One of the common rock-forming minerals is plagioclase feldspar which contains variable amounts of Na and Ca

Question:

One of the common rock-forming minerals is plagioclase feldspar which contains variable amounts of Na and Ca in addition to Al, Si, and 0. Pure Na-plagioclase has the formula NaA1Si3O3. Because the sizes of Nat+1 and Cat+2 ions are similar (see Table 2.1), Cat+2 can substitute for Na+1. Would the formula of Ca-plagioclase be CaAlSi3O8? Explain.