Here's a breakdown:

* Hematite (Fe2O3) is a common iron oxide mineral, known for its reddish-brown color.

* Ilmenite (FeTiO3) is a titanium iron oxide mineral.

Solid solutions occur when two or more minerals have similar crystal structures and can mix together in varying proportions. The resulting material has properties that are a blend of the original minerals.

The formula Fe2O3 - FeTiO3 indicates a range of compositions between pure hematite (where the proportion of FeTiO3 is zero) and pure ilmenite (where the proportion of Fe2O3 is zero).

Applications:

These solid solutions are important in several industries:

* Pigments: The color of the solid solution varies with the proportion of hematite and ilmenite, providing a range of colors for paints, ceramics, and other applications.

* Iron and titanium extraction: These minerals are mined as sources of iron and titanium.

* Gemstones: Certain compositions of the solid solution form gemstones, such as ilmenite-hematite.

Note: The exact composition of a specific sample can vary, and the formula Fe2O3 - FeTiO3 is used as a general representation of this solid solution series.