Do read and take seriously the parts about fire hazard - and the simple ways to circumvent that. Ignore and be prepared to get the scare of your life - a rapidly blossoming brilliant white fire impossible to put out - and making it tenfold worse - in the most spectacular way - if you douse it with water
There are (or were fifty years ago) purpose made dry compounds (think floor dry similar) useful in smothering the conflagration
Might be thinking of the GI-ubiquitous "Purple-K"?
Standard-issue around combat airfields & crash trucks "back in the day". Or our Oxygen & Acetylene plants. Still widely used. Works rather well.
Usta bee, big bare-iron hayrake wheels, tank of the powder, Nitrogen cylinder to heave it a fair distance was our "nuclear option".
ISTR is it sort of ablative?
Purple-K - Wikipedia
Anyway, it can leave an air-excluding crust that's a lot more useful than porous sand, kitty-litter, diatomaceous Earth, or vermiculite.
Even if it pretty well trashes whatever survives of what it is used on, it has saved a LOT of lives and avoided worse disasters, yet.
That said, Magnesium has gotten plenty of NEW attention since it is more recently found all around us, not just in warbirds or wormbirds (VW beetle engines..).
Firefighter Training: Extinguishing Magnesium Fires
Worth preparing to reduce the risk of more "Bad News".
I'd far rather grumble about extinguishers bought and never needed than the other outcome.
The day
twenty-two OTHER extinguisher had failed to even slow-down a damaged
Acetylene production plant hopper fire. Until one brave man went right into the fireball with an Axe to get the hopper wreckage clear to pour the Purple-K to it.