Accidental freak strong acid created

[g666]

Upon adding approximately 15 mL aqueous 31% HCl (by volume) to 1.5 g of what I think to be red phosphorus (from match-books) the solution heated up. I never took a temperature measurement, but it didn't heat up too much - I could still hold it, it was luke-warm. I then poured the HCl off on to a granite slate. The liquid instantly effervesced, foaming all over the slab and turning yellow. I then washed the effervesced liquid off with water, to find a red discoloration in the granite slab and a slight depression where I first poured the solution onto it.

Now, plain muriatic acid is NOT strong enough to do this - and I dont believe that such a minimal temperature change could have affected the reaction. Could someone explain to me what happened with the red phosphorus? I didn't think a reaction would take place.

Thanks in advance  

[fierceness]

you acid washed your granite with the muriatic acid.. no big deal