The general formula of an amphibole mineral is A₀₋ₓB₂C₅T₃O₂₂(OH)₂, where A, B, C, and T are cationic sites with different co-ordination numbers as stated below: A=12, B=6-8, C=6, T=4. The amount of octahedral Al in an amphibole of composition Na₀.₆Ca₂Mg₃Al₃Si₆O₂₂(OH)₂ is ......
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Correct Answer: 1.2
Solution and Explanation
Step 1: Identify the octahedral sites in the amphibole formula.
The general amphibole formula is A₀₋ₓB₂C₅T₃O₂₂(OH)₂. In this formula, A, B, and C represent cationic sites, where B is typically octahedral. The cations in the B-site are generally the ones that contribute to octahedral coordination. We are asked to determine how much Al (aluminum) occupies the octahedral site in the given amphibole composition.
The given composition is: Na₀.₆Ca₂Mg₃Al₃Si₆O₂₂(OH)₂.
- The cations in the B-site are Ca, Mg, and Al, and these occupy the octahedral positions.
- The amount of octahedral Al is simply the number of Al atoms in the B-site, which is 3.
Thus, the amount of octahedral Al in the given amphibole is 3.
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