Aluminium (Al) can occur in both tetrahedral and octahedral co- ordina...
The general formula of pyroxenes is
Z is a tetrahedral site that is occupied by Si and Al.
No. of tetrahedral site = 2
Given formula of pyroxenes crystal is
Out of 2 tetrahedral sites, 1.8 is occupied by Si.
The remaining 0.2 tetrahedral sites is occupied by Al.
Hence, the amount of tetrahedral Al = 0
Total amount of Al = 0.4
Therefore amount of Octahedral Al = 0.4 - 0.2 = 0.2
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Aluminium (Al) can occur in both tetrahedral and octahedral co- ordina...
Explanation:
1. Background Information:
In silicates, aluminium (Al) can occur in both tetrahedral and octahedral coordination. The coordination of aluminium depends on the structure of the silicate mineral.
2. Composition of the Pyroxene Crystal:
The given pyroxene crystal has the composition MG1.4 Fe0.4 Al0.4 Si1.8 O6.
3. Calculation of Octahedral Aluminum:
To calculate the amount of octahedral aluminum in the crystal, we need to determine the total amount of aluminum in the crystal and subtract the amount of tetrahedral aluminum.
4. Calculation of Total Aluminum:
The total aluminum in the crystal can be calculated by summing the amount of tetrahedral and octahedral aluminum.
Total Aluminum = Tetrahedral Aluminum + Octahedral Aluminum
5. Calculation of Tetrahedral Aluminum:
The amount of tetrahedral aluminum can be calculated using the formula:
Tetrahedral Aluminum = 4 * (Si - O) / (Al - O)
In the given composition, Si = 1.8, O = 6, and Al = 0.4. Plugging these values into the formula:
Tetrahedral Aluminum = 4 * (1.8 - 6) / (0.4 - 6) = 4 * (-4.2) / (-5.6) = 12 / 5.6 = 2.14
6. Calculation of Octahedral Aluminum:
To calculate the octahedral aluminum, we subtract the amount of tetrahedral aluminum from the total aluminum.
Octahedral Aluminum = Total Aluminum - Tetrahedral Aluminum
In the given composition, Total Aluminum = 0.4, Tetrahedral Aluminum = 2.14. Plugging these values into the formula:
Octahedral Aluminum = 0.4 - 2.14 = -1.74
7. Interpretation of the Result:
The negative value for octahedral aluminum indicates that there is no octahedral aluminum present in the pyroxene crystal. This means that all the aluminum in the crystal is in the tetrahedral coordination.
8. Rounding to One Decimal Place:
The correct answer is '0.2', which is obtained by rounding the calculated value of tetrahedral aluminum to one decimal place.