Ferromagnesium silicates, as the name suggests, primarily contain the following elements:

* Iron (Fe)

* Magnesium (Mg)

* Silicon (Si)

* Oxygen (O)

These elements combine to form various silicate minerals, with common examples including:

* Olivine: (Mg,Fe)₂SiO₄

* Pyroxene: (Mg,Fe)SiO₃

* Amphibole: (Ca,Na)₂(Mg,Fe,Al)₅Si₈O₂₂(OH)₂

* Biotite: K(Mg,Fe)₃AlSi₃O₁₀(OH)₂

The specific ratios of these elements vary depending on the mineral and its formation conditions.

It's worth noting that ferromagnesium silicates can also contain trace amounts of other elements, such as:

* Aluminum (Al)

* Calcium (Ca)

* Sodium (Na)

* Potassium (K)

* Titanium (Ti)

These trace elements can influence the mineral's color, hardness, and other properties.