hydrocarbons along the Texas and Louisiana coast is associated with salt domes.

In rock shops, large halite crystal masses are often sold as decorative stones. Because the crystals can form quickly in modern evaporative settings, rock shop owners can place bones and other objects in evaporating salt ponds to encrust them with decorative masses of pastel-colored halite crystals. Buyers should be aware though, that their purchases might not hold up well under humid conditions.

Most motorists are aware of the negative consequences of this use as salt helps to corrode their vehicles, but in areas where salt is extensively used on winter roads, the long term environmental impacts on fresh water systems and their associated plant/animal communities can be far more important.

In arid areas, salt can also buildup in soils to greatly decrease its productivity. When Rome finally defeated Carthage in the Third Punic War, the Romans sowed salt in the fields around Carthage so that the area could never again support a large population. Modern irrigation in arid and semi-arid areas can have the same effect. If the salts are not flushed from the soil by large amounts of water, evaporation of irrigation water can result in the slow buildup of salt in the soil and a long-term decrease in its fertility. In agricultural areas along both the Colorado River and Nile River, the buildup of salt in irrigated soils now threatens to decrease the lands’ productivity.

Although we need some salt to survive, consuming too much salt may also lead to increased blood pressure and a higher incidence of strokes and heart attacks.

Historically, one of the most destructive legacies of salt came from its role as being our society’s largest source of chlorine. Although chlorine provides many benefits to our modern world, chlorine gas is a deadly poison. Chlorine was the first poisonous gas used as a weapon and contributed greatly to the torment of trench warfare during World War I. Fortunately, most nations now ban its use as a weapon, but society still has to contend with the threat of chlorine gas leaks from chemical factories or accidental spills during its transport. In households it is important to never mix chlorine bleach with acid-containing or ammonia-containing cleansers or use it to clean cats’ litter pans, as the chlorine in the bleach can react to produce dangerous fumes.

There are no outcrops of halite in the Upper Midwest region, but immense subsurface deposits occur just to the east in the Michigan Basin. At first this relative lack of halite might seem surprising, since so much of the region’s Paleozoic bedrock formed as shallow marine deposits in a tropical setting. However, any halite that originally formed with these deposits has long since been lost as a result of the region’s very active groundwater systems.

Out of Minnesota’s 15,293 lakes, only one is an active salt lake. Located on the Minnesota-South Dakota border, Salt Lake Wildlife Management Area in Lac qui Parle County has frequently dried up completely and is never deeper than four feet. During the summer months, evaporated halite gives its shoreline a distinctive dusty white appearance.

Historically salt played a crucial, if indirect, role in the expansion of Euro-American settlers into the Upper Midwest. One of the main goals of the Erie Canal’s construction was to transport salt from important salt works in Upper New York that had first been used by the Iroquois. Although built to ease the transport and trade of salt and other goods, the canal also facilitated and spurred the immigration of Euro-American families into the Upper Midwest.

The Boundary Waters Canoe Area of northern Minnesota also provides a interesting side note on the importance of salt for animals. During winter months, despite snowy and icy conditions, there is a ban on salting roads in the Boundary Waters area as the salt attracts moose and increases the possibility of moose-automobile collisions!

Because of its transparent to translucent crystals, halite may initially be mistaken as other common minerals. However, its distinctive taste, combined with its perfect cubic cleavage, low hardness, and the ease with which small fragments dissolve in water should make it easy to identify.

Calcite:

Calcite is also transparent and translucent like halite, but its cleavage is rhombohedreal rather than cubic. Calcite also reacts easily with dilute acid, and lacks halite’s salty taste. Do not test the sample’s taste after testing it with dilute acid however. The dilute acid will not hurt you, but its bitter taste can be mistaken as being salty.

Quartz:

Transparent and translucent like halite, quartz is harder than glass or a nail, has no taste, and is never found as cubes. Quartz also lacks any cleavage.

Gypsum:

Gypsum can form as translucent crystals and may be associated with halite, but gypsum only has one well-developed cleavage plane, rather than two. Combining this with halite’s distinctive taste should distinguish the two.

Sylvite:

Sylvite forms under the same conditions as halite, but requires much greater concentrations of brine to precipitate, so it is not as common. Like halite, it forms clear, transparent crystals that are much softer than glass and exhibit good cubic cleavage. The only easy way to distinguish the two is by their taste. Halite is salty, while sylvite is quite bitter. You should be careful when testing the samples’ taste however, as sylvite in large quantities is poisonous, although there is little risk if care is taken.