Strontium (Sr, atomic number 38) is a highly reactive alkaline earth metal. Because it reacts vigorously with oxygen and water vapor, it is never found alone in nature. Strontium shares many properties with its neighbors, calcium and barium. Strontium’s appearance varies dramatically depending on its chemical state—whether it is a metal, a compound, or an excited vapor.
The Appearance of Pure Strontium Metal
When freshly refined, pure strontium appears as a soft, silvery-white metal with a subtle pale yellow tint and a bright metallic luster. It is soft enough that it can be cut with a knife. Strontium is a solid at room temperature, but its melting point is relatively low for a metal, sitting at 777 degrees Celsius.
The bright appearance is fleeting due to its high affinity for oxygen. Upon exposure, the surface rapidly oxidizes, forming a layer of strontium oxide. This reaction causes the metal to quickly lose its sheen, turning into a dull, yellowish or dark film. To preserve its elemental form, strontium must be stored under an inert atmosphere or submerged in a liquid hydrocarbon, such as mineral oil or kerosene, which prevents contact with air and moisture. Finely powdered strontium metal is even more reactive, capable of igniting spontaneously in the presence of air.
Strontium’s Signature Color in Pyrotechnics
The most visually recognized form of strontium is the brilliant color it produces when heated. When strontium compounds are introduced into a flame, they emit a vivid, intense crimson or deep red color. This characteristic has made strontium salts indispensable for creating the red hues in fireworks, signal flares, and road flares.
This color emission is a result of electron excitation. When strontium compounds, such as strontium nitrate or strontium carbonate, are heated, the absorbed energy causes electrons to jump to higher energy levels. Since these electrons are unstable, they immediately fall back to their original, lower energy levels. In doing so, they release the absorbed energy as light at a specific wavelength, which our eyes perceive as red.
The red color can be enhanced by incorporating a chlorine donor into the pyrotechnic mixture. When a chlorine source is present, molecular species like strontium monochloride (SrCl) are formed in the flame. This molecule produces the deepest, most saturated blood-red color sought after in firework displays. Without these specialized compounds, the red color produced by simple strontium oxide (SrO) or strontium monohydroxide (SrOH) tends to be a more orange-red hue.
Strontium in Compound Form and Natural Minerals
Strontium is most commonly encountered as a stable compound. Industrial strontium compounds, like strontium carbonate (\(\text{SrCO}_3\)) and strontium sulfate (\(\text{SrSO}_4\)), are generally white or gray crystalline powders or solids. These materials are odorless, stable, and used in various applications, including specialized glass, ceramics, and the manufacturing of permanent magnets.
In nature, strontium is primarily sourced from two main minerals. The first is celestine (\(\text{SrSO}_4\)), named for its occasional pale blue color. Celestine crystals are often found as transparent to translucent masses, though they can also be colorless, white, pink, or brown. This mineral is a widespread source for commercial strontium production.
The second primary source mineral is strontianite (\(\text{SrCO}_3\)). Strontianite typically presents as radiating aggregates of fibrous crystals or crystalline masses. While it can be colorless or white, it also appears in shades of gray, light yellow, or green. Both celestine and strontianite represent the stable, non-metallic appearance of strontium bound within the Earth’s crust.