If you want beautiful fireworks bursting in the sky, you’re going to need to mine the Earth first. Here’s the geology of the minerals that give fireworks their vibrant colours.
Fireworks get their colour from metal salts. A salt is a chemical compound formed when an acid and base neutralize each other, resulting in a new compound where the elements are bound together through ionic bonds. Many of the salts include an oxidizer like nitrates, chlorates, or perchlorates. Along with imparting colour, these oxidizers provide oxygen, allowing the fireworks to burn. The metals or salts can also be stabilizers, keeping the colour-imparting elements stable until showtime.
While not explicitly a colour-inducing element, phosphorous is also commonly included in fireworks as a fuel since it spontaneously burns in air, but also as a glowing component in darker fireworks effects. Zinc can be added to create smoke effects.
A lighter medium red is created by lithium salts like lithium carbonate (Li2CO3) or lithium chloride (LiCl). Neither occur naturally [correction: they occur in brines, creating evaporite deposits as they dry out], but lithium can be found in nearly every igneous rock in the minerals lepidolite, spodumene, petalite, or amblygonite. Both lithium salts are used in industry, as a brazing flux for aluminum, as a desiccant, or an additive in glazes. It also has biochemical uses.
The brilliant, deep red is created by strontium salts like strontium nitrate (Sr(NO3)2) and strontium carbonate (SrCO3). These metal salts do not naturally occur as a geological mineral, but the strontium is usually found in celestite. Strontium can also be used as a stabilizer for other fireworks effects.
Celestite. Image credit: USGS
About a third of all strontium nitrate in the United States is used for pyrotechnics, but it is also used in alloys that make aluminum more tractable to casting, as an additive in glass, to make paint corrosion-resistant, and as part of drilling mud. It also finds its way into common households as a component in constructing ceramic ferric magnets (fridge magnets). Other strontium compounds star as the active ingredients in toothpaste for temperature-sensitive teeth.
Orange is the result of calcium salts, usually calcium chloride (CaCl2) or calcium sulfates (CaSO4·xH2O). Calcium can also be mixed into other fireworks to enhance the colours, while other calcium salts make pretty pale pinks (namely CaCO3, CaSO4, or CaC2O4). Calcium sulfate occurs naturally as gypsum, an evaporate mineral, while calcium chlorides form as the far more rare as sinjarite or antarcticite minerals but can be easily extracted from limestone.
A gypsum layer in the Spearfish Formation of South Dakota. Image credit: USGS
Calcium sulfate is used for producing sulfuric acid. Calcium chloride has far more uses — a desiccant, changing freezing points, increasing water hardness in aquariums, or as a firming agent in foods like tofu.
Orange can also be created by a mix of strontium and sodium.
Sodium makes such a bright, overwhelming yellow that it can outright hide other, more subtle colours of cross-contamination occurs. The classic salts are sodium nitrate (NaNO3) or cryolite (Na3AlF6). Sodium nitrate (NaNO3) is a metal salt which naturally occurs as the mineral nitratine. The sedimentary rocks of the Atacama Desert in South America are the largest deposit of natural sodium nitrate in the world.
Sodium-rich plagioclase feldspar. Image credit: USGS
While other sodium salts are more widespread (sodium chloride finds its way into your home under its common name, table salt), sodium nitrate is infamous in its own right as saltpeter. Saltpeter is a fertilizer, a rocket propellent, a food preservative, and as an enamel. As most sodium salts, including sodium nitrate, can absorb large quantities of heat and release it slowly over time, it’s also being adopted for use in thermal energy storage.
The brilliant emerald green fireworks are created by the barium salts barium nitrate (Ba(NO3)2), barium chlorate (Ba(ClO3)2), barium chlorite (Ba(ClO2)2), or barium carbonate (BaCO3). These metal salts do not naturally occur as minerals. The barium within the compounds originates as barite, a barium sulfide mineral. Barium can also be used as a stabilizer for other more volatile elements.
A barite rose. Image credit: USGS
Barium blocks radiation, making it perfect for everything from a contrast-inducing milkshake prior to X-rays of the digestive track to mixing into concrete as radiation shielding for laboratories. It’s also used in oil and gas drilling, increasing density to suppress high pressure that could lead to blowouts, or in the manufacture of paints, plastic, and rubber.
Copper chloride is a metal salt that can be made from either of copper’s primary oxidation states, cuprous or cupric. The copper (I) chloride (CuCl) makes a beautiful greenish torquoise firework, while the copper (II) chloride (CuCl2) generates a rich blue. More complicated compounds of copper and chlorine create intermediate shades of blue to green: copper acetoarsenite creates a striking paris green, while the mess of Cu3As2O3Cu(C2H3O2)2 is a brilliant deep blue. Copper chloride fireworks are at a relatively lower temperature than other fireworks.
Native copper. Image credit: USGS
The metal salts rarely occurs in mineral form, but can be found in eriochalcite, nantokite, or tolbachite. Alternately, copper can also be extracted from the minerals chalcopyrite, azurite or malachite, or even as pure native copper. Copper is malleable and fantastic at conducting electricity, making it in high demand for power generation and transmission on all scales.
Native copper. Image credit: USGS
Alternately, a deeper indigo blue-purple is created by cesium nitrate (CsNO3). Cesium is also an oxidizer.
The combination of copper and strontium, extracted from their relative source ores, creates the pale lavender of fireworks. A darker violet is created by potassium nitrate (KNO3), which naturally occurs as niter, another form of saltpeter. Like sodium nitrate saltpeter, potassium-based saltpeter is a fertilizer, a rocket propellent, as a food preservative, and to make gunpowder.
A rich red-violet colour can be made from rubidium nitrate (RbNO3), although it is rarely used. Rubidium is also an oxidizer. Rubidium doesn’t naturally occur as the primary metal in any mineral, but is trace element as a potassium substitute in some common minerals like feldspar and mica. Rubidium is more commonly used as a source material for catalysts and scintillation counters.
The searing white is created by barium oxides (BaO), a compound formed by heating barium carbonate with coke, or by thermally decomposing barium nitrate. Sparkling white can also be created by aluminum or beryllium powders, while a more silvery white is more likely from titanium, zirconium, magnesium.
Aluminum is the most common component of the bright flashes of sparkler fireworks and mag stars, producing silver or white flames. Aluminum is lightweight, malleable, and resists corrosion, making it useful in manufacturing everything from vehicles to soda cans. Aluminum rarely occurs by itself naturally, instead more commonly forming in oxides and silicates like feldspar.
Aluminum. Image credit: USGS
Magnesium is also sometimes used for brilliant white flashes or to enhance the intensity of other fireworks, but is less common than aluminum because it can’t form a protective oxide layer.
For more glittery effects, antimony is used instead.
Sparklers are made from iron filings and charcoal fragments, with the temperature strongly influencing the colour of the sparks. The inclusion of potassium can tint the sparks from the typical warm gold to a paler violet-pink.
While an extremely abundant element in the Earth’s crust, iron doesn’t have a naturally-occurring native mineral. Instead, it is extracted from iron ores like hematite and magnetite. When the oxygen of iron ores is removed through heat and carbon, the result is steel, an incredibly strong and versatile material.
Iron filings. Image credit: USGS
Want more? Check out the chemistry of fireworks here.
Top image credit: peaceful-jp-scenery