Cohesive forces in solid solutions I have searched through printed books and the Internet, but I cannot find a definitive answer on an issue that fascinates me: what forces keep the various compounds belonging to a solid solution together?

If I correctly understand, a rock is a solid solution, and I know that the atoms of a single mineral species are bound by its particular covalent and ionic bonds, but what keeps different minerals together in a rock? Do they mix by sharing "neighbouring" cations and anions? I am not sure whether intermolecular forces such as Van der Waals forces are present, but I would definitely expect stronger forces in a solid solution or between the minerals of a rock.
It seems you were on right path - for example common feldspars are solid solutions and they are mixed because they share cations, while they easily substitute one another, otherwise they exsolve - become separate phases. Also you're right that van der Waals forces aren't important here as they are ionic compounds.