An explosive mineral formation on Gamma Trianguli VI
An exploding asteroid in range of the Enterprise
Explosive mineral formations were naturally-occurring minerals found throughout the galaxy that had the ability became explosive when impacted.
In 2267, the landing party from the USS Enterprise encountered explosive mineral formations on the planet Gamma Trianguli VI, which contained uraninite, hornblende and quartz. These highly unstable formations had an extremely low specific gravity, were extremely fragile, had good cleavage. Upon accidentally discovering the minerals, Spock noted that their discovery was of some importance, noting that in large quantities, the material could be a considerable source of power. In response, Captain James T. Kirk noted on the irony of these formations, describing their discovery as the "Garden of Eden, with land mines."
Lieutenant Mallory was later killed after stepping on one of these formations while running back to the landing party following the discovery of a native's village. (TOS: "The Apple")
In 2270, while in pursue of an Orion vessel, the USS Enterprise discovered an asteroid belt composed of a highly unstable mineral combination that also exploded upon impact with another object. Montgomery Scott theorized that if the power of those rocks could be harnessed, they would make a fantastic source of energy, or as Captain Kirk added, quite a weapon. (TAS: "The Pirates of Orion")