Calcium carbonate $(CaC{O}_3)$ has two common crystalline forms, calcite and aragonite, with

the following thermodynamic data:

calcite: $V=36\mathrm{.}93c{m}^3,{}_{f}G=-1128\mathrm{.}8kJ$

aragonite: $V=34\mathrm{.}15c{m}^3,{}_{f}G=-1127\mathrm{.}8kJ$

$($a$)$ Which form is stable at Earth's surface and why?

$($b$)$ At room temperature, calculate the pressure at which the other form should become

stable.