Here's why:

* Carbonate minerals are indeed composed of oxygen. The fundamental unit of a carbonate mineral is the carbonate ion (CO₃²⁻) which has three oxygen atoms.

* However, hydrogen is not a constituent of the carbonate ion itself.

* While some carbonate minerals *can* contain hydrogen in their chemical formula, this is not a defining characteristic. It's more common for hydrogen to be present in the form of water molecules within the mineral's structure (known as water of hydration). For example, calcite (CaCO₃) is a common carbonate mineral, but its hydrated form, hydrocalcite (CaCO₃·H₂O), contains hydrogen.

So, in summary:

* All carbonate minerals contain oxygen.

* Some carbonate minerals can contain hydrogen, usually in the form of water molecules.