What does the dot indicate in the molecular formula for white lead?

Question

I came across the following molecular formula for white lead: $2\mathrm{Pb}\mathrm{CO3}·\mathrm{Pb(OH)_2}$. What does the center dot indicate here? Is this a non-covalent interaction of some kind? https://en.wikipedia.org/wiki/White_lead

Answer 1

It could be as well written as $\ce{Pb3(OH)2(CO3)2}$.

The formula $\ce{2 PbCO3 . Pb(OH)2}$ is used as better visualization of composition being an intermediate between lead carbonate and lead hydroxide in ratio 2:1.

The dot has similar meaning as in formulas of salt hydrates like $\ce{CuSO4 . 5 H2O}$. Two substances incorporated into the single one in particular ratio, at level of molecular entities (general term involving molecules, atoms, ions, ionic pairs and any other kinds of the smallest building blocks), i.e. not just a mixture.

Another example are minerals malachite and azurite, containing cupric carbonate and cupric hydroxide in ratio 1:1 or 2:1, respectively.

These substances are members of a larger group called basic salts, where anions of acids are complemented with hydroxide or oxide entities, like basic chlorides of magnesium or zinc, or substances in chlorated lime.