Antimony

Antimony is a chemical element with the symbol Sb (from the Latin stibium) and atomic number 51. A lustrous, silvery-gray metalloid belonging to group 15 of the periodic table (the pnictogens), it is relatively rare in the Earth's crust and occurs in nature chiefly as the sulfide mineral stibnite (Sb₂S₃). Its compounds were exploited by the earliest civilizations—most famously ground into the cosmetic and medicinal powder known as kohl in ancient Egypt—while the elemental metal itself is brittle, crystalline, and a comparatively poor conductor of electricity. In the modern economy, antimony is indispensable above all as antimony trioxide, a synergist in flame-retardant systems, and as a hardening agent in lead alloys used in batteries and ammunition. It also finds uses in semiconductors, glass, catalysts, pyrotechnics and, historically, in medicine. Global supply is heavily concentrated in China, and the element is widely classified by governments as a critical raw material owing to supply risks and its strategic military applications.

Etymology

The English name antimony derives from medieval Latin antimonium, a term that appears in translated Arabic medical texts of the eleventh century and became standard in later alchemical literature. Its ultimate origin is disputed. One widely cited explanation derives it from the Greek anti-monos, "against solitude" or "not alone," a reference to the fact that native metallic antimony rarely occurs uncombined in nature. A popular but legendary folk etymology, repeated by the seventeenth-century chemist Nicolas Lémery, holds that a monk named Basil Valentine experimented with the substance, poisoned his fellow monks, and discarded the metal from the monastery—hence "anti-monk"; modern scholarship regards this story as apocryphal. The chemical symbol Sb, by contrast, comes from stibium, the Latin name for the sulfide ore, itself borrowed from the Greek stibi or stimmi, ultimately traceable to an ancient Egyptian word for the cosmetic mineral. The related Arabic term kuḥl is the origin of the English words "kohl" and, by a separate route through alchemy, "alcohol."

History

Antimony sulfide was among the first mineral substances systematically used by humankind. Ground stibnite served as the dark eye cosmetic kohl in Egypt and the Near East from at least the early third millennium BC, and small vessels and artifacts of metallic antimony from Mesopotamia and Egypt date to a similar or earlier period. The Hebrew Bible alludes to the practice of coloring the eyelids with the mineral, most famously in the account of Jezebel. Classical authors, including Dioscorides and Pliny the Elder, described the ore stibium and its use in cosmetics and medicine, although Pliny's account confusingly blends antimony with lead and its ores.

Islamic alchemists of the early medieval period, notably the tradition associated with Jabir ibn Hayyan, treated antimony as a significant substance and described methods for purifying it. The first detailed European descriptions of the preparation of elemental antimony appeared in the metallurgical literature of the Renaissance, most notably in Vannoccio Biringuccio's De la pirotechnia (1540), with further accounts by Georgius Agricola. In alchemical circles antimony acquired a distinctive symbol and an outsized reputation; the Triumphal Chariot of Antimony, attributed to the pseudonymous Basil Valentine around 1604, was among the most influential early monographs on a single substance, and Lémery's Treatise on Antimony (1707) continued the tradition. From the seventeenth century onward, antimony preparations such as tartar emetic (potassium antimonyl tartrate) were widely, and often recklessly, used in mainstream medicine as emetics and purgatives. The metal also entered technology early: its slight expansion on solidification made lead–antimony–tin type metal superb for casting sharp, legible printing type from the fifteenth century onward. Industrial demand grew dramatically in the late nineteenth and twentieth centuries with lead-acid batteries, ammunition, and, from the 1960s, halogen-based flame retardants in plastics and electronics; the Xikuangshan deposit in Hunan, China, worked on a large scale since the 1890s, became the world's dominant source.

Physical Properties

Antimony in its common gray, or metallic, form is a brittle, flaky, silvery-white crystalline solid with a layered, rhombohedral structure in which atoms are strongly bonded within corrugated sheets and more weakly linked between them. Its Mohs hardness is about 3, so it is readily powdered. It melts at 630.63 °C and boils at approximately 1,635 °C, and its density at room temperature is about 6.70 g/cm³. Although classified as a metalloid and sometimes described as a semimetal, it conducts heat and electricity much more poorly than typical metals. Like water, bismuth and gallium, antimony expands slightly upon solidification—a rare property that historically proved decisive for casting printing type and remains exploited in certain alloys.

Antimony also exhibits several unstable allotropes. Black antimony is an amorphous form obtained by rapid cooling of the vapor and reverts to the gray form on heating. Yellow antimony, produced by the oxidation of the gas stibine, is stable only below about −90 °C. So-called explosive antimony, formed electrolytically from antimony trichloride solutions, detonates violently when scratched or heated.

Chemical Properties and Compounds

Chemically, antimony sits between arsenic and bismuth, and its behavior echoes both. It displays oxidation states of −3, +3 and +5, with the +3 state generally the most stable. Massive antimony is reasonably resistant to oxidation at ordinary temperatures and is not attacked by water or dilute acids, but the powdered metal burns in air with a brilliant bluish-white flame to give antimony trioxide (Sb₂O₃), white fumes of which have long been used to camouflage fireworks displays and in flame retardancy.

