Peridotites, in particular, with lots of olivine. Olivine weathers particularly quickly because its silicon atoms are in SiO4(4-) tetrahedra that are not covalently linked to each other (a "nesosilicate").
A problem with just spreading this out on beaches and the like is that these rocks are also often enriched in nickel vs. the average crustal rock, and this would release many millions of tonnes of nickel into the environment. There is some evidence that nickel release was part of the terrible events at the Permian-Triassic extinction. Nickel is crucial to the enzymes in the metabolic pathways leading to the production of methane in anaerobic environments. There's an idea that nickel injection into the oceans by fallout from the PT megaeruption in Siberia led to massive methane production.
A problem with just spreading this out on beaches and the like is that these rocks are also often enriched in nickel vs. the average crustal rock, and this would release many millions of tonnes of nickel into the environment. There is some evidence that nickel release was part of the terrible events at the Permian-Triassic extinction. Nickel is crucial to the enzymes in the metabolic pathways leading to the production of methane in anaerobic environments. There's an idea that nickel injection into the oceans by fallout from the PT megaeruption in Siberia led to massive methane production.