If the atmospheric pressure changes, there could be changes to chemical reactions. There are several factors controlling chemical reactions. One being, obviously, the identity of the reactants. The other one is temperature. But also important is pressure. Because pressure on the surface of the earth is always 1 atm, no one really thinks or cares about pressure. No one, until it changes. That said, for any substantial change in chemistry, pressure has to change quite a lot, which would make earth uninhabitable for humans, so there would be no one to see that chemistry changed.
Silicates are not the only phases affected by the gravitationally generated pressure inside Earth. Carbon is affected too, leading to the (natural) formation of diamonds -- which often have inclusions of those high-pressure silicate phases trapped inside, thus providing verification of these gravity/pressure-induced phases.