In alkali feldspar, sodium (Na) is a common element that can substitute for potassium (K). This substitution occurs due to the similarity in ionic size and valence of potassium and sodium. Both are alkali metals and have a +1 charge, allowing sodium to occupy the same crystallographic sites within the feldspar structure that potassium would normally occupy. The presence of sodium instead of potassium can lead to variations in the mineral's properties, such as its melting temperature, crystal structure, and the overall composition of the rock in which the feldspar is found.
What other common element might substitute for potassium in an alkali feldspar? Why?
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