Antimony trioxide is amphoteric, dissolving in strong alkalis to form antimonites and in acids to give antimony salts. Higher oxides include the mixed-valence Sb₂O₄ and the oxidizing Sb₂O₅. The halides are numerous and industrially significant: antimony trichloride, once known as "butter of antimony," was a staple of early pharmacy; antimony pentachloride is a powerful chlorinating agent; and antimony pentafluoride is among the strongest Lewis acids known, forming with hydrogen fluoride the "magic acid" superacid systems of physical organic chemistry. The sulfides include the black-gray mineral stibnite (Sb₂S₃) and the orange-red amorphous trisulfide used in matches and pyrotechnics, as well as antimony pentasulfide employed in rubber vulcanization. The hydride stibine (SbH₃), an unstable and highly toxic gas, decomposes thermally to deposit a metallic antimony mirror, an analytical reaction analogous to the Marsh test for arsenic. Organometallic chemistry provides stibines R₃Sb and related pentavalent species, while the metal forms numerous intermetallic semiconductors, or antimonides, including indium antimonide, gallium antimonide and the thermoelectric cobalt antimonide CoSb₃.

Isotopes

Antimony occurs in nature as a mixture of two stable isotopes, ¹²¹Sb (about 57.4 percent) and ¹²³Sb (about 42.6 percent), a ratio that has found use in geochemical and nuclear studies. Around three dozen radioactive isotopes are known, with mass numbers from roughly 103 to 140; the longest-lived, ¹²⁵Sb, has a half-life of about 2.76 years and is released as a fission and activation product in nuclear facilities, where it serves as an environmental tracer. Antimony-124, produced by neutron irradiation, is combined with beryllium in compact photoneutron sources that emit neutrons when the gamma rays from ¹²⁴Sb knock them out of beryllium nuclei. Shorter-lived isotopes appear among the products of nuclear fission and reactor activation.

Occurrence, Production and Supply

Antimony is a scarce element, with an estimated crustal abundance of only about 0.2 to 0.5 parts per million. Geochemically it is chalcophile, concentrating in sulfide phases, and more than one hundred antimony minerals are known. The principal ore is stibnite; other significant minerals include the oxides valentinite and senarmontite, the oxy-sulfide kermesite, and sulfosalts such as tetrahedrite and jamesonite, while native metallic antimony is rare. Deposits typically form from hydrothermal fluids and range from simple stibnite veins to complex antimony–gold systems and tungsten–antimony–mercury provinces, the latter exemplified by south-central China.

Estimated global reserves are on the order of two million tonnes, with China holding the largest share; other resources lie in Russia, Bolivia, Tajikistan, Myanmar, Turkey and Australia. China has long dominated mine output, typically accounting for well over half of world production, centered on the Xikuangshan field in Hunan—often called the antimony capital of the world—followed by Tajikistan, Russia, Myanmar, Turkey and Bolivia. Metallurgical extraction commonly proceeds by volatilization roasting of stibnite, which converts the sulfide to the volatile trioxide, followed by carbothermic reduction with carbon; alternative processes reduce the sulfide directly with iron or employ hydrometallurgical and electrolytic refining. Secondary production from the recycling of lead-acid battery scrap supplies a meaningful fraction of refined demand, and antimony is recovered as a by-product of gold processing at some deposits.

Applications

Flame retardancy consumes roughly half of the world's antimony, almost entirely as antimony trioxide used in synergy with halogenated flame retardants in plastics, textiles, rubber and electronic housings. In a fire, the combination generates volatile antimony halides that quench the flame's radical chain reactions and promote protective char formation. The second major use is in lead alloys: antimony hardens lead for battery grids, bullets and shot, cable sheathing and plumbing, and it remains a constituent of bearing metals such as Babbitt alloys, of pewter and of solders.

In electronics, antimony compounds are foundational materials for infrared technology. Indium antimonide, with its narrow band gap, is the sensing element of many thermal-imaging and night-vision detectors, while gallium antimonide serves as substrate and laser material; antimony is also used to dope silicon n-type, and antimonide thermoelectrics such as CoSb₃ are studied for power generation from waste heat. Antimony trioxide acts as a polycondensation catalyst in the manufacture of polyethylene terephthalate (PET), where trace residues have attracted health scrutiny. In glass and ceramics it serves as an opacifier, fining (degassing) and decolorizing agent, notably in some photovoltaic glass. Pigment chemistry historically featured lead antimonate "Naples yellow" and antimony vermilion. Antimony trisulfide is a fuel in match-head compositions and contributes the glitter effect in pyrotechnics and a lubricating function in friction materials. In medicine, pentavalent antimonials such as sodium stibogluconate and meglumine antimoniate remain standard treatments for leishmaniasis in much of the world, although toxicity and emerging resistance have narrowed their use.

Biological Effects and Toxicity

Antimony has no known biological function in humans, and its chemistry parallels that of arsenic: the trivalent form binds strongly to sulfhydryl groups in enzymes, disrupting cellular metabolism, while the pentavalent form is considerably less toxic. The gas stibine is exceptionally poisonous, causing severe hemolysis of red blood cells. Occupational exposure, chiefly by inhalation of dust and fumes in mining, smelting and flame-retardant manufacturing, is associated with antimony pneumoconiosis, dermatitis and eye irritation, and the International Agency for Research on Cancer classifies antimony trioxide as possibly carcinogenic to humans (Group 2B). Workplace limits such as the OSHA permissible exposure level of 0.5 mg/m³ and the World Health Organization drinking-water guideline