The value of n in the molecular formula BenAl2Si6O18 is:

To determine the value of n in the molecular formula BenAl2Si6O18, we need to consider the charge balance. Beryllium (Be) has a +2 charge, Aluminum (Al) has a +3 charge, Silicon (Si) has a +4 charge, and Oxygen (O) has a -2 charge.

The total positive charge from the cations is: n(+2) + 2(+3) + 6(+4) = 2n + 6 + 24 = 2n + 30

The total negative charge from the anions is: 18(-2) = -36

For charge neutrality, the total positive charge must equal the total negative charge:

2n + 30 = -36

2n = -36 - 30

2n = -66

n = -33

However, a negative value for n is not physically possible. There must be an error in the provided formula or charges. Let's assume the formula is actually BenAl2Si6O18 and that the charges are correct. Then we would expect a positive value of n. Let's use a different approach. We can find the value of n by balancing the charges. Let's examine the common aluminosilicate minerals. Beryl has the formula Be3Al2Si6O18, which means n=3. This means the total charge of the cations is:
3(+2) + 2(+3) + 6(+4) = 6 + 6 + 24 = 36
The total charge of the anions is 18(-2) = -36. Therefore, the charges balance correctly.

Therefore, the value of n is 3.