Natron

Natron

[ney-tron, -truhn]
Natron, Lake, c.35 mi (60 km) long and 15 mi (20 km) wide, in the Great Rift Valley, E Africa, on the Kenya-Tanzania border. It has soda, salt, and magnesite deposits.

Natron is a naturally occurring mixture of sodium carbonate decahydrate (Na2CO3·10H2O, a naturally occurring form of soda ash) and about 17% sodium bicarbonate (also called nahcolite or baking soda, NaHCO3) along with small quantities of household salt (halite, sodium chloride) and sodium sulfate. Natron is white to colorless when pure, varying to gray or yellow with impurities. Natron deposits occur naturally as a part of saline lake beds in arid environments. Throughout history, natron has had many practical applications, which still resonate in the wide range of modern uses of its constituent mineral components.

In modern mineralogy the term natron has come to mean only the sodium carbonate decahydrate (hydrated soda ash) which makes up most of the historical salt.

Etymology

The English word natron is a French cognate derived from the Spanish natrón through the Arabic natrun from Greek nitron, which derived from the Ancient Egyptian word netjeri, stemming from Wadi El Natrun, Egypt. The modern chemical symbol for sodium, Na, is an abbreviation of that element's New Latin name natrium, which was derived from natron.

Importance in antiquity

Historical natron was harvested directly as a salt mixture from dry lake beds in ancient Egypt and has been used for thousands of years as a cleaning product for both the home and body. Blended with oil, it was an early form of soap. It softens water whilst removing oil and grease. Undiluted, natron was a cleanser for the teeth and an early mouthwash. The mineral was mixed into early antiseptics for wounds and minor cuts. Natron can be used to dry and preserve fish and meat. It was also an ancient household insecticide, was used for making leather and as a bleach for clothing.

The mineral was used in Egyptian mummification because it absorbs water and behaves as a drying agent. Moreover, when exposed to moisture the carbonate in natron increases pH, which creates a hostile environment for bacteria. In some cultures, natron was thought to enhance spiritual safety for both the living and the dead. Natron was added to castor oil to make a smokeless fuel, which allowed Egyptian artisans to paint elaborate artworks inside ancient tombs without staining them with soot.

Natron is an ingredient for the making of a distinct color called Egyptian blue. It was used along with sand and lime in ceramic and glass-making by the Romans and others at least until 640 CE. The mineral was also employed as a flux to solder precious metals together.

Declining use

Most of natron's uses both in the home and by industry were gradually replaced with closely related sodium compounds and minerals. Natron's detergent properties are now commercially supplied by soda ash, the mixture's chief compound ingredient, along with other chemicals. Soda ash also replaced natron in glass-making. Some of its ancient household roles are also now filled by ordinary baking soda, natron's other meaningful ingredient.

Chemistry of hydrated sodium carbonate

Natron is also the mineralogical name for the compound sodium carbonate decahydrate (Na2CO3·10H2O), which is the main component in historical natron. Sodium carbonate decahydrate has a specific gravity of 1.42 to 1.47 and a Mohs hardness of 1. It crystallizes in the monoclinic-domatic crystal system, typically forming efflorescences and encrustations.

The term hydrated sodium carbonate is commonly used to encompass the monohydrate (Na2CO3·H2O), the decahydrate and the heptahydrate (Na2CO3·7H2O), but is often used in industry to refer to the decahydrate only. Both the hepta- and the decahydrate effloresce (loose water) in dry air and are partially transformed into the monohydrate thermonatrite Na2CO3·H2O.

As a source of soda ash

Sodium carbonate decahydrate is stable at room temperature but recrystallizes at only 32°C (90°F) to sodium carbonate heptahydrate, Na2CO3·7H2O, then above 37-38°C (98-100°F) to sodium carbonate monohydrate, Na2CO3·H2O. This recrystallization from decahydrate to monohydrate releases much crystal water in a mostly clear, colorless salt solution with little solid thermonatrite. The mineral natron is often found in association with thermonatrite, nahcolite, trona, halite, mirabilite, gaylussite, gypsum, and calcite. Most industrially produced sodium carbonate is soda ash, sodium carbonate anhydrate Na2CO3, which is obtained by calcination (dry heating at temperatures of 150 to 200°C) of sodium bicarbonate, sodium carbonate monohydrate, or trona.

Geological occurrence

Geologically, the mineral natron as well as the historical natron are formed as transpiro-evaporite minerals, i.e. crystallizing during the drying up of salt lakes rich in sodium carbonate. The sodium carbonate is usually formed by absorption of carbon dioxide from the atmosphere by a highly alkaline, sodium-rich lake brine, according to the following reaction scheme:

NaOH(aq) + CO2 -> NaHCO3(aq)

NaHCO3(aq) + NaOH(aq) -> Na2CO3(aq) + H2O

Pure deposits of sodium carbonate decahydrate are rare, due to the limited temperature stability of this compound and due to the fact, that the absorption of carbon dioxide usually produces mixtures of bicarbonate and carbonate in solution. From such mixtures, the mineral natron (and also the historical one) will only be formed, if the brine temperature during evaporation is maximally ca. 20°C (68°F) - or the alkalinity of the lake is so high, that little bicarbonate is present in solution (see reaction scheme above) - in which case the maximum temperature is increased to ca. 30°C (86°F). In most cases the mineral natron will form together with some amount of nahcolite (sodium bicarbonate), resulting in salt mixtures like the historical natron. Otherwise, the minerals trona or thermonatrite and nahcolite are commonly formed. As the evaporation of a salt lake will occur over geological time spans, during which also part or all of the salt beds might redissolve and recrystallize, deposits of sodium carbonate can be composed of layers of all these minerals.

The following list may include geographical sources of either natron or other hydrated sodium carbonate minerals:

References

External links

See also

Saltpeter

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