a)
The anionic group that occurs in carbonate minerals is the carbonate ion, which is represented chemically as CO₃²⁻. This group typically bonds with various cations, which can include elements such as calcium (Ca), magnesium (Mg), and iron (Fe). The two most common carbonates that exist in minerals are calcite (CaCO₃) and dolomite (CaMg(CO₃)₂).
b)
The four primary elements that commonly join silicate structures, alongside iron (Fe), are:
- Silicon (Si)
- Aluminum (Al)
- Magnesium (Mg)
- Sodium (Na)
c)
In alkali feldspar, sodium (Na) can commonly substitute for potassium (K). This substitution occurs due to the similar ionic sizes and charges of sodium and potassium ions, which allows them to fit into the crystal lattice structure of feldspar minerals without significantly disrupting the overall structure. The ability to substitute one for the other depends on the local chemical environment and the specific conditions under which the mineral formed. Sodium is generally more abundant in certain geological settings, allowing it to replace potassium in some feldspar minerals like albite (NaAlSi₃O₈), which is part of the alkali feldspar group.