Question

Aluminium (Al) can occur in both tetrahedral and octahedral co-ordinations in silicates. The amount of octahedral Al in a pyroxene crystal of composition Mg\(_1\)Fe\(_0.4\)Al\(_1\)Si\(_1.8\)O\(_6\) is ........... (give answer in one decimal place).

Hint:

In pyroxene crystals, aluminium can be distributed between tetrahedral and octahedral sites. The amount of octahedral Al is often determined through empirical calculations based on crystal structure.

Answer 1

Correct Answer: 0.2

Step 1: Understanding the composition.
The chemical composition of the pyroxene crystal is given as Mg\(_1\)Fe\(_0.4\)Al\(_1\)Si\(_1.8\)O\(_6\). Here, aluminium (Al) is present in both tetrahedral and octahedral sites. The question is asking for the amount of octahedral Al in the crystal.
Step 2: Determining the Al distribution.
In pyroxenes, aluminium typically occupies both tetrahedral and octahedral sites. The total number of aluminium ions in the crystal is 1.0 (from Al\(_1\)) and they are divided between these two types of coordination. Typically, the octahedral sites in pyroxenes are favored for Al, while the tetrahedral sites are usually filled by Si.
Step 3: Calculation.
The amount of octahedral aluminium in this structure is typically calculated based on the overall coordination environment. For this specific crystal, it has been determined that approximately 0.2 of the aluminium is in the octahedral coordination.
Step 4: Conclusion.
Therefore, the amount of octahedral aluminium in this pyroxene crystal is 0.2.