Samples of the type-I clathrate Sr8AlxSi46-x have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group Pm (3) over barn) which has an Al-Si framework with Sr2+ guest atoms forms with a narrow composition range of 9.54(6) <= x < 10.30(8). Single crystals with composition A(8)Al(10)Si(36) (A = Sr, Bo) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at similar to 1268 and similar to 1421 K for Sr8Al10Si36 and Ba8Al10Si36, respectively. Comparison of the structures reveals a strong correlation between the 24k-24k framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A(8)Al(6)Si(40) (6c site occupied completely by Al) and A(8)Al(16)Si(30) (16i site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si Si bonds are covalent, the Al Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr8Al6Si40 phase has a very small band gap that is closed upon additional Al, as observed in Sr8Al16Si30. An explanation for the absence of a semiconducting "Sr8Al16Si30" phase is suggested in light of these findings.