During the Spring Festival, a variety of fireworks displays are staged (Figure 1). Colorful fireworks burst one after another in the night sky, igniting the authentic flavor of the New Year in our memories. The night sky is full of brilliant fireworks, and it seems difficult for us to associate it with the underground "mine". However, without a wide variety of minerals, there would be no fireworks in all their colours.
Figure 1 Colorful fireworks.
As early as the Sui and Tang dynasties, our ancestors had figured out the recipe for fireworks: a specific proportion of saltpeter, carbon and sulfur were packed into a bamboo tube and launched into the sky, which could produce sound and fire due to violent. However, fireworks in ancient times were monotonous, because compared to today's fireworks recipes, it also lacked the "additives" that blindly made itself colorful and gorgeous—salt compounds containing special metal cations.
The principle of the colorful color of fireworks is the flame color reaction: when different metal ions are burned in the flame, electronic transitions in specific bands occur, causing the flame to take on different colors (Figure 2). For example, when the gas stove in the kitchen is fully burned, the flame will appear blue, but when dipped in a little salt (sodium chloride, NaCl), it will turn bright yellow. We can also guess the color of the original fireworks according to this principle: there were no additional additives in ancient times, but the composition of saltpeter was potassium nitrate (kno), so the ancient fireworks were purple-red! Sorry, wrong answer – because potassium and sodium are chemically similar, potassium compounds are often inevitably mixed with some sodium. The purple color produced by the burning of potassium is easily masked by the yellow light produced by sodium, so it is likely that ancient fireworks were the same yellow as the fireworks we see most often today.
Fig.2. Flame color reaction of different metals.
To see today's colorful fireworks, we need to add different kinds of metal elements; In order to obtain different types of metal elements, we need to understand that the mining of different types of metal minerals - calcium (Ca) can make fireworks orange-red, and calcium sulfate mineral gypsum (calcium sulfate dihydrate, caso·2ho) is its most common industrial raw material. In nature, gypsum is often produced in the form of prismatic or fibrous forms, which is colorless and transparent (Figure 3), and has a very low hardness, leaving scratches when scratched with fingernails. In desert areas, gypsum can be produced in flake aggregates to form ornamental stones, known as "desert roses" (Figure 4). The use of gypsum is too wide, whether it is fertilizer, building materials, or medical and medicinal, it can be seen everywhere. Of course, there are many other ** for calcium, such as: limestone (calcium carbonate), dolomite (calcium magnesium carbonate), etc., which will not be introduced.
Fig.3 Gypsum.
Fig.4 "Desert Rose" ornamental stone.
Strontium (SR) gives fireworks a brilliant red color. It is in the next row of the periodic table and has similar properties to calcium. A common raw material for the production of strontium compounds also comes from its sulfate, lazurite (strontium sulfate, SRSO). Completely different from the red light emitted by strontium salts when burned, lapis lazuli gets its name from its rather attractive blue color, azure (Figure 5). "After the rain is azure**, this color will be the future", the color azure was originally used to describe the slightly gloomy hue of the sky at the beginning of the sunny day after the rain; It was later used to describe Ru kilns with this aesthetic color (Fig. 6); In recent years, it has been widely known because it has been misused in "azure and other smoke and rain" (Jay Chou's "Blue and White Porcelain").
Fig.5 Lapis lazuli crystal clusters.
Fig.6 Ru kiln azure glaze (**from the "Palace Museum").
Also in the same column of the periodic table (a main group), the barium element (ba) gives fireworks a green color. It is not difficult to guess that the raw material for the production of barium salts in nature is also its sulfate mineral, barite (barium sulfate, baso). Barite gets its name from the fact that it has a specific gravity that far exceeds that of other common minerals: common rocks are often 2-3 times denser than water; Barite, on the other hand, is 4-5 times more potent than water. Barite is often produced in nature as diamond-shaped crystals, with different colors depending on the impurities mixed in (Figure 7). Due to the similar properties of barium and calcium, barite can also form a "barium" version of "desert rose" (Figure 8).
Fig.7 Golden barite.
Fig.8 Barite "Desert Rose".
The presence of copper (Cu) gives fireworks a blue-green color, a color similar to the (basic) carbonates of copper found in nature: azurite (Fig. 9) and malachite (Fig. 10). These two minerals were used as precious mineral pigments in ancient China for their bright colors. The "green" in "Danqing" is another name for azurite, "stone blue". The "blue" of blue and white porcelain should also be this stone blue, not the azure of the lazuli above. In addition to the use of pigments, two minerals, azurite and malachite, are far from satisfying human needs for copper, an important metal. Most of the copper we currently use comes from various sulfides of copper, such as chalcopyrite (CUFES), bornite (CUFES), chalcocite (CUS), chalcocite (CUS), chalcocite (CUS), blue chalcocite (CUS) and so on.
Fig.9 Azurite.
Fig.10 Malachite.
In addition to the above-mentioned elements, more and more elements such as manganese (violet), cadmium (yellow light), cobalt (blue light) and so on have been gradually developed and added to the materials list of fireworks. In order to achieve a better visual effect and take into account the needs of environmental protection, today's high-end fireworks technology will also add a small amount of "industrial vitamins" - rare earth elements to the formula. The addition of different rare earth elements can significantly change the optical properties of fireworks: they can change the color of the spark or make it last for a long time (Figure 11). At the 2008 Beijing Olympic Games, it was this new technology that produced fireworks that left a deep impression on guests at home and abroad (Figure 12).
Fig.11 The fire of copper + rare earth element ytterbium is brighter and persistent (according to Memmel et al.)., 2022）
Fig.12 Fireworks display at the opening ceremony of the 2008 Beijing Olympic Games.
Naturally, the production of such rare earth fireworks also requires the use of corresponding rare earth minerals, such as monazite, bazite, yttrium phosphate (Fig. 13), etc.
Fig. 13 shows monazite, bathronite, yttrium phosphate, and the newly reported new mineral Nipeiite.
Without all kinds of minerals, there would be no brilliant fireworksIt is the abundant and diverse means of production of nature that allow human beings to yearn for and practice "beauty" in addition to material pursuits. On the other hand, we should also take the responsibility of protecting ecological nature: while the earth ignites the gorgeous night sky for us, we should use human wisdom to preserve the clear water and blue sky of the earth.
Editor: Xu Hongxi.
Proofreading: Wu Shuyu, Jiang Shumin.
*The content represents the author's views only.
It does not represent the position of the Institute of Physics of the Chinese Academy of Sciences.
If you need it, please contact the original***
*: Institute of Geology and Geosciences, Chinese Academy of Sciences.
Editor: Yue Yue.