Mineral Chemistry

Garnet has 3 divalent cations in the X-site:

Given:

So Ca = 0.2 atoms per formula unit.

Mole % of grossular (Ca₃Al₂Si₃O₁₂):

Answer: 6.67

To determine the number of silicon atoms in a plagioclase of composition based on 8 oxygen atoms, we must first understand the composition structure of the minerals involved in plagioclase:

• Albite ( ): Composed of 1 Na, 1 Al, 3 Si, and 8 O.
• Anorthite ( ): Composed of 1 Ca, 2 Al, 2 Si, and 8 O.

The composition indicates that the mineral is made up of 20% albite and 80% anorthite. We now calculate the total number of silicon atoms:

• In 1 mole of Albite, there are 3 silicon atoms. Therefore, in 20% Albite: silicon atoms per 8 oxygen atoms.
• In 1 mole of Anorthite, there are 2 silicon atoms. Therefore, in 80% Anorthite: silicon atoms per 8 oxygen atoms.

Summing these, we have the total number of silicon atoms in the plagioclase mineral composition:

silicon atoms.