Strontium

Strontium is a soft, silvery-yellow alkaline earth metal with the chemical symbol Sr and atomic number 38. It is chemically similar to calcium and barium and is known for producing bright red flames when burned, a property that has made it widely used in fireworks, flares, and signal lights. Beyond its visual appeal, strontium has significant industrial, medical, and technological applications — ranging from ceramics and glass production to radiological medicine and advanced materials. Economically, it plays an important role as a supporting industrial element and as a by-product of celestine and strontianite mining.

Discovery and Properties

Strontium was first identified in 1790 by Adair Crawford, who discovered a new mineral in the Scottish village of Strontian, from which the element takes its name. It was isolated in metallic form by Humphry Davy in 1808 through electrolysis of strontium chloride.
Key physical and chemical properties include:

• Atomic number: 38
• Atomic mass: 87.62 u
• Density: 2.64 g/cm³
• Melting point: 777°C
• Boiling point: 1,382°C
• Reactivity: Reacts readily with water and air, forming strontium hydroxide and oxide
• Flame colour: Bright crimson red
• Common oxidation state: +2
• Isotopes: Four stable isotopes (⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, ⁸⁸Sr) and several radioactive ones, including ⁹⁰Sr, a by-product of nuclear fission

Because of its reactivity, strontium metal is stored under mineral oil or in an inert atmosphere to prevent oxidation.

Occurrence and Extraction

Strontium is a moderately abundant element, ranking about 15th in the Earth’s crust. It occurs mainly in two mineral forms:

• Celestine (SrSO₄) – the primary commercial source
• Strontianite (SrCO₃) – less common but easier to process

The largest deposits are found in China, Spain, Mexico, Argentina, Iran, and Turkey. Global extraction involves mining celestine ore followed by chemical reduction or precipitation to produce strontium carbonate (SrCO₃), the most widely used industrial compound of strontium.

Everyday Applications

Strontium appears in various products and technologies encountered in daily life:

• Fireworks and signal flares: Strontium salts, particularly strontium nitrate (Sr(NO₃)₂) and strontium carbonate (SrCO₃), are used to create the distinctive red colour in fireworks, emergency flares, and tracer ammunition.
• Glass for electronics: Strontium oxide strengthens glass and improves its resistance to X-rays. Strontium compounds were once vital in cathode ray tube (CRT) television and computer screens.
• Ceramics and pigments: Strontium is used in producing glazes, ferrites, and pigments for ceramic tiles, paints, and porcelain, improving durability and colour stability.
• Dental care: Strontium chloride and strontium acetate are common ingredients in toothpaste for sensitive teeth, as they block nerve transmission in exposed dentinal tubules.

Although not visible to consumers, strontium enhances safety, quality, and functionality in many household and recreational products.

Industrial and Technological Applications

Strontium plays a critical role in several high-value industrial processes and advanced materials:

• Pyrotechnics: The most iconic use of strontium compounds, producing vivid red emissions in fireworks, warning lights, and military signals.
• Glass and ceramics manufacturing: Strontium carbonate is used to refine glass composition for television tubes, optical glass, and electronic displays, replacing toxic lead compounds in eco-friendly glass formulations.
• Metallurgy: Strontium improves the machinability and strength of aluminium and magnesium alloys used in aerospace, automotive, and electronics industries.
• Magnets and electronics: Strontium ferrite (SrFe₁₂O₁₉) is a hard magnetic material used in permanent magnets, small motors, microwave devices, and data storage systems.
• Chemical industry: Strontium compounds serve as catalysts, stabilising agents, and desulphurisation agents in refining and polymer processing.
• Energy storage: Emerging applications involve strontium titanate (SrTiO₃) and strontium ferrite in semiconductors, dielectric materials, and fuel cells.

Medical and Biological Applications

Strontium has interesting parallels with calcium, allowing it to interact with biological systems. Although not essential for human health, certain strontium compounds have therapeutic and diagnostic value:

• Bone health: Strontium ranelate has been used in the treatment of osteoporosis, as it enhances bone formation and reduces bone resorption.
• Medical imaging: Strontium-82 is used to generate rubidium-82, a radioactive tracer employed in positron emission tomography (PET) for cardiac imaging.
• Dental desensitisation: As mentioned, strontium chloride and acetate reduce dental sensitivity in toothpastes and mouth rinses.
• Radiotherapy: Strontium-89 chloride is administered for palliative treatment of bone cancer, as it selectively targets bone tissue and alleviates pain caused by metastases.

While beneficial in controlled doses, excessive intake of radioactive strontium isotopes—particularly strontium-90, a nuclear fission by-product—can be hazardous, as it accumulates in bones and emits beta radiation.

Economic Importance

Strontium’s economic significance arises from its diverse industrial roles and dependence on specific geological resources:

• Production and trade: Annual world production of strontium minerals exceeds 400,000 tonnes, with China as the leading producer, followed by Spain and Mexico.
• Market demand: The shift from CRT glass has reduced demand in traditional electronics, but new uses in permanent magnets, pyrotechnics, and medical isotopes sustain market growth.
• Price trends: Strontium carbonate and nitrate remain the most traded forms, typically valued between £1,000–£3,000 per tonne, depending on purity and form.
• By-product recovery: Strontium is often obtained as a by-product in mining operations involving barite and celestine, providing an additional economic incentive in those industries.
• Strategic applications: The element is considered a critical mineral in some nations due to its uses in defence technologies, electronics, and medical imaging.

Environmental and Safety Considerations

While stable strontium compounds are relatively safe, radioactive isotopes present significant health and environmental concerns:

• Stable strontium: Non-toxic in small quantities and sometimes beneficial in medical applications.
• Radioactive strontium-90: A major environmental pollutant resulting from nuclear weapons testing and reactor accidents. It mimics calcium, entering bones and teeth, where it can cause radiation-induced damage and leukaemia.
• Industrial handling: Strontium compounds require protective handling to prevent dust inhalation and environmental contamination.
• Waste management: Strontium-contaminated waste from nuclear facilities is treated and isolated under strict radiological safety protocols.

Scientific and Research Applications

Strontium contributes significantly to modern science and materials research:

• Isotopic analysis: Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) are used in geology, archaeology, and forensics to trace the origin of rocks, food, or human remains.
• Quantum physics: Strontium atoms are used in optical atomic clocks, some of the most precise timekeeping devices known, surpassing traditional cesium clocks in accuracy.
• Materials science: Strontium-based compounds such as strontium titanate are studied for their dielectric, superconducting, and optoelectronic properties, applicable in sensors and advanced electronics.
• Nuclear research: Strontium isotopes are used to study radiation shielding, nuclear decay, and reactor safety.

These scientific roles place strontium at the intersection of fundamental research and practical innovation.