large amount of carbon on the surface (66%), and the longstanding
mystery of the "very unusual" strontium and barium in the Ubatuba
magnesium samples, at a level totaling about 1,000 ppm or 0.1%, may be
explained as due to pyrotechnic contamination. Fireworks may use
large amounts of barium (to make a green color) and strontium (to make
a red color). Barium and strontium are common chemical components
of fireworks whereas they are completely unknown in magnesium metal
manufacture. Strontium is the standard red fire color in
fireworks and signal flares used everywhere. Barium is the
standard green color in fireworks all over the world.
Possibly an unusual firework display was designed with highly pure magnesium metal pieces which were intended to burn up, but instead a few pieces of magnesium survived but with strontium and barium contamination, plus a heavy carbon deposit on the surface, which now makes perfect sense if coming from a firework. Magnesium and titanium metals are often used in fireworks because they burn brightly and raise the combustion temperatures.
Purity of materials is important in firework manufacture, especially professional products quite unlike the cheap home consumer type. This might account for the unusual purity of the Ubatuba magnesium. One online article on the "Chemistry of Firework Colors" by a PhD scientist states in part:
"Pure colors require pure ingredients. Even trace amounts of ... impurities ... are sufficient to overpower or alter other colors. Careful formulation is required so that too much smoke or residue doesn't mask the color. With fireworks, as with other things, cost often relates to quality. Skill of the manufacturer and date the firework was produced greatly affect the final display (or lack thereof)."